Energy Efficiency in Electrical Utilities

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JNU, JaipurFirst Edition 2013

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Index

ContentI. ...................................................................... II

List of FiguresII. ......................................................VIII

List of TablesIII. ........................................................... X

AbbreviationsIV. ........................................................XI

Case StudyV. ............................................................. 182

BibliographyVI. ........................................................ 185

Self Assessment AnswersVII. ................................... 189

Book at a Glance

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Contents

Chapter I ...................................................................................................................................................... 1Electrical System .......................................................................................................................................... 1Aim ................................................................................................................................................................ 1Objectives ...................................................................................................................................................... 1Learning outcome .......................................................................................................................................... 11.1 Introduction .............................................................................................................................................. 21.2 Generation, Transmission and Distribution of Electricity ....................................................................... 31.3 IE (Indian Electricity) Rules .................................................................................................................... 41.4 Important Equipments .............................................................................................................................. 41.5 Electrical Symbols and SLD .................................................................................................................... 51.6 Electricity Billing ..................................................................................................................................... 61.7 Electrical Load Management and Maximum Demand Control ............................................................... 71.8 Maximum Demand .................................................................................................................................. 71.9 Contracted Maximum Demand (CMD) ................................................................................................... 81.10 Connected Load ..................................................................................................................................... 81.11 Power Factor .......................................................................................................................................... 91.12 Selection of Power Factor Correction Capacitors .................................................................................. 91.13 Leading and Lagging Power Factor ......................................................................................................111.14 Position of Power Factor Correction Capacitors ..................................................................................111.15 Performance Assessment of Power Factor Correction Capacitors ...................................................... 121.16 Transformer .......................................................................................................................................... 131.17 Rating and Location of the Transformer .............................................................................................. 131.18 Losses and Efficiency of a Transformer .............................................................................................. 141.19 Control Used for Voltage Fluctuation .................................................................................................. 141.20 The Parallel Operation of Transformers............................................................................................... 15Summary ..................................................................................................................................................... 16References ................................................................................................................................................... 16Recommended Reading ............................................................................................................................. 16Self assessment ........................................................................................................................................... 17

Chapter II ................................................................................................................................................... 19Electric Motors ........................................................................................................................................... 19Aim .............................................................................................................................................................. 19Objectives .................................................................................................................................................... 19Learning outcome ........................................................................................................................................ 192.1 Introduction ............................................................................................................................................ 202.2 Types of Motors ..................................................................................................................................... 20 2.2.1 Direct Current Motors (DC Motors) ...................................................................................... 20 2.2.2 Synchronous Motors .............................................................................................................. 21 2.2.3 Induction Motors .................................................................................................................... 222.3 The Power Factor ................................................................................................................................... 232.4 Name Plate ............................................................................................................................................. 232.5 Motor Load ............................................................................................................................................ 242.6 Motor Efficiency and its Losses............................................................................................................. 252.7 Factors Affecting Motor Performance ................................................................................................... 262.8 Rewinding and Motor Replacement Issues ............................................................................................ 272.9 Energy Saving Opportunities with Energy Efficient Motors ................................................................. 27Summary ..................................................................................................................................................... 29References ................................................................................................................................................... 29Recommended Reading ............................................................................................................................. 30Self Assessment ........................................................................................................................................... 31

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Chapter III .................................................................................................................................................. 33Compressed Air System ............................................................................................................................. 33Aim .............................................................................................................................................................. 33Objectives .................................................................................................................................................... 33Learning outcome ........................................................................................................................................ 333.1 Introduction ............................................................................................................................................ 343.2 Category of Compressors ....................................................................................................................... 343.3 Efficiency of a Compressor.................................................................................................................... 363.4 Compressed Air System Components .................................................................................................... 373.5 Efficient Operation of Compressor ........................................................................................................ 383.6 Capacity Assessment of A Compressor .................................................................................................. 393.7 Factors Affecting Performance and Efficiency ...................................................................................... 403.8 Load Unload Versus On/Off Control ..................................................................................................... 40Summary ..................................................................................................................................................... 41References ................................................................................................................................................... 41Recommended Reading ............................................................................................................................. 41Self Assessment ........................................................................................................................................... 42

Chapter IV .................................................................................................................................................. 44HVAC and Refrigeration System.............................................................................................................. 44Aim .............................................................................................................................................................. 44Objectives .................................................................................................................................................... 44Learning outcome ........................................................................................................................................ 444.1 Introduction ............................................................................................................................................ 45 4.1.1 Air-Conditioning System ....................................................................................................... 46 4.1.2 Refrigeration Systems (for processes) ................................................................................... 46 4.1.3 Capacity Measurement .......................................................................................................... 464.2 Types of Refrigeration System .............................................................................................................. 47 4.2.1 Vapour Compression Refrigeration........................................................................................ 47 4.2.2 Alternative Refrigerants for Vapour Compression Systems .................................................. 48 4.2.3 Absorption Refrigeration ....................................................................................................... 49 4.2.4 Evaporative Cooling .............................................................................................................. 514.3 Common Refrigerants and their Properties ............................................................................................ 514.4 Types of Compressor and Their Applications ........................................................................................ 52 4.4.1 Centrifugal Compressors ....................................................................................................... 52 4.4.2 Reciprocating Compressors ................................................................................................... 53 4.4.3 Screw Compressors ................................................................................................................ 54 4.4.4 Scroll Compressors ................................................................................................................ 544.5 Selection of a Suitable Refrigeration System ........................................................................................ 554.6 Performance Assessment of Refrigeration Plants .................................................................................. 59 4.6.1 Integrated Part Load Value (IPLV) ........................................................................................ 614.7 Factors Affecting Performance and Energy Efficiency of Refrigeration Plants .................................... 61 4.7.1 The Design of Process Heat Exchangers ............................................................................... 61 4.7.2 Maintenance of Heat Exchanger Surfaces ............................................................................. 63 4.7.3 Multi-staging for Efficiency .................................................................................................. 63 4.7.4 Matching Capacity to System Load ....................................................................................... 64 4.7.5 Capacity Control and Energy Efficiency ............................................................................... 64 4.7.6 Multi-level Refrigeration for Plant Needs ............................................................................. 64 4.7.7 Chilled Water Storage ............................................................................................................ 65 4.7.8 System Design Features ......................................................................................................... 654.8 Energy Saving Opportunities ................................................................................................................. 66Summary ..................................................................................................................................................... 67References ................................................................................................................................................... 67Recommended Reading ............................................................................................................................. 67Self Assessment ........................................................................................................................................... 68

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Chapter V .................................................................................................................................................... 70Fans and Blowers ....................................................................................................................................... 70Aim .............................................................................................................................................................. 70Objectives .................................................................................................................................................... 70Learning outcome ........................................................................................................................................ 705.1 Introduction ............................................................................................................................................ 715.2 Difference Between Fans, Blowers and Compressors ........................................................................... 715.3 Types of Fans and Blowers .................................................................................................................... 72 5.3.1 Types of Fan ........................................................................................................................... 72 5.3.2 Types of Blowers ................................................................................................................... 765.4 Fan Performance Evaluation and Efficient System Operation .............................................................. 78 5.4.1 System Characteristics ........................................................................................................... 785.5 Fan Characteristics ................................................................................................................................. 785.6 System Characteristics and Fan Curves ................................................................................................. 795.7 Fan Laws ................................................................................................................................................ 805.8 Fan Design and Selection Criteria ......................................................................................................... 80 5.8.1 Fan Performance and Efficiency ............................................................................................ 81 5.8.2 Safety Margin......................................................................................................................... 82 5.8.3 Installation of the Fan ............................................................................................................ 83 5.8.4 System Resistance Change .................................................................................................... 835.9 Flow Control Strategies ......................................................................................................................... 83 5.9.1 Pulley Change ........................................................................................................................ 84 5.9.2 Damper Controls .................................................................................................................... 84 5.9.3 Variable Speed Drives ............................................................................................................ 85 5.9.4 Series and Parallel Operation ................................................................................................. 855.10 Fan Performance Assessment .............................................................................................................. 87 5.10.1 Air flow Measurement ......................................................................................................... 87 5.10.2 Measurements and Calculations .......................................................................................... 885.11 Energy Savings Opportunities .............................................................................................................. 90Summary ..................................................................................................................................................... 91References ................................................................................................................................................... 91Recommended Reading ............................................................................................................................. 92Self Assessment ........................................................................................................................................... 93

Chapter VI .................................................................................................................................................. 95Pumps and Pumping System..................................................................................................................... 95Aim .............................................................................................................................................................. 95Objectives .................................................................................................................................................... 95Learning Outcome ....................................................................................................................................... 956.1 Introduction ............................................................................................................................................ 966.2 Types of Pumps ...................................................................................................................................... 96 6.2.1 Centrifugal Pump ................................................................................................................... 97 6.2.2 Hydraulic Power, Pump Shaft Power and Electrical Input Power ......................................... 996.3 System Characteristics ........................................................................................................................... 996.4 Pump Curves ........................................................................................................................................ 102 6.4.1 Pump Operating Point .......................................................................................................... 1026.5 Factors Affecting Pump Performance .................................................................................................. 103 6.5.1 Matching Pump and System Head-flow Characteristics ..................................................... 103 6.5.2 Effect of Over Sizing the Pump ........................................................................................... 103 6.5.3 Energy Loss in Throttling .................................................................................................... 1046.6 Efficient Pumping System Operation .................................................................................................. 105 6.6.1 Effect of Speed Variation ..................................................................................................... 106 6.6.2 Effects of Impeller Diameter Change .................................................................................. 107 6.6.3 Pump Suction Performance (NPSH) .................................................................................... 1086.7 Flow Control Strategies ....................................................................................................................... 109

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6.7.1 Pump Control by Varying Speed.......................................................................................... 109 6.7.2 Pumps in Parallel Switched to Meet Demand ......................................................................111 6.7.3 Stop/Start Control .................................................................................................................113 6.7.4 Flow Control Valve ...............................................................................................................113 6.7.5 By-pass Control ....................................................................................................................1146.8 Fixed Flow Reduction ...........................................................................................................................114 6.8.1 Impeller Trimming ................................................................................................................114 6.8.2 Meeting Variable Flow Reduction ........................................................................................1166.9 Steps for Energy Efficiency in Pumping System ..................................................................................117Summary ....................................................................................................................................................118References ..................................................................................................................................................118Recommended Reading ............................................................................................................................118Self Assessment ..........................................................................................................................................119

Chapter VII .............................................................................................................................................. 121Cooling Tower ........................................................................................................................................... 121Aim ............................................................................................................................................................ 121Objectives .................................................................................................................................................. 121Learning outcome ...................................................................................................................................... 1217.1 Introduction .......................................................................................................................................... 122 7.1.1 Cooling Tower Types ........................................................................................................... 122 7.1.2 Mechanical Draft Towers ..................................................................................................... 122 7.1.3 Components of a Cooling Tower ......................................................................................... 123 7.1.4 Tower Materials ................................................................................................................... 1247.2 Cooling Tower Performance ................................................................................................................ 125 7.2.1 Factors Affecting Cooling Tower Performance ................................................................... 1267.3 A Typical Comparison Between Various Fill Media ............................................................................ 1307.4 Choosing a Cooling Tower .................................................................................................................. 1307.5 Efficient System Operation .................................................................................................................. 130 7.5.1 Cooling Water Treatment ..................................................................................................... 130 7.5.2 Drift Loss in the Cooling Towers ......................................................................................... 131 7.5.3 Cooling Tower Fans ............................................................................................................. 131 7.5.4 Performance Assessment of Cooling Towers ...................................................................... 1317.6 Flow Control Strategies ....................................................................................................................... 1337.7 Energy Saving Opportunities in Cooling Towers ................................................................................ 133Summary ................................................................................................................................................... 135References ................................................................................................................................................. 135Recommended Reading ........................................................................................................................... 135Self Assessment ......................................................................................................................................... 136

Chapter VIII ............................................................................................................................................. 138Lighting System ........................................................................................................................................ 138Aim ............................................................................................................................................................ 138Objectives .................................................................................................................................................. 138Learning outcome ...................................................................................................................................... 1388.1 Introduction .......................................................................................................................................... 1398.2 Basic Terms in Lighting Systems and Features ................................................................................... 1398.3 Lamp Types and their Features ............................................................................................................ 1408.4 Recommended Illuminance Levels for Various Tasks / Activities / Locations .................................... 1418.5 Methodology of Lighting System Energy Efficiency Study ................................................................ 1428.6 Case Examples ..................................................................................................................................... 143 8.6.1 Energy Efficient Replacement Options................................................................................ 1438.7 Some Good Practices in Lighting ........................................................................................................ 144 8.7.1 Installation of Compact Fluorescent Lamps (CFL's) in Place of Incandescent Lamps ....... 144 8.7.2 Installation of Metal Halide Lamps in Place of Mercury/Sodium Vapour Lamps .............. 144

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8.7.3 Installation of High Pressure Sodium Vapour (HPSV) Lamps for Applications where Colour Rendering is not Critical .......................................................... 144

8.7.4 Installation of LED Panel Indicator Lamps in Place of Filament Lamps ............................ 144 8.7.5 Light Distribution ................................................................................................................ 145 8.7.6 Light Control ........................................................................................................................ 145Summary ................................................................................................................................................... 147References ................................................................................................................................................. 147Recommended Reading ........................................................................................................................... 147Self Assessment ......................................................................................................................................... 148

Chapter IX ................................................................................................................................................ 150DG Set Systems ........................................................................................................................................ 150Aim ............................................................................................................................................................ 150Objectives .................................................................................................................................................. 150Learning outcome ...................................................................................................................................... 1509.1 Introduction .......................................................................................................................................... 151 9.1.1 The Four Stroke Diesel Engine ............................................................................................ 151 9.1.2 The DG Set as a System....................................................................................................... 152 9.1.3 Selection Considerations ...................................................................................................... 152 9.1.4 Diesel Engine Power Plant Developments .......................................................................... 1539.2 Selection and Installation Factors ........................................................................................................ 154 9.2.1 Sizing of a Genset ................................................................................................................ 154 9.2.2 High Speed Engine or Slow/Medium Speed Engine ........................................................... 155 9.2.3 Capacity Combinations ........................................................................................................ 155 9.2.4 Air Cooling Vs. Water Cooling ............................................................................................ 155 9.2.5 Safety Features ..................................................................................................................... 155 9.2.6 Parallel Operation with Grid ................................................................................................ 156 9.2.7 Maximum Single Load on a DG Set .................................................................................... 156 9.2.8 Unbalanced Load Effects ..................................................................................................... 156 9.2.9 Neutral Earthing ................................................................................................................... 156 9.2.10 Site Condition Effects on Performance Derating ............................................................... 1569.3 Operational Factors .............................................................................................................................. 157 9.3.1 Load Pattern and DG Set Capacity ...................................................................................... 157 9.3.2 Sequencing of Loads ............................................................................................................ 157 9.3.3 Load Pattern ......................................................................................................................... 158 9.3.4 Load Characteristics ............................................................................................................ 1589.4 Energy Performance Assessment of DG Sets ...................................................................................... 1609.5 Energy Saving Measures for DG Sets.................................................................................................. 161Summary ................................................................................................................................................... 162References ................................................................................................................................................. 162Recommended Reading ........................................................................................................................... 162Self Assessment ......................................................................................................................................... 163

Chapter X ................................................................................................................................................. 165Energy Efficient Technologies in Electrical Systems ............................................................................ 165Aim ............................................................................................................................................................ 165Objectives .................................................................................................................................................. 165Learning outcome ...................................................................................................................................... 16510.1 Maximum Demand Controllers ......................................................................................................... 16610.2 Automatic Power Factor Controllers ................................................................................................. 166 10.2.1 Voltage Control .................................................................................................................. 166 10.2.2 Kilovar Control .................................................................................................................. 166 10.2.3 Automatic Power Factor Control Relay ............................................................................. 167 10.2.4 Intelligent Power Factor Controller (IPFC) ....................................................................... 16710.3 Energy Efficient Motors ..................................................................................................................... 167

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10.3.1 The Technical Aspects of Energy Efficient Motors ........................................................... 16910.4 Soft Starter ......................................................................................................................................... 16910.5 Variable Speed Drives ........................................................................................................................ 170 10.5.1 Speed Control of Induction Motors ................................................................................... 170 10.5.2 The Variable Frequency Drive ........................................................................................... 171 10.5.3 Variable Torque Vs. Constant Torque ................................................................................ 171 10.5.4 Why Variable Torque Loads Offer Greatest Energy Savings ............................................ 171 10.5.5 Tighter Process Control with Variable Speed Drives ......................................................... 171 10.5.6 Extended Equipment Life and Reduced Maintenance ....................................................... 172 10.5.7 Eddy Current Drives .......................................................................................................... 172 10.5.8 Slip Power Recovery Systems ........................................................................................... 173 10.5.9 Fluid Coupling ................................................................................................................... 173 10.5.10 Construction ..................................................................................................................... 173 10.5.11 Operating Principle .......................................................................................................... 174 10.5.12 Characteristics .................................................................................................................. 17410.6 Energy Efficient Transformers ........................................................................................................... 17410.7 Electronic Ballast ............................................................................................................................... 175 10.7.1 Role of Ballast ................................................................................................................... 175 10.7.2 Conventional vs. Electronic Ballasts ................................................................................. 17510.8 Energy Efficient Lighting Controls .................................................................................................... 176 10.8.1 Occupancy Sensors ............................................................................................................ 176 10.8.2 Timed Based Control ......................................................................................................... 176 10.8.3 Daylight Linked Control .................................................................................................... 177 10.8.4 Localised Switching ........................................................................................................... 178Summary ................................................................................................................................................... 179References ................................................................................................................................................. 179Recommended Reading ........................................................................................................................... 179Self Assessment ......................................................................................................................................... 180

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List of Figures

Fig. 1.1 Typical electric power supply systems ............................................................................................. 3Fig. 1.2 Graphical symbols for diagrams ....................................................................................................... 5Fig. 1.3 Typical single line diagram ............................................................................................................... 6Fig. 1.4 Power factor triangle ........................................................................................................................ 9Fig. 1.5 Power factor before and after improvement ................................................................................... 10Fig. 1.6 Power distribution diagram illustrating capacitor locations ........................................................... 12Fig. 1.7 View of a transformer ..................................................................................................................... 13Fig. 1.8 Losses in a transformer ................................................................................................................... 14Fig. 2.1 Classification of the main types of electric motors ........................................................................ 20Fig. 2.2 Direct current motors (DC motors) ................................................................................................ 21Fig. 2.3 Synchronous motors ....................................................................................................................... 22Fig. 2.4 Types of induction motors. ............................................................................................................. 23Fig. 2.5 Efficiency and power factor vs. percent load ................................................................................. 27Fig. 2.6 Comparison of efficiency - high efficiency motor vs. standard motor ........................................... 28Fig. 3.1 Types of compressors ..................................................................................................................... 34Fig. 3.2 A Typical reciprocating compressor ............................................................................................... 35Fig. 3.3 A typical rotary compressor ............................................................................................................ 35Fig. 3.4 A typical compressed air system components and network ........................................................... 38Fig. 4.1 Schematic representation of refrigeration system .......................................................................... 45Fig. 4.2 Heat transfer loops in refrigeration system ..................................................................................... 46Fig. 4.3 Schematic representation of the vapour compression refrigeration cycle ...................................... 47Fig. 4.4 Schematic diagram of a basic vapour compression refrigeration system ....................................... 48Fig. 4.5 Schematic diagram absorption refrigeration system ...................................................................... 49Fig. 4.6 An Evaporative cooling unit ........................................................................................................... 51Fig. 4.7 Centrifugal compressor ................................................................................................................... 52Fig. 4.8 Schematic diagram of reciprocating compressors .......................................................................... 53Fig. 4.9 Screw compressor ........................................................................................................................... 54Fig. 4.10 Scroll compressor ......................................................................................................................... 54Fig. 4.11 Effect of evaporator temperature on chiller COP ......................................................................... 60Fig. 4.12 Effect of condensing temperature on chiller COP ........................................................................ 60Fig. 5.1 Typical fan system components ...................................................................................................... 71Fig. 5.2 Centrifugal flow fan ....................................................................................................................... 73Fig. 5.3 Axial flow fan ................................................................................................................................. 74Fig. 5.4 Types of centrifugal and axial fans ................................................................................................. 76Fig. 5.5 Centrifugal blowers ........................................................................................................................ 77Fig. 5.6 Positive-displacement blowers ....................................................................................................... 77Fig. 5.7 System characteristics ..................................................................................................................... 78Fig. 5.8 Fan characteristics curve by manufacturer ..................................................................................... 79Fig. 5.9 System curve ................................................................................................................................... 79Fig. 5.10 Speed, pressure and power of fans .............................................................................................. 80Fig. 5.11 Fan static pressure and power requirements for different fans ..................................................... 81Fig. 5.12 Fan performance characteristics based on fans/ impellers ........................................................... 81Fig. 5.13 Pulley change ................................................................................................................................ 84Fig. 5.14 Damper control ............................................................................................................................. 84Fig. 5.15 Inlet guide vanes ........................................................................................................................... 85Fig. 5.16 Series and parallel operation ......................................................................................................... 86Fig. 5.17 Comparison of various volume control methods .......................................................................... 86Fig. 5.18 Static, total and velocity pressure ................................................................................................. 87Fig. 5.19 Velocity measurement using pitot tube ......................................................................................... 88Fig. 5.20 Traverse points for circular duct ................................................................................................... 89Fig. 6.1 Different types of pumps ................................................................................................................ 97Fig. 6.2 Centrifugal pump ............................................................................................................................ 98Fig. 6.3 Pump performance curve ................................................................................................................ 98

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Fig. 6.4 Static head ....................................................................................................................................... 99Fig. 6.5 Static head vs. flow ....................................................................................................................... 100Fig. 6.6 Friction head vs. flow ................................................................................................................... 100Fig. 6.7 System with high static head ........................................................................................................ 101Fig. 6.8 System with low static head ......................................................................................................... 101Fig. 6.9 Head-flow curve ........................................................................................................................... 102Fig. 6.10 Pump operating point.................................................................................................................. 102Fig. 6.11 Typical centrifugal pump performance curve ............................................................................. 103Fig. 6.12 Effect on system curve with throttling........................................................................................ 104Fig. 6.13 Pump characteristic curves ......................................................................................................... 105Fig. 6.14 Example of speed variation effecting centrifugal pump performance ........................................ 107Fig. 6.15 Example effect of impeller diameter reduction on centrifugal pump performance .................... 108Fig. 6.16 Example of the effect of pump speed change in a system with only friction loss .......................110Fig. 6.17 Example for the effect of pump speed change for a system with high static head ......................111Fig. 6.18 Typical head-flow curves for pumps in parallel ..........................................................................112Fig. 6.19 Typical head-flow curves for pumps in parallel, with system curve illustrated ..........................112Fig. 6.20 Control of pump flow by changing system resistance using a valve ...........................................113Fig. 6.21 (a) Before impeller trimming .......................................................................................................115Fig. 6.21 (b) After impeller trimming .........................................................................................................115Fig. 6.22 Effect of VFD ..............................................................................................................................116Fig. 7.1 Cooling water system ................................................................................................................... 122Fig. 7.2 Cooling tower types ...................................................................................................................... 123Fig. 7.3 Range and approach ...................................................................................................................... 125Fig. 9.1 Schematic diagram of a four stroke diesel engine ........................................................................ 151Fig. 9.2 Diesel generator system ................................................................................................................ 152Fig. 9.3 Turbocharger ................................................................................................................................. 154Fig. 10.1 Maximum demand controller ..................................................................................................... 166Fig. 10.2 Reactive power control relay ...................................................................................................... 167Fig. 10.3 Energy efficient motors .............................................................................................................. 168Fig. 10.4 Efficiency range for standard and high efficiency motors .......................................................... 169Fig. 10.5 Soft Starter .................................................................................................................................. 170Fig. 10.6 Soft Starter: Starting current, Stress profile during starting ....................................................... 170Fig. 10.7 Eddy current drive ...................................................................................................................... 173Fig. 10.8 Fluid coupling ............................................................................................................................. 173Fig. 10.9 1600 kVA amorphous core transformer ...................................................................................... 175Fig. 10.10 Electronic ballasts ..................................................................................................................... 176Fig. 10.11 Timed-turn off switch ............................................................................................................... 177

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List of Tables

Table 1.1 List important electrical equipment ............................................................................................... 4Table 1.2 Multipliers to determine capacitor kVAr requirements for power factor correction ....................11Table 2.1 Name plate parameters of a motor ............................................................................................... 24Table 2.2 Efficiency improvement areas used in EEM (source BEE india, 2004) ...................................... 28Table 3.1 Compressor selection chart .......................................................................................................... 36Table 4.1 Typical schematic representation of the absorption refrigeration concept ................................... 50Table 4.2 Properties of commonly used refrigerants ................................................................................... 51Table 4.3 Performance of commonly used refrigerants* ............................................................................. 52Table 4.4 Comparison of different types of refrigeration plants .................................................................. 58Table 4.5 Effect of variation in evaporator temperature on compressor power consumption ..................... 62Table 4.6 Effects of variations in condenser temperature on compressor power consumption ................... 62Table 4.7 Effect of poor maintenance on compressor power consumption ................................................. 63Table 5.1 Differences between fans, blowers and compressors ................................................................... 72Table 5.2 Fan efficiencies ............................................................................................................................ 72Table 5.3 Types of centrifugal fans ............................................................................................................. 74Table 5.4 Types of axial fans........................................................................................................................ 76Table 6.1 Symptoms indicating potential opportunity for energy savings ................................................ 106Table 7.1.Approach vs. cooling tower size ................................................................................................ 128Table 7.2 Flow vs. approach for a given tower .......................................................................................... 128Table 7.3 Typical comparisons between various fill media ....................................................................... 130Table 7.4 Typical comparison of cross flow splash fill, counter flow tower with film fill and splash fill 130Table 7.5 Typical problems and trouble shooting for cooling towers problem ......................................... 134Table 8.1 Luminous performance characteristics of commonly used luminaries ...................................... 140Table 8.2 Recommended illuminance range for different tasks and activities for the chemical sector ..... 142Table 8.3 Device rating, population and use profile .................................................................................. 142Table 8.4 Lighting transformer/rating and population profile ................................................................... 142Table 8.5 Savings by use of high efficacy lamps ....................................................................................... 143Table 8.6 Saving potential by use of high efficacy lamps for street lighting ............................................. 144Table 8.7 Types of luminaire with their gear and controls used in different industrial locations .............. 146Table 9.1 Comparison of different types of captive power plants ............................................................. 153Table 9.2 Altitude and intake temperature corrections .............................................................................. 157Table 9.3 Derating due to air inter cooler water inlet temperature ............................................................ 158Table 9.4 Typical flue gas temperature and flow pattern in a 5-mw dg set at various loads ..................... 160Table 9.5 Typical format for DG set monitoring........................................................................................ 161Table 10.1 Watt loss area and efficiency improvement ............................................................................. 168

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Abbreviations

AC - Alternating CurrentACSR - Aluminium Conductor with Steel ReinforcementAHU - Air Handling UnitsANSI - American National Standards InstituteAS - Australian StandardASME - American Society of Mechanical EngineersBEE - Bureau of Energy EfficiencyBEP - Best Efficiency PointBHP - Brake HorsepowerBIS - Bureau of Indian StandardsCFC - Chlorinated FluorocarbonCFL - Compact Fluorescent LampsCMD - Contracted Maximum DemandCOC - Cycles of ConcentrationCOP - Coefficient of PerformanceDC - Direct CurrentDG - Diesel GeneratingEEM - Energy Efficient Motors FAD - Free Air DeliveryFCU - Fan Coil UnitsFD - The Forced DraftFIFO - First in First OutFILO - First in Last OutFTL - Fluorescent Tube LampsGLS - General Lighting ServiceGRP - Glass Reinforced PlasticHCFC - Hydrochloro FluorocarbonHFC - Hydro FluorocarbonHP - HorsepowerHPMV - High Pressure Mercury HPSV - High Pressure Sodium VapourHT - High TensionHVAC - Heating, Ventilation, and Air ConditioningID - Induced DraftIE - Indian ElectricityIEC - International Electro Technical CommissionIEEE - Institute of Electrical and Electronics EngineersIGBTs - Insulated Gate Bi Polar TransistorsIPFC - Intelligent Power Factor ControllerIPLV - Integrated Part Load ValueIS - Indian StandardkV - KilovoltKVA - Kilo Volts-AmpereskVAr - Kilo Volt-Amperes ReactivekW - Kilo WattsLEDs - Light Emitting DiodesLPSV - Low Pressure Sodiumlx - LuxMCC - Motor Control CentreMD - Maximum DemandMERC - Maharashtra State Electricity Regulatory Commission

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MT - Metric TonNEMA - National Electrical Manufacturers AssociationNPSH - Net Positive Suction Head NPSHA - Net Positive Suction Head AvailableNPSHR - Net Positive Suction Head RequiredOLTC - On Load Tap ChangerPA - Primary AirPCC - Power Control CentrePF - Power FactorPID - Proportional Integral Differential (PID)PLC - Programmable Logic ControllerR - ResistanceRH - Relative HumidityRI - Colour Rendering IndexRMS - Root Mean SquareRVSS - Reduced Voltage Soft Starters SLD - Single Line DiagramSP - Static PressureTOD - Time of the DayTR - Tons of RefrigerationV - VoltageVAR - Vapour Absorption RefrigerationVCR - Vapour Compression RefrigerationVFDs - Variable Frequency DrivesVSDs - Variable Speed DrivesW - WattWBT - Wet Bulb TemperatureWHR - Waster Heat Recovery

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Chapter I

Electrical System

Aim

The aim of this chapter is to:

define the concept of generation, transmission and distribution of energy•

explain the important electrical equipments•

explicate the electrical symbols and SLD•

Objectives

The objectives of this chapter are to:

explain the concept of generation, transmission and distribution of energy•

explicate important electrical equipments, electrical symbols and SLD•

elucidate the factor improvement and transformer distribution•

Learning outcome

At the end of this chapter students will be able to:

define and identify the concept of generation, transmission and distribution of energy•

understand the concept of power factor improvement and its benefit•

comprehend the importance of distri• bution and transformer losses

Energy Efficiency in Electrical Utilities

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1.1 IntroductionElectricity (from the New Latin ēlectricus, meaning "amber-like", from the Greek ήλεκτρον [(electron) meaning • amber] is a general term encompassing a variety of phenomena resulting from the presence and flow of electric charge. These include many easily recognisable phenomena, such as lightening and static electricity, but in addition, less familiar concepts, such as the electromagnetic field and electromagnetic induction.Electric Power Supply System is an aggregate of equipments used to transmit and distribute electricity from • sources to consumers. General-purpose power supply systems transmit and distribute approximately 98% of the total electricity generated. They link electric power plants with consumers of electricity within electric power systems and interconnect individual systems by overhead or cable power transmission lines. Electric power supply systems ensure reliable, centralised power supply with the required quality and excellent economy to widely dispersed locations of consumption. Some electric power supply systems are self-contained and are not connected to transmission lines, for example, the systems used in aircraft, ships, and automobiles.Electric power supply systems may be classified according to several characteristics. Depending on their • purpose, they may be classified as feeder systems:

used to transmit electric power �used to transmit distribution systems, which distribute power from central substations to consumers (urban, �industrial, agricultural, and other users)

Classification by voltage divides systems into two groups:those carrying voltages up to 1 kilovolt (kV) �those carrying voltages of more than 1 kV �

Electric power supply systems may also be classified as:by type of current (direct and alternating) �by plant location (overhead and cable) �by layout (circular and radial) �by normal operating mode (open and closed) �

In addition to transmission lines, electric power supply systems also have power substations, which are used • for the conversion and distribution of electric power and for controlling operation of the system.Electric power supply system in a country comprises of the following: •

generating units that produce electricity �high voltage transmission lines that transport electricity over long distances �distribution lines that deliver the electricity to consumers �substations that connect the pieces to each other �energy control centres to coordinate the operation of the components �

Today electricity is the most widely used form of energy mainly because;• it is easy to generate, transmit and distribute �it is easy to control �it can be easily converted to other forms of energy like mechanical motion, heating etc. with very accurate �controls required for processesit is the cleanest form of energy �

Because of these four reasons, we find that electricity is a more preferrable form of energy almost everywhere • whether it is household, commercial, shop, hospital, hotel, small or large industry or railway traction.The Figure 1.1 shows a simple electric supply system with transmission and distribution networks and linkages • from electricity sources to end-user.

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GENERATION

FuelPower Plant

Power Lines

Substation

Substation

CUSTOMERS DISTRIBUTION

TRANSMISSION

Fig. 1.1 Typical electric power supply systems

1.2 Generation, Transmission and Distribution of ElectricityThe fundamental principles of electricity generation were discovered during the 1820s and early 1830s by the British scientist, Michael Faraday. His basic method is still used; i.e. electricity is generated by the movement of a loop of wire or disc of copper between the poles of a magnet. For electric utilities, it is the first process in the delivery of electricity to consumers. The other processes, electricity transmission, distribution, and electrical power storage and recovery using pumped storage methods are normally carried out by the electrical power industry.

Electricity generationElectricity is generated in a number of ways, the most prominent of all are thermal power plants, hydroelectric power plants, nuclear power plants, etc. There are other ways of producing electricity which are called Non-Conventional Energy sources; which include windmill, solar systems, tidal energy etc. The electricity thus generated is needed to be sent to a place where it can be utilized; such a place could be a household consumer using electricity for his microwave oven or it could be an industry manufacturing cement, paper, etc. The process of sending electricity for utilization is called transmission.

Electricity transmissionThe term transmission is used for the process of transporting electricity at a very high voltage and the transmission is in bulk amount. Normally this is done at different voltages. In addition to transmission lines, electric power supply systems also have power substations, which are used for the conversion and distribution of electric power and for controlling operations of the system (raising and lowering voltages, converting three-phase alternating current to direct current and vice versa, and providing a number of outgoing lines that differ from incoming lines).Voltage is usually lowered or raised in several steps. For each step, there is a separate network of transmission lines and substations through which the electricity is fed to the network operated at the next voltage step. The resulting multistage system consists of several interconnected networks carrying different voltages. Transmission is a link between Generation and Distribution.

Electricity distributionDistribution of electricity denotes sending electricity from substations where it is transmitted and received followed by its distribution to various points of utilisation. The substation could be a small place in a village receiving electricity and then distributing it to nearby consumers or a place where bulk power is received by an industry like aluminium, steel processing, etc.

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1.3 IE (Indian Electricity) RulesIndian Electricity Rules 1956 are the guidelines for electrical standards followed in our country. They classify • various voltages into different grades. These are classified as follows:

Low: the voltage need not exceed 250V �Medium: the voltage need not exceed 650V �High: the voltage need not exceed 33000V (33KV) �Extra High: the voltage exceeds 33000V (33KV) �

It is important to know why there are different voltages for different stages. Generation at power plants is • generally at 11KV. This 11KV is then converted into 132KV, 220KV or 400 KV using various transformers. Again when power is to be utilized at the user end, the voltages are reduced to the required voltages first at 33KV, 22KV or 11 KV for distribution and then finally to 415V three phases or 230V single phase. Generation is at 11KV because that is considered optimum based on its cost. If one tries to go to a higher voltage, the cost of insulation is high and if he tries to go below, the current rating increases, thus demanding a higher size of conductor in generator winding, thereby also increasing the cost. Hence 11KV is considered the most optimum voltage level for generation.On the other hand, transmission is done at voltage levels of 132KV and above. Here the obvious advantage is • the use of air as an insulator and the negligible cost of insulation of the conductor. The conductor size reduces drastically because; current value comes down for the same power handled. Again if one tries to reduce voltage for transmission, the conductor cost will go up. Hence transmission is always preferred at extra high voltage.Distribution voltages of course again have to be brought down because the voltage has to match the voltage of • utilizing equipment like motors, furnaces, etc. Since a majority of the equipment is rated for medium and low voltages, these voltages have to be brought down.

1.4 Important Equipments

One can classify the equipment used in electrical systems into the following categories:Equipment FunctionGenerator This equipment converts mechanical energy of rotation to electrical energy.Cables These are insulated conductors used to provide a flow path to electricity.Breaker Breaker is protects electrical circuits under any abnormal conditions like faults,

accidents, etc.Transformer This equipment converts the voltage of one level to another level, also changing

the current level simultaneously. If the voltage is increased, it is called a Step Up Transformer. If the voltage is reduced, it is called a Step Down Transformer.

CT/PT CT stands for Current Transformer and PT stands for Potential Transformer. This equipment is used for measurement and protection in the electrical circuits.

Motor This equipment converts electrical energy into mechanical energy. There are various types of motors depending on the type of application. This equipment is most widely used in electrical circuits.

ACSR conductor ACSR stands for Aluminium Conductor with Steel Reinforcement. This is used as overhead conductor in transmission and distribution.

Transmission Tower This equipment is used as a mechanical supporting structure for the ACSR conductor used for the transmission of electrical energy.

Table1.1 List important electrical equipment

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1.5 Electrical Symbols and SLDElectrical engineers use standard symbols for various equipments to represent them in a drawing. The most • important drawing is called the Single Line Diagram (SLD), which represents the power flow in a system along with various protection equipment, cable sizes, and relevant details.There are several national and international standards for graphical symbols in circuit diagrams, in particular • the following ones:

International Electro technical Commission[IEC 60617] (also known as British Standard BS 3939) �American National Standards Institute[ANSI standard Y32] (also known as IEEE Std 315) �Australian Standard [AS 1102] �

The symbols are covered in IS 12032 entitled “Graphical Symbols for Diagrams in the Field of Electro • technology”. A typical page from the standard is shown below:

COAXIAL CABLE

SHIELDED WIRE

TIEPOINT

GROUND CONNECTION

CHASSIS GROUND

CONNECTOR

ILLUMINATION OR INDICATING LAMP, LETTERS ADDED WITHIN

SYMBOL DENOTE LAMP COLOUR

PUSHBUTTON INDICATING LAMP, LETTERS ADDED WITHIN HALF CIRCLE DENOTES SIDE OF

SYSTEM IN OPERATION

ELEMENT OF ANY MANUAL MECHANICALLY OPERATED

SWITCH. NORMALLY OPEN OR CLOSED AS INDICATED

CONTACTS OR MICROSWITCH OR RELAY, NORMALLY OPEN

OR CLOSED AS INDICATED

CIRCUIT BREAKER

COIL OF A SOLENOID OR RELAY

TRANSFORMER

FUSE

CAPACITOR

RESISTOR

NPN TRANSISTOR

POTENTIOMETER

PNP TRANSISTOR

DIODE

ZENER DIODE

KLIP-SEL TRANSIENT

SUPPRESSOR

TRIAC

SYNCHRO

TACHOMETER

ELECTRIC MOTOR

LOUDSPEAKER

TELEPHONE JACK

BATTERY

Fig. 1.2 Graphical symbols for diagrams

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Fig. 1.3 Typical single line diagram

1.6 Electricity BillingElectricity Billing varies from State to State. Till the enactment and implementation of the Electricity Act 2003, the State Electricity Boards had the monopoly for supplying Electricity to consumers and their own formats for electricity billing. With the implementation of this act, each state has to have a number of distribution companies.In order to bring uniformity amongst all the distribution companies, the task of Standardizing has been entrusted to the respective State Electricity Regulatory Commissions. Recently the state of Maharashtra MERC (Maharashtra State Electricity Regulatory Commission) has brought out a standard format of the bill applicable to the state. It states as follows:

The bill to the consumer shall include all charges, deposits, taxes and duties due and payable by the consumer to • the Distribution Licensee for the period billed, in accordance with the provisions of the Act. These regulations and the schedule of charges are as approved by the Commission under Regulation 18.The Distribution Licensee shall, upon request by the consumer, explain the detailed basis of computation of • the consumer's bill.Unless otherwise agreed between the distribution licensee and the consumer, the bill shall be in Marathi and / • or in the English language.The bill shall include, inter alia (among other things), the following information•

Consumer No., name and address �Name of office of the distribution licensee having jurisdiction over the supply �(i) Type of supply (i.e. single phase, three-phase, LT or HT) �

Contract demand / Sanctioned Load• Category of the consumer (i.e. domestic, commercial etc.) �Meter No �Billing period (dates to be mentioned) �Previous meter reading of the billing period / cycle with date �Present meter reading of the billing period / cycle with date �Multiplying factor of the meter �

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Number of units (e.g. Kwh, kVArh, kVAh, etc.) consumed during the billing period and, where relevant �determination of charges, during different time slots in the billing periodMaximum demands during the billing period �Average power factor during the billing period �Last six months consumption �Date of the bill and due date of payment �Billing details- Details for the current month demand and arrears shall be furnished in the bills �Security deposit details �Table showing the various components of applicable tariff �Details of subsidy, if any, under Section 65 of the Act �Mode of payment and collection facilities �Telephone number and address of the Customer Service Centre where the consumer can make a bill-related �complaintTelephone numbers and address of the Forum constituted in accordance with the Grievance Redressal �RegulationsIn case of cheques and bank drafts, the receiving authority in whose favour the amount should be drawn �

Similarly for other states, one can find out details from the websites of the respective electricity regulatory commissions.

1.7 Electrical Load Management and Maximum Demand Control

In the utilisation of electrical energy, there are three fundamental parameters which one has to understand. • They are:

Load Factor �Diversity Factor �Utilisation Factor �

Normally for low voltage consumers, only energy consumed is measured. But for a bulk electricity consumer • with a high voltage connection, the meter used is called a trivector meter. This meter measures the maximum demand (kva), energy consumed (kwh), and total apparent energy consumed (kvah). from these measurements, one can calculate the average power factor as (kWh/kVAH).Previously, these meters were electromagnetic, but with advancements in electronics, new meters have been • developed. They are called TOD (Time of the Day) meters. These meters not only record all the above mentioned quantities but they also record the time these quantities were measured.

1.8 Maximum DemandMaximum Demand is kVAH measured during a prefixed time duration of either 15 minutes or 30 minutes and then multiplied by either four or two respectively to give KVAH per hour i.e. KVA. Thus, at the end of each time cycle, the timer is reset and fresh measurement starts. Such measurement also ensures that an instantaneous load like starting of the motor does not affect the MD measurement.Normally a 15 minute timer is used for CMD exceeding 5 MVA whereas a 30 minute timer is used for CMD less than 5 MVA. This provision was applicable till recently but as the per latest regulations by MERC for the state of Maharashtra, all the consumers are covered by 30 minute timers. Similarly this provision varies from state to state.

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As example, in an industry, if the drawl over a recording cycle of 30 minutes is:2500 kVA for 4 minutes• 3600 kVA for 12 minutes• 4100 kVA for 6 minutes• 3800 kVA for 8 minutes•

The MD recorder will be computing MD as:(2500x4) +(3600 x 12) + (4100 x 6) + (3800 x 8) = 3606.7 kVA30

1.9 Contracted Maximum Demand (CMD)Contracted maximum demand (CMD) is the demand mutually agreed between the supply company and the • consumer by way of a signed contract.This demand forms the basis for working out of various capacities for the supply company and the consumer. • Thus the supply company makes arrangements to supply the required demand and expects the consumer to restrict his demand within that limit.If a consumer exceeds that demand, the supply company charges him a penalty. If the consumer draws less • than 80% of CMD the supply company charges him for 80% of CMD called Billing Demand. Thus the billing demand is the higher of the two:

80% of CMD �Actual maximum demand established by the meter �

This figure of 80% may again vary from state to state but the principle remains same.•

1.10 Connected LoadConnected load is the sum of the nameplate ratings of all the equipments utilising electricity inside the consumer • installation. Normally, when the figure is worked out, standby equipments is not considered since only one of them is running at a time. Also the figure is based on end utilising equipment and intermediate equipment like distribution transformers, motor control centres, etc. are not considered.

Average load is energy consumption recorded divided by the operating hours of the plant.• Load Factor = (Average Load)/ (Maximum Demand) always less than 1. �Diversity Factor = (Connected Load)/ (Maximum Demand) always more than 1. �Utilisation Factor=(Average Load)/(Connected Load) always less than 1. �Utilisation Factor = (Load Factor) / (Diversity Factor) �Utilisation factor can easily be derived by multiplying both the numerator and the denominator by maximum �demand.

It is important to note here that all the quantities must be worked out on the same unit basis of KW or KVA. • Example, if maximum demand is measured in KVA, then using a power factor, it should be converted to KW. Similarly in a connected load, if the rating of any particular equipment is given in KVA, then using its rated power factor it should be converted to KW.

From the above it will be amply clear that by reducing maximum demand one can save lot of money and hence • the control of maximum demand forms an essential part of the energy conservation programme. Similarly achieving a load factor as close to unity as possible ensures that demand is uniform and energy is uniformly and well utilised. This also ensures that distribution losses are reduced.

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There are various methods of controlling maximum demand. They can be:• Shift non essential loads to off peak hours: For example, if one is working in a bottling plant working in �two shifts, then loads like the water treatment plant, the effluent treatment plant can be made during the third shift alone.Better co-ordination amongst departments: For example, in a cement plant where mines work only from �sunrise to sunset, the colony water pumps, water treatment plants, etc. can be run only during sunset to sunrise.Improvement of the power factor gives a great relief to MD. This is because for the given kW, the unity �power factor records the same kVA. But a 0.5 power factor records double the kVA. Since billing is done on the basis of kVA, a better power factor helps in controlling maximum demand.

The above methods indicated are dependent on manual systems and hence are not reliable. The most modern • method is to provide an automatic maximum demand controller. This is a microprocessor - based instrument which monitors the Maximum Demand by iterative projections and cuts off automatically nonessential loads as per priorities decided earlier.

1.11 Power FactorIn case of pure resistive loads, the voltage(V),current (I), resistance (R)relations are linearly related, i.e. V = I • × R and Power (kW) = V × IActive power is measured in kW (Kilo Watts). Reactive power is measured in kVAr (Kilo Volt-Amperes • Reactive)The vector sum of the active power and reactive power make up the total (or apparent) power used. This is the • power generated by the SEBs for the user to perform a given amount of work. Total Power is measured in KVA (Kilo Volts-Amperes)Power Factor is a ratio of kW to KVA which is always less than or equal to unity. This is represented by a famous • triangular relation as shown below:

Fig.1.4 Power factor triangle

The active power (shaft power required or true power required) in kW and the reactive power required (kVAr) • are 900 apart vectorically in a pure inductive circuit i.e., reactive power kVAr lagging the active kW. The vector sum of the two is called the apparent power or kVA, as illustrated above and the kVA reflects the actual electrical load on distribution system.

1.12 Selection of Power Factor Correction CapacitorsFrom the triangle of KW, KVA and kVAr, it is seen that capacitor rating in kVAr should be equal to the kVArR • causing the power factor to deviate from Unity. This will ensure that the power factor is properly compensated. But in practice where automatic power factor correction is used extensively now-a-days, these are selected in steps of eight or ten and brought in the circuit as and when required through cutting in and cutting out devices.

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Example• A chemical industry had installed a 1500 kVA transformer. The initial demand of the plant was 1160 kVA with power factor of 0.70. The % loading of transformer was about 78% (1160/1500 = 77.3%). To improve the power factor and to avoid the penalty, the unit had added about 410 kVAr in motor load end. This improved the power factor to 0.89, and reduced the required kVA to 913, which is the vector sum of kW and kVAr (see Fig. 1.5).

Fig. 1.5 Power factor before and after improvement

Figure 1.5 Effect of power factor improvement for a typical plant where active load is 812 kW and the power factor is improved from 0.7 to 0.89 by the addition of the 418 kVAr capacitor, the obvious advantage is reduction in the kVA demand from 1160 kVA to 913 kVA.

After improvement the plant had avoided penalty and the 1500 kVA transformer now loaded only to 60% of • capacity. This will allow the addition of more load in the future to be supplied by the transformer.If the power factor of an installation is improved from PF• 1 to PF2, percentage distribution losses will come down as shown by the formula:

[1 – (PF1/PF2)2] X 100Direct relation for capacitor sizing;•

kVAr Rating = kW [tan φ1 – tan φ2] where,kVAr rating is the size of the capacitor needed, �kW is the average power drawn, �tan φ1 is the trigonometric ratio for the present power factor, and �tan φ2 is the trigonometric ratio for the desired PF. �φ1 = Existing (Cos-1 PF � 1) and φ2 = Improved (Cos-1 PF � 2)

Alternatively the Table 1.2 can be used for capacitor sizing.• The figures given in table are the multiplication factors which are to be multiplied with the input power (kW) • to give the kVAr of capacitance required to improve present power factor to a new desired power factor.Example: The utility bill shows an average power factor of 0.72 with an average KW of 627. How much kVAr • is required to improve the power factor to 0.95?

Using formula; Cos Φ1 = 0.72, tan Φ1 = 0.963 Cos Φ2 = 0.95, tan Φ2 = 0.329 kVAr required = P (tanφ1 - tanφ2) = 627 (0.964 – 0.329) = 398 kVAr Using table (see Table 1.2);

Locate 0.72 (original power factor) in column (1). �Read across desired power factor to 0.95 column. We find 0.635 multiplier �Multiply 627 (average kW) by 0.635 = 398 kVAr. �Install 400 kVAr to improve power factor to 95%. �

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The advantages of PF improvement by capacitor addition are as follows:• Reactive component of the network is reduced and so also the total current in the system from the source �end.I2R power losses are reduced in the system because of reduction in current. �Voltage level at the load end is increased. �kVA loading on the source generators as also on the transformers and lines upto the capacitors reduces giving �capacity relief. A high power factor can help in utilising the full capacity of your electrical system.

Table 1.2 Multipliers to determine capacitor kVAr requirements for power factor correction

1.13 Leading and Lagging Power Factor

Leading Power Factor Lagging Power FactorWhen the Current in an AC Circuit is leading the voltage in waveform, the power factor is called the leading power factor.

When the Current in an AC Circuit is lagging the voltage in waveform, the power factor is called the lagging power factor.

The leading power factor is caused when the net load is capacitive in nature.

The lagging power factor is caused when the net load is inductive in nature.

The inductive and capacitive loads cancel each other while responding to an electric supply. Since a majority • of the loads by nature are inductive, the best way to improve the power factor is to add capacitors. The obvious advantages of improving the power factor are: •

Reduction in distribution losses. �Improvement in voltage. �

1.14 Position of Power Factor Correction CapacitorsThe ideal location for capacitors is to provide them as close to the point of utilisation as possible.•

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Locations C1A, C1B and C1C of Figure 1.6 indicate three different arrangements at the load. Note that in all • three locations extra switches are not required, since the capacitor is either switched with the motor starter or the breaker before the starter. Case C1A is recommended for new installation, since the maximum benefit is derived and the size of the motor thermal protector is reduced. In Case C1B, as in Case C1A, the capacitor is energized only when the motor is in operation. Case C1B is recommended in cases where the installation already exists and the thermal protector does not need to be re-sized. In position C1C, the capacitor is permanently connected to the circuit but does not require a separate switch, since capacitor can be disconnected by the breaker before the starter.

Fig. 1.6 Power distribution diagram illustrating capacitor locations

But there are certain practical difficulties.• Introduction of capacitors demands a cutting out device which will disconnect them from the live circuit as �soon as the main equipment is switched off. In short, if one is using a power factor correction capacitor for a furnace, as soon as the furnace is switched off, the capacitor also should be switched off.If not done, this may result in either the power factor going to the leading side but less than unity again. �Moreover it may also result in voltage surges which may damage the installation.Hence installing capacitors for each individual equipment becomes expensive and may work out to be �uneconomical.

On the other hand, installing capacitors right at the receiving point will benefit only in reducing Maximum • Demand and the distribution losses down the line will not be reduced. Hence, one has to judiciously work out a cost benefit analysis for each location of the capacitors and take an appropriate decision.

1.15 Performance Assessment of Power Factor Correction CapacitorsOnce the power factor capacitors are installed, they continuously need to be monitored for their performance. • Their performance depends on voltage as well as ambient temperature. They too have an internal loss called Tan Delta loss.The capacitor before failing totally, gives a number of indications showing the deterioration of their performance. • This can be monitored by recording the daily reading or hourly reading of the consumption and power factor by the user. However specialised testing can be done by the manufacturer to know the exact reason for the failure.

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1.16 TransformerThe transformer is one of the most widely used electrical equipment. The main function of the transformer is to • either increase voltage or to reduce voltage. The Indian Standard IS 2026 (part-1) of 1997 covers transformers in detail. There are a few parameters of a transformer which need to be understood. The voltage ratio is the ratio of Output Voltage/Input Voltage. This is also called Secondary Voltage/Primary Voltage. When voltage is reduced, the current output increases with the same ratio. Alternatively when voltage is increased, the current output decreases with the same ratio.A typical transformer is shown below.•

Fig. 1.7 View of a transformer

A transformer requires cooling which is either by using oil or air. The following types of cooling are generally • indicated on transformer nameplates.

ONAN: Means oil is cooled by natural convection and air cooling is also cooled by natural convection. �ONAF: Means oil is cooled by natural convection and air cooling is by forced circulation. �OFAF : Means both oil and air cooling are by forced circulation. �

Transformer performance depends on losses it suffers during no load operation and full load operation.• Transformers are classified in two categories:• Power transformers are used in the transmission network for higher voltages, deployed for step-up and step-• down transformer applications (400 kV, 200 kV, 110 kV, 66 kV, 33kV).

Distribution transformers are used for lower voltage distribution networks as a means to end user connectivity �(11kV, 6.6 kV, 3.3 kV, 440V, 230V).

1.17 Rating and Location of the TransformerRating of the transformer:Rating of the transformer is calculated based on the connected load and applying the diversity factor on the connected load, applicable to the particular industry and arriving at the kVA rating of the Transformer. The diversity factor is defined as the ratio of overall maximum demand of the plant to the sum of individual maximum demand of various equipments.

The diversity factor varies from industry to industry and depends on various factors such as individual loads, • load factor and future expansion needs of the plant. The diversity factor should always be less than one.

Location of the Transformer:Location of the transformer is very important as far as distribution losses are concerned. A transformer receives HT voltage from the grid and steps it down to the required voltage. Transformers should be placed close to the load centre, considering other features like optimisation needs for centralised control, operational flexibility, etc. This brings down distribution losses in cables.

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1.18 Losses and Efficiency of a TransformerThe efficiency varies anywhere between 96 to 99 percent. The efficiency of transformers not only depends on • the design, but also, on the effective operating load.Transformer losses consist of two parts:•

No-load loss �Load loss �

No-load loss (also called core loss) is the power consumed to sustain the magnetic field in the transformer's steel • core. Core loss occurs whenever the transformer is energised; core loss does not vary with load. Core losses are caused by two factors: hysteresis and eddy current losses. Hysteresis loss is that energy lost by reversing the magnetic field in the core as the magnetising AC rises and falls and reverses direction. Eddy current loss is a result of induced currents circulating in the core. Load loss (also called copper loss) is associated with a full-load current flow in the transformer windings. Copper • loss is power lost in the primary and secondary windings of a transformer due to the ohmic resistance of the windings. Copper loss varies with the square of the load current (P=I2R).Transformer losses as a percentage of load are given below:•

Fig. 1.8 Losses in a transformer

For a given transformer, the manufacturer can supply values for no-load loss, PNO-LOAD, and load loss, • PLOAD. The total transformer loss, P TOTAL , at any load level can then be calculated from:PTOTAL = P NO-LOAD + (%Load/100)2 x PLOAD• Where transformer loading is known, the actual transformer loss at a given load can be computed as;• No Load Loss + kVALoad/Rated kVA)2 x (Full Load Loss)•

1.19 Control Used for Voltage FluctuationThe control of voltage in a transformer is important due to frequent changes in the supply voltage level. • Whenever the supply voltage is less than the optimal value, there is a chance of nuisance tripping of voltage sensitive devices. Voltage regulation in transformers is done by altering the voltage transformation ratio with the help of tapping.

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There are two types of tap changing facility available:• the Off-circuit tap changer �the On-load tap changer �

The Off-circuit tap changer:• It is a device fitted in the transformer, used to vary the voltage transformation ratio. Here the voltage levels can be varied only after isolating the primary voltage of the transformer.

The On-load tap changer (OLTC)• Voltage levels can be varied without isolating the connected load to the transformer. To minimise the �magnetisation losses and to reduce the nuisance tripping of the plant, the main transformer (the transformer that receives supply from the grid) should be provided with an On-load Tap Changing facility at the design stage.

Down stream distribution transformers can be provided with an off-circuit tap changer.• The On-load gear can be put on the auto mode or the manual, depending on the requirement. The OLTC can be • arranged for transformers of size 250 kVA onwards. However, the necessity for an OLTC below 1000 kVA can be considered after calculating the cost economics.

1.20 The Parallel Operation of TransformersThe design of the Power Control Centre (PCC) and the Motor Control Centre (MCC) of any new plant should • have the provision for operating two or more transformers in parallel. Additional switchgears and bus couplers should be provided at the design stage.Whenever two transformers are operating in parallel, both should be technically identical in all aspects and • more importantly should have the same impedance level. This minimises the circulating current between transformers.Where the load is fluctuating in nature, it is preferable to have more than one transformer running in parallel, • so that the load can be optimised by sharing it between transformers. The transformers can be operated close to the maximum efficiency range by this operation.

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SummaryElectricity is generated from various sources such as thermal power stations, hydroelectric power stations, nuclear • power stations and renewable sources. The performance of a Thermal Power station is measured in terms of the specific heat ratio kCal per kWH generated. Lower the ratio better is the performance of the plant.Electricity is the most popular form of energy because it can be easily transmitted, controlled, measured and • utilised. It is also the cleanest form of energy.The different equipment used in electricity generation are turbines, alternators, etc. The transmission equipment • used is Step up Transformers, Transmission Towers, Overhead Conductors, Insulators, Circuit Breakers and other protective equipment, etc. Distribution equipment includes Distribution Transformers, Circuit breakers, Overhead wires and Cables, etc.Electrical engineers use a graphical representation to express power flow and various equipments used to indicate • the complete electrical installation. This is represented by one line though most of the circuits are three phase. These diagrams are called single line diagrams.The power factor represents the utilisation rate of power transferred from one place to the other. The power • transferred is measured in terms of kVA whereas utilisation is measured in terms of kW. The ratio of kW to kVA is given as power factor. Most of the transmission equipments such as transformer, circuit breakers are designated by the kVA capacity whereas utilisation equipment such as motors, etc. is designated by kW. The unity power factor represents that utilization is at the maximum with its best performance and hence the power factor should be as close to unity as possible.Capacitors improve the power factor, but their location has to be very judicious. The transformer is electrical • equipment which helps to either increase voltage or to reduce voltage. The Indian Standard IS 2026 (part-1) of 1997 covers transformers in detailThe voltage control in a transformer is important due to frequent changes in the supply voltage level. Voltage • regulation in transformers is done by altering the voltage transformation ratio with the help of tapping.The last part of the chapter talks about the parallel operation of transformers. Whenever two transformers are • operating in parallel, both should be technically identical in all aspects and more importantly should have the same impedance level. This minimises the circulating current between transformers.

ReferencesDunlop, C., 2003. • Electrical Systems, Dearborn Financial Publishing, Inc.Giridharan, M.K., • Electrical Systems Design:Data Handbook, I.K.International Pvt. Ltd.Electrical System • [Pdf] Available at: <http://www.enercon.gov.pk/images/pdf/3ch1.pdf> [Accessed 5 July 2013].Introduction to Electrical Design Systems• [Pdf] Available at: < http://www.ecs.umass.edu/ece/hollot/ECE497DS06/ESD_1.pdf> [Accessed 5 July 2013].2012.Electrical Systems-Part 1• [Video online] Available at: <https://www.youtube.com/watch?v=tUul6kB9slo> [Accessed 5 July 2013].2011.Electrical Systems• [Video online] Available at: < https://www.youtube.com/watch?v=ffP8t7F3l_I> [Accessed 5 July 2013].

Recommended ReadingChapman, S., 2001. • Electric Machinery and Power System Fundamentals, 1st ed., McGraw-Hill Science/Engineering/Math Publication.Rustebakke, H. M., 1983. • Electric Utility Systems and Practices, 4rth ed., Wiley-Interscience Publication.Casazza, J. & Delea. F.,2003. • Understanding electric power systems an overview of the technology and the marketplace. (Volume 13). Wiley-IEEE Publication.

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Self assessment

_______________is the sum of the nameplate ratings of all the equipments utilising electricity inside the 1. consumer installation

Associated loada. Coupled loadb. Connected loadc. Reconnected loadd.

Electrical engineers use a graphical representation to express power flow and various equipment used to indicate 2. the complete electrical installation. This is represented by one line though most of the circuits are three phase. These diagrams are called ___________.

block diagramsa. flat diagramsb. bold line diagramsc. single line diagramsd.

Which of the following sentences is false?3. Capacitors improve the power factor, but their location has to be very judicious.a. Capacitors degenerates the power factor, but their location has to be very judicious.b. Capacitors deteriorates the power factor, but their location has to be very reckless.c. Capacitors worsen the power factor, but their location has to be very inattentive.d.

The electrical equipment which helps to either increase voltage or to reduce voltage is called ________.4. a capacitora. a transformer b. an alarmc. a meterd.

The maximum demand of an industry, if trivector motor records 3600 kVA for 15 minutes and 3000 kVA for 5. next 15 minutes over a recording cycle of 30 minutes is__________.

3600 kVAa. 3000 kVAb. 3300 kVAc. 600 kVAd.

___________is a term used for sending electricity from substations where it is transmitted and is received and 6. then distributed to various points of utilisation.

Distributiona. Generationb. Transmissionc. Productiond.

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The demand mutually agreed between the supply company and the consumer by way of a signed contract is 7. called ____________.

Contracted Minimum Demand (CMD)a. Contracted Maximum Demand (CMD)b. Convention Maximum Demand (CMD)c. Convention Minimum Demand (CMD)d.

Power Factor is a ratio of _________.8. reactive power to total powera. active power to total powerb. total power to reactive powerc. active power to total powerd.

Match the following9.

1. Leading power A. The net load is inductive in nature

2. Power factor B. The net load is capacitive in nature

3. Transformer C. Always less than or equal to unity

4. Lagging power D. Used to either increase voltage or to reduce voltage

1-A, 2-B, 3-C, 4-Da. 1-C, 2-D, 3-A, 4-Bb. 1-B, 2-C, 3-D, 4-Ac. 1-D, 2-A, 3-B, 4-Cd.

The equipment used as a mechanical supporting structure for the ACSR conductor used for the transmission of 10. electrical energy is known as _________.

transmission towera. CT/PTb. transformerc. breakerd.

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Chapter II

Electric Motors

Aim

The aim of this chapter :

enlist the types of electric motors•

explain the losses in induction motor•

explicate motor efficiency•

Objectives

The objectives of the chapter are:

explain the losses in motor efficiency•

enlist the factors affecting motor performance, rewinding and motor replacement issues•

explicate energy saving opportunities with energy efficient motors•

Learning outcome

At the end of this chapter, you will be able to:

define the types of electric motors•

understand the concept of rewinding and motor replacement issues•

comprehend the importance of energy saving opportunities with energy efficient m• otors

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2.1 IntroductionMotors convert electrical energy into mechanical energy by the interaction between the magnetic fields set up in the stator and rotor windings. Electric motors are undoubtedly the largest prime movers used. Industrial electric motors can be broadly classified as:

induction motors• direct current motors• synchronous motors•

All motor types have the same four operating components which are:stator (stationary windings)• rotor (rotating windings)• bearings• frame (enclosure)•

2.2 Types of MotorsThere are various types of motors depending on types of construction. Due to difference in construction and their design their applications also vary from process to process. There is a continuous technological development in the motor technology and hence many types get outdated too.As we have seen there are two types of electric supply. Similarly to suit each of them there are two types of motors:

DC Motors• AC Motors•

Industrial electric motors are generally classified as:direct current motors (DC motors)• synchronous motors (AC motors)• induction motors (induced magnetic field)•

Electric Motors

Alternating Current(AC) Motors

Synchronous

Single-Phase Three-Phase Series Compound Shunt

Self Excited

SeparatelyExcited

Induction

Direct Current(DC) Motors

Fig. 2.1 Classification of the main types of electric motors

2.2.1 Direct Current Motors (DC Motors)DC motors were the first to be developed. They have a stationary field winding housed in a stator and rotating armature called rotor. The rotor winding rotate in front of alternating rotor poles, north and south poles. This tries to reverse the current in the winding. To overcome this problem, a commutator and brush arrangement is provided.Depending on the field and armature connections, the motors are classified into different categories:

Separately Excited Motor• Self excited motor•

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Series Motor �Compound motor �Shunt Motor �

Since these motors give different torque and speed characteristics, they are used for different applications. Example, series motors are invariably used in DC traction systems. Separately excited motors are used for applications where a high starting torque is required and also where jamming is likely to take place and sudden torque requirement is likely to go up to 250% of full load torque. The speed control of DC Motors is quite linear and accurate. The torque offered is uniform over the entire speed range. Because of these characteristics, they are still preferred in some of the applications.

The main advantage of DC motors is speed control, which does not affect the quality of power supply. It can be controlled by adjusting:

The armature voltage – increasing the armature voltage will increase the speed �The field current – reducing the field current will increase the speed �

The disadvantages of these motors are mainly their initial cost and maintenance of brushes and the commutator. Moreover they require a separate cooling arrangement.

BatteryS

Rotor

SPLIT RING COMMUTATOR

BATTERY ARMATURE

BRUSH

S

N

S

N N

Brush

Commutator

Fig. 2.2 Direct current motors (DC motors)

2.2.2 Synchronous Motors

In these motors, the stator is given a three phase A.C. supply. The Rotor is given a DC supply through brushes • and slip rings. The stator produces a rotating magnetic field and the rotor field is locked into the synchronism of the rotating magnetic field.The speed of rotation is equal to:•

Synchronous Speed (RPM) = (120 x Frequency) ÷ No. of polesThe synchronous motor is not a self-starting motor. It has to be prepared to a speed near the synchronous • speed so that the rotor gets locked with the stator rotator magnetic field. At one point of time in the history of technology, this motor was quite popular.The main advantage of this motor was that by controlling the rotor excitation, the power factor of the motor • could be controlled and the motor could be made to operate with a leading power factor. Hence in the olden days when a combination of the induction motor and the static capacitor was not available, one big synchronous motor was provided to control the power factor of the entire electrical installation. They were even referred to as synchronous condensers rather than motors.The evident disadvantages of this motor were:•

the slip ring and brush maintenance and provision for the prime mover �the initial cost of these motors also was high. But these days nobody opts to use these motors. �

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Field

AC in

Single-phase synchronous converter

DC out

Slip Rings

N

Commutator Brushes

S

Fig.2.3 Synchronous motors

2.2.3 Induction Motors

Induction motors are the most commonly used prime mover for various equipments in industrial applications. • In induction motors, the induced magnetic field of the stator winding induces a current in the rotor. This induced rotor current produces a second magnetic field, which tries to oppose the stator magnetic field, and this causes the rotor to rotate.There are two types of Induction Motors.•

the Slip Ring Induction Motor �the Squirrel Cage Induction Motor �

The slip ring induction motorIn this motor, the stator is given a three phase A.C. supply. The rotor is shorted outside through slip rings and • brushes. Since the rotor connection is brought outside, an initial resistance for the higher starting torque can be introduced and as the motor picks up speed, the resistance is slowly reduced and eventually shorted to make it zero.This arrangement ensures that a starting torque of as much as 250% of full load torque can be obtained. This • type of an arrangement is very common for loads like ball mill, hammer mills, etc.The disadvantage of this motor is the slip ring and brush maintenance.•

The squirrel cage induction motorA squirrel cage rotor is the rotating part used in the most common form of AC induction motor. An electric motor • with a squirrel cage rotor is termed a squirrel cage motor. By far, this type of motor has the largest population in industry.In overall shape, it is a cylinder mounted on a shaft. Internally it contains longitudinal conductive bars (usually • made of aluminium or copper) set into grooves and connected together at both ends by shorting rings forming a cage-like shape. The name is derived from the similarity between this rings-and-bars winding and a squirrel cage.In this motor the stator is given a three phase A.C. supply. The rotor is shorted inside through copper rings at • either ends. When the stator is given a supply, it produces a rotating magnetic field which rotates at synchronous speed. The rotor then starts rotating; trying to achieve the synchronous speed but it can never achieve it. The difference between actual speed and the synchronous speed is called slip. The slip, when represented in terms of percentage with respect to synchronous speed is called percent slip.

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The formula is as follows:

Slip (%) = Synchronous speed - Full load rated speed × 100Synchronous speed

From the above points, one must remember the following facts:the number of poles available in a machine will be 2,4,6,8, and so on.• since the Indian Standard for frequency is 50 Hz, the synchronous speeds available are 3000, 1500, 1000,750 • RPM.if a 4 Pole Induction Motor is running at 1470 RPM, the slip will be 30 and percent slip will be 2%.• the advantages of these motors are simple construction, wide variety in application, practically no • maintenance.

Squirrel Cage Induction motor

Slip Ring induction motors

Squirrel Cage Induction motor

Fig.2.4 Types of induction motors.Squirrel cage induction; (b) Slip ring induction motor•

2.3 The Power FactorThe power factor of the motor is given as:•

Power factor = Cosφ = kW/kVAAs the load on the motor comes down, the magnitude of the active current reduces. However, there is no • corresponding reduction in the magnetizing current, which is proportional to supply voltage with the result that the motor power factor reduces, with a reduction in applied load. Induction motors, especially those operating below their rated capacity, are the main reason for low power factor in electric systems.

2.4 Name PlateEvery motor leaves the factory with a specific nameplate. The nameplate contains information about the • motor.

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The parameters mentioned which are useful to understand the performance of the motor are:• rated voltage �rated current �rated power output �rated speed �number of poles �connection �rated power factor �rated efficiency �

Since each parameter is measured at a different point, one has to understand the nameplate well. The following • Table 2.1 will be quite helpful.

Parameter PointRated Voltage Stator Input SupplyRated Current Stator Input Supply Current. This will occur when the motor is loaded fully to its rated

output and rated supply voltage is appliedRated Power Output This is shaft power. This will occur when the full mechanical load is applied to the

shaftRated Speed This is shaft speed and will occur when the motor is fully loaded.Number of Poles This will decide synchronous speedConnection Star or Delta. This will decide the stator winding connections.Rated Power Factor This will occur when the motor is loaded fully and will occur at the stator inputRated Efficiency This will occur when the motor is loaded fully and will indicate Mechanical Output

Power at Shaft / Input Electrical Power

Table 2.1 Name plate parameters of a motor

Since it is not possible to load every motor at all times to operate at its rated capacity, most of the motors operate at • partial loading and hence it becomes necessary to study motor performances when they are partially loaded.It is at this stage of partial load that the performance of the induction motor is highly unpredictable and needs • proper analysis to come to a definite conclusion so that energy can be saved.

2.5 Motor LoadBecause the efficiency of a motor is difficult to assess under normal operating conditions, the motor load can • be measured as an indicator of the motor’s efficiency. As loading increases, the power factor and the motor efficiency increase to an optimum value at around full load.

The following equation is used to determine the load:

Load = Pi x η/HP x 0.7457Where,

η = Motor operating efficiency in %• HP = Nameplate rated horse power• Load = Output power as a % of rated power• Pi = Three phase power in kW•

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The load is measured in three steps:Step 1. Determine the input power using the following equation:•

Pi = V x I x PF x √31000

Where,Pi = Three phase power in kW �V = RMS (root mean square) voltage, mean line to line of 3 phases �I = RMS current, mean of 3 phases �PF = Power factor as a decimal �

It is noted that power analysers give the power value directly. Industries that do not have a power analyser can use multi-meters or tong-testers to measure voltage, current and power factor separately to calculate the input power.

Step 2. Determine the rated power by taking the nameplate value or by using the following equation:• Pr = (hp×0.7457)/nr

Where, �Pr = Input power at full-rated load in kW �HP = Nameplate rated horse power �ηr = Efficiency at full-rated load (nameplate value or from motor efficiency tables) �

Step 3. Determine the percentage load using the following equation:• Load = (Pi /Pr)×100%

Where,Load = Output power as a % of rated power �Pi = Measured three phase power in kW �Pr = Input power at full-rated load in kW �

2.6 Motor Efficiency and its LossesElectrical Motor Efficiency when Shaft Output is measured in Watt is given as; •

If power output is measured in Watt (W), efficiency can be expressed as:ηm = Pout / PinWhere

ηm = motor efficiency �Pout = shaft power out (Watt, W) �Pin = electric power in to the motor (Watt, W) �

Thus, if one can concentrate on assessing losses and trying to reduce them, then too efficiency can be increased. • Hence it is pertinent to study what losses do occur in a motor.The losses can be generally classified into two categories:•

Fixed losses �Variable losses �

Fixed losses are those which occur in the motor irrespective of the quantum of load. These are also called no • load losses.Variable losses are those which are dependent on the quantum of the load. Since the current drawn by the motor • is a function of the load, both stator and rotor copper losses are variable losses. They occur by way of heat loss equal to I2R where I represent the current and R represents the resistance. Hence they are dependent on stator and rotor resistance. The value of stator resistance in turn is dependent on temperature.

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The variation of resistance with respect to temperature is governed by the relation:•

Where,t1 = ambient temperature in oCt2 =operating temperature in oC

The ambient temperature can be measured by an accurate multimeter or Wheatstone bridge. There are two • important tests carried out on the motor, based on which its performance at partial load can be calculated with the help of a circle diagram.No Load Test : This test gives the fixed losses of the motor. The motor is run in a decoupled condition by applying • the rated voltage and parameters like current and wattage are measured.Blocked Rotor Test:•

For this test, the rotor is blocked and with reduced voltage a full load rated current is circulated through the �stator. At this time, voltage applied is measured along with the power drawn i.e. wattage. These parameters are extrapolated for full load voltage and based on the extrapolated parameters; a diagram is drawn called the Circle Diagram.Once the circle diagram is drawn, the performance of the motor at any partial load can be assessed. But this �method is used in the design office or at the test bed and is rarely used at site.The mechanical losses such as friction and windage losses are fixed losses dependent on bearings and �aerodynamic losses associated with the ventilation fan and other rotating parts.Stray load losses are difficult to measure and are taken as a thumb rule based on certain standards. �

2.7 Factors Affecting Motor PerformanceMotor performance depends on:•

Partial load operation �Correct Application of the drive to suit the application �Application of rated voltage and frequency �

The performance of an induction motor on partial load can be seen from the following graph. It will be observed • that the best performance of the motor is nearer the rated output. The power factor is also improved as the load approaches the rated load. Hence the performance of the motor depends very much on the load on the motor. Thus loading a 10 HP motor with 9 HP load is far superior than loading a 15 HP motor with 9HP load. In that case, it is worthwhile to replace a 15 HP motor with a 10 HP Motor. The economics of replacement will • always be in favour of replacement which will ensure near rated load operations.

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Fig.2.5 Efficiency and power factor vs. percent load

2.8 Rewinding and Motor Replacement IssuesIt is a general exercise in industry to rewind burnt-out motors. Many a times during the maintenance of motors, • one has to encounter rewinding of motors. The rewound motors seldom give the same performance as original motors. This is mainly because:

The gauge of the wire used for rewinding is rarely the same �The characteristics do not remain the same after rewinding �No load motor and full load current also undergo change �

Due to this every time a motor is rewound, its useful life is affected and the next rewinding is advanced. Hence • a stage comes when replacement is more economical that rewinding. Hence motors rewound must be checked thoroughly before loading them to the original loading pattern.One standard method is to measure the stator winding resistance before and after rewinding. It is also a good • practice to keep a record of the no. load current prior to and after rewinding. These two parameters do speak about the condition of the rewound motor. Infact in many factories, the payment of the rewinding contractor is linked with these two parameters and the motor is rejected after rewinding if the no load current exceeds the predefined value.

2.9 Energy Saving Opportunities with Energy Efficient MotorsEnergy-efficient motors (EEM) are the ones in which, design improvements are incorporated specifically to • increase operating efficiency over motors of standard design. It is important to know what an EEM is, as it takes care of the shortfall in efficiency. Energy Efficient Motors were defined by National Electrical manufacturers association (NEMA) in the U.S.A, first under the Energy Policy Act and then the trend was followed by other countries.Bureau of Indian Standards (BIS) defines EEM as those which operate without loss of efficiency from 75% load • to 100% load. Moreover the mounting and overall dimensions are designed as per IS 1231, making it easier to replace the standard performance motors with EEM.While selecting a motor for any application, an engineer should keep a margin of about 20% in motor rating. • EEM automatically takes care of this shortfall in efficiency. Many a times economics has proved that even an existing running standard performance motor can be replaced with an EEM with a payback period of as low as 18 months.

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The figure below shows the comparison of EE and SP motors:•

EFFICIENCY/POWER FACTOR vs LOAD(Typical 3-Phase Induction motor)

Fig.2.6 Comparison of efficiency - high efficiency motor vs. standard motor

It is important to know how EEM are different from SP motors. As we have seen, efficiency can be improved • by reducing standard losses. Thus EEM are designed so that these losses are reduced.Table 2.2 describes the improvement opportunities that are often used in the design of energy efficient • motors.

Power Loss Area Efficiency ImprovementStator and Rotor Copper Losses

These losses are reduced by using copper instead of aluminium wherever possible.

Magnetic Losses of the Stator are reduced by reducing the air gapCore losses Reduced by using thinner material. Also low loss silicon steel is adopted.Friction and Windage losses Reduced by using better bearings and better cooling fans.Stray losses Reduced by better designing geometry of the motor

Table 2.2 Efficiency improvement areas used in EEM (source BEE india, 2004)

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SummaryMotors convert electrical energy into mechanical energy by the interaction between the magnetic fields set up • in the stator and rotor windings. DC Motors were the first to be developed. They have a stationary field winding housed in a stator and rotating • armature called rotor. DC motors are classified into different categories: Separately Excited Motor and Self excited motor. Self excited • motors are of three types.The synchronous motor is not a self-starting motor. It has to be prepared to a speed near the synchronous • speed so that the rotor gets locked with the stator rotator magnetic field. At one point of time in the history of technology, this motor was quite popular.Induction motors are the most commonly used prime mover for various acquirements in industrial • applications.The Slip Ring Induction motor, the stator is given a three phase A.C. supply. The rotor is shorted outside through • slip rings and brushes. A squirrel cage rotor is the rotating part used in the most common form of AC induction motor. An electric motor with a squirrel cage rotor is termed a squirrel cage motor. By far, this type of motor has the largest population in industry.The difference between actual speed and the synchronous speed is called slip. The slip, when represented in • terms of percentage with respect to synchronous speed is called percent slip.Every motor leaves the factory with a specific nameplate. The nameplate contains information about the • motor.Because the efficiency of a motor is difficult to assess under normal operating conditions, the motor load can • be measured as an indicator of the motor’s efficiency. As loading increases, the power factor and the motor efficiency increase to an optimum value at around full load.The losses can be generally classified into two categories; fixed losses and Variable losses.• Fixed losses are those which occur in the motor irrespective of the quantum of load. These are also called no • load losses.It is a general exercise in industry to rewind burnt-out motors. Many a times during the maintenance of motors, • one has to encounter rewinding of motors. The rewound motors seldom give the same performance as original motors.Energy-efficient motors (EEM) are the ones in which, design improvements are incorporated specifically to • increase operating efficiency over motors of standard design. It is important to know what an EEM is, as it takes care of the shortfall in efficiency.

ReferencesHughes, A. & Drury. B., 2013. • Electric Motors and Drives: Fundamentals, Types and Applications, 4rth ed., Elsevier Ltd.Moczola, H., 1998. • Small Electrical Motors, The Institution of Electrical Engineers.Moyer, E. J. & Chicago, U., 2010.• Basics on electric motors [Pdf] Available at: <http://geosci.uchicago.edu/~moyer/GEOS24705/Readings/ElecReadingII_Motors.pdf> [Accessed 5 July 2013].Types of Electric Motors• [Pdf] Available at: <http://www.ece.uah.edu/courses/material/EE410-Wms2/Electric%20motors.pdf> [Accessed 5 July 2013].2009. • How electric motors work, [Video online] Available at: <https://www.youtube.com/watch?v=Q2mShGuG4RY> [Accessed 5 July 2013].2012. • Build an Electric Motor [Video online] Available at: < https://www.youtube.com/watch?v=elFUJNodXps> [Accessed 5 July 2013].

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Recommended ReadingHughes, A., 2006. • Electric motors and drives: fundamentals, types and applications, 3rd ed., Newnes publication.Meade, N. G.,1908. • Electric motors: their installation, control, operation and maintenance, McGraw publishing company.Crocker, F. B., 2009. • Electric Motors: Their Action, Control and Application, 2nd ed., BiblioBazaar, LLC, Publication.

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Self Assessment

Which motors have the disadvantages of their initial cost and maintenance of brushes and the commutator?1. DC motorsa. AC motorsb. Induction motorsc. Magnetic motorsd.

___________convert electrical energy into mechanical energy by the interaction between the magnetic fields 2. set up in the stator and rotor windings.

Compressorsa. Transistorsb. Motorsc. Circuitsd.

The difference between actual speed and the synchronous speed is called a _________.3. slidea. glideb. tripc. slipd.

____________are the ones in which, design improvements are incorporated specifically to increase operating 4. efficiency over motors of standard design.

Energy-efficient motors a. Bureau of Indian Standardsb. National Electrical manufacturers associationc. Partial load operationd.

The synchronous speed of a motor with 6 poles and operating at 50 Hz frequency is _______.5. 1500a. 1000b. 3000c. 750d.

The motor load can be measured as an indicator of the __________.6. motor’s competencea. motor’s proficiencyb. motor’s abilityc. motor’s efficiencyd.

Which of the following statements is true?7. No Load Test gives the fixed gain of the motora. Load Test gives the fixed losses of the motorb. No Load Test gives the variable losses of the motorc. No Load Test gives the fixed losses of the motord.

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Motor performance does not depend on which of the following factor8. Correct Application of the drive to suit the applicationa. Erroneous Application of the drive to suit the applicationb. Application of rated voltage and frequencyc. Partial load operationd.

Fixed losses are those which occur in the motor irrespective of the ________.9. quantum of loada. resistance of loadb. maintenance of loadc. efficiency of loadd.

It is a general exercise in industry to ___________burnt-out motors.10. generatea. produceb. rewindc. redrawd.

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Chapter III

Compressed Air System

Aim

The aim of this chapter are:

explain compressed air systems•

enlist the types of air compressors•

explicate compressor efficiency and its components•

Objectives

The objectives of the chapter are:

define compressed air systems •

explicate efficient compressor operation, capacity assessment and leakage test•

enlist factors affecting performance and efficiency•

Learning outcome

At the end of this chapter, you will be able to:

identify a compressed air systems •

define the types of air compressors•

understand the concept of efficient compressor operation, capacity assessment a• nd leakage test

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3.1 IntroductionAir compressors account for significant amount of electricity used in Indian industries. Compressed air is an • essential but costly utility and its use must be made wisely. Compressed air is generated from compressors which are largely driven by electricity. If efficiency is calculated, only 10% useful energy reaches the end point through compressed air. Thus there is a vast scope for energy saving through proper understanding of the functions of this utility and avoiding its wastage.The applications of compressed air are plenty. A few of them can be listed as:•

operation of solenoid valves plunger �operation of pneumatic cylinders �instrumentation �pneumatic tool �

In the olden days, vast areas of instrumentation including modulating actuators, etc. were operated using • compressed air especially 3-15 PSI standards. But slowly, these are being replaced by electronic and electrical drives mainly because of their accuracy, repeatability, maintainability and cost.However, there are certain hazardous areas, like the petroleum industry, the mining industry, etc. where even • small electrical sparks are not permissible. In such areas it is compulsory to use pneumatic devices.

3.2 Category of CompressorsCompressors are broadly classified as:•

Positive Displacement Compressors �Dynamic (Centrifugal) Compressors �

The flow and pressure requirements of a given request determine the suitability of a particular type of compressor. • These are further classified as:

Fig. 3.1 Types of compressors

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Positive displacement compressorsThe compressors which increase the pressure of the gas by reducing the volume are called positive displacement compressors. These compressors are further classified into:

reciprocating compressors �rotary compressors �

Reciprocating compressorsReciprocating compressors are the most widely used compressors. They operate on the cylinder and piston principle. Their flow output remains constant over a wide range of discharge pressures. The capacity is directly proportional to the speed of the prime mover. The output however, is pulsating since in one cycle, air is allowed to enter and in the other it is compressed and discharged. To make this output smooth, a receiver is invariably used. Reciprocating compressors come in variety of types; such as: lubricated and non lubricated, single or multiple cylinder, water or air cooled, single or multi stage reciprocating compressors.

Suction chamber

Discharge chamber

ReedValve orifice

CylinderPiston

Fig. 3.2 A Typical reciprocating compressor

Rotary compressors Rotary compressors unlike reciprocating compressors give a uniform flow. They are directly coupled to the prime mover and need less starting torque. Their outputs are higher compared to reciprocating compressors. Mechanically, reciprocating compressors give an imbalance, a thrust and vibrations, hence need heavy foundation. On the other hand, rotary compressors need a simple foundation. These compressors can give a discharge pressure upto 10 bar.

TRAPPED AIRHOUSING

ROTOR

DISCHARGE PORT

INTAKE PORT

Fig. 3.3 A typical rotary compressor

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Dynamic compressorsDynamic compressors increase the air velocity, which is then converted to increased pressure at the outlet. They are basically centrifugal compressors and are further divided into:

radial type• axial flow types•

Dynamic compressors operate on similar principles as centrifugal pumps. But one fundamental principle is to be understood. Pumps deal with liquid which is an incompressible fluid. Hence if you throttle output, the discharge is reduced. For air, if you throttle output, the pressure goes up because it gets compressed. This is how a centrifugal compressor operates. Hence these are typically suitable for outputs above 12000 CFM.

Compressors should be selected on the basis of individual requirements but as a general guideline, the given table may be used.

Type of Compressor Capacity (m3/h) Pressure (bar)From To From To

Roots blower compressor single stage 100 30,000 0.1 1Reciprocating- Single / Two stage 100 12,000 0.8 12- Multi stage 100 1,200 12 700Screw- Single stage 100 2,400 0.8 13- Two stage 100 2,400 0.8 24Centrifugal 600 30,000 0.1 450

Table 3.1 Compressor selection chart

3.3 Efficiency of a CompressorCompressor capacity:Capacity of a compressor is the full rated volume of flow of gas compressed and delivered at conditions of total temperature, total pressure, and composition existing at the compressor inlet. It sometimes means actual flow rate, rather than rated volume of flow. This is also termed as Free Air Delivery (FAD) i.e. air at atmospheric conditions at any specific location. Because the altitude, barometer, and temperature may vary at different localities and at different times, it follows that this term does not mean air under identical or standard conditions.

Compressor efficiency :Several different measures of compressor efficiency are commonly used: volumetric efficiency, adiabatic efficiency, isothermal efficiency and mechanical efficiency.

Before going on to the formulae, one must understand that air behaves as per the gas equation:• PV = mRT

Where;P = PressureV = Volumem = Specific MassR = ConstantT = Absolute Temperature

Also, there are many thermodynamic processes like the isothermal, the adiabatic, the polytrophic, etc.Adiabatic and isothermal efficiencies are computed as the isothermal or adiabatic power divided by the actual • power consumption. The figure obtained indicates the overall efficiency of compressor and drive motor.

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As air is compressed, its temperature at the outlet tends to increase. If this temperature is accounted, the • calculations become complex and hence for simplicity, efficiency is calculated assuming the temperature remains constant. This efficiency is called Isothermal Efficiency.Isothermal power is calculated using the following:•

Isothermal power (kW) = P1 x Q1 x loger/36.7Where,P1 = Absolute intake pressure kg/ cm2

Q1 = Free air delivered m3/hr.r = Pressure ratio P2/P1

Since Actual Power can be measured on the electrical side, the Isothermal Efficiency is calculated as:• Isothermal Efficiency= Actual measured input power/Isothermal PowerNormally,manufacturers give the isothermal efficiency.

Volumetric Efficiency:• Volumetric efficiency = Free air delivered (m3/min)/Compressor displacement

Compressor Displacement = x D2 x L x S x χ x n

Where,D = Cylinder bore (metre)L = Cylinder stroke (metre)S = Compressor speed in rpmχ = 1 for single acting and 2 for double acting cylindersn = No. of cylinders

For practical purposes, the most effective guide in comparing compressor efficiencies is the specific power • consumption i.e. kW per volume flow rate , for different compressors.

3.4 Compressed Air System ComponentsApart from the compressor proper, there are certain system components in the compressed air system which • also should be understood properly. The most important of these are as follows:Intake air filters:•

They prevent dust from entering the compressor and are normally specified in terms of microns (indicating the size below which dust cannot be prevented). Example, a 5-micron filter can prevent dust particles above 5 microns size where as cannot stop dust particles lower than 5 microns. Hence before choosing a filter, it is worthwhile to assess the dust conditions in the surroundings.

Interstage coolers: • During compression the temperature of air increases, especially when multistage compressors are used, each stage needs cooling. Hence coolers are required and they are mostly water-cooled. They need specific attention as per the manufacturer's recommendations.

After Coolers: These are used to remove moisture. �Air Dryers: �

These are used to remove the rest of the moisture which may be left at various locations in the pipelines.Moisture Drain Traps:•

These are used at the end of piping sections to remove moisture in compressed air. Various types of moisture drain traps are available like manual drain cocks, timer based / automatic drain valves etc.

Receivers:• Air receivers are provided to be storage and smoothening vital air output - reducing pressure variations from the compressor.

Each of the components mentioned above, needs to be well-maintained for an overall good performance of the • system. See Fig.3.4 for a typical compressed air system components and network.

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Supplemental Aftercooler

Distribution System

Filter, Regulator and Lubricator

Lubricant/Air Separator

Compressor Air End

Control Panel

Compressor Package Enclosure

Aftercooler and Lubricant Cooler

Air Inlet Filter

Dryer Air Receiver

Air Filter

Motor

Pneumatic tool

Fig. 3.4 A typical compressed air system components and network

3.5 Efficient Operation of CompressorThere are a number of issues to be must be considered right at the stage of project planning and also during • operation. This will ensure the efficient operation of the compressor. One important issue is the centralised compressor house or distribution system.A typical case (example, a cement plant) would explain the situation. In a cement plant, compressed air is required • at various places. Apart from this, there is a vehicle workshop at the mines for vehicle maintenance. There is no point in running a compressed air pipeline from a centralised compressor house to the mines workshop. It is worth to provide an independent compressor for the mine vehicle maintenance workshop. Thus, locating a compressor at the proper location is most important since compressors once located cannot be shifted very easily.Cool Air Intake: Statistics show that every 40C rise in intake air results in additional 1% power consumption. • Hence cool ambient temperature must be provided as intake air. There should be no heat source like a Kiln, Furnace, etc. near the compressor house.Dust free air intake is most essential for a compressor. The dust accumulated in the filter should be removed • regularly and eventually, the filters must be replaced. There are manometers or pressure switches which will warn about the differential pressure across the filters so that their performance can be watched.Dry Air is equally important. Moisture in the air can get converted into water during compressor operations • and damage compressor parts.The compressor must be operated within the altitude recommended by the manufacturer else its performance • will be affected.The cooling water circuit should be properly maintained. As explained above, compression results in a rise • in temperature and many compressors require intercoolers. The intercoolers are water cooled. They need to be maintained well. Especially the quality and quantity of water should be as per the manufacturer's recommendations.Optimum pressure settings should be set for load and unload operations.•

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3.6 Capacity Assessment of A CompressorUnlike electricity, compressed air is not a continuous flowing energy. Example, in a plant if there is no operation of • any pneumatic device for one hour the compressed air supplied will only cater to leakages. If there are no leakages during this time, then the compressor will run without compressing air. This operation is called unload.During unload operations, the prime mover is running the compressor, but it does not compress air. Thus, it • meets only no load losses. The measurement of timings of load and unload operations and power consumption readings will give an idea about the efficiency of operation.The ideal method of compressor capacity assessment is through a nozzle test wherein a calibrated nozzle is used • as a load, to vent out the generated compressed air. Flow is assessed, based on the air temperature, stabilization pressure, and orifice constant.Actual Free air discharge (Q)•

Q = × Nm3/Minute

Where,P2 = Final pressure after filling (kg/cm2 a)P1 = Initial pressure (kg/cm2a) after bleedingP0 = Atmospheric Pressure (kg/cm2 a)V = Storage volume in m3 which includes receiver, after cooler, and delivery pipingT = Time take to build up pressure to P2 in minutes

The above equation is relevant where the compressed air temperature is same as the ambient air temperature, • i.e., perfect isothermal compression. In case the compressed air temperature at discharge, say t20C is higher than ambient air temperature say t10C (as is usual case), the compressor free air delivery test (FAD) is to be corrected by a factor (273 + t1) / (273 + t2).

Leakage testIf the compressor has prolonged load operations, one can come to the conclusion that there are a lot of leakages. This means apart from the pneumatic devices, there are certain places where air is continuously being drained. The best way is to attend to the leakage and find out the reduction in load time.

The Specific Power Consumption of a Compressor can be calculated in the following method:• If a compressor of capacity Qm3 FAD per minute is being operated with the following consumption, the �specific energy consumption can be worked out.

Load Cycle P1kW for t1 minutes and Unload P2kW for t2 minutes, then specific energy consumption =Air Delivered = Q x t1 minutes

Energy Consumed =

Hence,

Specific Energy Consumption = kWh/m3 of FAD

Note the sub-sequent time taken for ‘load’ and ‘unload’ cycles of the compressors. For accuracy, take ON & • OFF times for 8 – 10 cycles continuously. Then calculate total ‘ON’ Time (T) and Total ‘OFF’ time (t).The system leakage is calculated as:•

% leakage = T × 100 / (T + t)(Or)

System leakage (m3/minute) = Q × T / (T + t)Where,Q = Actual free air being supplied during trial, in cubic meters per minute (cmm)T = Time on load in minutest = Time on unload in minutes

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Example• In the leakage test in a process industry, following results were observed-Compressor capacity (m3/minute) = 35Cut in pressure, kg/cm2 = 6.8Cut out pressure, kg/cm2 = 7.5Load kW drawn = 188 kWUnload kW drawn = 54 kWAverage ‘Load’ time = 1.5 minutesAverage ‘Unload’ time = 10.5 minutesComment on leakage quantity and avoidable loss of power due to air leakages.

Leakage quantity (m3/minute) =

= 4.375 Leakage per day, (m3/day) = 6300

Specific power for compressed air generation =

= 0.0895 kWh/m3Energy lost due to leakages/day = 564 kWh

3.7 Factors Affecting Performance and EfficiencyOne of the • most important factors affecting efficiency is lack of general awareness amongst the plant personnel that compressed air is the costliest utility and the prevention of small leakages and misuse can result in great economical benefits.Example, let us take a simple case where a worker uses a compressed air pipe in the plant to fill air into his • bicycle tyre. Normally the pressure required by a bicycle tyre is 0.3-0.5 bar. For this, he wastes the compressed air which is worth Rs.5/-. The same job can be done in 50 paise or less outside the plant. The management can also afford free air filling well outside the plant from a roadside shop rather than allowing the utility air to be misused for this purpose.Another case is the wrong application and extensions of existing pipelines.• Example, if a new area is being added to the plant, a proper study is not undertaken to revise options on whether to • install a dedicated compressor or to extend the existing line. Many a times, the existing compressed air pipelines are extended. Then it is found that the pressure is not sufficient. Then the discharge pressure of the compressor is increased and the existing system components fail more frequently due to increased pressure, thus adding to leakages. Hence proper needs must always be assessed before undertaking such works.Also, proper choice should be exercised between a centralised and a distributed system.• Many a time’s compressed air is required for aeration. At such places blowers can do the job of compressors. • Hence pressure requirements of each application should be done from the point of view of energy savings.Another consideration is pneumatic conveying. In the • 70's and 80's this process was quite popular because it does not involve mechanical maintenance. But now again with energy costs rising, economic viability is in favour of bucket elevators, etc.

3.8 Load Unload Versus On/Off ControlIn earlier days starting and stopping large motors was not easy. They used to create a lot of stress on the motor • and starting devices. Hence it was worthwhile to operate compressors with the Load/Unload Control where the motor continues to run but the compressor does not compress air. During these operations, motors will have no load losses and compared to the cost of stress on the motor and starting devices, this was affordable.But now-a-days, excellent electronic devices are available to give a smooth soft start to the motor. This avoids • stress on the motor and starting devices. Their cost is much less compared to the savings achieved in avoiding no load losses of the motor. So now a days most of the compressors are operated with the On/Off controls rather than Load/Unload Control.

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SummaryCompressed air is generated from compressors which are largely driven by electricity. If efficiency is calculated, • only 10% useful energy reaches the end point through compressed air. Pneumatic tool and instrumentation are few of the applications of compressed air.Compressors are broadly classified as Positive Displacement Compressors and Dynamic (Centrifugal) • Compressors.The compressors which increase the pressure of the gas by reducing the volume are called positive displacement • compressors. These compressors are further classified as reciprocating compressors and rotary compressors.Dynamic compressors increase the air velocity, which is then converted to increased pressure at the outlet. These • are basically centrifugal compressors and are further divided into radial type and axial flow types.Capacity of a compressor is the full rated volume of flow of gas compressed and delivered at conditions of total • temperature, total pressure, and composition existing at the compressor inlet. Sometimes, it means actual flow rate rather than rated volume of flow. This is also termed as Free Air Delivery (FAD) i.e. air at atmospheric conditions at any specific location.The most commonly used compressor efficiency are volumetric efficiency, adiabatic efficiency, isothermal • efficiency and mechanical efficiency.The compressed air system is composed of certain system components like intake air filters, interstage coolers, • after coolers, air dryers, moisture drain traps and receivers.If there are no leakages at this time, the compressor will run without compressing air. This operation is called • unload.The ideal method of compressor capacity assessment is done through a nozzle test wherein a calibrated nozzle • is used as a load to vent out the generated compressed air. Flow is assessed, based on the air temperature, stabilization pressure and orifice constant.Factors affecting performance and efficiency include the lack of general awareness among the plant personnel • that compressed air is the costliest utility and the prevention of small leakages and misuse can result in great economical benefits. Also the wrong application and extensions of existing pipelines and proper choice should be exercised between a centralised and a distributed system.These days most of the compressors are operated with the On/Off controls rather than Load/Unload Control.•

ReferencesElliott, B., 2006. • Compressed Air Operations Manual, 1st ed., McGraw-Hill Professional publication.Simons, T., 1914. • Compressed air: a treatise on the production, transmission and use of compressed air, McGraw-Hill Book Co. Publication.Compressed Air System• [Pdf] Available at: < http://www.energymanagertraining.com/GuideBooks/3Ch3.pdf> [Accessed 5 July 2013].Compressed Air System Study Guidelines• [Pdf] Available at: <http://www.vectren.com/cms/assets/pdfs/conservation/compressed-air-guidelines.pdf> [Accessed 5 July 2013].2011.• Compressed Air System Basics, [Video online] Available at: <https://www.youtube.com/watch?v=2KKCwfvqoNs> [Accessed 5 July 2013].2012. • Compressed Air System [Video online] Available at: < https://www.youtube.com/watch?v=HUcHHIrm9CI> [Accessed 5 July 2013].

Recommended ReadingRamli, Y., 2010. • Introduction to Compressed Air Systems.Talbott, E. M., 1993. • Compressed air systems: a guidebook on energy and cost savings, 2nd ed. The Fairmont Press, Inc.Doty, S. & Turner, W. C., 2009. • Energy management handbook, 7th ed., The Fairmont Press.

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Self AssessmentThe ratio of actual measured input power of a compressor to isothermal power is known as_______.1.

Isothermal efficiencya. Volumetric Efficiencyb. Barometric efficiencyc. Mechanical efficiencyd.

The compressor capacity of a reciprocating compressor is directly proportional to ______.2. pressurea. speedb. volumec. timed.

Reciprocating compressors are based on which principle?3. Pyramid and piston principlea. Triangle and piston principleb. Cylinder and piston principlec. Circular and piston principled.

Dynamic compressors increase the air velocity, which is then converted to increased pressure at the outlet. They 4. are basically______.

pressure compressorsa. volume compressorsb. density compressorsc. centrifugal compressorsd.

______ prevent dust from entering the compressor.5. After coolersa. Receiversb. Interstage coolersc. Intake air filtersd.

For every 40C rise in air intake, the power consumption in turn increase by ____.6. 1%a. 2%b. 3%c. 4%d.

Which of the following statements is true?7. Dryness in the air can get converted into water during compressor operations and damage compressor a. parts.The dust accumulated in the filter should be removed regularly and eventually the filters must never be b. replaced.Cool ambient temperature must be provided as intake air.c. Most terrible pressure settings should be set for load and unload operations.d.

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The ideal method of compressor capacity assessment is through a ______.8. needle testa. nozzle testb. puzzle testc. riddle testd.

Now a days most of the compressors are operated with the ________.9. in/out controla. on/off controlb. left/right controlc. up/down controld.

The compressors which increase the pressure of the gas by reducing the volume are called _______.10. Dynamic compressorsa. Centrifugal compressorsb. Positive displacement compressorsc. Negative displacement compressorsd.

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Chapter IV

HVAC and Refrigeration System

Aim

The aim of this chapter is:

explain HVAC and refrigeration systems•

elucidate vapour compression refrigeration cycle•

explicate refrigerants, coefficient of performance and capacity•

Objectives

The objectives of the chapter are:

explain the factors affecting refrigeration•

explain air conditioning system performance, savings opportunities and potential•

enlist types and comparisons with vapour compression system•

Learning outcome

At the end of this chapter, you will be able to:

identify a HVAC and refrigeration systems•

understand the concept of vapour absorption refrigeration system and working principle•

enlist the types of refrigeration system•

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4.1 IntroductionThe Heating, Ventilation, and Air Conditioning (HVAC) and refrigeration system transfer heat energy from • one atmosphere to the other. One of them is a closed environment, while the other is the open atmosphere of the Earth.HVAC includes the bi-directional flow of heat, in the sense that when earth's atmospheric temperature is • too low, then the requirements of a closed atmosphere are to be maintained. Heat is injected into the closed atmosphere.Refrigeration on the other hand, has a unidirectional flow of heat. It always extracts heat from the closed • atmosphere with the help of a low boiling point refrigerant and dispels it into the open atmosphere of the earth.

Condenser may be water-cooled or air-cooled Vapor

Evaporator

Cold air

Fan

Warmair

Expansion

ValveLiquid + Vapour Liquid

CompressorVapor

Condenser

Fig. 4.1 Schematic representation of refrigeration system

According to thermodynamic laws, heat cannot flow from a cold object to a hot object unless external work is • done. The external work is done by the equipment used in HVAC/Refrigeration using electricity as the energy source.There are several heat transfer loops in the refrigeration system as described below. In Fig. 4.2, thermal energy • moves from the left to the right as it is extracted from the space and expelled to the outdoors through five loops of heat transfer:

Indoor air loopIn the leftmost loop, indoor air is driven by the supply air fan through a cooling coil, where it transfers its heat • to chilled water. The cool air then cools the building space.

Chilled water loopDriven by the chilled water pump, water returns from the cooling coil to the chillers’ evaporator to be re-• cooled.

Refrigerant loopUsing a phase-change refrigerant, the chillers’ compressor pumps heat from the chilled water to the condenser • water.

Condenser water loopWater absorbs heat from the chillers’ condenser, and the condenser water pump sends it to the cooling tower.•

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Cooling tower loopThe cooling tower fan drives air across an open flow of the hot condenser water, transferring the heat to the • outdoors.

Fig. 4.2 Heat transfer loops in refrigeration system

4.1.1 Air-Conditioning SystemDepending on applications, there are several options / combinations, which are available for use as given below:

Air Conditioning (for comfort / machine)• Split air conditioners• Fan coil units in a larger system• Air handling units in a larger system•

4.1.2 Refrigeration Systems (for processes)

Small capacity modular units of direct expansion type similar to domestic refrigerators, small capacity • refrigeration units.Centralised chilled water plants with chilled water as a secondary coolant for temperature range over 50˚C • typically. They can also be used for ice bank formation.Brine plants, which use brines as lower temperature, secondary coolant, for typically sub zero temperature • applications, which come as modular unit capacities as well as large centralised plant capacities.The plant capacities upto 50 TR are usually considered as small capacity, 50 – 250 TR as medium capacity and • over 250 TR as large capacity units.A large industry may have a bank of such units, often with common chilled water pumps, condenser water • pumps, cooling towers, as an off site utility.

The same industry may also have two or three levels of refrigeration & air conditioning such as:Comfort air conditioning (200 – 250 � 0C)Chilled water system (80 – 100 � 0C)Brine system (sub-zero applications) �Two principle types of refrigeration plants found in industrial use are: Vapour Compression Refrigeration �(VCR) and Vapour Absorption Refrigeration (VAR). VCR uses mechanical energy as the driving force for refrigeration, while VAR uses thermal energy as the driving force for refrigeration.

4.1.3 Capacity MeasurementThe capacity of household refrigerators is measured in litres of volume of the enclosed chamber which is cooled. The capacity of the air conditioning system is measured in terms of TR. The term 1 TR means the rate at which heat is extracted. 1 ton signifies that amount of heat required to melt one ton of ice in 24 hours. This can be derived as given below.1 Ton = 907 Kg. Please note Ton means 1 metric ton (MT).

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The latent heat of Ice = 80 kCal/kg.Hence the rate of extraction of heat equivalent to 1 TR will be:(907 x 80)/24 = 3024 kCal/hr = (3024/0.252) BTU per hour = 12000 BTU/hrIn terms of kW, it will be 3.5169988 kW.Since this is an ideal conversion assuming no losses, the operating efficiency criterion of any HVAC/Refrigeration system will be judged from kW/TR.

4.2 Types of Refrigeration System The two principle types of refrigeration plants found in industry include:

Vapour Compression Refrigeration (VCR) and• Vapour Absorption Refrigeration (VAR)•

VCR uses mechanical energy as the driving force for refrigeration, while VAR uses thermal energy as the driving force for refrigeration.

4.2.1 Vapour Compression Refrigeration

Condenser Evaporator

Compressor

Capillary Tube (Expansion Device)Mass

Flow Meter

1

3

2 4

Fig. 4.3 Schematic representation of the vapour compression refrigeration cycle

Heat flows naturally from a hotter to colder body. In refrigeration system, the opposite must occur i.e. heat flows • from a colder to hotter body. This is achieved by using a substance called a refrigerant, which absorbs heat and hence boils or evaporates at a low pressure to form a gas. This gas is then compressed to a higher pressure, such that it transfers the heat it has gained to ambient air or water and turns back (condenses) into a liquid. In this way, heat is absorbed or removed from a low temperature source and transferred to a higher temperature source.The refrigeration cycle can be broken down into the following stages (see Figure 4.4):•

1–2 - Low pressure liquid refrigerant in the evaporator absorbs heat from its surroundings, usually air, �water or some other process liquid. During this process it changes its state from a liquid to a gas, and at the evaporator exit is slightly superheated.2–3 - the superheated vapour enters the compressor where its pressure is raised. There will also be a big �increase in temperature because a proportion of the energy input into the compression process is transferred to the refrigerant.3–4 - the high pressure superheated gas passes from the compressor into the condenser. The initial part of �the cooling process (3–3a) desuperheats the gas before it is then turned back into liquid (3a–3b). The cooling

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for this process is usually achieved by using air or water. A further reduction in temperature happens in the pipe work and liquid receiver (3b–4) so that the refrigerant liquid is sub-cooled as it enters the expansion device.4–1 - The high-pressure sub-cooled liquid passes through the expansion device, which both reduces its �pressure and controls the flow into the evaporator.It can be seen that the condenser has to be capable of rejecting the combined heat inputs of the evaporator �and the compressor; i.e. (1–2) + (2–3) has to be the same as (3–4). There is no heat loss or gain through the expansion device.

Fig. 4.4 Schematic diagram of a basic vapour compression refrigeration system

4.2.2 Alternative Refrigerants for Vapour Compression Systems

The use of CFCs is now beginning to be phased out due to their damaging impact on the protective ozone layer • present in the troposphere around the earth.The Montreal Protocol of 1987 and the subsequent Copenhagen agreement of 1992 mandated a reduction in • the production of ozone depleting Chlorinated Fluorocarbon (CFC) refrigerants in a phased manner, with an eventual stop to all production by the year 1996.In response, the refrigeration industry has developed two alternative refrigerants;•

based on Hydrochloro Fluorocarbon (HCFC) �another based on Hydro Fluorocarbon (HFC) �

The HCFCs have a 2–10% ozone depleting potential as compared to CFCs and also have an atmospheric • lifetime between 2–25 years as compared to 100 or more years for CFCs (Brandt, 1992). However, even HCFCs are mandated to be phased out by 2005, and only the chlorine free (zero ozone depletion) HFCs would be acceptable.Until now, only one HFC based refrigerant, HFC 134a, has been developed. HCFCs are comparatively simpler • to produce and the three refrigerants 22, 123, and 124 have been developed. The use of HFCs and HCFCs results in slightly lower efficiencies as compared to CFCs, but this may change with increasing efforts being made to replace CFCs.

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4.2.3 Absorption Refrigeration

HEAT IN

HEAT OUT

FROM MILK COOLER

CHILLED WATER

TO MILK COOLER

A – GENERATORB – CONDENSERC – EVAPORATORD – ABSORBERE – HEAT EXCHANGERF – PUMP

HEAT OUT

D C

E

B

Fig. 4.5 Schematic diagram absorption refrigeration system

The absorption chiller is a machine which produces chilled water by using heat such as steam, hot water, gas, oil etc. Chilled water is produced by the principle that a liquid (refrigerant), which evaporates at low temperature, absorbs heat from the surroundings when it evaporates. Pure water is used as refrigerant and lithium bromide solution is used as the absorbent

Heat for the vapour absorption refrigeration system can be provided by waste heat extracted from process, diesel generator sets, etc. Absorption systems require electricity only to run pumps. Depending on the temperature required and the power cost, it may even be economical to generate heat/steam to operate the absorption system.A description of the absorption refrigeration concept is given in Table 4.1

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EvaporatorThe refrigerant (water) evaporates at around 40• 0C under a high vacuum condition of 754mmHg in the evaporator.Chilled water goes through heat exchanger tubes in the evaporator and transfers • heat to the evaporated refrigerant.The evaporated refrigerant (vapour) turns into liquid again, while the latent • heat from this vaporization process cools the chilled water (in the diagram from 120C to 70C). The chilled water is then used for cooling purposes.

AbsorberIn order to keep evaporating, the refrigerant vapour must be discharged from • the evaporator and refrigerant (water) must be supplied. The refrigerant vapour is absorbed into lithium bromide solution, which is • convenient to absorb the refrigerant vapour in the absorber. The heat generated in the absorption process is continuously removed from • the system by cooling water. The absorption also maintains the vacuum inside the evaporator.•

High Pressure GeneratorAs lithium bromide solution is diluted, the ability to absorb the refrigerant • vapour reduces. In order to keep the absorption process going, the diluted lithium bromide • solution must be concentrated again.An absorption chiller is provided with a solution concentrating system, called • a generator. Heating media such as steam, hot water, gas or oil perform the function of concentrating solutions.The concentrated solution is returned to the absorber to absorb refrigerant • vapour again.

CondenserTo complete the refrigeration cycle, and thereby ensuring the refrigeration • takes place continuously, the following two functions are required,To concentrate and liquefy the evaporated refrigerant vapour, which is • generated in the high pressure generator.To supply the condensed water to the evaporator as refrigerant (water).• For these two functions a condenser is installed.•

Table 4.1 Typical schematic representation of the absorption refrigeration concept

Absorption refrigeration systems that use Li-Br-water as a refrigerant have a Coefficient of Performance (COP) in the range of 0.65–0.70 and can provide chilled water at 6.70C with a cooling water temperature of 300C. Systems capable of providing chilled water at 300C are also available. Ammonia based systems operate at above atmospheric pressures and are capable of low temperature operation (below 0oC). Absorption machines are available with capacities in the range of 10-1500 tons. Although the initial cost of an absorption system is higher than that of a compression system, operational costs are much lower if waste heat is used.

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4.2.4 Evaporative Cooling

Fig. 4.6 An Evaporative cooling unit

There are occasions where air conditioning, which stipulates control of humidity up to 50 % for human comfort or for process, can be replaced by a much cheaper and less energy intensive evaporative cooling.The concept is very simple and is the same as that used in a cooling tower. Air is brought in close contact with water to cool it to a temperature close to the wet bulb temperature. The cool air can be used for comfort or process cooling. The disadvantage is that the air is rich in moisture. Nevertheless, it is an extremely efficient means of cooling at a very low cost. Large commercial systems employ cellulose filled pads over which water is sprayed. The temperature can be controlled by controlling the airflow and the water circulation rate. The possibility of evaporative cooling is especially attractive for comfort cooling in dry regions. This principle is practiced in textile industries for certain processes.

4.3 Common Refrigerants and their PropertiesA variety of refrigerants are used in vapour compression systems. The choice of the fluid is determined largely by the cooling temperature required. Commonly used refrigerants are in the family of chlorinated fluorocarbons (CFCs, also called Freons), R-11, R-12, R-21, R-22 and R-502. The properties of these refrigerants are summarised in Table 4.2 and the performance of these refrigerants is given in Table 4.3.

Refrigerant BoilingPoint**

(0C)

FreezingPoint(0C)

VapourPressure*

(kPa)

Vapour Volume* (m3/kg)

Enthalpy*

Liquid (kJ/kg) Vapour (kJ/kg)R 11 -23.82 -111.0 25.73 0.61170 191.40 385.43R 12 -29.79 -158.0 219.28 0.07702 190.72 347.96R 22 -40.76 -160.0 354.74 0.06513 188.55 400.83R 502 -45.40 414.30 0.04234 188.87 342.31R 7 (Ammonia) -33.30 -77.7 289.93 0.41949 808.71 487.76

Table 4.2 Properties of commonly used refrigerants* At 100C** At Standard Atmospheric Pressure (101.325 kPa)

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Refrigerant Evaporating Press (kPa)

Condensing Press (kPa)

Pressure Ratio Vapour Enthalpy (kJ/kg)

COP**Carnot

R 11 20.4 125.5 6.15 155.4 5.03R 12 182.7 744.6 4.08 116.3 4.70R 22 295.8 1192.1 4.03 162.8 4.66R 502 349.6 1308.6 3.74 106.2 4.37R 717 236.5 1166.5 4.93 103.4 4.78

Table 4.3 Performance of commonly used refrigerants*

* At -150C Evaporator Temperature, and 300C Condenser Temperature

**COP = Coefficient of Performance =

The choice of the refrigerant and the required cooling temperature and load determine the choice of the compressor, as well as the design of the condenser, evaporator, and other auxiliaries. Additional factors, such as ease of maintenance, physical space requirements and the availability of utilities for auxiliaries (water, power, etc.) also influence component selection.

4.4 Types of Compressor and Their ApplicationsFor industrial use, open type systems (compressor and motor as separate units) are normally used, though hermetic systems (motor and compressor in a sealed unit) also find service in some low capacity applications. Hermetic systems are used in refrigerators, air conditioners, and other low capacity applications. Industrial applications largely employ reciprocating, centrifugal and more recently, screw compressors, and scroll compressors. Water-cooled systems are more efficient than air-cooled alternatives because the temperatures produced by refrigerant condensation are lower with water than with air.

4.4.1 Centrifugal Compressors

AIR OUT

IMPELLERAIR ENTERS IN CENTRE

HOUSING

AIR

AIR THROWN OFF AT AIM OF IMPELLER

Fig. 4.7 Centrifugal compressor

Centrifugal compressors are the most efficient types (see Figure 4.7), when they operate near full load. Their • efficiency advantage is the greatest in large sizes and they offer considerable economy of scale, so they dominate the market for large chillers. They are able to use a wide range of refrigerants efficiently, so they will probably continue to be the dominant types in large sizes.Centrifugal compressors have a single major moving part -an impeller that compresses the refrigerant gas by • the centrifugal force. The gas is given kinetic energy as it flows through the impeller. This kinetic energy is not useful in itself, so it must be converted to pressure energy. This is done by allowing the gas to slow down smoothly in a stationary diffuser surrounding the impeller.

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To minimise efficiency loss at reduced loads, centrifugal compressors typically throttle output with inlet guide • vanes located at the inlet to the impeller(s). This method is efficient down to about 50% load, but the efficiency of this method decreases rapidly below 50% load.The older centrifugal machines are unable to reduce load much below 50%. This is because of “surge” in the • impeller. As the flow through the impeller is choked off, the gas does not acquire enough energy to overcome the discharge pressure. Flow drops abruptly at this point and an oscillation begins as the gas flutters back and forth in the impeller. Efficiency drops abruptly, and the resulting vibration can damage the machine. Many older centrifugal machines deal with low loads by creating a false load on the system, such as by using hot gas bypass. This wastes the portion of the cooling output that is not required.Another approach is to use variable speed drives in combination with inlet guide vanes. This may allow the • compressor to throttle down to about 20% of full load, or less, without false loading. Changing the impeller speed causes a departure from optimum performance, so efficiency still declines badly at low loads.A compressor that uses a variable-speed drive reduces its output in the range between full load and approximately • half load by slowing the impeller speed.At lower loads, the impeller cannot be slowed further, because the discharge pressure would become too low • to condense the refrigerant. Below the minimum load provided by the variable-speed drive, inlet guide vanes are used to provide further capacity reduction.

4.4.2 Reciprocating Compressors

Fig. 4.8 Schematic diagram of reciprocating compressors

The maximum efficiency of reciprocating compressors (see Fig. 4.8) is lower than that of centrifugal and screw • compressors. Efficiency is reduced by the clearance volume (the compressed gas volume that is left at the top of the piston stroke), throttling losses at the intake and discharge valves, abrupt changes in gas flow, and friction. Lower efficiency also results from the smaller sizes of reciprocating units, because motor losses and friction account for a larger fraction of energy input in smaller systems.Reciprocating compressors suffer less efficiency loss at partial loads than other types, and they may actually • have a higher absolute efficiency at low loads than the other types. Smaller reciprocating compressors control output by turning on and off. This eliminates all part-load losses, except for a short period of inefficient operation when the machine starts.Larger multi-cylinder reciprocating compressors commonly reduce output by disabling (“unloading”) individual • cylinders. When the load falls to the point that even one cylinder provides too much capacity, the machine turns off. Several methods of cylinder unloading are used, and they differ in efficiency. The most common is holding open the intake valves of the unloaded cylinders. This eliminates most of the work of compression, but a small amount of power is still wasted in pumping refrigerant gas to-and-fro through the unloaded cylinders. Another method is blocking gas flow to the unloaded cylinders, which is called “suction cut-off.”Variable-speed drives can be used with reciprocating compressors, eliminating the complications of cylinder • unloading. This method is gaining popularity with the drastic reduction in costs of variable speed drives.

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4.4.3 Screw Compressors

Fig. 4.9 Screw compressor

Screw compressors, sometimes called “helical rotary” compressors, compress the refrigerant by trapping it in • the “threads” of a rotating screw-shaped rotor (see Figure 4.7). Screw compressors have increasingly taken over from reciprocating compressors of medium sizes and large sizes, and they have even entered the size domain of centrifugal machines. Screw compressors are applicable to refrigerants that, such as HCFC-22 and ammonia.They are especially compact. A variety of methods are used to control the output of screw compressors. There • are major efficiency differences among the different methods. The most common is a slide valve that forms a portion of the housing that surrounds the screws.Using a variable-speed drive is another method of capacity control. It is limited to oil-injected compressors, • because slowing the speed of a dry compressor would allow excessive internal leakage. There are other methods of reducing capacity, such as suction throttling that are inherently less efficient than the previous two.

4.4.4 Scroll Compressors

Fig. 4.10 Scroll compressor

The scroll compressor is an old invention that has finally come to the market. The gas is compressed between • two scroll-shaped vanes. One of the vanes is fixed, and the other moves within it. The moving vane does not rotate, but its centre revolves with respect to the centre of the fixed vane, as shown in Fig. 4.10.This motion squeezes the refrigerant gas along a spiral path, from the boundaries of the vanes toward the centre, • where the discharge port is located. The compressor has only two moving parts, the moving vane and a shaft with an off-centre crank to drive the moving vane.Scroll compressors have only recently become practical, because close machining tolerances are needed to • prevent leakage between the vanes, and between the vanes and the casing.

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4.5 Selection of a Suitable Refrigeration SystemA clear understanding of the cooling load to be met is the first and most important part of designing/selecting the • components of a refrigeration system. Important factors to be considered in quantifying the load are the actual cooling need, heat (cool) leaks, and internal heat sources (from all heat generating equipment).Consideration should also be given to process changes and/or changes in ambient conditions that might affect the • load in the future. Reducing the load, e.g. through better insulation, maintaining as high a cooling temperature as practical, etc. is the first step towards minimising electrical power required to meet refrigeration needs.With a quantitative understanding of the required temperatures and the maximum, minimum, and average • expected cooling demands, selection of the appropriate refrigeration system (single-stage/multi-stage, economised

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Energy Efficiency in Electrical Utilities

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59/JNU OLE

compression, compound/cascade operation, direct cooling/secondary coolants) and equipment (type of refrigerant, compressor, evaporator, condenser, etc.) can be undertaken.

4.6 Performance Assessment of Refrigeration PlantsThe cooling effect produced is quantified as tons of refrigeration (TR).1 TR of refrigeration = 3024 kCal/hr heat rejected.The refrigeration TR is assessed as

TR=

Where, Q = mass flow rate of coolant in kg/hr Cp= coolant specific heat in kCal/kg0C Ti = inlet temperature of coolant to evaporator (chiller) in 0C T0= outlet temperature of coolant from evaporator (chiller) in 0C The above TR is also called as chiller tonnage

Specific power consumption kW/TR is a useful indicator of the performance of a refrigeration system. By • measuring the refrigeration duty performed in TR and the kiloWatt inputs, kW/TR is used as a reference energy performance indicator.In a centralised chilled water system, apart from the compressor unit, power is also consumed by the chilled • water (secondary) coolant pump as well as condenser water (for heat rejection to cooling tower) pump and cooling tower fan in the cooling tower. Effectively, the overall energy consumption would be towards:

Compressor kW �Chilled water pump kW �Condenser water pump kW �Cooling tower fan kW, for induced / forced draft towers �

The specific power consumption for certain TR output would therefore have to include:• Compressor kW/TR �Chilled water pump kW/TR �Condenser water pump kW/TR �Cooling tower fan kW/TR �

The overall kW/TR is the sum of the above.

The theoretical Coefficient of Performance (Carnot),COPCarnot -a standard measure of refrigeration efficiency of an ideal refrigeration system- depends on two key system temperatures namely;

the evaporator temperature Te �the condenser temperature Tc �

With COP being given as:

COPCarnot =

This expression also indicates that a higher COPCarnot is achieved with higher evaporator temperature and lower condenser temperature. But COPCarnot is only a ratio of temperatures, and hence does not take into account the type of compressor. Hence the COP normally used in the industry is given by

COP =

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where, the cooling effect is the difference in enthalpy across the evaporator and expressed as kW. The effect of evaporating and condensing temperatures are given in Figure 4.11 and Figure 4.12 below:

Fig. 4.11 Effect of evaporator temperature on chiller COP

Fig. 4.12 Effect of condensing temperature on chiller COP

In field performance assessment, accurate instruments for inlet and outlet chilled water temperature and condenser • water temperature measurement are required, preferably with a least count of 0.1 C. Flow measurements of chilled water can be made by an ultrasonic flow meter directly or inferred from pump duty parameters. An adequacy check of chilled water is needed often and most units are designed for a typical 0.68 m3/hr per TR (3gpm/TR) chilled water flow. Condenser water flow measurements can also be made by a non-contact flow meter directly or inferred from pump duty parameters. An adequacy check of condenser water is also often needed, and most units are designed for a typical 0.91 m3/hr per TR (4 gpm/TR) condenser water flow.In case of air conditioning units, the airflow at the Fan Coil Units (FCU) or the Air Handling Units (AHU) can • be measured with an anemometer. Dry bulb and wet bulb temperatures are measured at the inlet and outlet of the AHU or the FCU and the refrigeration load in TR is assessed as:

TR =

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Where,Q = the air flow in m3/hρ = density of air kg/m3

hin= enthalpy of inlet air kCal/kghout= enthalpy of outlet air kCal/kg

Use of psychometric charts can help to calculate h and h in out from dry bulb, wet bulb temperature values • which are, in-turn measured during trials, by a whirling psychrometer.Power measurements at, compressor, pumps, AHU fans, cooling tower fans can be accomplished by a portable • load analyser.Estimation of the air conditioning load is also possible by calculating various heat loads, sensible and latent • based on inlet and outlet air parameters, air ingress factors, air flow, number of people and the type of materials stored.

An indicative TR load profile for air conditioning is presented as follows:Small office cabins = 0.1TR/m � 2

Medium size office i.e., 10–30 people = 0.06TR/m � 2 occupancy with central A/CLarge multi-storeyed office = 0.04TR/m � 2 complexes with central A/C

4.6.1 Integrated Part Load Value (IPLV)

Although the kW/ TR can serve to be an initial reference, it should not be taken as an absolute since this value • is derived from 100% of the equipment's capacity level and is based on design conditions that are considered the most critical.These conditions occur may be, example, during only 1% of the total time the equipment is in operation throughout • the year. Consequently, it is essential to have data that reflects how the equipment operates with partial loads or in conditions that demand less than 100% of its capacity.To overcome this, an average of kW/TR with partial loads i.e. Integrated Part Load Value (IPLV) have to be • formulated. The IPLV is the most appropriate reference, although not considered the best, because it only captures four points within the operational cycle: 100%, 75%, 50% and 25%.Furthermore, it assigns the same weight to each value, and most equipment usually operates at between 50 % • and 75% of its capacity. This is why it is so important to prepare specific analysis for each case that addresses the four points already mentioned, as well as developing a profile of the heat exchanger's operations during the year.

4.7 Factors Affecting Performance and Energy Efficiency of Refrigeration PlantsThe various factors which affect the performance and energy efficiency of refrigeration plants are as follows:

4.7.1 The Design of Process Heat Exchangers

There is a tendency of the process group to operate with high safety margins which influences the compressor • suction pressure/evaporator set point. For instance, a process cooling requirement of 150C would need chilled water at a lower temperature, but the range can vary from 60C to say 100C. At 100C chilled water temperature, the refrigerant side temperature has to be lower, say 50C to +50C.The refrigerant temperature again sets the corresponding suction pressure of the refrigerant which decides the • inlet duty conditions for the work of compression of the refrigerant compressor.Having an optimum/ minimum driving force (temperature difference) can, thus, help to achieve the highest • possible suction pressure at the compressor, thereby leading to less energy requirement.This requires proper sizing of heat transfer areas of process heat exchangers and evaporators as well as • rationalising the temperature requirement to the highest possible value. A 10C raise in evaporator temperature can help save almost 3% on power consumption. The TR capacity of the same machine will also increase with the evaporator temperature, as given in Table 4.4.

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Evaporator Temperature(0C)

Refrigeration Capacity* (tons)

Specific Power Consumption

Increase in kW/ton (%)

5.0 67.58 0.81 -0.0 56.07 0.94 16.0-5.0 45.98 1.08 33.0-10.0 37.20 1.25 54.0-20.0 23.12 1.67 106.0

Table 4.5 Effect of variation in evaporator temperature on compressor power consumption

* Condenser temperature 400CTowards rationalising the heat transfer areas, the heat transfer coefficient on the refrigerant side can be considered to range from 1400–2800 watts/m2K.The refrigerant side heat transfer areas provided are of the order of 0.5 Sqm./TR and above in evaporators.

Condensers in a refrigeration plant are critical equipment that influence the TR capacity and power consumption • demands. Given a refrigerant, the condensing temperature and the corresponding condenser pressure depend upon

the heat transfer area provided �effectiveness of heat exchange �the type of cooling chosen �

A lower condensing temperature, pressure, in the best of combinations would mean that the compressor has to • work between a lower pressure differential as the discharge pressure is fixed by the design and the performance of the condenser.The choices of condensers in practice range from air cooled, air cooled with water spray, and heat exchanger • cooled. Generously sized shell and tube heat exchangers as condensers, with good cooling tower operations help to operate with low discharge pressure values and the TR capacity of the refrigeration plant also improves.With the same refrigerant, R22, a discharge pressure of 15 kg/cm• 2 with a water cooled shell and tube condenser and 20kg/cm2 with an air cooled condenser indicate the kind of additional work of compression duty and almost 30% additional energy consumption required by the plant.One of the best options at the design stage would be to select generously sized (0.65m• 2/TR and above) shell and tube condensers with water-cooling as against cheaper alternatives like air cooled condensers or water spray atmospheric condenser units.

The effect of condenser temperature on refrigeration plant energy requirements is given in Table 4.5.

Condensing Temperature(0C)

Refrigeration Capacity (tons)

Specific Power Consumption (kW/TR)

Increase in kW/TR (%)

26.7 31.5 1.17 -35.0 21.4 1.27 8.540.0 20.0 1.41 20.5

Table 4.6 Effects of variations in condenser temperature on compressor power consumption

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* Reciprocating compressor using R-22 refrigerant. Evaporator temperature-100C

4.7.2 Maintenance of Heat Exchanger Surfaces

After ensuring procurement, effective maintenance holds the key to optimising power consumption.• Heat transfer can also be improved by ensuring proper separation of the lubricating oil and the refrigerant, timely • defrosting of coils, and increasing the velocity of the secondary coolant (air, water, etc.).However, increased velocity results in larger pressure drops in the distribution system and higher power • consumption in pumps/fans. Therefore, careful analysis is required to determine the most effective and efficient option.Fouled condenser tubes force the compressor to work harder to attain the desired capacity. For example, a • 0.8mm scale build-up on condenser tubes can increase energy consumption by as much as 35%. Similarly, fouled evaporators (due to residual lubricating oil or infiltration of air) result in increased power consumption. Equally important is proper selection, sizing, and maintenance of cooling towers. A reduction of 0.550C temperature in water returning from the cooling tower reduces the compressor power consumption by 3.0% (see Table 4.6).

• Condition Evap.

Temp (0C)

Cond. Temp (0C)

Refrigeration Capacity* (tons)

kW/Ton

Specific Power Consumption

Increase in (kW/ton) (%)

Normal 7.2 40.5 17.0 0.69 -Dirty condenser

7.2 46.1 15.6 0.84 20.4

Dirty evaporator

1.7 40.5 13.8 0.82 18.3

Dirty condenser and evaporator

1.7 46.1 12.7 0.96 38.7

Table 4.7 Effect of poor maintenance on compressor power consumption

*15 ton reciprocating compressor based system.

The power consumption is lower than that for systems typically available in India. However, the percentage • change in power consumption is indicative of the effect of poor maintenance.

4.7.3 Multi-staging for Efficiency

Efficient compressor operation requires that the compression ratio be kept low to reduce discharge pressure and • temperature. For low temperature applications involving high compression ratios, and for wide temperature requirements, it is preferable (due to equipment design limitations) and often economical to employ multi-stage reciprocating machines or centrifugal / screw compressors.Multi-staging systems are of two-types:•

compound �cascade �

They are applicable to all types of compressors.With reciprocating or rotary compressors, two-stage compressors are preferable for load temperatures from • 20–580C, and with centrifugal machines for temperatures around 430C.In multi-stage operation, a first-stage compressor, sized to meet the cooling load, feeds into the suction of a • second-stage compressor after inter-cooling of the gas. A part of the high-pressure liquid from the condenser is flashed and used for liquid sub-cooling. The second compressor, therefore, has to meet the load of the evaporator and the flash gas.

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A single refrigerant is used in the system, and the work of compression is shared equally by the two compressors. • Therefore, two compressors with low compression ratios can in combination provide a high compression ratio.For temperatures in the range of 46–101• 0C, cascaded systems are preferable. In this system, two separate systems using different refrigerants are connected such that one provides the means of heat rejection to the other.The chief advantage of this system is that a low temperature refrigerant which has a high suction temperature • and low specific volume can be selected for the low-stage to meet very low temperature requirements.

4.7.4 Matching Capacity to System Load

During part-load operation, the evaporator temperature rises and the condenser temperature falls, effectively • increasing the COP. But at the same time, deviation from the design operation point and the fact that mechanical losses form a greater proportion of the total power negate the effect of improved in COP, resulting in lower part-load efficiency.Therefore, consideration of part-load operation is important, because most refrigeration applications have varying • loads. The load may vary due to variations in temperature and process cooling needs.Matching refrigeration capacity to the load is a difficult exercise, requiring knowledge of compressor performance, • and variations in ambient conditions, and detailed knowledge of the cooling load.

4.7.5 Capacity Control and Energy Efficiency

The capacity of compressors is controlled in a number of ways. Capacity control of reciprocating compressors • through cylinder unloading results in incremental (step-by-step) modulation as against continuous capacity modulation of centrifugal through vane control and screw compressors through sliding valves. Therefore, temperature control requires a careful system design.Usually, when using reciprocating compressors in applications with widely varying loads, it is desirable to • control the compressor by monitoring the return water (or other secondary coolant) temperature rather than the temperature of the water leaving the chiller. This prevents excessive on-off cycling or unnecessary loading/unloading of the compressor.However, if load fluctuations are not high, the temperature of the water leaving the chiller should be monitored. • This has the advantage of preventing operation at very low water temperatures, especially when flow reduces at low loads. The leaving water temperature should be monitored for centrifugal and screw chillers.Capacity regulation through speed control is the most efficient option. However, when employing speed control • for reciprocating compressors, it should be ensured that the lubrication system is not affected. In the case of centrifugal compressors, it is usually desirable to restrict speed control to about 50% of the capacity to prevent surging. Below 50%, vane control or hot gas bypass can be used for capacity modulation.The efficiency of screw compressors operating at part load is generally higher than either centrifugal compressors • or reciprocating compressors, which may make them attractive in situations where part-load operation is common.Screw compressor performance can be optimised by changing the volume ratio. In some cases, this may result • in higher full-load efficiencies as compared to reciprocating and centrifugal compressors. Also, the ability of screw compressors to tolerate oil and liquid refrigerant slugs makes them preferred in some situations.

4.7.6 Multi-level Refrigeration for Plant Needs

The selection of refrigeration systems also depends on the range of temperatures required in the plant. For • diverse applications requiring a wide range of temperatures, it is generally more economical to provide several packaged units (several units distributed throughout the plant) instead of one large central plant.Another advantage would be the flexibility and reliability accorded. The selection of packaged units could also • be made depending on the distance at which cooling loads need to be met. Packaged units at load centres reduce distribution losses in the system.

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Despite the advantages of packaged units, central plants generally have lower power consumption since at reduced • loads; power consumption can reduce significantly due to the large condenser and evaporator surfaces.Many industries use a bank of compressors at a central location to meet the load. Usually chillers feed into a • common header from which branch lines are taken to different locations in the plant. In such situations, operation at part-load requires extreme care.For efficient operation, the cooling load, and the load on each chiller must be monitored closely. It is more • efficient to operate a single chiller at full load than to operate two chillers at part-load.The distribution system should be designed such that individual chillers can feed all branch lines. Isolation • valves must be provided to ensure that chilled water (or other coolant) does not flow through chillers not in operation.Valves should also be provided on branch lines to isolate sections where cooling is not required. This reduces • pressure drops in the system and reduces power consumption in the pumping system.Individual compressors should be loaded to their full capacity before operating the second compressor. In some • cases it is economical to provide a separate smaller capacity chiller, which can be operated on an on-off control to meet peak demands, with larger chillers meeting the base load.Flow control is also commonly used to meet varying demands. In such cases the savings in pumping at reduced • flow should be weighed against the reduced heat transfer in coils due to reduced velocity.In some cases, operation at normal flow rates, with subsequent longer periods of no-load (or shut-off) operation • of the compressor, may result in larger savings.

4.7.7 Chilled Water Storage

Depending on the nature of the load, it is economical to provide a chilled water storage facility with very good • cold insulation. Also, the storage facility can be fully filled to meet the process requirements so that chillers need not be operated continuously.This system is usually economical if small variations in temperature are acceptable. It has the added advantage • of allowing the chillers to be operated at periods of low electricity demand to reduce peak demand charges.Low tariffs offered by some electric utilities for operation at night time can also be taken advantage of by using • a storage facility. An added benefit is that lower ambient temperature at night lowers condenser temperature and thereby increases the COP.If temperature variations cannot be tolerated, it may not be economical to provide a storage facility since the • secondary coolant would have to be stored at a temperature much lower than required to provide for heat gain.The additional cost of cooling to a lower temperature may offset benefits. The solutions are case specific. For • example, in some cases it may be possible to employ large heat exchangers, at a lower cost burden than low temperature chiller operation, to take advantage of the storage facility even when temperature variations are not acceptable. The Ice bank systems which store ice rather than water are often economical.

4.7.8 System Design Features

In overall plant design, adoption of good practices improves the energy efficiency significantly. Some areas for • consideration are:

Design of cooling towers with FRP impellers and film fills, PVC drift eliminators, etc. �Use of softened water for condensers in place of raw water. �Use of economic insulation thickness on cold lines, heat exchangers, considering cost of heat gains and �adopting practices like infrared thermography for monitoring -applicable especially in large chemical / fertiliser / process industry.Adoption of roof coatings / cooling systems, false ceilings / as applicable, to minimise refrigeration load. �Adoption of energy efficient heat recovery devices like air to air heat exchangers to pre-cool the fresh air �by indirect heat exchange; control of relative humidity through indirect heat exchange rather than use of duct heaters after chilling.

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Adopting of variable air volume systems; adopting of sun film application for heat reflection; optimising �lighting loads in the air conditioned areas; optimising a number of air changes in the air conditioned areas are a few other examples.

4.8 Energy Saving OpportunitiesCold Insulation• Insulate all cold lines / vessels using economic insulation thickness to minimise heat gains; and to choose • appropriate (correct) insulation.Building Envelope• Optimise air conditioning volumes by measures such as use of false ceiling and segregation of critical areas • for air conditioning by air curtains.Building Heat Loads Minimisation• Minimise the air conditioning loads by measures such as roof cooling, roof painting, efficient lighting, pre-• cooling of fresh air by air- to-air heat exchangers, variable volume air system, optimal thermo-static setting of temperature of air conditioned spaces, sun film applications, etc.Process heat loads minimisation•

Minimise process heat loads in terms of TR capacity as well as refrigeration level;flow optimisation �heat transfer area increase to accept higher temperature coolant �avoiding wastages like heat gains, loss of chilled water, idle flows �frequent cleaning / de-scaling of all heat exchangers �

At the Refrigeration A/C Plant Area;• ensure regular maintenance of all A/C plant components as per manufacturer guidelines �ensure an adequate quantity of chilled water and cooling water flows, avoid bypass flows by closing valves �of idle equipmentminimise part load operations by matching loads and plant capacity on line; adopt variable speed drives �for varying process loadmake efforts to continuously optimise condenser and evaporator parameters for minimising specific energy �consumption and maximising capacityadopt VAR system where economics permit as a non-CFC solution �

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SummaryThe Heating, Ventilation, and Air Conditioning (HVAC) and refrigeration system transfers heat energy from • one atmosphere to the other. HVAC includes the bi-directional flow of heat, in the sense that when earth's atmospheric temperature is too low, then the requirements of a closed atmosphere are to be maintained. Heat is injected into the closed atmosphere.Depending on applications, there are several options / combinations, which are available for use are Air • Conditioning (for comfort / machine), split air conditioners, fan coil units in a larger system, air handling units in a larger system.In Vapour Compression Refrigeration (VCR) heat flows naturally from a hot to a colder body. In refrigeration • system the opposite must occur i.e. heat flows from a cold to a hotter body. This is achieved by using a substance called a refrigerant, which absorbs heat and hence boils or evaporates at a low pressure to form a gas. This gas is then compressed to a higher pressure, such that it transfers the heat it has gained to ambient air or water and turns back (condenses) into a liquid. In this way heat is absorbed, or removed, from a low temperature source and transferred to a higher temperature source.In Vapour Absorption Refrigeration (VAR), the absorption chiller is a machine which produces chilled water • by using heat such as steam, hot water, gas, oil etc. Chilled water is produced by the principle that a liquid (refrigerant), which evaporates at low temperature, absorbs heat from the surroundings when it evaporates. Pure water is used as refrigerant and lithium bromide solution is used as the absorbent.TR is a measure of refrigeration capacity. One TR means the heat rate that will melt one ton of ice in 24 hours. • 1 TR means 3024 kCal/hr.The various types of refrigeration systems and refrigerants are Compression Refrigeration, Absorption • Refrigeration. The energy efficiency of such systems depends a lot on the refrigerant use, leakages in the system, type and quality of insulation, etc. Each of them presents a number of ways and opportunities in energy savings.COP is the coefficient of Performance. It is the ratio of the cooling effect in kW to the Power Input to the • Compressor.

ReferencesWang, S., 2000. • Handbook of Air Conditioning and Refrigeration, 2nd ed., McGraw-Hill Professional publication.Stoecker, W., 1998.• Industrial Refrigeration Handbook, 1st ed., McGraw-Hill Professional Publication.hvac and refrigeration system• [Pdf] Available at: < http://www.beeindia.in/energy_managers_auditors/documents/guide_books/3Ch4.pdf> [Accessed 5 July 2013].Refrigeration System Accessories • [Pdf] Available at: < http://www.mavcc.org/pdffiles/ACRUnit14SG.pdf> [Accessed 5 July 2013].2009. • HVAC 101 evacuating AC unitand adding refrigerant [Video online] Available at: < https://www.youtube.com/watch?v=W6mzdUfdSNM> [Accessed 5 July 2013].2008. • Principles of Refrigeration [Video online] Available at: < https://www.youtube.com/watch?v=b527al9D_rY&list=PL95C8D5AC21D8955B> [Accessed 5 July 2013].

Recommended ReadingSmith, R. E., • Electricity for Refrigeration, 8th ed., CengageBrain.com.Rosaler, R. C., 1998. • HVAC maintenance and operations handbook. McGraw-Hill Professional publication.McDowall, R., 2007. • Fundamentals of HVAC systems. Academic Press Publication.

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Self AssessmentScrew compressor performance can be optimised by changing the _______.1.

volume ratioa. pressure ratiob. temperature ratioc. distance ratiod.

One ton of refrigeration (TR) is equal to_____.2. 3.62 kWa. 14000 BTU/hb. 3.75 BTU/hc. 3024 Kcal/hd.

HVAC includes the _____flow of heat.3. uni-directionala. bi-directionalb. multi-directionalc. mono-directionald.

Vapour Compression Refrigeration (VCR) and Vapour Absorption Refrigeration (VAR) are two types of 4. _____.

household AC systemsa. industrial AC systemsb. refrigeration plantsc. HVAC systemsd.

Which of the following statements is false?5. The HCFCs have a 2–10% ozone depleting potential as compared to CFCs.a. An atmospheric lifetime for CFCs is to 100 or more years.b. The use of CFCs is now beginning to be phased out due to their damaging impact on the protective ozone c. layer.A gain in the production of Chlorinated Fluorocarbon (CFC) refrigerants is made mandate as it protects d. ozone layer.

Centrifugal compressors have a single major moving part -_____ that compresses the refrigerant gas by the 6. centrifugal force

a compellera. a moverb. an impellerc. a rotord.

Screw compressors also called_____.7. helical rotary compressorsa. vertical rotary compressorsb. horizontal rotary compressorsc. circular rotary compressorsd.

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The performance of a refrigeration system is indicated with the help of _________.8. Specific pressure consumptiona. Specific power consumptionb. Specific temperature consumptionc. Specific volume consumptiond.

The specific power consumption for certain TR output does not include which of the options given below.9. Compressora. Condenser water pumpb. Chilled water pumpc. Cooling rise fand.

Match the following:10.

1) Indoor air loop A. Water absorbs heat from the chillers’ condenser, and the condenser water pump sends it to the cooling tower

2) Chilled water loop B. Driven by the supply air fan through a cooling coil, where it transfers its heat to chilled water

3) Refrigerant loop C. Driven by the chilled water pump, water returns from the cooling coil to the chillers’ evaporator to be re-cooled

4) Condenser water loop D. The chillers’ compressor pumps heat from the chilled water to the condenser water

1-A, 2-B, 3-C, 4-Da. 1-B, 2-C, 3-D, 4-Ab. 1-C, 2-D, 3-A, 4-Bc. 1-D, 2-A, 3-B, 4-Cd.

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Chapter V

Fans and Blowers

Aim

The aim of this chapter is :

explain fans and blowers•

enlist types of fans and blowers•

explicate the performance evaluation•

Objectives

The objectives of the chapter are :

describe the important factors helping energy conservation opportunities•

explicate efficient system operation and flow control strategies•

explain energy conservation opportunities•

Learning outcome

At the end of this chapter, you will be able to:

define fans and blowers•

identify different types of fans and blowers•

understand the concept of e• fficient system operation and flow control strategies

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5.1 IntroductionFans and blowers provide air for ventilation and industrial process requirements. Fans generate a pressure to move air (or gases) against a resistance caused by ducts, dampers, or other components in a fan system. The fan rotor receives energy from a rotating shaft and transmits it to the air.

Industrial fans and blowers are machines whose primary function is to provide a large flow of air or gas to various processes of many industries. This is achieved by rotating a number of blades, connected to a hub and shaft and driven by a motor or turbine. The flow rates of these fans range from approximately 200-2,000,000 cubic feet (5.7 to 57000 cubic meters) per minute.

A blower is another name for a fan that operates where the resistance to the flow is primarily on the downstream side of the fan. Most manufacturing plants use fans and blowers for ventilation and for industrial processes that need an air flow. Fan systems are essential to keep manufacturing processes working, and consist of a fan, an electric motor, a drive system, ducts or piping, flow control devices, and air conditioning equipment (filters, cooling coils, heat exchangers, etc.). An example system is illustrated in Fig. 5.1.

Outlet Diffusers

Heat Exchanger

Baffles

Filter

Intel Vanes

Motor Controller

Variable Frequency DriveCentrifugal Fan

MotorBelt Drive

Turning Vanes (typically used on short-radius elbows)

Fig. 5.1 Typical fan system components

[Source: US Department of Energy (DOE), 1989]

5.2 Difference Between Fans, Blowers and CompressorsFans, blowers and compressors are differentiated by the methods used to move air, and by the system pressure they must operate against. As per the American Society of Mechanical Engineers (ASME) the specific ratio –the ratio of the discharge pressure over the suction pressure – is used for defining the fans, blowers and compressors (see Table 5.1).

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Equipment Specific Ratio Pressure rise (mmWg)Fans Up to 1.11 1136

Blowers 1.11 to 1.20 1136-2066Compressors More than 1.20 -

Table 5.1 Differences between fans, blowers and compressors

5.3 Types of Fans and BlowersFan and blower selection depends on the volume flow rate, pressure, type of material handled, space limitations, and efficiency.

5.3.1 Types of FanFan efficiencies differ from design to design and also by types. Typical ranges of fan efficiencies are given in Table 5.2.

Type of fan Peak Efficiency RangeCentrifugal FanAirfoil, backward curved/ inclined

79-83

Modified radial 72-79Radial 69-75Pressure blower 58-68Forward curved 60-65Axial fanVanaxial 78-85Tubeaxial 67-72Propeller 45-50

Table 5.2 Fan efficiencies

Fans are divided into two general categories:• centrifugal flow �axial flow �

In centrifugal flow, airflow changes direction twice – once when entering and secondly, while leaving (forward • curved, backward curved or inclined, radial) (see Fig. 5.2).

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Fan Wheel

Gas In

Housing

Gas Out

Fig. 5.2 Centrifugal flow fan

Centrifugal fans increase the speed of an air stream with a rotating impeller. The speed increases as the air • stream reaches the ends of the blades, which is then converted into pressure. These fans are able to produce high pressures, which makes them suitable for harsh operating conditions, such as systems with high temperatures, moist or dirty air streams and material handling. Centrifugal fans are categorized by their blade shapes as summarized in Table 5.3.

Type of fan and blade Advantages DisadvantagesRadial fans, withflat blades

Suitable for high static pressures (up to • 1400 mmWC) and high temperatures

Only suitable for low-medium airflow rates

Simple design allows custom build units • for special applicationsCan operate at low air flows without • vibration problemsHigh durability• Efficiencies up to 75%• Have large running clearances, which is • useful for airborne-solids (dust, wood chips and metal scraps) handling services

Forward curved fans, with forward curved blades

Can move large air volumes against • relatively low pressure

Relative small size �Low noise level (due to low speed) �and well suited for residential heating, ventilation, and air conditioning (HVAC) applications

Only suitable for clean service • applications but not for high pressure and harsh servicesFan output is difficult to adjust • accuratelyDriver must be selected carefully • to avoid motor overload because power curve increases steadily with airflowRelatively low energy efficiency • (55-65%)

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Backward inclined fan, with blades that tilt away from the direction of rotation: flat, curved, and airfoil

Can operate with changing static pressure • (as this does not overload the motor)Suitable when system behaviour at high air • flow is uncertain

Suitable for forced-draft services �Flat bladed fans are more robust �Curved blades fans are more efficient �(exceeding 85%)Thin air-foil blades fans are most �efficient

Not suitable for dirty air streams (as • fan shape promotes accumulation of dust)Airfoil blades fans are less stable • because of staff as they rely on the lift created by each bladeThin airfoil blades fans subject to • erosion

Table 5.3 Types of centrifugal fans (Source US DOE, 1989)

In axial flow, air enters and leaves the fan with no change in direction (propeller, tube axial, vane axial) (see • Fig. 5.3).

Fig. 5.3 Axial flow fan

Axial fans move an air stream along the axis of the fan. The way these fans work can be compared to a propeller • on an aeroplane; the fan blades generate an aerodynamic lift that pressurizes the air. They are popular in industry because of their cost effectiveness, compact form and lightness in weight. The main types of axial flow fans are summarised in Table 5.4.

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Type of fan Advantages DisadvantagesPropeller fan generate high airflow •

rates at low pressuresnot combined with • extensive ductwork (because they generate little pressure)inexpensive because • o f t h e i r s i m p l e constructionachieve maximum • e f f i c i e n c y, n e a r -free del ivery, and are of ten used in rooftop ventilation applicationscan generate flow in • reverse direction, which is helpful in ventilation applications

relative low Energy efficiency• comparatively noisy•

Tube-axial fan, essentially a propeller fan placed inside a cylinder

higher pressures • and better operating efficiencies than propeller fanssuited for medium-• pressure, high airflow rate applications, e.g. ducted HVAC installationscan quickly accelerate • to rated speed (because of their low rotating mass) and generate flow in reverse direction, which is useful in many ventilation applicationscreate sufficient • pressure to overcome duct losses and are relatively space efficient, which is useful for exhaust applications

relatively expensive• moderate airflow noise• relatively low energy efficiency • (65%)

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Vane-axial fan suited for medium- to • high pressureapplications (up to • 500 mmWC), such as induced draft service for a boiler exhaustcan quickly accelerate • to rated speech (because of their low rotating mass) and generate flow in reverse directions, which is useful in many ventilation applicationssuited for direct • connection to motor shaftsmost energy • efficient (up to 85% if equipped with airfoil fans and small clearances)

relatively expensive compared to • propeller fans

Table 5.4 Types of axial fans(Source from US DOE, 1989)

Fig. 5.4 Types of centrifugal and axial fans

5.3.2 Types of BlowersBlowers can achieve much higher pressures than fans, as high as 1.20 kg/cm2. They are also used to produce negative pressures for industrial vacuum systems.

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Major types are:centrifugal blower• positive-displacement blower•

Centrifugal blowersCentrifugal blowers look more like centrifugal pumps compared to fans. The impeller is typically gear-driven and rotates as fast as 15,000 rpm. In multi-stage blowers, air is accelerated as it passes through each impeller. In a single-stage blower, air does not take many turns, and hence it is more efficient.Centrifugal blowers typically operate against pressures of 0.35- 0.70 kg/cm2, but can achieve higher pressures. One characteristic is that, air-flow tends to drop drastically as system pressure increases. This can be a disadvantage in material conveying systems that depend on a steady air volume. As a result of which, they are most often used in applications that are not prone to clogging.

Fig. 5.5 Centrifugal blowers

Positive-displacement blowersPositive-displacement blowers have rotors, which "trap" air and push it through the housing. Positive-displacement blowers provide a constant volume of air even if the system pressure varies. They are especially suitable for applications prone to clogging, since they can produce enough pressure -typically up to 1.25 kg/cm2 - to blow clogged materials free. They turn much slower than centrifugal blowers (e.g. 3,600 rpm), and are often belt driven to facilitate speed changes.

ShaftCompressed wheel

Inlet

Compressor housing

Centre bearing Turbine wheel

Exhaust gases exit alter spinning turbine

Turbine housing

Compresses outlet

Waste exhaust gases enter here

Fig. 5.6 Positive-displacement blowers

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5.4 Fan Performance Evaluation and Efficient System Operation

5.4.1 System Characteristics

The term “system resistance” is used while referring to the static pressure. The system resistance is the sum • of static pressure losses in the system. System resistance is a function of the configuration of ducts, pickups, elbows and the pressure drops across equipment-for example bagfilter or cyclone.The system resistance varies with the square of the volume of air flowing through the system. For a given • volume of air, the fan with narrow ducts and multiple short radius elbows in a system is going to work harder to overcome a greater system resistance than it would have had in a system with larger ducts and a minimum number of long radius turns.Long narrow ducts with many bends and twists will require more energy to pull air through them. Consequently, • for a given fan speed, the fan will be able to pull less air through this system than through a short system with no elbows. Thus, system resistance increases substantially as the volume of air flowing through the system increases; square of air flow.Conversely, resistance decreases as flow decreases. To determine what volume the fan will produce, it is necessary • to know the system resistance characteristics.In existing systems, system resistance can be measured. In systems that have been designed, but not built, it • must be calculated. Typically a system resistance curve (see Fig. 5.7) is generated for various flow rates on the X-axis and the associated resistance on the Y-axis.

Fig. 5.7 System characteristics

5.5 Fan CharacteristicsFan characteristics can be represented in the form of fan curve(s). The fan curve is a performance curve for • the particular fan under a specific set of conditions. The fan curve is a graphical representation of a number of interrelated parameters.Typically, a curve will be developed for a given set of conditions usually including fan volume, system static • pressure, fan speed and brake horsepower required to drive the fan under the stated conditions.Some fan curves will also include an efficiency curve so that a system designer will know where on that curve • the fan will be operating under the chosen conditions (see Fig.5.8). In the many curves shown in the figure, the curve static pressure (SP) vs. flow is especially important.The intersection of the system curve and the static pressure curve defines the operating point. When the system • resistance changes, the operating point also changes. Once the operating point is fixed, the power required could be found by following a vertical line that passes through the operating point to an intersection with the power (BHP) curve.

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A horizontal line drawn through the intersection with the power curve will lead to the required power on the • right vertical axis. In the depicted curves, the fan efficiency curve is also presented.

Fig. 5.8 Fan characteristics curve by manufacturer

5.6 System Characteristics and Fan CurvesIn any fan system, the resistance to air flow (pressure) increases when the flow of air is increased. As mentioned before, it varies as the square of the flow. The pressure required by a system over a range of flows can be determined and a "system performance curve" can be developed (shown as SC) (see Fig. 5.9).

Fig. 5.9 System curve

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This system curve can then be plotted on the fan curve to show the fan's actual operating point at "A" where the • two curves (N1 and SC1) intersect. This operating point is at air flow Q1 delivered against pressure P1.A fan operates along a performance given by the manufacturer for a particular fan speed. (The fan performance • chart shows performance curves for a series of fan speeds.) At fan speed N1, the fan will operate along the N1 performance curve as shown in Fig. 5.9. The fan's actual operating point on this curve will depend on the system resistance; fan’s operating point at “A” is flow (Q1) against pressure (P1).Two methods can be used to reduce air flow from Q• 1 to Q1:

First method is to restrict the air flow by partially closing a damper in the system. This action causes a new �system performance curve (SC2) where the required pressure is greater for any given air flow. The fan will now operate at "B" to provide the reduced air flow Q2 against higher pressure P2.Second method to reduce air flow is by reducing the speed from N � 1 to N2, keeping the damper fully open. The fan would operate at "C" to provide the same Q2 air flow, but at a lower pressure P3.

Thus, reducing the fan speed is a much more efficient method to decrease airflow since less power is required • and less energy is consumed.

5.7 Fan LawsThe fans operate under a predictable set of laws concerning speed, power and pressure. A change in speed (rpm) of any fan will predictably change the pressure rise and power necessary to operate it at the new RPM. This is shown in Fig. 5.10.

Flow ∞ Speed Pressure ∞ (Speed)2 Power ∞ (Speed)3

Varying the RPM by 10% decreases or increases air delivery by 10%.

Reducing the RPM by 10% decreases the static pressure by 19% and an increase in RPM by 10% increases the static pressure by 21%.

Reducing the RPM by 10% decreases the power requirement by 27% and an increase in RPM by 10% increases the power requirement by 33%

Where,Q – flowSP – Static PressurekW – PowerN – speed (RPM)

Fig. 5.10 Speed, pressure and power of fans (Source - Bureau of Energy Efficiency India, 2004)

5.8 Fan Design and Selection CriteriaThe precise determination of air-flow and required outlet pressure is the most important step in the proper • selection of fan type and size. The air-flow required depends on the process requirements; normally determined from heat transfer rates or combustion air or flue gas quantity to be handled.The system pressure requirement is usually more difficult to compute or predict. A detailed analysis should • be carried out to determine pressure drop across the length, bends, contractions and expansions in the ducting system, pressure drop across filters, drop in branch lines, etc.

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These pressure drops should be added to any fixed pressure required by the process (in the case of ventilation • fans there is no fixed pressure requirement). Frequently, a very conservative approach is adopted allocating large safety margins, resulting in over-sized fans which operate at flow rates much below their design values and, consequently, at very poor efficiency.Once the system flow and pressure requirements are determined, the fan and impeller type are then selected. • For best results, values should be obtained from the manufacturer for specific fans and impellers.The choice of fan type for a given application depends on the magnitudes of the required flow and static pressure. • For a given fan type, the selection of the appropriate impeller depends additionally on rotational speed.The speed of operation varies with the application. High speed small units are generally more economical • because of their higher hydraulic efficiency and relatively low cost. However, at low pressure ratios, large, low-speed units are preferable.

5.8.1 Fan Performance and EfficiencyTypical static pressures and power requirements for different types of fans are given in the Figure 5.11. Also fan performance characteristics based on the fan and impeller type (See Fig. 5.12).

Fig. 5.11 Fan static pressure and power requirements for different fans

Fig .5.12 Fan performance characteristics based on fans/ impellers

In the case of centrifugal fans, the hub-to-tip ratios (ratio of inner-to-outer impeller diameter), the tip angles • (angle at which forward or backward curved blades are curved at the blade tip - at the base the blades are always oriented in the direction of flow), and the blade width determine the pressure developed by the fan.Forward curved fans have large hub-to-tip ratios compared to backward curved fans and produce lower • pressure.Radial fans can be made with different heel-to-tip ratios to produce different pressures.• At both design and off-design points, backward-curved fans provide the most stable operation. Also, the power • required by most backward curved fans will decrease at a flow higher than the design values.

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A similar effect can be obtained by using inlet guide vanes instead of replacing the impeller with different tip • angles. Radial fans are simple in construction and are preferable for high-pressure applications.Forward curved fans however, are less efficient than backward curved fans and power rises continuously with • flow. Thus, they are generally more expensive to operate despite their lower first cost.Among centrifugal fan designs, aerofoil designs provide the highest efficiency (upto 10% higher than backward • curved blades), but their use is limited to clean, dust-free air.Axial-flow fans produce lower pressure than centrifugal fans, and exhibit a dip in pressure before reaching the • peak pressure point. Axial-flow fans equipped with adjustable / variable pitch blades are also available to meet varying flow requirements.Propeller-type fans are capable of high-flow rates at low pressures. Tube-axial fans have medium pressure, high • flow capability and are not equipped with guide vanes.Vane-axial fans are equipped with inlet or outlet guide vanes, and are characterised by high pressure, medium • flow-rate capabilities.Performance is also dependent on the fan enclosure and duct design. Spiral housing designs with inducers, • diffusers are more efficient as compared to square housings. The density of inlet air is another important consideration, since it affects both volume flow-rate and capacity of the fan to develop pressure.Inlet and outlet conditions (whirl and turbulence created by grills, dampers, etc.) can significantly alter • fan performance curves from that provided by the manufacturer (which are developed under controlled conditions).Bends and elbows in the inlet or outlet ducting can change the velocity of air, thereby changing fan characteristics • (the pressure drop in these elements is attributed to system resistance). All these factors, termed the System Effect Factors, should, therefore, be carefully evaluated during fan selection since they would modify the fan performance curve.Centrifugal fans are suitable for low to moderate flow at high pressures, while axial-flow fans are suitable for • low to high flows at low pressures. Centrifugal fans are generally more expensive than axial fans. Fan prices vary widely based on the impeller type and the mounting (direct-or-belt-coupled, wall-or-ductmounted).Among centrifugal fans, aerofoil and backward-curved blade designs tend to be somewhat more expensive than • forward-curved blade designs and will typically provide more favourable economics on a life-cycle basis.Reliable cost comparisons are difficult since costs vary with a number of application-specific factors. A careful • technical and economic evaluation of the available options is important in identifying the fan that will minimise life-cycle costs in any specific application.

5.8.2 Safety MarginThe choice of the safety margin also affects the efficient operation of the fan. In all cases where fan requirement is linked to the process/other equipment, the safety margin is to be decided, based on the discussions with the process equipment supplier. In general, the safety margin can be 5% over the maximum requirement on flow rate.In the case of boilers, the induced draft (ID) fan can be designed with a safety margin of 20% on the volume and 30% on the head. The forced draft (FD) fans and primary air (PA) fans do not require any safety margins. However, safety margins of 10% on volume and 20% on pressure are maintained for FD and PA fans.

Some Pointers on Fan Specifications• The right specifications of the parameters of the fan at the initial stage are a prerequisite for choosing the appropriate and energy efficient fan.

The user should specify the following information to the fan manufacturer to enable the right selection:design operating point of the fan- volume and pressure• normal operating point volume and pressure• maximum continuous rating•

Low load operation: This is particularly essential for units, which in the initial few years may operate at lower capacities, with plans for up-gradation at a later stage. The initial low load and the later higher load operational

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requirements need to be specified clearly, so that, the manufacturer can supplies a fan which can meet both the requirements, with different sizes of the impeller.The maximum temperature of the gas at the fan during upset conditions should be specified to the supplier. This will enable the choice of the right material of the required creep strength. In addition, the following data should be furnished to the supplying the fan for proper design.

The density of the gas at different temperatures at the fan outlet.• Composition of the gas -- This is very important for choosing the material of construction of the fan.• Dust concentration and nature of dust. Dust concentration and the nature of dust (e.g. bagasse- soft dust, coal- • hard dust) should be clearly specified.

The proposed control mechanisms that are going to be used to control the fan. �Operating frequency varies from plant-to-plant, depending on the source of the power supply. Since this �has a direct effect on the speed of the fan, the frequency prevailing or being maintained in the plant also needs to be specified to the supplier.Altitude of the plant. �The choice of speed of the fan can be best left to the fan manufacturer. This will enable him to design the �fan of the highest possible efficiency. However, if the plant has some preferred speeds on account of any operational need, the same can be communicated to the fan supplier.

5.8.3 Installation of the Fan

The installation and mechanical maintenance of the fan play a critical role with efficiency. The following • clearances (typical values) should be maintained for the efficient operation of the impeller.Impeller Inlet Seal Clearances•

Axial overlap 5 to 10 mm for 1 metre plus dia impeller �Radial clearance 1 to 2 mm for 1 metre plus dia impeller �Back plate clearance 20 to 30 mm for 1 metre plus dia impeller �Labyrinth seal clearance 0.5 to 1.5 mm �

The inlet damper positioning is also to be checked regularly so that the "full open" and "full close" conditions • are satisfied. The fan user should get all details of the mechanical clearances from the supplier at the time of installation. These should be strictly adhered to, for the efficient operation of the fan.A checklist on these clearances should be prepared and checked after every maintenance activity so that the • efficient operation of the fan is ensured on a continuous basis.

5.8.4 System Resistance Change

System resistance has a major role in determining the performance and efficiency of a fan. System resistance • also changes depending on the process. Example, the formation of the coatings / erosion of the lining in the ducts, changes system resistance marginally.In some cases, the change of equipment (e.g. Replacement of Multi-cyclones with ESP / Installation of low • pressure drop cyclones in the cement industry), duct modifications, drastically shift the operating point, resulting in lower efficiency. In such cases, to maintain efficiency as before, the fan has to be changed.Hence, system resistance has to be periodically checked, more so when modifications are introduced and actions • taken accordingly, for the efficient operation of the fan.

5.9 Flow Control StrategiesTypically, once a fan system is designed and installed, the fan operates at a constant speed. There may be occasions when a speed change is desirable, i.e., when adding a new run of duct that requires an increase in air flow (volume) through the fan. There are also instances when the fan is oversized and flow reductions are required.Various ways to achieve a change in flow are: pulley change, damper control, inlet guide vane control, variable speed drive and the series and parallel operation of fans.

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5.9.1 Pulley ChangeWhen a fan volume change is required on a permanent basis, and the existing fan can handle the change in the capacity, volume change can be achieved with a speed change. The simplest way to change the speed is with a pulley change.

For this, the fan must be driven by a motor through a v-belt system. The fan speed can be increased or decreased with a change in the drive pulley or the driven pulley or in some cases, both pulleys.As shown in Figure 5.13, a higher sized fan operating with damper control was downsized by reducing the motor (drive) pulley size from 8” to 6”. The power reduction was 12 kW.

Fig.5.13 Pulley change

5.9.2 Damper ControlsSome fans are designed with damper controls (see Figure 5.14). Dampers can be located at the inlets or outlets. Dampers provide a means of changing the air volume by adding or removing system resistance. This resistance forces the fan to move up or down along its characteristic curve, generating more or less air without changing fan speed. However, dampers provide a limited amount of adjustment, and they are not particularly energy efficient.

Fig. 5.14 Damper control

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Inlet guide vanes are another mechanism that can be used to meet the variable air demand (see Fig. 5.15).

Fig. 5.15 Inlet guide vanes

Guide vanes are curved sections that lay against the inlet of the fan when they are open. But when closed, the vanes extend out into the air stream. As they are closed, guide vanes pre-swirl the air entering the fan housing. This changes the angle at which air is presented to the fan blades, which, in turn, changes the characteristics of the fan curve. Guide vanes are energy efficient for modest flow reductions from 100 percent flow to about 80 percent. Below 80 % flow, energy efficiency drops sharply.

Axial-flow fans can be equipped with variable pitch blades, which can be hydraulically or pneumatically controlled to change blade pitch, while the fan is stationary. Variable-pitch blades modify fan characteristics substantially and thereby provide dramatically higher energy efficiency than the other options discussed thus far.

5.9.3 Variable Speed Drives

Although variable speed drives are expensive, they provide almost infinite variability in speed control. Variable • speed operation involves reducing the speed of the fan to meet reduced flow requirements.Fan performance can be predicted at different levels of speed using the fan laws. Since the power input to the • fan changes as the cube of the flow, this will usually be the most efficient form of capacity control. However, variable speed control may not be economical for systems which have infrequent flow variations.When considering variable speed drive, the efficiency of the control system (fluid coupling, eddy-current, VFD, • etc.) should be accounted for in the analysis of power consumption.

5.9.4 Series and Parallel Operation

Parallel operation of fans is another useful form of capacity control. Fans in parallel can be additionally equipped • with dampers, variable inlet vanes, variable-pitch blades, or speed controls to provide a high degree of flexibility and reliability.Combining fans in series or parallel can achieve the desired air flow without greatly increasing the system package • size or fan diameter. Parallel operation is defined as having two or more fans blowing together side by side.The performance of two fans in parallel will result in doubling the volume flow, but only at free delivery.•

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Fig.5.16 Series and parallel operation

As Figure 5.16 shows, when a system curve is overlaid on the parallel performance curves, the higher the system • resistance, the less increase in flow results with parallel fan operation. Thus, this type of application should only be used when the fans can operate at a low resistance almost in a free delivery condition.Series operation can be defined as using multiple fans in a push-pull arrangement. By staging two fans in series, • the static pressure capability at a given airflow can be increased, but again, not to double at every flow point, as the above figure displays.In series operation, the best results are achieved in systems with high resistance. In both series and parallel • operation, particularly with multiple fans, certain areas of the combined performance curve will be unstable and should be avoided. This instability is unpredictable and is a function of the fan and motor construction and the operating point.Factors to be considered in the selection of flow control methods are a comparison of various volume control • methods with respect to power consumption (%) required power are shown in Fig. 5.17.

Fig.5.17 Comparison of various volume control methods

All methods of capacity control mentioned above have turn-down ratios (ratio of maximum to minimum flow • rate) determined by the amount of leakage (slip) through the control elements.

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Example, even with dampers fully closed, the flow may not be zero due to leakage through the dampers. In the • case of variable-speed drives, the turn-down ratio is limited by the control system.In many cases, the minimum possible flow will be determined by the characteristics of the fan itself. The stable • operation of a fan requires that it operates in a region where the system curve has a positive slope and the fan curve has a negative slope.The range of operation and the time duration at each operating point also serve as a guide to the selection of the • most suitable capacity control system. The outlet damper control due to its simplicity, ease of operation, and low investment cost, is the most prevalent form of capacity control.However, it is the most inefficient of all methods and is best suited for situations where only small, infrequent • changes are required, example, minor process variations due to seasonal changes.The economic advantage of one method over the other is determined by the time duration over which the fan • operates at different operating points. The frequency of flow change is another important determinant.For systems requiring frequent flow control, the damper adjustment may not be convenient. Indeed, in many • plants, dampers are not easily accessible and are left at some intermediate position to avoid frequent control.

5.10 Fan Performance AssessmentFans are tested for field performance by the measurement of flow, head and temperature on the fan side and electrical motor kW input on the motor side.5.10.1 Air flow Measurement

Static Pressure• Static pressure is the potential energy put into the system by the fan. It is given up to friction in the ducts �and at the duct inlet as it is converted to velocity pressure. At the inlet to the duct, static pressure produces an area of low pressure

Velocity Pressure• Velocity pressure is the pressure along the line of the flow that results from the air flowing through the duct. �Velocity pressure is used to calculate air velocity.

Total Pressure• Total pressure is the sum of the static and velocity pressures. Velocity pressure and static pressure can change �as the air flows though different size ducts accelerating and de-accelerating the velocity. The total pressure stays constant, changing only with friction losses. The illustration that follows shows how the total pressure changes in a system (see Figure 5.18).

Fig.5.18 Static, total and velocity pressure

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The fan flow is measured using the pitot tube manometer combination, or a flow sensor (the differential • pressure instrument) or an accurate anemometer. Care needs to be taken regarding the number of traverse points, straight length sections (to avoid turbulent flow regimes of measurement) upstream and downstream of the measurement location. The measurements can be on the suction or discharge side of the fan and preferably both where feasible.Measurement by Pitot tube.•

Fig.5.19 Velocity measurement using pitot tube

Fig. 5.19 shows how velocity pressure is measured using a pitot tube and a manometer. Total pressure is measured using the inner tube of the pitot tube and static pressure is measured using the outer tube of the pitot tube. When the inner and outer tube ends are connected to a manometer, we get the velocity pressure. To measure low velocities, it is preferable to use an inclined tube manometer instead of the U tube manometer.

5.10.2 Measurements and Calculations

Velocity Pressure/Velocity CalculationWhen measuring velocity pressure, the duct diameter (or the circumference from which the diameter is calculated) should be measured as well. This will allow calculating the velocity and the volume of air in the duct. In most cases, velocity must be measured at several places in the same system.

Velocity pressure varies across the duct. Friction slows the air near the duct walls, so velocity is greater at the centre of the duct. Velocity is affected by changes in the ducting configuration such as bends and curves. The best place to take measurements is in a section of the duct that is straight for at least 3-5 diameters after any elbows, branch entries or duct size changes.

To determine the average velocity, it is necessary to take a number of velocity pressure readings across the cross-section of the duct. Velocity should be calculated for each velocity pressure reading, and the average of the velocities should be used. Do not average the velocity pressure; average the velocities. For round ducts over 6 inches diameter, the following locations will give areas of equal concentric area (see Fig. 5.20).

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.974 Diameters.918 Diameters

.856 Diameters

.774 Diameters

.685 Diameters

.026 Diameters

.082 Diameters

.146 Diameters

.226 Diameters

.342 Diameters

Fig.5.20 Traverse points for circular duct

For the best results, one set of readings should be taken in one direction and another set at a 900 angle to the first. For square ducts, the readings can be taken in 16 equally spaced areas. If it is impossible to traverse the duct, an approximate average velocity can be calculated by measuring the velocity pressure at the centre of the duct and calculating it. This value is reduced to an approximate average by multiplying by 0.9.

Air density calculationThe first calculation is to determine the density of the air. To calculate the velocity and volume from the velocity pressure measurements, it is necessary to know the density of the air. Density is dependent on altitude and temperature.

Gas Density Where,t0C temperature of gas/air at site condition

Velocity calculationOnce the air density and velocity pressure have been established, the velocity can be determined from the equation:

Velocity (v), m/s = Where,Cp = Pitot tube constant, 0.85 (or) as given by the manufacturer

=Average differential pressure measured by the pitot tube by taking measurements at a number of points over the entire cross section of the duct

= Density of air or gas at test conditions

Volume calculationThe volume in a duct can be calculated for the velocity using the equation:Volumetric flow (Q), m3/sec = Velocity, v (m/sec) x Area (m2 ).

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Fan efficiencyFan manufacturers generally mention fan efficiency in two ways: mechanical efficiency (sometimes called the total efficiency) and static efficiency. Both measure how well the fan converts horsepower into flow and pressure.The equation for determining mechanical efficiency is:

The static efficiency equation is the same except that outlet velocity pressure is not added to the fan static pressure

Drive motor kW can be measured by a load analyser. This kW multiplied by motor efficiency gives the shaft power to the fan.

5.11 Energy Savings OpportunitiesMinimising demand on the fan.•

Minimising excess air level in combustion systems to reduce FD fan and ID fan load. �Minimising air in-leaks in the hot flue gas path to reduce the ID fan load, especially in case of kilns, boiler �plants, furnaces, etc. Cold air in-leaks increase ID fan load tremendously due to the density increase of flue gases and in-fact choke up the capacity of the fan, resulting in a bottleneck for the boiler / furnace.In-leaks / out-leaks in air conditioning systems also have a major impact on energy efficiency and fan power �consumption and need to be minimised.

The findings of performance assessment trials will automatically indicate potential areas for improvement, • which could be one or more of the following:

change of impeller by a high efficiency impeller along with cone �change of fan assembly as a whole, by a higher efficiency fan �impeller derating (by a smaller dia impeller) �change of metallic/Glass Reinforced Plastic (GRP) impeller by the more energy efficient hollow FRP impeller �with aerofoil design, in case of axial flow fans, where significant savings have been reportedfan speed reduction by pulley dia modifications for derating �option of two speed motors or variable speed drives for variable duty conditions. �option of energy efficient flat belts, or, cogged, raw edged V belts in place of conventional V belt systems, �for reducing transmission lossesadopting inlet guide vanes in place of the discharge damper control �minimising system resistance and pressure drops by improvements in the duct system �

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SummaryFans generate a pressure to move air (or gases) against a resistance caused by ducts, dampers, or other components • in a fan system. The fan rotor receives energy from a rotating shaft and transmits it to the air.A blower is another name for a fan that operates where the resistance to the flow is primarily on the downstream • side of the fan. Most manufacturing plants use fans and blowers for ventilation and for industrial processes that need an air flow.Fans and blowers provide air for ventilation and industrial process requirements.• Fan and blower selection depends on the volume flow rate, pressure, type of material handled, space limitations, • and efficiency. Fans are divided into two general categories:

centrifugal flow �axial flow �

In centrifugal flow, airflow changes direction twice – once when entering and then when leaving (forward curved, • backward curved or inclined, radial. In axial flow, air enters and leaves the fan with no change in direction (propeller, tube axial, vane axial).Blowers can achieve much higher pressures than fans, as high as 1.20 kg/cm• 2. Major types are centrifugal blower and positive-displacement blower. Centrifugal blowers look more like centrifugal pumps than fans. The impeller is typically gear-driven and rotates as fast as 15,000 rpm. In multi-stage blowers, air is accelerated as it passes through each impeller.Positive-displacement blowers have rotors, which "trap" air and push it through the housing. Positive-displacement • blowers provide a constant volume of air even if the system pressure varies.The term “system resistance” is used when referring to the static pressure. The system resistance is the sum • of static pressure losses in the system. System resistance is a function of the configuration of ducts, pickups, elbows and the pressure drops across equipment-for example bagfilter or cyclone. Fan characteristics can be represented in the form of fan curve(s). The fan curve is a performance curve for • the particular fan under a specific set of conditions. The fan curve is a graphical representation of a number of interrelated parameters. The fans operate under a predictable set of laws concerning speed, power and pressure. A change in speed (RPM) • of any fan will predictably change the pressure rise and power necessary to operate it at the new RPM. The choice of fan type for a given application depends on the magnitudes of the required flow and static pressure. For a given fan type, the selection of the appropriate impeller depends additionally on rotational speed. When a fan volume change is required on a permanent basis, and the existing fan can handle the change in the • capacity, volume change can be achieved with a speed change. The simplest way to change the speed is with a pulley change.Dampers can be located at the inlets or outlets. Dampers provide a means of changing the air volume by adding • or removing system resistance. This resistance forces the fan to move up or down along its characteristic curve, generating more or less air without changing fan speed.Fans are tested for field performance by the measurement of flow, head and temperature on the fan side and • electrical motor kW input on the motor side.Energy saving opportunities include, minimising demand on the fan and the findings of performance assessment • trials will automatically indicate potential areas for improvement

ReferencesYahya, S. M., 2005. • Turbines compressors and fans, 3rd ed., Tata McGraw-Hill Publication.Bleier, F., 1997. • Fan Handbook: Selection, Application, and Design, 1st ed., McGraw-Hill Professional Publication.Fans and Blowers• [Pdf] Available at: <http://www.enercon.gov.pk/images/pdf/3ch5.pdf> [Accessed 5 July 2013].

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How to Select a Fan or Blower • [Pdf] Available at: <http://www.cincinnatifan.com/manuals/HowToSelectAFanOrBlower.pdf> [Accessed 5 July 2013].2011. • Fans and Blowers [Video online] Available at: <https://www.youtube.com/watch?v=M0ZmidNA520> [Accessed 5 July 2013].2008. • Blowers and Industrial Fans [Video online] Available at: <https://www.youtube.com/watch?v=Ua1vQKWL2P8> [Accessed 5 July 2013].

Recommended ReadingAbbi, Y. P. & Jain, S., 2006. Handbook on Energy Audit and Environment Management, TERI Press.• Heumann, W. L., 1997. • Industrial air pollution control systems. McGraw-Hill Professional publication.Srinivasulu, P. & Vaidyanathan, C. V., 1977. • Handbook of machine foundations. Tata McGraw-Hill Publication.

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Self AssessmentThe efficiency of backward curved fans compared to forward curved fans is______.1.

highera. lowerb. samec. noned.

Which of the following axial fan types is most efficient?2. Propellera. Tube axialb. Vane axialc. Radiald.

Centrifugal fans increase the speed of an air stream with a_____________impeller.3. vibratinga. rotatingb. circulatingc. movingd.

Which of the following is not a centrifugal fan type?4. Vane axiala. Radialb. Airfoil, backwardc. Forward curvedd.

Axial fans are best suitable for________ application.5. large flow, low heada. low flow, high headb. high head, large flowc. low flow, low headd.

The efficiency of forward curved fans compared to backward curved fans is ________.6. highera. lowerb. samec. mediumd.

Varying the RPM of a fan by 10% varies the pressure by7. 19%a. 29%b. 10%c. 11%d.

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Varying the RPM of a fan by 10% varies the flow by _______.8. 20%a. 30%b. 40%c. 10%d.

Varying the RPM of a fan by 10% varies the power by __________.9. 37%a. 10%b. 27%c. 35%d.

The intersection of system curve with fan operating curve is called __________.10. design point a. operating pointb. selection pointc. shut off pointd.

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Chapter VI

Pumps and Pumping System

Aim

The aim of this chapter is:

explain pumps and pumping system•

enlist types of pumps•

explicate efficient system operation•

Objectives

The objectives of the chapter are:

explain centrifugal pump•

explicate pump curves•

elucidate flow control strategies and energy conservation opportunities•

Learning Outcome

At the end of this chapter, you will be able to:

identify a pumping system•

understand the concept of flow control strategies•

comprehend the important factor• s affecting energy conservation opportunities

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6.1 IntroductionA pump is a device used to move fluids, such as liquids, gases or slurries. A pump displaces a volume by physical • or mechanical action. Pumps fall into five major groups:

direct lift �displacement �velocity �buoyancy �gravity pumps �

Their names are self-explanatory, describing methods for moving a fluid.Pumps have two main purposes:•

transfer of liquid from one place to another place (e.g., water from an underground aquifer into a water �storage tank)circulate liquid around a system (e.g., cooling water or lubricants through machines and equipment) �

The main components of a pumping system are:• pumps �prime movers: electric motors, diesel engines or air system �piping; used to carry the fluid �valves; used to control the flow in the system �other fittings, controls and instrumentation �end-use equipment; which have different requirements (e.g., pressure, flow) and therefore, determine the �pumping system components and configuration

Examples include heat exchangers, tanks and hydraulic machines.The pump and the prime mover are typically the most energy inefficient components.•

6.2 Types of PumpsPumps have a variety of sizes for a wide range of applications. They can be classified according to their basic • operating principles as;

dynamic pumps �positive-displacement pumps �

Dynamic pumps are also characterized by their mode of operation;a rotating impeller converts kinetic energy • into pressure or velocity that is needed to pump the fluid. Dynamic pumps can be sub-classified into the following:

Centrifugal pumps �These are the most common pumps used for pumping water in industrial applications. Typically, more than 75% of the pumps installed in an industry are centrifugal pumps. This pump is further described below on account of its vast use.

Special effect pumps. �These are particularly used for specialized conditions at an industrial site.

Positive displacement pumps are distinguished by the way they operate: liquid is taken from one end and • positively discharged at the other end for every revolution. Positive displacement pumps are widely used for pumping fluids other than water, mostly viscous fluids. Positive-displacement pumps can be sub-classified into the following:

Rotary pumps �

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If the displacement within a pump is by rotary action of a gear, cam or vanes in a chamber of diaphragm in a fixed casing, it is called rotary pumps. Rotary pumps are further classified into internal gear, external gear, lobe and slide vane etc. These pumps are used for special services with particular conditions existing in industrial sites.

Reciprocating pumps �If the displacement within a pump is by reciprocation of a piston plunger, it is called reciprocating pumps. Reciprocating pumps are used only for pumping viscous liquids and oil wells.

Based on principle, any liquid can be handled by any of the pump designs. Based on the use of different pump • designs, the centrifugal pump is generally considered to be the most economical, followed by the rotary and reciprocating pumps. Although, positive displacement pumps are generally more efficient than centrifugal pumps, the benefit of higher efficiency tends to be offset by the increased maintenance costs.

Since, worldwide, centrifugal pumps account for a major part of electricity used by pumps, the focus of this chapter is on the centrifugal pump.

Fig. 6.1 Different types of pumps

6.2.1 Centrifugal Pump

A centrifugal pump is of a very simple design. The two main parts of the pump are:• impeller �diffuser �

The impeller, which is the only moving part, is attached to a shaft and driven by a motor. Impellers are generally • made of bronze, polycarbonate, cast iron, stainless steel as well as other materials.The diffuser (also called volute) houses the impeller and captures and directs the water off the impeller.• Water enters the centre (eye) of the impeller and exits the impeller with the help of centrifugal force. As water • leaves the eye of the impeller, a low-pressure area is created, causing more water to flow into the eye. Atmospheric pressure and centrifugal force cause this to happen. Velocity is developed as the water flows through the impeller spinning at high speed.The water velocity is collected by the diffuser and converted to pressure by specially designed passageways • that direct the flow to the discharge of the pump or to the next impeller, in case the pump has a multi-stage configuration.The pressure (head) that a pump will develop is directly related to the impeller diameter, the number of impellers, • the size of the impeller eye and shaft speed.Capacity is determined by the exit width of the impeller. The head and capacity are the main factors which • affect the horsepower size of the motor to be used. The more the quantity of water to be pumped, the more will be the energy required.

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Volute casing

Discharge

Vanes

Suction EyeImpeller

Fig. 6.2 Centrifugal pump

A centrifugal pump is not positive acting as it will not pump the same volume always. The greater the depth of • the water, the lesser is the flow from the pump.Also, when it pumps against increasing pressure, the lesser will be pumped. For these reasons, it is important • to select a centrifugal pump that is designed to do a particular job.Since the pump is a dynamic device, it is convenient to consider the pressure in terms of head i.e., metres of • liquid column.The pump generates the same head of liquid irrespective of the density of the liquid being pumped.• The actual contours of the hydraulic passages of the impeller and the casing are extremely important in order • to attain the highest efficiency possible.The standard convention for the centrifugal pump is to draw the pump performance curves showing flow on the • horizontal axis and head generated on the vertical axis. Efficiency, Power and NPSH Required (described later), are also all conventionally shown on the vertical axis, plotted against Flow, as illustrated in Fig. 6.3.

Fig. 6.3 Pump performance curve

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Given the significant amount of electricity attributed to pumping systems, even small improvements in pumping • efficiency could yield very significant savings of electricity.The pump is among the most inefficient components that comprise a pumping system, including the motor, • transmission drive, piping and valves.

6.2.2 Hydraulic Power, Pump Shaft Power and Electrical Input Power

Hydraulic power Ph = Where,hd-discharge headhs-suction headρ-density of the fluidg-acceleration due to gravity

Pump shaft power Ps = Hydraulic power, Ph / pump efficiency, ηpump

Electrical input power =

6.3 System Characteristics

In a pumping system, the objective, in most cases, is either to transfer a liquid from a source to a required • destination, e.g., filling a high level reservoir or to circulate liquid around a system, e.g., as a means of heat transfer in the heat exchanger.Pressure is needed to make the liquid flow at the required rate and this must overcome head ‘losses' in the • system. Losses are of two types: static and friction head.The static head is simply the difference in the height of the supply and destination reservoirs, as in Fig.6.4. In • this illustration, the flow velocity in the pipe is assumed to be very small.

Fig. 6.4 Static head

Another example of a system with only static head includes, pumping into a pressurised vessel with short pipe • runs. The static head is independent of flow and graphically would be shown as in Fig.6.5.

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Fig. 6.5 Static head vs. flow

The friction head (sometimes called dynamic head loss) is the friction loss on the liquid being moved, in pipes, • valves and equipment in the system. Friction tables are universally available for various pipe fittings and valves.These tables show friction loss per 100 feet (or metres) of a specific pipe size at various flow rates. In case of • fittings, friction is stated as an equivalent length of pipe of the same size. The friction losses are proportional to the square of the flow rate.A closed loop circulating system without a surface open to atmospheric pressure, would exhibit only friction • losses and would have a system friction head loss vs. flow curve as Fig.6.6.

Fig. 6.6 Friction head vs. flow

Most systems have a combination of the static and the friction head. The system curves for two cases are • shown in Fig.6.7 and Fig.6.8. The ratio of static to friction head over the operating range influences the benefits achievable from variable speed drives which shall be discussed later.

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Fig. 6.7 System with high static head

Fig. 6.8 System with low static head

Static head is a characteristic of the specific installation and reducing this head where this is possible generally • helps both the cost of the installation and the cost of pumping the liquid.Friction head losses must be minimised to reduce the pumping cost. But after eliminating unnecessary pipe • fittings and length, further reduction in the friction head will require a larger diameter pipe, which adds to the installation cost.

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6.4 Pump CurvesThe performance of a pump can be expressed graphically as head against flow rate. The centrifugal pump has a curve where the head falls gradually with increasing flow. This is called the pump characteristic curve (Head Flow curve) see Fig.6.9

Fig. 6.9 Head-flow curve

6.4.1 Pump Operating Point

When a pump is installed in a system, the effect can be illustrated graphically by superimposing pump and • system curves. The operating point will always be where the two curves intersect. Refer fig.6.10.

Fig. 6.10 Pump operating point

If the actual system curve is different in reality to that calculated one, the pump will operate at a flow and head • different to that expected.For a centrifugal pump, an increasing system resistance will reduce the flow eventually to zero, but the • maximum head is limited as shown. Even so, this condition is only acceptable for a short period without causing problems.

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An error in the system curve calculation is also likely to lead to a centrifugal pump selection, which is less than • optimal for the actual system head losses.Adding safety margins to the calculated system curve to ensure that a sufficiently large pump is selected, will • generally result in installing an oversized pump, which will operate at an excessive flow rate or in a throttled condition, which increases energy usage and reduces pump life.

6.5 Factors Affecting Pump PerformanceFactors which affect the performance of the pump are discussed in detail below:

6.5.1 Matching Pump and System Head-flow Characteristics

Centrifugal pumps are characterised by the relationship between the flow rate (Q) they produce and the pressure • (H) at which the flow is delivered. Pump efficiency varies with flow and pressure, and it is the highest at one particular flow rate.Fig.6.11 mentioned below shows a typical vendor-supplied head-flow curve for a centrifugal pump. Pump head-• flow curves are typically given for clear water. The choice of pump for a given application depends largely on how the pump head-flow characteristics match the requirement of the system downstream of the pump.

ITT Industries

NRSHR

Goulds Pumps

Fig. 6.11 Typical centrifugal pump performance curve

6.5.2 Effect of Over Sizing the PumpAs mentioned earlier, pressure losses to be overcome by the pumps are a function of flow. The system • characteristics are also quantified in the form of head-flow curves.The system curve is basically a plot of system resistance i.e., head to be overcome by the pump versus various • flow rates. The system curves change with the physical configuration of the system; example, the system curve depends upon the height or the elevation, diameter and length of the piping, the number and type of fittings and pressure drops across various equipment for example a heat exchanger.A pump is selected based on how well the pump curve and system head-flow curves match. The pump operating • point is identified as the point, where the system curve crosses the pump curve when they are superimposed on each other.

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The Fig.6.12 shows the effect on system curve with throttling.•

Fig. 6.12 Effect on system curve with throttling

In the system under consideration, water has to be first lifted to a height. This represents the static head. �Then, we make a system curve considering the friction and pressure drops in the system- this is shown as �the curve PA.Suppose, the operating conditions are estimated to be 500 m � 3/hr flow and 50m head, a pump curve is chosen, which intersects the system curve (Point A) at the pump's best efficiency point (BEP). But, in actual operation, we find that 300 m3/hr is sufficient. The reduction in the flow rate has to be effected by a throttle valve. In other words, we are introducing an artificial resistance in the system.Due to this additional resistance, the frictional part of the system curve increases and thus, the new system �curve will shift to the left, as shown as the curve PB.

So the pump has to overcome additional pressure in order to deliver the reduced flow. Now, the new system curve will intersect the pump curve at point B. The revised parameters are 300 m /hr at 70 m head. The red double arrow line shows the additional pressure drop due to throttling.

Note -The best efficiency point has shifted from 82% to 77% efficiency.So what we want is to actually operate at point C which is 300 m /hr on the original system curve. The head �required at this point is only 42 metres.What we now need is a new pump which will operate with its best efficiency point at C. But there are other �simpler options rather than replacing the pump. The speed of the pump can be reduced or the existing impeller can be trimmed (or new lower size impeller). �The pump curve QD represents either of these options.

6.5.3 Energy Loss in ThrottlingConsider a case (see fig.6.13) where we need to pump 68 m3/hr of water at 47m head. The pump characteristic curves (A…E) for a range of pumps are given in Fig.6.13.

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Fig. 6.13 Pump characteristic curves

If we select E, then the pump efficiency is 60%.Hydraulic Power = Q (m3/s) x Total head, hd - hs (m) x ρ (kg/m3) x g (m/s2) / 1000 = (68/3600) x 47 x 1000 x 9.81 1000 = 8.7 kWShaft Power = 8.7 / 0.60 = 14.5 KwMotor Power = 14.5 / 0.9 = 16.1Kw (considering a motor efficiency of 90%)

If we select A, the pump efficiency is 50% (drop from earlier 60%).Obviously, this is an oversize pump. Hence, the pump has to be throttled to achieve the desired flow. Throttling increases the head to be overcome by the pump. In this case, head is 76 metres.Hydraulic Power = Q (m3/s) x Total head, hd - hs(m) x ρ(kg/m3) x g(m/s2) / 1000 = (68/3600) x 76 x 1000 x 9.81 1000 = 14 kWShaft Power =14 / 0.50 = 28 KwMotor Power = 28 / 0.9 = 31 Kw (considering a motor efficiency of 90%)Hence, additional power drawn by A over E is

31 -16.1 = 14.9 kWExtra energy used = 8760 hrs/yr x 14.9 = 1, 30,524 kwh/annum = Rs. 5, 22, 096/annumIn this example, the extra cost of the electricity is more than the cost of purchasing a new pump.

6.6 Efficient Pumping System OperationTo understand a pumping system, one must realise that all of its components are interdependent. While examining • or designing a pump system, the process demands must be established first and the most energy efficient solution should be introduced.

Example, does the flow rate have to be regulated continuously or in steps? Can on-off batch pumping be used? What are the needed flow rates and how are they distributed in time?

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The first step to achieve energy efficiency in the pumping system is to target the end-use. A plant water balance • would establish a usage pattern and highlight areas where water consumption can be reduced or optimised. Good water conservation measures alone may eliminate the need for some pumps.Once flow requirements are optimised, then the pumping system can be analysed for energy conservation • opportunities. Basically this means matching the pump to the requirements by adopting the proper flow control strategies.Common symptoms that indicate opportunities for energy efficiency in pumps are given in Table 6.1.•

Symptom Likely Reason Best SolutionsThrottle valve-controlled systems

Oversized pump Trim impeller, smaller impeller, variable speed drive, two speed drive, lower rpm

Throttle valve-controlled systems

Oversized pump Trim impeller, smaller impeller, variable speed drive, two speed drive, lower rpm

Multiple parallel pump system with the same number of pumps always operating

Pump use not monitored or controlled

Install controls

Constant pump operation in a batch environment

Wrong system design

On-off controls

High maintenance cost (seals, bearings)

Pump operated far away from BEP

Match pump capacity with system requirement

Table 6.1 Symptoms indicating potential opportunity for energy savings

6.6.1 Effect of Speed Variation

As stated above, a centrifugal pump is a dynamic device with the head generated from a rotating impeller. There • is therefore a relationship between the impeller peripheral velocity and the generated head.Peripheral velocity is directly related to the shaft rotational speed for a fixed impeller diameter and so varying • the rotational speed has a direct effect on the performance of the pump.All the parameters shown in Fig.6.3 will change if the speed is varied and it is important to have an appreciation • of how these parameters vary in order to safely control a pump at different speeds. The equations relating to the rotodynamic pump performance parameters of flow, head and power absorbed, to speed are known as the Affinity Laws.According to the Affinity Law,•

Q α NH α N2

P α N3

Where,Q = Flow rateH = HeadP = Power absorbedN = Rotating speedEfficiency is essentially independent of speed

Flow: Flow is proportional to the speed and is given as; Q1 / Q2 = N1/ N2

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Example: 100 / Q2 = 1750/3500 Q2 = 200 m3/hr

Head : Head is proportional to the square of speed H1 / H2 = (N12) / (N22)Example: 100 / H2 = 17502 /35002

H2 = 400mPower (kW): Power is proportional to the cube of speed (kW)1/kW2 = (N13)/(N23)Example : 5/kW2 = 17503/ 35003 kW2 = 40

As seen from the above laws, doubling the speed of the centrifugal pump will increase the power consumption • by eight times. Conversely a small reduction in speed will result in drastic reduction in power consumption.This forms the basis for energy conservation in centrifugal pumps with varying flow requirements. The implications • of this can be better understood as shown in an example of a centrifugal pump in Fig.6.14 below.

Fig. 6.14 Example of speed variation effecting centrifugal pump performance

Points of equal efficiency on the curves for the three different speeds are joined to make the iso-efficiency lines, • showing that efficiency remains constant over small changes of speed, providing the pump continues to operate at the same position related to its best efficiency point (BEP).Affinity laws give a good approximation of how pump performance curves change with speed, but in order to • obtain the actual performance of the pump in a system, the system curve also has to be taken into account.

6.6.2 Effects of Impeller Diameter Change

Changing the impeller diameter gives a proportional change in the peripheral velocity, so it follows that there • are equations similar to the affinity laws, for the variation of performance with the impeller diameter D:

Q α DH α D2

P α D3

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Efficiency varies when the diameter is changed within a particular casing. Note the difference in iso-efficiency • lines in Fig.6.15 compared with Fig.6.14.

Fig. 6.15 Example effect of impeller diameter reduction on centrifugal pump performance

The relationships shown here apply to the case for changing only the diameter of an impeller within a fixed • casing geometry, which is a common practice for making small permanent adjustments to the performance of a centrifugal pump.Diameter changes are generally limited to reducing the diameter to about 75% of the maximum, i.e., a head • reduction to about 50%. Beyond this, efficiency and NPSH are badly affected. However, speed change can be used over a wider range without seriously reducing efficiency.Example, reducing the speed by 50% typically results in a reduction of efficiency by 1 or 2 percentage points. • The reason for the small loss of efficiency with the lower speed is that mechanical losses in seals and bearings, which generally represent <5% of total power, are proportional to speed, rather than speed cubed.It should be noted that if change in diameter is more than 5%, the accuracy of the squared and cubic relationships • can fall off and the pump manufacturer's performance curves should be referred to for precise calculations.The illustrated curves are typical of most centrifugal pump types. Certain high flow, low head pumps have • performance curve shapes somewhat different and have a reduced operating region of flows. This requires additional care in matching the pump to the system, when changing speed and diameter.

6.6.3 Pump Suction Performance (NPSH)

Liquid entering the impeller eye turns and is split into separate streams by the leading edges of the impeller • vanes, an action which locally drops the pressure below that in the inlet pipe to the pump.If the incoming liquid is at a pressure with insufficient margin above its vapour pressure, then vapour cavities • or bubbles appear along the impeller vanes just behind the inlet edges. This phenomenon is known as cavitation and has three undesirable effects mentioned below;

the collapsing cavitation bubbles can erode the vane surface, especially when pumping water-based �liquids

noise and vibration are increased, with possible shortened seal and bearing life•

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the cavity areas will initially partially choke the impeller passages and reduce pump performance. In extreme • cases, a total loss of pump developed head occursThe value by which the pressure in the pump suction exceeds the liquid vapour pressure, it is expressed as • a head of liquid and referred to as Net Positive Suction Head Available (NPSHA). This is a characteristic of system design.The value of NPSH needed at the pump suction to prevent the pump from cavitating is known as NPSH Required • (NPSHR). This is a characteristic of the pump design.The three undesirable effects of cavitation described above begin at different values of NPSHA and generally • there is cavitation erosion before there is a noticeable loss of pump head.However for a consistent approach, manufacturers and industry standards, usually define the onset of cavitation • as the value of NPSHR when there is a head drop of 3% compared to the head with cavitation free performance. At this point, cavitation is present and a prolonged operation at this point usually lead to damage. So a margin is usually applied by which NPSHA should exceed NPSHR.As would be expected, the NPSHR increases as the flow through the pump increases.(see fig 6.3). In addition, • as flow increases in the suction pipe work, friction losses also increase, giving a lower NPSHA at the pump suction, both of which give a greater chance that cavitation will occur.NPSHR also varies approximately with the square of speed the same way as pump head and conversion of • NPSHR from one speed to another can be made using the following equations;

Q α NNPSHR α N2

It should be noted however, that at very low speeds there is a minimum NPSHR plateau, NPSHR does not tend • to zero at zero speed. It is therefore essential to carefully consider NPSH in variable speed pumping.

6.7 Flow Control StrategiesDifferent strategies to control flow are:

6.7.1 Pump Control by Varying Speed

To understand how speed variations change the duty point, the pump and system curves are over-laid. Two • systems which are considered:

one with only friction loss �another where the static head is high in relation to the friction head �It will be seen that the benefits are different. �

The drop in pump efficiency during speed reduction in a system with static head reduces the economic benefits • of variable speed control. There may still be overall benefits but economics should be examined on a case-by-case basis.Usually, it is advantageous to select the pump such that the system curve intersects the full speed pump curve • to the right of best efficiency in an order such that the efficiency first increases as the speed is reduced and then decreases.This can extend the useful range of variable speed operation in a system with a static head. The pump manufacturer • should be consulted on the safe operating range of the pump.It is relevant to note that flow control by speed regulation is always more efficient than by the control valve. In • addition to energy savings, there could be other benefits of lower speed.The hydraulic forces on the impeller created by the pressure profile inside the pump casing reduce approximately • with the square of speed. These forces are carried by the pump bearings and hence reducing the speed increases the bearing life.It can be shown that for a centrifugal pump, bearing life is inversely proportional to the 7th power of speed. In •

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addition, vibration and noise are reduced and the seal life is increased providing the duty point remains within the allowable operating range.In corollary to the above statement, small increase in the speed of a pump significantly increases the power • absorbed, shaft stress and bearing loads. It should be remembered that the pump and motor must be sized for the maximum speed at which the pump set will operate.At higher speed, the noise and vibrations from both the pump and motor will increase, although for small increase, • the change will be small. If the liquid contains abrasive particles, increasing speed will give a corresponding increase in the surface wear in the pump and pipe work.The effect on the mechanical seal of the change in seal chamber pressure should be reviewed with the pump or • seal manufacturer, if the speed increase is large. Conventional mechanical seals operate satisfactorily at very low speeds and generally there is no requirement for a minimum speed to be specified, however due to their method of operation, gas seals require a minimum peripheral speed of 5m/s.

Fig. 6.16 Example of the effect of pump speed change in a system with only friction loss

According to the Fig.6.16,reducing speed in the friction loss system moves the intersection point on the system curve along a line of • constant efficiencythe operating point of the pump, relative to its best efficiency point, remains constant and the pump continues • to operate in its ideal regionAffinity laws are obeyed, which means that there is a substantial reduction in the power absorbed accompanying • the reduction in the flow and head, making variable speed the ideal control method for systems with friction loss

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Fig. 6.17 Example for the effect of pump speed change for a system with high static head

Based on the Fig.6.17,in a system where the static head is high, as illustrated, the operating point for the pump moves relative to the • lines of constant pump efficiency when the speed is changedthe reduction in flow is no longer proportional to speed• a small turn down in speed could give a big reduction in the flow rate and pump efficiency, which could result • in the pump operating in a region where it could be damaged if it ran for an extended period of time even at the lower speedat the lowest speed illustrated, (1184 rpm), the pump does not generate sufficient head to pump any liquid into • the system, i.e. pump efficiency and flow rate are zero and with energy still being input to the liquid, the pump becomes a water heater and damaging temperatures can quickly be reached

6.7.2 Pumps in Parallel Switched to Meet Demand

Another energy efficient method of flow control, particularly for systems where the static head is a high proportion • of the total, is to install two or more pumps to operate in parallel. Variation of flow rate is achieved by switching on and off additional pumps to meet demand. The combined pump curve is obtained by adding the flow rates at a specific head. The head/flow rate curves for two and three pumps are shown in Fig.6.18.

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Fig. 6.18 Typical head-flow curves for pumps in parallel

The system curve is usually not affected by the number of pumps that are running. For a system with a combination • of static and friction head loss, it can be seen in Fig.6.19, the operating point of the pumps on their performance curves moves to a higher head and hence, a lower flow rate per pump, as more pumps are started.It is also apparent that the flow rate with two pumps running is not double that of a single pump. If the system • head were static, the flow rate would be proportional to the number of pumps operating.

FLOW RATE

Fig. 6.19 Typical head-flow curves for pumps in parallel, with system curve illustrated

It is possible to run pumps of different sizes in parallel provided their closed valve heads are similar. By arranging • different combinations of pumps running together, a larger number of different flow rates can be provided into the system.Care must be taken while running pumps in parallel to ensure that the operating point of the pump is controlled • within the region deemed as acceptable by the manufacturer.It can be seen from Fig.6.19 that if 1 or 2 pumps were stopped then the remaining pump(s) would operate well • along the curve, where the NPSH is higher and vibration level increased, giving an increased risk of operating problems.

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6.7.3 Stop/Start Control

In this control method, the flow is controlled by switching pumps on or off. It is necessary to have a storage • capacity in the system e.g., a wet well, an elevated tank or an accumulator type pressure vessel.The storage can provide a steady flow to the system with an intermittent operating pump. When the pump runs, • it does so at the chosen (presumably optimum) duty point and when it is off, there is no energy consumption.If intermittent flow, stop/start operation and the storage facility is acceptable, this is an effective approach to • minimise energy consumption.The stop/start operation leads to additional loads on the power transmission components and increased heating • in the motor. The frequency of the stop/start cycle should be within the motor design criteria and checked with the pump manufacturer.It may also be used to benefit from “off peak” energy tariffs by arranging the run times during the low tariff • periods.To minimise energy consumption with stop start control, it is better to pump at flow rate as low as possible for • the process to permit. This minimises friction losses in the pipe and an appropriately small pump can be installed. Example, pumping at half the flow rate for twice as long can reduce energy consumption to a quarter.

6.7.4 Flow Control Valve

With this control method, the pump runs continuously and a valve in the pump discharge line is opened or closed • to adjust the flow to the required value.

Fig. 6.20 Control of pump flow by changing system resistance using a valve

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To understand how the flow rate is controlled, see Fig.6.20. With the valve fully open, the pump operates at • “Flow 1”. When the valve is partially closed, it introduces an additional friction loss in the system, which is proportional to the flow squared.The new system curve cuts the pump curve at “Flow 2”, which is the new operating point. The head difference • between the two curves is the pressure drop across the valve.It is usual practice with valve control to have the valve 10% shut even at maximum flow. Energy is therefore • wasted, overcoming the resistance through the valve at all flow conditions.There is some reduction in pump power absorbed at the lower flow rate (see Fig.6.19), but the flow multiplied by • the head drop across the valve, is wasted energy. It should also be noted that, while the pump will accommodate changes in its operating point as far as it is able within its performance range, it can be forced to operate high on the curve, where its efficiency is low and its reliability is affected.Maintenance cost of control valves can be high, particularly on corrosive and solids-containing liquids.•

Therefore, the lifetime cost could be unnecessarily high.

6.7.5 By-pass Control

With this control approach, the pump runs continuously at the maximum process demand duty, with a permanent • by-pass line attached to the outlet. When a lower flow is required, the surplus liquid is bypassed and returned to the supply source.An alternative configuration may have a tank supplying a varying process demand, which is kept full by a fixed • duty pump running at the peak flow rate.Most of the time the tank overflows and recycles back to the pump suction. This is even less energy efficient • than a control valve because there is no reduction in power consumption with a reduced process demand.The small by-pass line sometimes installed to prevent a pump running at zero flow, is not a means of flow • control, but required for the safe operation of the pump.

6.8 Fixed Flow Reduction6.8.1 Impeller Trimming

Impeller trimming refers to the process of machining the diameter of an impeller to reduce the energy added • to the system fluid.Impeller trimming offers a useful correction to pumps that, through overly conservative design practices or • changes in system loads are oversized for their application.Trimming an impeller provides a level of correction below buying a smaller impeller from the pump manufacturer. • But in many cases, the next smaller size impeller is too small for the pump load.Also, smaller impellers may not be available for the pump size in question and impeller trimming is the only • practical alternative short of replacing the entire pump/motor assembly. (See Fig.6.21 (a) & (b) for before and after impeller trimming).

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Fig. 6.21(a) Before impeller trimming

Fig. 6.21(b) After impeller trimming

Impeller trimming reduces tip speed, which in turn directly lowers the amount of energy imparted to the system • fluid and lowers both the flow and pressure generated by the pump.The Affinity Laws, which describe centrifugal pump performance, provide a theoretical relationship between • the impeller size and the pump output (assuming constant pump speed).

Where,Q = flowH = headBHP = brake horsepower of the pump motorSubscript 1 = original pumpSubscript 2 = pump after impeller trimmingD = Diameter

Q2 = (D1/D2)Q1H2 = (D1/D2)

2 H1BHP2 = (D1/D2)

3 BHP1

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Trimming an impeller changes its operating efficiency, and the non-linearties of the Affinity Laws with respect to impeller machining complicate the prediction of pump performance. Consequently, impeller diameters are rarely reduced below 70 percent of their original size.

6.8.2 Meeting Variable Flow ReductionVariable Speed Drives (VSDs)

In contrast, pump speed adjustments provide the most efficient means of controlling pump flow. By reducing • pump speed, less energy is imparted to the fluid and less energy needs to be throttled or bypassed.There are two primary methods of reducing pump speed;•

Multiple-speed pump motors �Variable speed drives (VSDs) �

Although both directly control pump output, multiple-speed motors and VSDs serve entirely separate applications. • Multiple-speed motors contain a different set of windings for each motor speed; consequently, they are more expensive and less efficient than single speed motors. Multiple speed motors also lack subtle speed changing capabilities within discrete speeds.VSDs allow pump speed adjustments over a continuous range avoiding the need to jump from speed to speed • as with multiple-speed pumps. VSDs control pump speeds using several different types of mechanical and electrical systems.Mechanical VSDs include hydraulic clutches, fluid couplings and adjustable belts and pulleys. Electrical VSDs • include eddy current clutches, wound-rotor motor controllers, and variable frequency drives (VFDs).VFDs adjust the electrical frequency of the power supplied to a motor to change the motor's rotational speed. • VFDs are by far the most popular type of VSD.However, pump speed adjustment is not appropriate for all systems. In applications with high static head, slowing • a pump, risks inducing vibrations and creating performance problems that are similar to those found when a pump operates against its shutoff head.For systems in which the static head represents a large portion of the total head, caution should be used in • deciding whether to use VFDs. Operators should review the performance of VFDs in similar applications and consult VFD manufacturers to avoid the damage that can result when a pump operates too slowly against a high static head.

BEP

Fig. 6.22 Effect of VFD

For many systems, VFDs offer a means to improve the pump operating efficiency despite changes in operating • conditions. The effect of slowing pump speed on pump operation is illustrated by the three curves in Fig.6.23.

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When a VFD slows a pump, its head/flow and brake horsepower (BHP) curves drop down and its efficiency • curve shifts to the left. This efficiency response provides an essential cost advantage; by keeping the operating efficiency as high as possible across variations in the system's flow demand, the energy and maintenance costs of the pump can be significantly reduced.VFDs may offer operating cost reductions by allowing a higher pump operating efficiency, but the principal • savings derive from the reduction in frictional or bypass flow losses. Using a system perspective to identify areas in which fluid energy is dissipated in non-useful work often reveals opportunities for operating cost reductions.Example, in many systems, increasing flow through by-pass lines does not impact the backpressure on a pump • noticeably. Consequently, in these applications pump efficiency does not necessarily decline during periods of low flow demand. By analysing the entire system, however, the energy lost in pushing fluid through bypass lines and across throttle valves can be identified.Another system benefit of VFDs is a soft start capability. During start-up, most motors experience in-rush • currents that are 5 -6 times higher than normal operating currents.This high current fades when the motor spins up to normal speed. VFDs allow the motor to be started with a • lower start-up current (usually only about 1.5 times the normal operating current). This reduces wear on the motor and its controller.

6.9 Steps for Energy Efficiency in Pumping SystemEnsure adequate NPSH at site of installation.• Ensure availability of basic instruments at pumps like pressure gauges, flow meters.• Operate pumps near the best efficiency point.• Modify pumping systems and pump losses to minimise throttling.• Adapt to wide load variation with variable speed drives or sequenced control of multiple units.• Stop running multiple pumps - add an auto-start for an on-line spare or add a booster pump in the problem • area.Use booster pumps for small loads requiring higher pressures.• Increase fluid temperature differentials to reduce pumping rates in case of heat exchangers. Repair seals and • packing to minimise water loss by dripping.Balance the system to minimise flows and reduce pump power requirements.• Avoid pumping head with a free-fall return (gravity); Use siphon effect to advantage• Conduct water balance to minimise water consumption.• Avoid cooling water re-circulation in DG sets, air compressors, refrigeration systems, cooling towers, feed water • pumps, condenser pumps and process pumps.In multiple pump operations, carefully combine the operation of pumps to avoid throttling.• Provide booster pump for few areas of higher head.• Replace old pumps by energy efficient pumps.• In the case of over designed pump, provide variable speed drive or downsize /replace impeller or replace with • correct sized pump for efficient operation.Optimise the number of stages in the multi-stage pump in case of head margins. Reduce system resistance by • pressure drop assessment and pipe size optimisation.

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SummaryA pump is a device used to move fluids, such as liquids, gases or slurries.• Centrifugal pumps are the most common pumps used for pumping water in industrial applications. Typically, • more than 75% of the pumps installed in an industry are centrifugal pumps.Affinity laws states the relations between Power, Pressure, Flow and Speed.• System resistance is an obstruction to flow. This is mainly caused by various components like pipes, valves, • flanges, bends, etc. Each of these presents resistance in the form of a pressure drop across it and consumes energy. Hence, it is essential to reduce the system resistance to the bare minimum possible.The performance of a pump can be expressed graphically as head against flow rate. The centrifugal pump has • a curve where the head falls gradually with increasing flow. This is called the pump characteristic curve (Head Flow curve).Flow can be controlled by various methods. The best method depends on the type of application and the operating • point. The strategy should be well planned so that the maximum efficiency can be achieved.Many opportunities exist for energy savings in pumping systems. These include reduction in the system resistance, • preventing leakages, checking the foot valve, impeller trimming, speed control. Again, speed can be controlled by pulley change, dual speed motor, variable speed drive etc.The best operating point for a pump is the intersection of the Pump Performance Curves and the operating • characteristics of the pump as given by manufacturer.The value by which the pressure in the pump suction exceeds the liquid vapour pressure, and is expressed as a • head of liquid, referred to be the Net Positive Suction Head Available (NPSHA).In stop/start control method, the flow is controlled by switching pumps on or off. It is necessary to have a storage • capacity in the system e.g., a wet well, an elevated tank or an accumulator type pressure vessel.Impeller trimming refers to the process of machining the diameter of an impeller to reduce the energy added • to the system fluid.Ensure adequate NPSH at site of installation; ensure availability of basic instruments at pumps like pressure • gauges, flow meters, operate pumps near the best efficiency point, modify pumping systems and pump losses to minimise throttling are few steps for energy efficiency in pumping system.

ReferencesRishel, J., Durkin, T. & Kincaid, B., 2006. • HVAC Pump Handbook, 2nd ed., McGraw-Hill Professional publication.Menon, E. S., 2009. • Working Guide to Pump and Pumping Stations: Calculations and Simulations,1st ed., Gulf Professional Publishing.Pumps and Pumping System• [Pdf] Available at: < http://www.beeindia.in/energy_managers_auditors/documents/guide_books/3Ch6.pdf> [Accessed 5 July 2013].Chapter 6 Introduction to Pumping Systems• [Pdf] Available at: < dec.alaska.gov/water/opcert/Docs/Chapter6.pdf‎> [Accessed 5 July 2013].2011. • System Head Curves:How to have a successful pumping system [Video online] Available at: < https://www.youtube.com/watch?v=okKKZiRqrPI> [Accessed 5 July 2013].2012. • Progressing Cavity Pumping System [Video online] Available at: < https://www.youtube.com/watch?v=v5VnnBtXtlc> [Accessed 5 July 2013].

Recommended Reading2004. • Variable Speed Pumping: A Guide to Successful Applications, Elsevier Ltd.Mackay, R. C., 2004. • The Practical Pumping Handbook, Elsevier Science publication.Nourbakhsh, A., Jaumotte, A., Hirsch, C. & Parizi, H. B., 2007. • Turbopumps and Pumping Systems, 1st ed., Springer Publication.

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Self AssessmentIf the speed of the pump is doubled, power goes up by _____.1.

2 timesa. 6 times b. 8 times c. 4 timesd.

Friction losses in a pumping system is _________ .2. proportional to Qa. proportional to Qb. 2

proportional to Qc. 3 proportional to Qd. 4

The first step to achieve energy efficiency in the pumping system is to target ________.3. the end-usea. the start-useb. the middle-usec. the begin-used.

Power is proportional to _______.4. square of speeda. cube of speedb. square root of speedc. cube root of speedd.

Flow is proportional to ________.5. square of speeda. cube of speedb. square root of speedc. speedd.

If the incoming liquid is at a pressure with insufficient margin above its vapour pressure, then vapour cavities 6. or bubbles appear along the impeller vanes just behind the inlet edges. This phenomenon is known as _____.

gravitationa. cavitationb. invitationc. vaporisationd.

The value, by which the pressure in the pump suction exceeds the liquid vapour pressure, is expressed as a head 7. of liquid and referred to as _______.

NPSHa. NPSHRb. NPSHAc. NPPSHRd.

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The process of machining the diameter of an impeller to reduce the energy added to the system fluid is 8. called_______.

impeller fillinga. impeller edgingb. impeller borderingc. impeller trimmingd.

A device used to move fluids, such as liquids, gases or slur is called _______.9. pumpa. motorb. blowerc. fand.

Relations between Power, Pressure, Flow and Speed is known as _______.10. centrifugal lawa. axial lawb. affinity lawc. parallel lawd.

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Chapter VII

Cooling Tower

Aim

The aim of this chapter is:

explain cooling towers and types•

explicate efficient system operation of cooling towers•

define assessment of cooling towers•

Objectives

The objectives of the chapter are:

explain performance evaluation of cooling towers•

elucidate flow control strategies•

describe energy saving opportunities of cooling towers•

Learning outcome

At the end of this chapter, you will be able to:

define a cooling tower•

identify types of cooling tower•

understand the concept of effi• cient system operation, flow control strategies and energy saving opportunities

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7.1 IntroductionCooling towers are a very important part of many chemical plants. The primary task of a cooling tower is to reject heat into the atmosphere. They represent a relatively inexpensive and dependable means of removing low-grade heat from cooling water. The make-up water source is used to replenish water lost to evaporation. Hot water from heat exchangers is sent to the cooling tower. The water exits the cooling tower and is sent back to the exchangers or to other units for further cooling. A typical closed loop cooling tower system is shown in Fig. 7.1.

Fig. 7.1 Cooling water system

7.1.1 Cooling Tower Types

Cooling towers fall into two main categories;• natural draft �mechanical draft �

Natural draft towers use very large concrete chimneys to introduce air through the media. Due to the large size • of these towers, they are generally used for water flow rates above 45,000 m /hr. These types of towers are used only by utility power stations.Mechanical draft towers utilise large fans to force or suck air through circulated water. The water falls downward • over fill surfaces, which helps to increase the contact time between the water and the air - this helps to maximise heat transfer between the two.Cooling rates of Mechanical draft towers depend upon their fan diameter and speed of operation. Since, the • mechanical draft cooling towers are much more widely used, let us learn about it more.

7.1.2 Mechanical Draft Towers

Mechanical draft towers are available in the following airflow arrangements:• counter flow induced draft �counter flow forced draft �cross flow induced draft �

In the counter flow induced draft design, hot water enters at the top, while air is introduced from the bottom • and exits at the top. Both forced and induced draft fans are used.In cross flow induced draft towers, the water enters at the top and passes over the fill. Air however, is introduced • at the side either on one side (single-flow tower) or opposite sides (double-flow tower).An induced draft fan draws air across the wetted fill and expels it through the top of the structure. Fig. 7.2 • illustrates various cooling tower types.

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Fig. 7.2 Cooling tower types

Mechanical draft towers are available in a large range of capacities. Normal capacities range from approximately • 10 tons, 2.5 m3/hr flow to several thousand tons and m3/hr. Towers can be factory built or field erected; example, concrete towers are only field erected.Many towers are constructed so that they can be grouped together to achieve the desired capacity. Thus, many • cooling towers are assemblies of two or more individual cooling towers or “cells.”The number of cells they have e.g., an eight- cell tower, often refers to such towers. Multiple-cell towers can be • linear, square, or round depending upon the shape of the individual cells and whether the air inlets are located at the sides or bottoms of the cells.

7.1.3 Components of a Cooling TowerThe basic components of an evaporative tower are as follows:

frame and casing• fill �cold water basin �drift eliminators �air inlet �louvers �nozzles �fans �

Frame and casing: Most towers have structural frames that support the exterior enclosures (casings), motors, • fans and other components. With some smaller designs such as, some glass fibre units, the casing may essentially be the frame.

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Fill• Most towers employ fills (made of plastic or wood) to facilitate heat transfer by maximising water and air contact. The fill can either be splash or film type.

With splash fill, water falls over successive layers of horizontal splash bars, continuously breaking into �smaller droplets, while also wetting the fill surface. The plastic splash fill promotes better heat transfer than the wood splash fill.Film fill consists of thin, closely spaced plastic surfaces over which the water spreads, forming a thin film in �contact with the air. These surfaces may be flat, corrugated, honeycombed, or other patterns. The film type of fill is the more efficient and provides the same heat transfer in a smaller volume than the splash fill.

Cold water basin:The cold water basin located at or near the bottom of the tower receives the cooled water that flows down through the tower and fills. The basin usually has a sump or low point for the cold water discharge connection. In many tower designs, the cold water basin is beneath the entire fill. In some forced draft counter flow designs however, the water at the bottom of the fill is channelled to a perimeter trough that functions as the cold water basin. Propeller fans are mounted beneath the fill to blow the air up through the tower. With such a design, the tower is mounted on legs to providing easy access to the fans and their motors.

Drift eliminators: These capture water droplets entrapped in the air stream that otherwise would be lost to the • atmosphere.Air inlet: This is the point of entry for the air entering a tower. The inlet may take up an entire side of a tower • cross flow design or be located low on the side or the bottom of counter flow designs.Louvers: Generally, cross-flow towers have inlet louvers. The purpose of louvers is to equalise airflow into the • fill and retain the water within the tower. Many counter flow tower designs do not require louvers.Nozzles: These provide the water sprays to wet the fill. Uniform water distribution at the top of the fill is essential • to achieve proper wetting of the entire fill surface. Nozzles can either be fixed in place and have either round or square spray patterns or can be part of a rotating assembly as found in some circular cross-section towers.Fans: Both axial (propeller type) and centrifugal fans are used in towers. Generally, propeller fans are used in • induced draft towers and both propeller and centrifugal fans are found in forced draft towers. Depending upon their size, propeller fans can either be fixed or variable pitch.

A fan having non-automatic adjustable pitch blades permits the same fan to be used over a wide range of kW with the fan adjusted to deliver the desired air flow at the lowest power consumption.Automatic variable pitch blades can vary air flow in response to changing load conditions.

7.1.4 Tower Materials

In the early days of cooling tower manufacture, towers were constructed primarily of wood. Wooden components • included the frame, casing, louvers, fill and often the cold water basin. If the basin was not of wood, it likely was of concrete.Today tower manufacturers fabricate towers and tower components from a variety of materials. Often several • materials are used to enhance corrosion resistance, reduce maintenance and promote reliability and long service life.Galvanized steel, various grades of stainless steel, glass fibre and concrete are widely used in tower construction • as well as aluminium and various types of plastics for some components.Wood towers are still available but they have glass fibre rather than wood panels (casing) over the wood • framework. The inlet air louvers may be glass fibre, the fill may be plastic, and the cold water basin may be steel.Larger towers sometimes, are made of concrete. Many tower casings and basin are constructed of galvanised • steel or where corrosive atmosphere is a problem, stainless steel.Sometimes a galvanised tower has a stainless steel basin. Glass fibre is also widely used for cooling tower • casings and basins giving long life and protection from the harmful effects of many chemicals.

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Plastics are widely used for fill, including PVC, polypropylene and other polymers. Treated wood splash fill • is still specified for wood towers, but plastic splash fill is also widely used when water conditions mandate the use of splash fill. Film fill, because it offers greater heat transfer efficiency, is the fill of choice for applications where the circulating water is generally free of debris that could plug the fill passageways.Plastics also find wide use as nozzle materials. Many nozzles are being made of PVC, ABS, polypropylene and • glass-filled nylon. Aluminium, glass fibre and hot-dipped galvanized steel are commonly used fan materials. Centrifugal fans are often fabricated from galvanized steel. Propeller fans are fabricated from galvanized, aluminium or moulded glass fibre reinforced plastic.

7.2 Cooling Tower PerformanceThe important parameters, from the point of determining the performance of cooling towers, are:

Fig. 7.3 Range and approach

“Range” is the difference between the cooling tower water inlet and outlet temperature. (see fig. 7.3).• “Approach” is the difference between the cooling tower outlet cold water temperature and the ambient wet bulb • temperature. Although, both range and approach should be monitored, the “Approach” is a better indicator of cooling tower performance (see fig.7.3).Cooling tower effectiveness (in percentage) is the ratio of range, to the ideal range, i.e., difference between • cooling water inlet temperature and ambient wet bulb temperature, or in other words it is = Range / (Range + Approach).Cooling capacity is the heat rejected in kCal/hr or TR, given as a product of the mass flow rate of water, specific • heat and temperature difference.Evaporation loss is the water quantity evaporated for cooling duty and theoretically, for every 10, 00,000 kCal • heat rejected, evaporation quantity works out to 1.8 m3. An empirical relation used often is:

Evaporation loss (m3/hr) = 0.00085 x 1.8 x circulation rate (m3/hr) x (T1 –T2)where,T1 –T2 = Temperature difference between inlet and outlet waterSource: Perry's Chemical Engineers Handbook (Page: 12-17)

Cycles of concentration (C.O.C) is the ratio of dissolved solids in circulating water to the dissolved solids in • make up water.

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Blow down losses depend upon cycles of concentration and the evaporation losses and is given by relation:• Blow Down = Evaporation Loss / (C.O.C. – 1)

Liquid/Gas (L/G) ratio of a cooling tower is the ratio between the water and the air mass flow rates. Against • design values, seasonal variations require adjustment and tuning of water and air flow rates to get the best cooling tower effectiveness through measures like water box loading changes, blade angle adjustments.Thermodynamics also dictate that the heat removed from the water must be equal to the heat absorbed by the • surrounding air:

L = G

Where, L/G = liquid to gas mass flow ratio (kg/kg)T1 = hot water temperature (0C)T2 = cold water temperature (0C)h2 = enthalpy of air-water vapour mixture at exhaust wet-bulb temperature (same units as above)h1 = enthalpy of air-water vapour mixture at inlet wet-bulb temperature (same units as above)

7.2.1 Factors Affecting Cooling Tower Performance

Capacity• Heat dissipation (in kCal/hour) and circulated flow rate (m � 3/hr) are not sufficient to understand cooling tower performance. Other factors which we will see, must be stated along with the flow rate m3/hr.

Example, a cooling tower sized to cool 4540 m• 3/hr through a 13.90C range might be larger than a cooling tower to cool 4540 m3/hr through 19.50C range.Range•

Range is determined not by the cooling tower, but by the process it is serving. The range at the exchanger �is determined entirely by the heat load and the water circulation rate through the exchanger and on to the cooling water.

Range 0C = Heat Load in kcals/hour / Water Circulation Rate in LPHThus, range is a function of the heat load and the flow circulated through the system. Cooling towers are �usually specified to cool a certain flow rate from one temperature to another temperature at a certain wet bulb temperature.

Example, the cooling tower might be specified to cool 4540 m3/hr from 48.90C to 32.20C at 26.70C wet bulb temperature.

Cold Water Temperature (32.20C) Wet Bulb Temperature(26.70C) = Approach(5.50C)

Factors that affect cooling tower sizeCooling tower size is affected by the heat load, range, approach, and WBT. When three of these four quantities are held constant, tower size varies in the following manner:

directly with the heat load • inversely with the range • inversely with the approach • inversely with the entering WBT•

As a generalisation, the closer the approach to the wet bulb, the more expensive the cooling tower due to increased size. Usually a 2.80C approach to the design wet bulb is the coldest water temperature that cooling tower manufacturers will guarantee. If the flow rate, range, approach and wet bulb had to be ranked in the order of their importance in sizing a tower, approach would be first with flow rate closely following, the range and wet bulb would be of lesser importance.

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Heat Load•

The heat load imposed on a cooling tower is determined by the process being served. The degree of cooling required is controlled by the desired operating temperature level of the process. In most cases, a low operating temperature is desirable to increase the process efficiency or to improve the quality or quantity of the product. In some applications (e.g. internal combustion engines) however, high operating temperatures are desirable. The size and cost of the cooling tower is proportional to the heat load. If heat load calculations are low, undersized equipment will be purchased. If the calculated load is high, oversized and more costly equipment will result.

Process heat loads may vary considerably depending upon the process involved. Determination of accurate process heat loads can become very complex but proper consideration can produce satisfactory results. On the other hand, air conditioning and refrigeration heat loads can be determined with greater accuracy.

Information is available for the heat rejection requirements of various types of power equipment. A sample list is as follows:

Air Compressor• Single-stage -129 kCal/kW/hr• Single-stage with after cooler -862 kCal/kW/hr• Two-stage with intercooler -518 kCal/kW/hr• Two-stage with intercooler and after cooler -862 kCal/kW/hr• Refrigeration, Compression -63 kCal/min/TR• Refrigeration, Absorption -127 kCal/min/TR• Steam Turbine Condenser -555 kCal/kg of steam• Diesel Engine, Four-Cycle, Supercharged -880 kCal/kW/hr• Natural Gas Engine, Four-cycle -1523kCal/kW/hr(18 kg/cm• 2 compression)

Wet bulb t emperatureWet bulb temperature is an important factor in the performance of evaporative water cooling equipment. It is a controlling factor from the aspect of the minimum cold water temperature to which water can be cooled by the evaporative method. Thus, the wet bulb temperature of the air entering the cooling tower determines the operating temperature levels throughout the plant, process, or system. Theoretically, a cooling tower will cool water to the entering wet bulb temperature, when operating without a heat load. However, a thermal potential is required to reject heat, so it is not possible to cool water to the entering air wet bulb temperature, when a heat load is applied.

The approach obtained is a function of the thermal conditions and tower capability.Initial selection of towers with respect to the design wet bulb temperature must be made on the basis of conditions existing at the tower site. The temperature selected is generally close to the average maximum wet bulb for the summer months. An important aspect of wet bulb selection is whether it is specified as ambient or inlet. The ambient wet bulb is the temperature which exists generally in the cooling tower area, whereas the inlet-wet bulb is the wet bulb temperature of the air entering the tower. The latter can be, and often is, affected by discharge vapours being recirculated into the tower.

Recirculation raises the effective wet bulb temperature of the air entering the tower with corresponding increase in the cold water temperature. Since there is no initial knowledge or control over the recirculation factor, the ambient wet bulb should be specified. The cooling tower supplier is required to furnish a tower of sufficient capability to absorb the effects of the increased wet bulb temperature peculiar to his own equipment.It is very important to have the cold water temperature low enough to exchange heat or to condense vapours at the optimum temperature level. By evaluating the cost and size of heat exchangers versus the cost and size of the cooling tower, the quantity and temperature of the cooling tower water can be selected to get the maximum economy for the particular process.

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Table 7.1 illustrates the effect of approach on the size and cost of a cooling tower. The towers included were sized to cool 4540 m3 /hr through a 16.670C range at a 26.70C design wet bulb. The overall width of all towers is 21.65 metres; the overall height, 15.25 metres, and the pump head, 10.6 m approximately.

Approach 0C 2.77 3.33 3.88 4.44 5.0 5.55Hot Water 0C 46.11 46.66 47.22 47.77 48.3 48.88Cold Water 0C 29.44 30 30.55 31.11 31.66 32.22No. of Cells 4 4 3 3 3 3Length of Cells Mts. 10.98 8.54 10.98 9.76 8.54 8.54Overall Length Mts. 43.9 34.15 32.93 29.27 25.61 25.61No. of Fans 4 4 3 3 3 3Fan Diameter Mts. 7.32 7.32 7.32 7.32 7.32 6.71Total Fan kW 270 255 240 202.5 183.8 183.8

Table 7.1.Approach vs. cooling tower size(4540m3/hr; 16.670C Range 26.70C Wet Bulb; 10.7m Pump Head)

Approach and flowSuppose a cooling tower is installed that is 21.65 m wide, 36.9 m long, 15.24m high, has three 7.32 m diameter fans and each is powered by 25 kW motors, the cooling tower cools from 3632 m3/hr water from 46.10C to 29.40C at 26.70C WBT dissipating 60.69 million kCal/hr. Table 7.2 shows what would happen with additional flow but with the range remaining constant at 16.670C. The heat dissipated varies from 60.69 million kCal/hr to 271.3 million kCal/hr.

Flow m3/hr Approach 0C Cold Water 0C Hot Water 0C Million kCal/hr3632 2.78 29.40 46.11 60.6914086 3.33 29.95 46.67 68.3184563 3.89 30.51 47.22 76.255039 4.45 31.07 47.78 84.055516 5.00 31.62 48.33 92.176060.9 5.00 32.18 48.89 101.287150.5 5.56 33.29 50.00 119.488736 6.67 35.29 51.67 145.6311590 6.67 37.80 54.45 191.6313620 8.33 40.56 57.22 226.9116276 11.1 43.33 60.00 271.32

Table 7.2 Flow vs. approach for a given tower

(Tower is 21.65m × 36.9M; Three 7.32M Fans; Three 25kW Motors; 16.70C Range with 26.70C Wet Bulb)To meet the increased heat load, a few modifications would be needed to increase the water flow through the tower. However, at higher capacities, the approach would increase.

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Range, Flow and Heat LoadRange is a direct function of the quantity of water circulated and the heat load. Increasing the range as a result of added heat load does require an increase in the tower size. If the cold water temperature is not changed and the range is increased with a higher hot water temperature, the driving force between the wet bulb temperature of the air entering the tower and the hot water temperature is increased, the higher level heat is economical to dissipate.If the hot water temperature is left constant and the range is increased by specifying a lower cold water temperature, the tower size would have to be increased considerably. Not only would the range be increased, but the lower cold water temperature would lower the approach. The resulting change in both range and approach would require a much larger cooling tower.

Approach and Wet Bulb TemperatureThe design wet bulb temperature is determined by the geographical location. Usually the design wet bulb temperature selected is not exceeded over five percent of the time in that area. Wet bulb temperature is a factor in cooling tower selection; the higher the wet bulb temperature, the smaller the tower required to give a specified approach to the wet bulb at a constant range and flow rate.

A 4540 m3/hr cooling tower selected for a 16.670C range and a 4.450C approach to 21.110C wet bulb would be larger than a 4540m3/hr tower selected for a 16.670C range and a 4.450C approach to a 26.670C wet bulb. Air at the higher wet bulb temperature is capable of picking up more heat. Assume that the wet bulb he air temperature of the air is increased by approximately 11.10C. As air removes heat from the water in the tower, each kg of air entering the tower at 21.10C wet bulb would contain 18.86 kCals and if it were to leave the tower at 32.20C wet bulb, it would contain 24.17 kCal per kg of air.

In the second case, if each kg of air entering the tower at 26.670C wet bulb would contain 24.17 kCals and were to leave at 37.80C wet bulb, it would contain 39.67kCal per kg of air. In going from 21.10C to 32.20C, 12.1kCal per kg of air is picked up, while 15.5kCal/kg of air is picked up in going from 26.670C to 37.80C.

Fill Media EffectsIn a cooling tower, hot water is distributed above the fill media which flows down and is cooled due to evaporation with the intermixing air. Air draft is achieved with the use of fans. Thus some power is consumed in pumping the water to a height above the fill and also by fans creating the draft.

An energy efficient or low power consuming cooling tower is to have efficient designs of fill media with the appropriate water distribution, drift eliminator, fan, gearbox and motor. Power savings in a cooling tower, with the use of efficient fill design, is directly reflected as savings in fan power consumption and pumping head requirement.

Function of Fill media in a Cooling TowerHeat exchange between air and water is influenced by the surface area of the heat exchange, time of heat exchange (interaction) and turbulence in water effecting thoroughness of intermixing. Fill media in a cooling tower is responsible to achieving all of above.

Splash and Film Fill MediaAs the name indicates, splash fill media generates the required heat exchange area by the splashing action of water over the fill media and hence breaking into smaller water droplets. Thus, the surface of heat exchange is the surface area of the water droplets, which is in contact with air.

Film Fill and its AdvantagesIn a film fill, water forms a thin film on either side of fill sheets. Thus the area of heat exchange is the surface area of the fill sheets, which is in contact with air. Due to the fewer requirements of the air and pumping head, there is a tremendous saving in power with the invention of film fill.Recently, low-clog film fills with higher flute sizes have been developed to handle highly turbid waters. For sea water, low clog film fills are considered the best choice in terms of power savings and performance as compared to conventional splash type fills.

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7.3 A Typical Comparison Between Various Fill Media

Splash Fill Film Fill Low clog Film FillPossible L/G Ratio 1.1 -1.5 1.5 -2.0 1.4 -1.8Effective Heat Exchange Area

30 -45m2/m3 150m2/m3 85 -100m2/m3

Fill Height Required 5 - 10 m 1.2 -1.5 m 1.5 -1.8 mPumping Head Requirement

9 -12 m 5 -8 m 6 -9 m

Quantity of Air Required

High Much low Low

Table 7.3 Typical comparisons between various fill media

7.4 Choosing a Cooling TowerThe counter-flow and cross flows are two basic designs of cooling towers based on the fundamentals of heat exchange. It is well known that counter flow heat exchange is more effective as compared to cross flow or parallel flow heat exchange.

Cross-flow cooling towers are provided with splash fill of concrete, wood or perforated PVC. Counter-flow cooling towers are provided with both film fill and splash fill.Typical comparisons of Cross flow Splash Fill, Counter Flow Tower with Film Fill and Splash fill are shown in Table 7.4. The power consumption is least in Counter Flow Film Fill followed by Counter Flow Splash Fill and Cross-Flow Splash Fill.

Counter Flow Film Fill

Counter Flow Splash Fill

Cross-Flow Splash Fill

Fill Height, Metre 1.5 5.2 11.0Plant Area per Cell 14.4 ×14.4 14.4 ×14.4 12.64 × 5.49Number of Cells per Tower 6 6 5Power at Motor Terminal/Tower, kW 253 310 330Static Pumping Head, Metre 7.2 10.9 12.05

Table 7.4 Typical comparison of cross flow splash fill, counter flow tower with film fill and splash fill

Number of Towers: 2Water Flow: 16000 m3/hrHot Water Temperature: 41.50CCold Water Temperature: 32.50CDesign Wet Bulb Temperature: 27.60C

7.5 Efficient System Operation7.5.1 Cooling Water TreatmentCooling water treatment is mandatory for any cooling tower whether with splash fill or with film type fill for controlling suspended solids, algae growth etc. With increasing costs of water, efforts to increase Cycles of Concentration (COC), by Cooling Water Treatment would help to reduce make up water requirements significantly. In large industries, power plants, COC improvement is often considered as a key area for water conservation.

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7.5.2 Drift Loss in the Cooling TowersIt is very difficult to ignore the drift problem in cooling towers. Now-a-days most of the end user specifications call for 0.02% drift loss.With technological development and the processing of PVC, manufacturers have brought about a big change in the drift eliminator shapes and the possibility of making efficient designs of drift eliminators that enable the end user to specify the drift loss requirement to as low as 0.003–0.001%.

7.5.3 Cooling Tower FansThe purpose of a cooling tower fan is to move a specified quantity of air through the system, overcoming the system resistance which is defined as the pressure loss. The product of air flow and the pressure loss is the air power developed/work done by the fan; this may be also termed as the fan output and input kW which depends on fan efficiency.Fan efficiency in turn is greatly dependent on the profile of the blade. An aerodynamic profile with the optimum twist, taper and higher coefficient of lift to coefficient of drop ratio can provide the fan a total efficiency as high as 85–92 %. However, this efficiency is drastically affected by factors such as tip clearance, obstacles to airflow and inlet shape, etc.

As the metallic fans are manufactured by adopting either the extrusion or the casting process, it is always difficult to generate the ideal aerodynamic profiles. The FRP blades are normally hand moulded, which facilitates the generation of the optimum aerodynamic profile to meet specific duty conditions more efficiently. Cases reported where the replacement of metallic or Glass fibre reinforced plastic fan blades have been replaced by efficient hollow FRP blades, with resultant fan energy savings of the order of 20–30% and with a simple pay back period of 6–7 months.Also, due to their light weight, FRP fans need low starting torque resulting in the use of lower HP motors. The light weight of the fans also increases the life of the gear box, motor and bearings and allows for easy handling and maintenance.

7.5.4 Performance Assessment of Cooling TowersIn operational performance assessment, the typical measurements and observations involved are:

Cooling tower design data and curves to be referred to as the basis. �Intake air WBT and DBT at each cell at ground level using a whirling pyschrometer. �Exhaust air WBT and DBT at each cell using a whirling psychrometer. �CW inlet temperature at the risers or the top of the tower, using accurate mercury-in-glass or a digital �thermometer.CW outlet temperature at full bottom, using accurate mercury-in-glass or a digital thermometer. �Process data on heat exchangers, loads on line or power plant control room readings, as relevant. �CW flow measurements either direct or inferred from pump motor kW and pump head and flow �characteristics.CT fan motor amps, volts, kW and blade angle settings �TDS of cooling water. �Rated cycles of concentration at site conditions. �Observations on nozzle flows drift eliminators, condition of fills, splash bars, etc. �

The findings of one typical trial pertaining to the Cooling Towers of a Thermal Power Plant 3 x 200 MW are given below:

Observations• Unit Loan 1 and 3 of the Station = 398 MW �Mains Frequency = 49.3 �Inlet Cooling Water Temperature � 0C = 44 (Rated 430C)Outlet Cooling Water Temperature = 37.6 (Rated 33 � 0C)Air Wet Bulb Temperature near Cell � 0C = 29.3 (Rated 27.50C)Air Dry Bulb Temperature men Cell � 0C = 40.80C

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Number of CT Cells on line with water flow = 45 (Total 48) �Total Measured Cooling Water Flow m3/hr = 70426.75 �Measured CT Fan Flow m3/hr = 989544 �

Analysis• CT water Flow/Cell, m3/hr = 1565 m � 3/hr (1565000 kg/hr) (Rated 1875 m3/hr)CT Fan air Flow, m3/hr (Avg.) = 989544 m � 3/hr (Rated 997200 m3/hr)CT Fan air Flow kg/hr (Avg.) @ Density of 1.08kg/m � 3=1068708 kg/hrL/G Ratio of C.T. kg/kg = 1.46 (Rated 1.74 kg/kg) �CT Range = (44 – 37.6) = 6.4 � oC

CT Approach = (37.6 – 29.3) = 8.3• oC

% CT Effectiveness = �

= = 43.53

Rated % CT Effectiveness = 100 × (43 – 33) / (43 – 27.5) �= 64.5%

Cooling Duty Handled/Cell in kCal =1565×6.4×103(i.e. Flow*Temperature Difference in kCal/hr)=10016×103kCal/hr(Rated 18750×103kCal/hr)

Evaporation Losses in m3/hr = 0.00085 x 1.8 x circulation rate (m � 3/hr) x (T1-T2) = 0.00085 x 1.8 x 1565 x (44-37.6) = 15.32 m3/hr per cell

Percentage Evaporation Loss = [15.32/1565]×100 � = 0.97%

Blow down requirement for site COC of 2.7 = Evaporation losses / (COC–1) � =15.32/ (2.7–1) per cell i.e., 9.01 m3/hr

Make up water requirement/cell in m � 3/hr = Evaporation Loss + Blow down Loss = 15.32 + 9.01 = 24.33

Comments• Cooling water flow per cell is much lower, almost by 16.5%; need to investigate CW pump and system �performance for improvements. Increasing CW flow through cell was identified as a key result area for improving performance of cooling towers. Flow stratification in 3 cooling tower cells identified. �Algae growth identified in 6 cooling tower cells. �Cooling tower fans are of GRP type drawing 36.2kW average. Replacement by efficient hollow FRP fan �blades is recommended

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7.6 Flow Control StrategiesControl of tower air flow can be done by varying methods: starting and stopping (On-off) of fans, use of two or • three-speed fan motors, use of automatically adjustable pitch fans, use of variable speed fans.On-off fan operation of single speed fans provides the least effective control. Two-speed fans provide better • control with further improvement shown with three speed fans.Automatic adjustable pitch fans and variable-speed fans can provide even closer control of tower cold-water • temperature. In multi-cell towers, fans in adjacent cells may be running at different speeds or some may be switched on and others switched off depending upon the tower load and required water temperature.Depending upon the method of air volume control selected, control strategies can be determined to minimise • fan energy while achieving the desired control of the Cold water temperature.

7.7 Energy Saving Opportunities in Cooling TowersFollow the manufacturer's recommended clearances around cooling towers and relocate or modify structures • that interfere with the air intake or exhaust.Optimise cooling tower fan blade angle on a seasonal and/or load basis.• Correct excessive and/or uneven fan blade tip clearance and poor fan balance.• On old counter-flow cooling towers, replace old spray type nozzles with new square spray ABS practically • non-clogging nozzles.Replace splash bars with self-extinguishing PVC cellular film fill.• Install new nozzles to obtain a more uniform water pattern. • Periodically clean plugged cooling tower distribution nozzles. • Balance flow to cooling tower hot water basins. • Cover hot water basins to minimise algae growth that contribute to fouling. • Optimise blow down flow rate, as per COC limit.• Replace slat type drift eliminators with low pressure drop, self extinguishing, PVC cellular units.• Restrict flows through large loads to design values.• Segregate high heat loads like furnaces, air compressors; DG sets, and isolate cooling towers for sensitive • applications like A/C plants, condensers of captive power plant etc. A 10C cooling water temperature increase may increase A/C compressor kW by 2.7%. A 10C drop in cooling water temperature can give a heat rate saving of 5 kCal/kWh in a thermal power plant.Monitor L/G ratio, CW flow rates w.r.t. design as well as seasonal variations. It would help to increase the water • load during summer and times when the approach is high and the increase air flow during monsoon times and when the approach is narrow.Monitor approach, effectiveness and cooling capacity for continuous optimisation efforts, as per seasonal • variations as well as load side variations.Consider COC improvement measures for water savings.• Consider energy efficient FRP blade adoption for fan energy savings.• Consider possible improvements on CW pumps w.r.t. efficiency improvement.• Control cooling tower fans based on leaving water temperatures especially in the case of small units.• Optimise process CW flow requirements, to save on pumping energy, cooling load, evaporation losses (directly • proportional to circulation rate) and blow down losses.

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Some typical problems and their trouble shooting for cooling towers are given in Table 7.5.

Problem/ Difficulty Possible Causes Remedies/ Rectifying Action

Excessive absorbedcurrent/electrical load

1. Voltage Reduction Check the voltage2a. Incorrect angle of axial fan blades Adjust the blade angle2b. Loose belts on centrifugal fans (or speed reducers)

Check belt tightness

3. Overloading owing to excessive air flow-fill has minimum water loading per m2 of tower section

Regulate the water flow by means of the value

4. Low ambient air temperature The motor is cooled proportionately and hence delivers more than name plate power

Drift/ carry-over of water outside the unit

1. Uneven operation of spray nozzles Adjust the nozzle orientation and eliminate any dirt

2. Blockage of the fill pack Eliminate any dirt at the top of the fill3. Defective or displaced droplet eliminators

Replace or realign the eliminators

4. Excessive circulating water flow (possibly owing to too high pumping head)

Adjust the water flow-rate by means of the regulating valves. Check for absence of damage of the fill.

Loss of water frombasins/pans

1. Float-valve not at correct level Adjust the make-up valve2. Lack of equalising connections Equalise the basins of towers operating in

parallelLack of cooling and hence increase intemperatures owing to increased temperature range

1. Water flow below the design valve Regulate the flow by means of the valves2. Irregular airflow or lack of air Check the direction of rotation of the fans

and/or belt tension (broken belt possible)3a. Recycling of humid discharge air Check the air descent velocity3b. Intake of hot air from other sources Install deflectors4a. Blocked spray nozzles (or even blocked spray tubes)

Clean the nozzles and/or the tubes

4b. Scaling of joints Wash or replace the item

Table 7.5 Typical problems and trouble shooting for cooling towers problem

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SummaryCooling tower is a heat exchanger where heat from hot water is dissipated to the ambient and the cold water is • sent back to process. The performance of the cooling tower is the ratio of the range to approach.The different types of cooling towers are natural draft and forced draft.• Natural draft towers use very large concrete chimneys to introduce air through the media. Due to the large size • of these towers, they are generally used for water flow rates above 45,000 m /hr. These types of towers are used only by utility power stationsMechanical draft towers utilise large fans to force or suck air through circulated water. The water falls downward • over fill surfaces, which helps increase the contact time between the water and the air - this helps maximise heat transfer between the twoThe basic components of an evaporative tower are: Frame and casing, fill, cold water basin, drift eliminators, • air inlet, louvers, nozzles and fans.Tower manufacturers fabricate towers and tower components from a variety of materials. Often several materials • are used to enhance corrosion resistance, reduce maintenance, and promote reliability and long service life.Range is the difference of temperature between inlet and outlet water. Approach is the difference of temperature • between the outlet water temperature and the wet bulb temperature. Hence as the outlet water temperature approaches towards the wet bulb temperature, the effectiveness of the cooling tower approaches 100%.The counter-flow and cross flows are two basic designs of cooling towers based on the fundamentals of heat • exchange. It is well known that counter flow heat exchange is more effective as compared to cross flow or parallel flow heat exchangeEnergy saving opportunities exists based on fan blade material, material used for construction, piping, etc.•

ReferencesFrayne, C., 1999. • Cooling water treatment: Principles and practice, Chemical Pub. Co publication.Kroger, D., 2004. • Air-cooledHeatExchangersAndCoolingTowers:Thermal-flowerPerformanceEvaluationand Design, Volume 2. Pennwell Books Publication.Cooling Tower • [Pdf] Available at: < www.beeindia.in/energy_managers_auditors/documents/.../3Ch7.pdf‎> [Accessed 5 July 2013].Cooling Tower Fundamentals- SPX Cooling Technologies• [Pdf] Available at: < spxcooling.com/pdf/Cooling-Tower-Fundamentals.pdf‎‎> [Accessed 5 July 2013].2012. • What are Cooling Towers?[Video online] Available at: < https://www.youtube.com/watch?v=KbxHk7go7UU> [Accessed 5 July 2013].2012. • How a Cooling Tower Works? [Video online] Available at: < https://www.youtube.com/watch?v=z9-cVGrR9OE> [Accessed 5 July 2013].

Recommended ReadingGurney, J. D. & Cotter, I. A., 1966. Cooling towers, McMillan & sons.• Stanford, H. W., 2003. HVAC • Water Chillers and Cooling Towers: Fundamentals, Application, and Operation (Dekker Mechanical Engineering),1st ed., CRC Press publication.McCoy, J. W., 1983. • The Chemical Treatment of Cooling Water, 2nd ed., Chemical Publishing Company Publication

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Self AssessmentNatural draft cooling towers are mainly used in _______.1.

steel industry a. alumina industryb. fertilizer industryc. power stationsd.

Better indicator for cooling tower performance is ____.2. Wet bulb temperaturea. Dry bulb temperatureb. Rangec. Approachd.

Cooling tower effectiveness is the ratio of__________.3. Range/(range + approach)a. Approach/(range + approach)b. Range/Approachc. Approach/Ranged.

Cooling tower reduces circulation water temperature close to ________.4. dry bulb temperaturea. ambient wet bulb temperature (WBT)b. dew point temperaturec. wet bulb temperatured.

The ratio of dissolved solids in circulating water to the dissolved solids in make up water is termed as ____.5. liquid gas ratioa. cycles of concentrationb. cooling tower effectivenessc. dry bulb temperatured.

Which one of the following has maximum effect on cooling tower performance? 6. Fill mediaa. Driftb. Louversc. Casingd.

Use of very large concrete chimneys to introduce air through the media is seen in __________.7. Warming draft towersa. Mechanical draft towersb. Cooling draft towersc. Natural draft towersd.

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In the ____________________ draft design, hot water enters at the top, while air is introduced at the bottom 8. and exits at the top.

counter flow forced drafta. cross flow induced draftb. counter flow induced draftc. cross flow forced draftd.

Recirculation raises the effective wet bulb temperature of the air entering the tower with corresponding increase 9. in the ____________.

hot water temperaturea. warm water temperatureb. medium water temperaturec. cold water temperatured.

__________ is a direct function of the quantity of water circulated and the heat load10. Rangea. Flowb. Heat loadc. Approachd.

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Chapter VIII

Lighting System

Aim

The aim of this chapter is:

explain lighting systems•

elucidate the types of lamp and light source•

explicate the choice of lighting•

Objectives

The objectives of the chapter are:

explain the basic terms in lighting system•

enlist lamp types and their features•

describe energy conservation avenues•

Learning outcome

At the end of this chapter, you will be able to:

identify light distribution•

define the types of lamp and light source•

understand the choice of lighting and luminance requirements•

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8.1 IntroductionLighting is an essential service in all the industries. Power consumption by industrial lighting varies between 2–10% of the total power depending on the type of industry. Innovation and continuous improvement in the field of lighting, has given rise to tremendous energy saving opportunities in this area. Lighting is an area which provides a major scope to achieve energy efficiency at the design stage, by the incorporation of modern energy efficient lamps, luminaries and gears, apart from good operational practices.

8.2 Basic Terms in Lighting Systems and FeaturesLamps•

The lamp is an equipment, which produces light. The most commonly used lamps are described briefly as follows:

Incandescent Lamps• Incandescent lamps produce light by means of a filament heated to incandescence by the flow of electric current through it. The principal parts of an incandescent lamp, also known as GLS (General Lighting Service) lamp include the filament, the bulb, the fill gas and the cap.

Reflector Lamps• Reflector lamps are basically incandescent and are provided with a high quality internal mirror which exactly follows the parabolic shape of the lamp. The reflector is resistant to corrosion, thus making the lamp maintenance free and output efficient.

Gas Discharge Lamps• The light from a gas discharge lamp is produced by the excitation of gas contained in either a tubular or elliptical outer bulb.

The most commonly used discharge lamps are as follows:fluorescent tube lamps (FTL)• compact fluorescent lamps (CFL)• mercury vapour lamps• sodium vapour lamps• metal halide lamps•

Luminaire• The luminaire is a device that distributes filters or transforms the light emitted from one or more lamps. Luminaires include all the parts necessary for fixing and protecting the lamps, except the lamps themselves. In some cases, luminaires also include the necessary circuit auxiliaries, together with the means for connecting them to the electric supply. The basic physical principles used in optical luminaires are reflection, absorption, transmission and refraction.

Control Gear• The gears used in lighting equipment are as stated:

Ballast �A current limiting device, to counter the negative resistance characteristics of any discharge lamp. In case of fluorescent lamps, it aids the initial voltage build up, required for starting.

Ignitors �These are used for starting high intensity Metal Halide and Sodium vapour lamps.

Illuminance �This is the quotient of the luminous flux incident on an element of the surface at a point of surface containing the point, by the area of that element. The lighting level produced by a lighting installation is usually qualified by the illuminance produced on a specified plane. In most cases, this plane is the major plane of the tasks in the interior and is commonly called the working plane.

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The illuminance provided by an installation affects both the performance of tasks and the appearance of space.

Lux (lx)• This is the illuminance produced by a luminous flux of one lumen, uniformly distributed over a surface area of one square metre. One lux is equal to one lumen per square metre.

Luminous Efficacy (lm/W)• This is the ratio of luminous flux emitted by a lamp to the power consumed by the lamp. It is a reflection of the efficiency of energy conversion from electricity to light.

Colour Rendering Index (RI)• It is a measure of the degree to which the colours of surfaces illuminated by a given light source conform to those of the same surfaces under a reference illuminent; suitable allowance having been made for the state of Chromatic adaptation.

8.3 Lamp Types and their FeaturesTable 8.1 shows the various types of lamps available along with their features.

Type of LampLumens/ Watt

Colour Rendering

IndexTypical Application Typical Life

(hours)

Range Avg.Incandescent 8–18 14 Excellent Homes, restaurants,

general lighting, emergency lighting

1000

Fluorescent Lamps

46–60 50 Good w.r.t coating

Offices, shops, hospitals, homes

5000

Compact fluorescent

lamps (CFL)

40–70 60 Very good Hotels, shops, homes offices

8000–10000

High pressure mercury (HPMV)

44–57 50 Fair General lighting in factories garages, car

parking, flood lighting

5000

Halogen lamps

18–24 20 Excellent Display, flood lighting, stadium

exhibition grounds, construction areas

2000–4000

High pressure sodium

(HPSV) Son

67–121 90 Fair General lighting in factories, warehouses,

street lighting

6000–12000

Low pressure sodium

(LPSV) SOX

101–175 150 Poor Roadways, tunnels canals, street lighting

6000–12000

Incandescent 8–18 14 Excellent Homes, restaurants, general lighting,

emergency lighting

1000

Table 8.1 Luminous performance characteristics of commonly used luminaries

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8.4 Recommended Illuminance Levels for Various Tasks / Activities / LocationsRecommendations on Illuminance•

Scale of Illuminance: �The minimum illuminance for all non-working interiors has been mentioned as 20 Lux (as per IS 3646). A factor of approximately 1.5 represents the smallest significant difference in the subjective effect of illuminance Therefore, the following scale of illuminance is recommended.20-30-50-75-100-150-200-300-500-750-1000-1500-2000, ----------Lux

Illuminance Ranges: Because circumstances may be significantly different for different interiors used for the same application or for different conditions for the same kind of activity, a range of illuminance is recommended for each type of interior or activity intended of a single value of illuminance. Each range consists of three successive steps of the recommended scale of illuminance. For working interiors, the middle value (R) of each range represents the recommended service illuminance that would be used unless one or more of the factors mentioned below apply.

The higher value (H) of the range should be used in exceptional cases where low reflectances or contrasts are present in the task, errors are costly to rectify, visual work is critical, accuracy or higher productivity is of great importance and the visual capacity of the worker makes it necessary. Similarly, a lower value (L) of the range may be used when reflectances or contrasts are unusually high, speed and accuracy is not important and the task is executed only occasionally.

Recommended IlluminationThe following details give the recommended illuminance range for different tasks and activities for the chemical sector. The values are related to the visual requirements of the task, to the user's satisfaction, to practical experience and to the need for the cost effective use of energy (Source IS 3646 (Part I): 1992). For the recommended illumination in other sectors, you may refer to the Illuminating Engineers Society Recommendations Handbook.

Petroleum, Chemical and Petrochemical WorksExterior walkways, platforms, stairs and ladders 30-50-100Exterior pump and valve areas 50-100-150Pump and compressor houses 100-150-200Process plant with remote control 30-50-100Process plant requiring occasional manual intervention 50-100-150Permanently occupied work stations in process plant 150-200-300Control rooms for process plant 200-300-500Pharmaceuticals Manufacturer and Fine chemicalsManufacturerPharmaceutical ManufacturerGrinding, granulating, mixing, drying, tableting, sterilising washing, preparation of solutions, filling, capping, wrapping,hardening 300-500-750Fine Chemical ManufacturersExterior walkways, platforms, stairs and ladders 30-50-100Process plant 50-100-150Fine chemical finishing 300-500-750Inspection 300-500-750Soap manufactureGeneral area 200-300-500

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Automatic processes 100-200-300Control panels 200-300-500Machines 200-300-500Paint worksGeneral 200-300-500Automatic processes 150-200-300Control panels 200-300-500Special batch mixing 500-750-1000Colour matching 750-100-1500

Table 8.2 Recommended illuminance range for different tasks and activities for the chemical sector

8.5 Methodology of Lighting System Energy Efficiency StudyA step-by-step approach for assessing the energy efficiency of the lighting system is as stated:Step-1: Inventorise the Lighting System elements and transformers in the facility as per the following typical format (Table 8.3 and 8.4).

S. No. Plant Location

Lighting Device & Ballast Type

Rating in Watts Lamp & Ballast

Population Numbers No. of hours/Day

Table 8.3 Device rating, population and use profile

S.No Plant Location

Lighting Transformer Rating (kVA)

Numbers Installed

Meter Provisions Available Volts /Amps/kW/Energy

Table 8.4 Lighting transformer/rating and population profile

In case of distribution boards (instead of transformers) being available, fuse ratings may be inventorised along the above pattern in place of transformer kVA.

Step-2: With the aid of a lux meter, measure and document the lux levels at various plant locations at the working level, as day time lux and night time lux values alongside the number of lamps “ON” during measurement.

Step-3:With the aid of a portable load analyser, measure and document the voltage, current, power factor and power consumption at various input points, namely the distribution boards or the lighting voltage transformers at the same as that of the lighting level audit.

Step-4: Compare the measured lux values with standard values as reference and identify locations as under lit and over lit areas.

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Step-5:Collect and analyse the failure rates of lamps, ballasts and the actual life expectancy levels from past data.

Step-6: Based on careful assessment and evaluation bring out improvement options which could include:

Maximise sunlight use through use of transparent roof sheets, north light roof, etc. • Examine the scope for replacements of lamps by more energy efficient lamps, with due consideration to luminiare, • colour rendering index, lux level as well as expected life comparison.

Replace conventional magnetic ballasts by more energy efficient ballasts, with due consideration to the life �and power factor apart from watt loss. Select interior colours for light reflection. �Modify layout for optimum lighting. �Provide individual / group controls for lighting for energy efficiency such as: �On / off type, voltage regulation type (for illuminance control) �Group control switches / units �Occupancy sensors �Photocell controls �Timer operated controls �Pager operated controls �Computerised lighting control programmes �Install input voltage regulators / controllers for energy efficiency as well as longer life expectancy for lamps �where higher voltages, fluctuations are expected.Replace energy efficient displays like LED's in place of lamp type displays in control panels / instrumentation �areas, etc.

8.6 Case Examples8.6.1 Energy Efficient Replacement OptionsLamp efficacy is the ratio of light output in lumens to the power input to lamps in watts. Over the years, developments in lamp technology have led to improvements in the efficacy of lamps. However, the low efficacy lamps, such as incandescent bulbs, still constitute a major share of the lighting load. High efficacy gas discharge lamps suitable for different types of applications offer an appreciable scope for energy conservation. Typical energy efficient replacement options, along with the percent energy saving, are given in Table-8.5

Sector Lamp type Power savingExisting Proposed Watts %

Domestic/Commercial GLS 100 W *CFL 25 WIndustry GLS 13 W *CFL 9 W

GLS 200 W Blended 160 WTL 40 W TLD 36 W

HPMV 250 W HPSV 150 WHPMV 400 W HPSV 250 W

Table 8.5 Savings by use of high efficacy lamps(* Wattages of CFL includes energy consumption in ballasts.)

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Energy Saving Potential in Street LightingThe energy saving potential, in typical cases of replacement of inefficient lamps with efficient lamps in street lighting is given in Table 8.6Existing lamp Replaced units Saving

Existing lamp Replaced units SavingType W Life hrs. Type W Life W %GLS GLS TL HPMV HPMV HPMV 200 3002×40 125 250 400 1000 1000 5000 50005000 5000 ML ML TL HPSV HPSV HPSV160 250 2×36 70 150 250 5000 50005000 12000 12000 12000 40 50 8 25100 150 7 17 6 44 40 38

Table 8.6 Saving potential by use of high efficacy lamps for street lighting

8.7 Some Good Practices in LightingInstallation of energy efficient fluorescent lamps in place of “Conventional” fluorescent lamps. �Energy efficient lamps are based on the highly sophisticated tri-phosphor fluorescent powder technology. �They offer excellent colour rendering properties in addition to the very high luminous efficacy. �

8.7.1 Installation of Compact Fluorescent Lamps (CFL's) in Place of Incandescent LampsCompact fluorescent lamps are generally considered best for the replacement of lower wattage incandescent lamps. These lamps have an efficacy ranging from 55–65 lumens/Watt. The average rated lamp life is 10,000 hours, which is 10 times longer than that of a normal incandescent lamp. CFLs are highly suitable for places such as Living rooms, Hotel lounges, Bars, Restaurants, Pathways, Building entrances, Corridors, etc.

8.7.2 Installation of Metal Halide Lamps in Place of Mercury/Sodium Vapour LampsMetal halide lamps provide a high colour-rendering index when compared with mercury and sodium vapour lamps. These lamps offer an efficient white light. Hence, metal halide is the choice for colour critical applications where higher illumination levels are required. These lamps are highly suitable for applications such as assembly line, inspection areas, painting shops, etc. Metal halide lamps are recommended where the colour rendering is more critical.

8.7.3 Installation of High Pressure Sodium Vapour (HPSV) Lamps for Applications where Colour Rendering is not CriticalHigh-pressure sodium vapour (HPSV) lamps offer more efficacy. But the colour rendering property of HPSV is very low. Hence, they are recommended for applications such street lighting, yard lighting, etc.

8.7.4 Installation of LED Panel Indicator Lamps in Place of Filament LampsPanel indicator lamps are used widely in industries for monitoring, fault indication, signalling, etc. �Conventionally, filament lamps are used for these purposes, but have the disadvantages as stated:High energy consumption (15 W/lamp) �Failure of lamps is high (Operating life less than 10,000 hours) �Very sensitive to voltage fluctuations. Recently, the conventional filament lamps are being replaced with �Light Emitting Diodes (LEDs)

LEDs have the following merits over filament lamps.Lesser power consumption (Less than 1 W/lamp) �Withstand high voltage fluctuations in the power supply �Longer operating life (more than 1,00,000 hours) �LEDs are recommended to be installed for panel indicator lamps at the design stage. �

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8.7.5 Light DistributionEnergy efficiency cannot be obtained by the mere selection of more efficient lamps alone. Efficient luminaires along with lamps of high efficacy achieve the optimum efficiency. Mirror-optic luminaires with a high output ratio and bat-wing light distribution can save energy. For achieving better efficiency, luminaires that have light distribution characteristics appropriate for the task interior should be selected.It should be ensured that luminaires fitted with a lamp should minimise discomfort glare and that veiling reflections are minimised. The installation of suitable luminaires depends upon height - Low, Medium and High Bay. Luminaires for high intensity, discharge lamps are classified as follows:

Low bay, for heights less than 5 metres. �Medium bay, for heights between 5–7 metres. �High bay, for heights greater than 7 metres. �

The system layout and fixing of the luminaires play a major role in achieving energy efficiency. This also varies from application to application. Hence, fixing the luminaires at the optimum height and the usage of mirror optic luminaires leads to energy efficiency.

8.7.6 Light ControlThe simplest and the most widely used form of controlling a lighting installation is the "On-Off" switch. The initial investment for this set up is extremely low, but the resulting operational costs may be high. This does not provide the flexibility for the control of the lighting, where it is not required.Hence, a flexible lighting system has to be provided, which will offer switch-off or reduction in the lighting level,

when not needed. The following light control systems can be adopted at the design stage:Grouping of the lighting system, to provide greater flexibility in lighting control. �Grouping of the lighting system, which can be controlled manually or by timer control. �Installation of microprocessor based controllers �Another modern method is the usage of microprocessor/infrared controlled dimming or switching circuits. �Lighting control can be obtained by using logic units located in the ceiling, which can take pre-programme commands and activate specified lighting circuits. The advanced lighting control system uses movement detectors or lighting sensors, to feed signals to the controllers.

Optimum usage of day lightingWhenever the orientation of a building permits, daylight can be used in combination with electric lighting. This • should not introduce a glare or a severe imbalance of brightness in the visual environment. Usage of day lighting (in offices/air conditioned halls) will have to be very limited, because the air conditioning load will increase on account of the increased solar heat dissipation into the area. In many cases, a switching method, to enable reduction of electric light in the window zones during certain hours, has to be designed.

Installation of an "exclusive" transformer for lighting �In most of the industries, the lighting load varies between 2 to 10%. Most of the problems faced by the �lighting equipment and the "gear" are due to the "voltage" fluctuations. Hence, the lighting equipment has to be isolated from the power feeders. This provides a better voltage regulation for the lighting. This will reduce the voltage related problems, which in turn increases the efficiency of the lighting system.Installation of the servo stabiliser for a lighting feeder �

Wherever, the installation of an exclusive transformer for lighting is not economically attractive, the servo stabiliser can be installed for the lighting feeders. This will provide a stabilised voltage for the lighting equipment. The performance of "gear" such as chokes, ballasts, will also improve due to the stabilised voltage.

This set up also provides the option to optimise the voltage level fed to the lighting feeder. In many plants, during the non-peaking hours, voltage levels are on the higher side. During this period, voltage can be optimised without

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any significant drop in the illumination level.Installation of high frequency (HF) electronic ballasts in place of conventional ballasts �

New high frequency (28-32 kHz) electronic ballasts have the following advantages over the traditional magnetic ballasts:

energy savings up to 35%. �less heat dissipation, which reduces the air conditioning load �lights instantly. �improved power factor. �operates on low voltage load. �lighter in weight. �increases the life of the lamp. �

The advantage of HF electronic ballasts outweighs the initial investment (higher costs when compared with conventional ballast). In the past the failure rate of the electronic ballast in Indian Industries was high. Recently, many manufacturers have improved the design of the ballast leading to drastic improvements in their reliability. The life of the electronic ballast is high especially when used in a lighting circuit fitted with an automatic voltage stabiliser. Table 8.7 gives the type of luminaire, gear and controls used in the different areas of industry.

Location Source Luminaire Gear ControlsPlant HID/FTL Industrial rail reflector : High

bay Medium bay Low bayConventional/ low loss electronic ballast

Manual/electronic

Office FTL/CFL FTL/CFL Electronic/low loss Manual/autoYard HID Flood light Suitable ManualRoad peripheral

HID/PL Street light luminaire Suitable Manual

Table 8.7 Types of luminaire with their gear and controls used in different industrial locations

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SummaryPower consumption by industrial lighting varies between 2–10% of the total power depending on the type of • industry. Innovation and continuous improvement in the field of lighting, has given rise to tremendous energy saving opportunities in this area.The various terms used in Illumination Engineering include incandescent lamps, reflector lamps, gas discharge • lamps, luminaire, control gear, ballast, igniters, illuminance, lux, luminous efficacy, colour rendering index.The minimum illuminance for all non-working interiors has been mentioned as 20 Lux (as per IS 3646). A factor • of approximately 1.5 represents the smallest significant difference in the subjective effect of illuminanceIncandescent lamps, fluorescent lamps, Compact Fluorescent Lamps, High Pressure Mercury Vapour Lamps, • Halogen Lamps, High Pressure Sodium Vapour Lamps, and Low Pressure Sodium Vapour Lamps are the different types of luminaries.A step by step approach can be obtained to assess the performance of the Illumination System. The methodology • includes the measurement of lux levels and comparing these with the standards. Apart from this there are many steps involved.The biggest energy saving opportunity in Illumination Engineering is the replacement of conventional copper • wound chokes with electronic ballasts. The other opportunities include designing buildings to give the maximum sunlight during the day time.

ReferencesChen, K.,1999. • Energy Management in Illuminating Systems,1st ed., CRC Press Publication.Levermore, G., 2000. • Building Energy Management Systems: An Application to Heating, Natural Ventilation, Lighting and Occupant Satisfaction, 2nd ed., Spon Press Publication.Lighting System• [Pdf] Available at: <www.beeindia.in/energy_managers_auditors/documents/.../3Ch8.pdf‎‎> [Accessed 5 July 2013].Lighting Systems Made Easy • [Pdf] Available at: <www.leprecon.com/catalogs/280075BLightingMadeEasy.pdf‎> [Accessed 5 July 2013].2013.• Smart LED Lighting System [Video online] Available at: <https://www.youtube.com/watch?v=YHBaVmpcdso> [Accessed 5 July 2013].2010. • LED Mobile Lighting System [Video online] Available at: <https://www.youtube.com/watch?v=3CiOw04ZNT8> [Accessed 5 July 2013].

Recommended ReadingPatterson, E. G., 2001. Lighting Systems: Advanced Course, Thomson Learning.• Beggs, C., 2009. • Energy: Management, Supply and Conservation, 2nd ed., Butterworth-Heinemann publication.Lindsey, J. L., 1997. • Applied illumination engineering, 2nd ed., The Fairmont Press, Inc.Publication.

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Self AssessmentWhich of the following light source has least life?1.

Sodium vapoura. Mercury Vapourb. Halogenc. Incandescentd.

Colour rendering index of incandescent lamp is: 2. Fair when compared to HPSV lampa. Poor when compared to LPSV lampb. Same when compared to HPMV lampc. Excellent when compared to fluorescent lampd.

One lux is equal to ___.3. One lumen per metera. One lumen per m3b. One lumen per m2c. One lumend.

Colour rendering index of Halogen lamps compared to low pressure sodium vapour lamps is ___.4. Poora. Excellentb. Averagec. Very poord.

The colour rendering property of HPSV is very ___________ 5. higha. mediumb. lowc. dulld.

Which of the following is the best definition of illuminance?6. Time rate of flow of light energya. Luminous flux incident on an object per unit areab. Flux density emitted from an object without regard for directionc. Flux density emitted from an object in a given directiond.

The ratio of luminous flux emitted by a lamp to the power consumed by the lamp is ___.7. Illuminancea. Luxb. Luminous Efficacyc. CRId.

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Ignitors are used for starting____.8. FTLa. CFLb. Sodium vapour lampsc. HTLd.

A device that distributes and filters the light emitted from one or more lamps is ___.9. Control geara. Lampb. Luminairec. Starterd.

GLS stands for_____.10. General Lamp sourcea. General Lamp Serviceb. General Lighting Servicec. General Lighting Sourced.

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Chapter IX

DG Set Systems

Aim

The aim of this chapter is to:

explain the diesel generating system•

enlist factors affecting selection•

explicate motor efficiency•

Objectives

The objectives of the chapter are to:

explain the four stroke diesel engine•

explicate the DG set as a system•

elucidate energy performance assessment of diesel conservation avenues•

Learning outcome

At the end of this chapter, you will be able to:

define diesel generator captive power plants•

identify selection and installation factors•

understand the concept of motor efficiency•

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9.1 IntroductionThe Diesel engine is a prime mover, which drives an alternator to produce electrical energy. In the diesel engine, air is drawn into the cylinder and is compressed to a high ratio (14:1 to 25:1). During this compression, the air is heated to a temperature of 700– 9000C. A metered quantity of diesel fuel is then injected into the cylinder, which ignites spontaneously because of the high temperature. Hence, the diesel engine is also known as the compression ignition (CI) engine.

Diesel generating (DG) sets can be classified according to cycle types as:two stroke• four stroke• However, the bulk of IC engines use the four-stroke cycle. Let us look at the principle of operation of the four-• stroke diesel engine.

9.1.1 The Four Stroke Diesel EngineThe 4 stroke operations in a diesel engine are: induction stroke, compression stroke, ignition and power stroke and exhaust stroke.

1st: Induction stroke-while the inlet valve is open, the descending piston draws in fresh air.• 2nd: Compression stroke -while the valves are closed, the air is compressed to a pressure of upto 25 bar.• 3rd: Ignition and power stroke - fuel is injected, while the valves are closed (fuel injection actually starts at the • end of the previous stroke), the fuel ignites spontaneously and the piston is forced downwards by the combustion gases.4th: Exhaust stroke - the exhaust valve opens and the rising piston discharges the spent gases from the • cylinder.

Suction Compression Power Exhaust

Fig. 9.1 Schematic diagram of a four stroke diesel engine

Since power is developed only during one stroke, the single cylinder four-stroke engine has a low degree of uniformity. Smoother running is obtained with multi- cylinder engines because the cranks are staggered in relation to one another on the crankshaft. There are many variations of engine configurations, for example, 4 or 6 cylinders, in-line, horizontally opposed, vee or radial configurations.

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9.1.2 The DG Set as a SystemA diesel generating set should be considered as a system since its successful operation depends on the well-matched performance of the components, namely:

the diesel engine and its accessories. �the ac generator. �the control systems and switchgear. �the foundation and power house civil works. �the connected load with its own components like heating, motor drives, lighting, etc. �

It is necessary to select components with the highest efficiency and operate them at their optimum efficiency levels to conserve energy in this system.

Fig. 9.2 Diesel generator system9.1.3 Selection Considerations

To make a decision on the type of engine which is most suitable for a specific application, several factors need • to be considered. The two most important factors are: power and speed of the engine.

The power requirement is determined by the maximum load. The engine power rating should be 10-20 % �more than the power demand for the end use. This prevents overloading the machine by absorbing the extra load during the starting of the motors or switching on of some types of lighting systems or when wear and tear on the equipment pushes up its power consumption.Speed is measured at the output shaft and given in revolutions per minute (RPM). An engine will operate �over a range of speeds, with diesel engines typically running at lower speeds (1300–3000 RPM). There will be an optimum speed at which the fuel efficiency will be the greatest. Engines should be run as closely as possible to their rated speed to avoid poor efficiency and to prevent build up of engine deposits due to incomplete combustion -which will lead to higher maintenance and running costs. To determine the speed requirement of an engine, one has to again look at the requirement of the load.

For some applications, the speed of the engine is not critical, but for other applications such as a generator, it is important to get a good speed match. If a good match can be obtained, direct coupling of the engine and generator is possible; if not, then some form of gearing will be necessary - a gearbox or belt system, which will add to the cost and reduce the efficiency.

There are various other factors that have to be considered when choosing an engine for a given application. • These include the following: cooling system, abnormal environmental conditions (dust, dirt, etc.), fuel quality, speed governing (fixed or variable speed), poor maintenance, control system, starting equipment, drive type, ambient temperature, altitude, humidity, etc.The suppliers or manufacturers literature will specify the required information when purchasing an engine. The • efficiency of an engine depends on various factors, for example, load factor (percentage of full load), engine size, and engine type.

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Diesel Generator Captive Power PlantsDiesel engine power plants are most frequently used in small power (captive non-utility) systems. The main reason for their extensive use is the higher efficiency of the diesel engines compared to gas turbines and small steam turbines in the output range considered. In applications requiring low captive power, without much requirement of process steam, the ideal method of power generation would be by installing diesel generator plants. The fuels burnt in diesel engines range from light distillates to residual fuel oils. Most frequently used diesel engine sizes are between the ranges 4–15 MW. For continuous operation, the low speed diesel engine is more cost-effective than the high speed diesel engine.

The advantages of adopting Diesel Power Plants are:low installation cost,• short delivery and installation periods,• higher efficiency (as high as 43 -45 %),• more efficient plant performance under part loads,• suitable for different types of fuels such as low sulphur heavy stock and heavy fuel oil in case of large • capacities.

Minimum cooling water requirements.Adapted with the air cooled heat exchanger in areas where water is not available,Short start up time.

A brief comparison of the different types of captive power plants (combined gas turbine and steam turbine, conventional steam plant and diesel engine power plant) is given in Table 9.1. It can be seen from the table that the captive diesel plant wins over the other two in terms of thermal efficiency, capital cost, space requirements, auxiliary power consumption, plant load factor, etc.

Description Units GT & ST Combined Steam Plant

Conventional Power Plants

Diesel Engine

Thermal Efficiency % 40–46 33–36 43–45Initial Investment of Installed Capacity

Rs. / kW 8,500–10,000 15,000–18,000 7,500–9,000

Space requirement 125% (Approx.) 250% (Approx.) 100 % (Approx.)Construction time Months 24–30 42–48 12–15Project period Months 30–36 52– 60 12Auxiliary Power Consumption

% 2–4 8–10 1.3–2.1

Plant Load Factor kWh/kW 6000–7000 5000–6000 7200–7500Start up time from cold Minutes About 10 120–180 15–20

Table 9.1 Comparison of different types of captive power plants

9.1.4 Diesel Engine Power Plant Developments

Diesel engine developments have been steady and impressive. Specific fuel consumption has come down from • a value of 220 g/kWh in the 1970s to a value of around 160 g/kWh in the present times.The slow speed diesel engine, with its flat fuel consumption curve over a wide load range (50%–100%), • compares very favourably over other prime movers such as medium speed diesel engine, steam turbines and gas turbines. With the arrival of modern, high efficiency turbochargers, it is possible to use an exhaust gas driven turbine • generator to further increase the engine rated output. The net result lower fuel consumption per kWh and further increase in overall thermal efficiency.

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The diesel engine is able to burn the poorest quality fuel oils, unlike the gas turbine, which is able to do so with • only costly fuel treatment equipment.Slow speed dual fuel engines are now available using high-pressure gas injection, which give the same thermal • efficiency and power output as a regular fuel oil engine.

Fig. 9.3 Turbocharger

9.2 Selection and Installation Factors9.2.1 Sizing of a Genset

If the DG set is required for 100% standby, then the entire connected load in HP/kVA should be added. After • finding out the diversity factor, the correct capacity of a DG set can be found out.

Example:Connected Load = 650kW

Diversity Factor = 0.54 (Demand / Connected load) Max. Demand = 650 x 0.54 = 350 kW

% Loading = 70 Set rating = 350/0.7 = 500 kW At 0.8 PF, rating = 625 kVA

For an existing installation, record the current, voltage and power factor (kWh/kVAh) reading at the main • bus-bar of the system at every half-an-hour interval for a period of 2–3 days and during this period the factory should conduct its normal operations. Non-essential loads should be switched off to find the realistic current taken for running essential equipment. This will give a fair idea of current taken from which the rating of the set can be calculated.

For an existing installation: kVA = v3V I

kVA Rating = kVA / Load Factor where Load factor = Average kVA / Maximum kVAFor a new installation, an approximate method of estimating the capacity of a DG set is to add full load currents • of all the proposed loads to be run in the DG set. Then, applying a diversity factor depending on the industry, process involved and guidelines obtained from other similar units, the correct capacity can be arrived at.

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9.2.2 High Speed Engine or Slow/Medium Speed EngineThe normal accepted definition of the high-speed engine is 1500 rpm. The high-speed sets have been developed in India, whereas the slow speed engines of higher capacities are often imported. Other features and comparisons between high and medium / slow speed engines are mentioned below:Factor Slow speed engine High speed engineBreak mean effective pressure-therefore wear and tear and consumption of spares

Low High

Weight to power ratio-therefore sturdiness and life

More Less

Type of use Continuous use Intermittent usePeriod between overhauls* 8000 hours 3200Direct operating cost (includes lubricating oils, filters, etc.

Less High

* Typical recommendations from manufacturers

Keeping the above factors and available capacities of the DG sets in mind, the economic cost for both the engines should be worked out before arriving at a decision.

9.2.3 Capacity Combinations

From the point of view of space, operation, maintenance and initial capital investment, it is certainly economical • to go in for one large DG set than two or more DG sets in parallel.Two or more DG sets running in parallel can be an advantage when only the short-fall in power depending upon • the extent of power cut prevailing - needs to filled up. Also, flexibility of operation is increased since one DG set can be stopped, while the other DG set is generating • at least 50% of the power requirement. Another advantage is that one DG set can become 100% standby during lean and low power-cut periods.•

9.2.4 Air Cooling Vs. Water Cooling

The general feeling has been that a water-cooled DG set is better than an air cooled set, as most users are worried • about the overheating of engines during the summer months. This is to some extent true and precautions have to be taken to ensure that the cooling water temperature does not exceed the prescribed limits. However, from the performance and maintenance point of view, water and air cooled sets are equally good except • that proper care should be taken to ensure cross ventilation so that as much cool air as possible is circulated through the radiator to keep its cooling water temperature within limits.While, it may be possible to have air cooled engines in the lower capacities, it will be necessary to go in for • water cooled engines in larger capacities to ensure that the engine does not get over-heated during the summer months.

9.2.5 Safety Features

It is advisable to have short circuit, over load and earth fault protection on all the DG sets. However, in case • of smaller capacity DG sets, this may become uneconomical. Hence, installing circuit protection is strongly recommended. Other safety equipment like high temperature, low lube oil pressure cut-outs should be provided, so that in the • event of any of these abnormalities, the engine would stop and prevent damage.It is also essential to provide reverse power relay when DG sets are to run in parallel to avoid back feeding from • one alternator to another.

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9.2.6 Parallel Operation with Grid

Running the DG set in parallel with the mains from the supply undertakings can be done in consultation with the • concerned electricity authorities. However, some supply undertakings ask the consumer to give an undertaking that the DG set will not be run in parallel with their supply. The reasons stated are that the grid is an infinite bus and paralleling a small capacity DG set would involve • operational risks despite normal protections like reverse power relay, voltage and frequency relays.

9.2.7 Maximum Single Load on a DG Set

The starting current of squirrel cage induction motors is as much as six times the rated current for a few seconds • with direct-on-line starters. In practice, it has been found that the starting current value should not exceed 200 % of the full load capacity of the alternator. The voltage and frequency throughout the motor starting interval recovers and reaches the rated values usually • much before the motor has picked up full speed.In general, the HP of the largest motor that can be started with direct on-line starting is about 50 % of the kVA • rating of the generating set. On the other hand, the capacity of the induction motor can be increased, if the type of starting is changed over • to star delta or to the auto transformer starter, and with this starting, the HP of the largest motor can be upto 75 % of the kVA of the Genset.

9.2.8 Unbalanced Load EffectsIt is always recommended that the load be much balanced as much as possible, since unbalanced loads can cause heating of the alternator, which may result in unbalanced output voltages. The maximum unbalanced load between phases should not exceed 10% of the capacity of the generating sets.

9.2.9 Neutral EarthingElectricity rules clearly specify that two independent earths to the body and a neutral should be provided to give adequate protection to the equipment in case of an earth fault, and also to drain away any leakage potential from the equipment to the earth for safe working.

9.2.10 Site Condition Effects on Performance DeratingSite conditions with respect to altitude, intake temperature and cooling water temperature derate diesel engine output as shown in the following Tables: 9.2 and 9.3.

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Correction Factors For Engine OutputAltitude Correction Temperature Correction

Altitude Meters over MSL

Non Super Charged

Super Charged

Intake oC Correction Factor

610 0.980 0.980 32 1.000915 0.935 0.950 35 0.9861220 0.895 0.915 38 0.9741525 0.855 0.882 41 0.9621830 0.820 0.850 43 0.9502130 0.780 0.820 46 0.9372450 0.745 0.790 49 0.9252750 0.712 0.765 52 0.9133050 0.680 0.740 54 0.9003660 0.612 0.6854270 0.550 0.6304880 0.494 0.580

Table – 9.2 Altitude and intake temperature corrections

9.3 Operational Factors9.3.1 Load Pattern and DG Set Capacity

The average load can be easily assessed by logging the current drawn at the main switchboard on an average • day. The ‘over load' has a different meaning when referred to the DG set. Overloads, which appear insignificant and harmless on electricity board supply, may become detrimental to a • DG set, and hence overload on the DG set should be carefully analysed. Diesel engines are designed for 10% overload for 1 hour in every 12 hours of operation. A.C. generators are designed to meet 50% overload for 15 seconds as specified by standards.The DG set/s selection should be such that the overloads are within the above specified limits. It would be ideal • to connect steady loads on DG sets to ensure good performance. Alongside alternator loading, engine loading in terms of kW or BHP, needs to be maintained above 50%. Ideally, the engine and alternator loading conditions are both to be achieved to give high efficiency.Engine manufacturers offer curves indicating % Engine Loading vs. fuel Consumption in grams/BHP. Optimal • engine loading corresponding to the best operating point is desirable for energy efficiency.Alternators are sized for kVA ratings with the highest efficiency attainable at a loading of around 70% and • more. Manufacturers’ curves can be referred to for the best efficiency point and corresponding kW and kVA loading values.

9.3.2 Sequencing of Loads

The captive diesel generating set has certain limits in handling transient loads. This applies to both kW (as • reflected on the engine) and kVA (as reflected on the generator). In this context, the base load that exists before the application of the transient load brings down the transient load handling capability, and in case of A.C. generators, it increases the transient voltage dip.Hence, great care is required in sequencing the load on DG set/s. It is advisable to start the load with the highest • transient kVA first, followed by other loads in the descending order of the starting kVA. This will lead to optimum sizing and better utilisation of the transient load handling capacity of the DG set.

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Water Temperature 0C Flow % Derating %25 100 030 125 335 166 540 166 8

Table.9.3 Derating due to air inter cooler water inlet temperature

9.3.3 Load Pattern

In many cases, the load will not be constant throughout the day. If there is a substantial variation, then consideration • should be given for parallel operation of D.G.sets. In such a situation, additional D.G. set(s) are to be switched on when the load increases. A typical case may be • an establishment demanding substantially different powers in the first, second and third shifts. By parallel operation, D.G. sets can be run at optimum operating points near optimum, for optimum fuel • consumption and additionally, flexibility is built into the system. This scheme can be also be applied where loads can be segregated as critical and non-critical loads to provide • standby power to the critical load in the captive power system.

9.3.4 Load Characteristics

Some of the load characteristics influence the efficient use of a D.G.set. These characteristics are entirely load • dependent and cannot be controlled by the D.G.set. The extent of detrimental influences of these characteristics can be reduced in several cases.

Power Factor �The load power factor is entirely dependent on the load. The A.C. generator is designed for a power factor of 0.8 lag as specified by standards. A lower power factor demands higher excitation currents and results in increased losses. Over sizing A.C.generators for operations at lower power factors results in lower operating efficiency and higher costs. The economical alternative is to provide power factor improvement capacitors.

Unbalanced LoadUnbalanced loads on the A.C. generator lead to an unbalanced set of voltages and additional heating in the A.C. generator. When other connected loads like motor loads are fed with an unbalanced set of voltages, additional losses occur in the motors as well. Hence, the load on the A.C. generators should be balanced as far as possible. Where single-phase loads are predominant, consideration should be given to procuring a single phase A.C. generator.

Transient LoadingOn many occasions, to contain the transient voltage dip arising due to the transient load application, a specially designed generator may have to be selected. Many times non-standard combination of the engine and the A.C. generator may have to be procured. Such a combination ensures that the prime mover is not unnecessarily over sized who add to capital cost and running cost.

Special LoadsSpecial loads like the rectifier/thyristor loads, welding loads, furnace loads need an application check. The manufacturer of the diesel engine and AC generator should be consulted for proper recommendations so that the desired utilisation of the DG set is achieved without any problem. In certain cases of loads, which are sensitive to voltage, frequency regulation, voltage wave form, consideration should be given to segregate the loads, and feed it by a dedicated power supply which usually assumes the form of a DG motor driven generator set. Such an alternative ensures that special design of the AC generator is restricted to that portion of the load which requires high purity rather than increasing the price of the DG set by a specially designed AC generator for complete load.

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Waste Heat Recovery in DG SetsA typical energy balance in a DG set indicates the following break-up:Input : 100% Thermal EnergyOutputs : 35% Electrical Output 4% Alternator Losses 33% Stack Loss through Flue Gases 24% Coolant Losses 4% Radiation Losses

Among these, stack losses through fine gases or the exhaust flue gas losses on account of existing flue gas temperature of 3500C to 550C, constitute the major area of concern towards operational economy. It would be realistic to assess the Waster Heat Recovery (WHR) potential in relation to quantity, temperature margin, in kcals/ hour as:

Potential WHR=(kWh Output/Hour) x (8 kg Gases/kWh Output)x 0.25 kcal/kg0C×(tg -1800C)Where tg is the gas temperature after Turbocharger, (the criteria being that limiting exit gas temperature cannot be less than 1800C, to avoid acid dew point corrosion), 0.25 being the specific heat of flue gases and kWh output being the actual average unit generation from the set per hour. For a 1100 kVA set, at 800kW loading, and with 4800C exhaust gas temperature, the waste heat potential works out to:800kWh x 8 kg gas generation / kWh output x 0.25 kCal/kg0C X (480 - 180), i.e., 4, 80,000 kCal/hr

While the above method only yields the potential for heat recovery, the actual realisable potential depends upon various factors and if applied judiciously, a well configured waste heat recovery system can tremendously boost the economics of captive DG power generation.The factors affecting Waste Heat Recovery from flue gases are:

DG Set loading, temperature of exhaust gases �Consistent DG set loading (to over 60% of rating) would ensure a reasonable exit flue gas quantity and temperature. Fluctuations and gross under loading of the DG set results in erratic flue gas quantity and temperature profile at entry to the heat recovery unit, thereby leading to possible cold end corrosion and other problems.

Hours of operation �The number of hours of operation of the DG set has an influence on the thermal performance of the waste heat recovery unit. With continuous DG Set operations, cost benefits are favourable

Back pressure on the DG set �

Back pressure in the gas path caused by an additional pressure drop in the waste heat recovery unit is another key factor. Generally, the maximum back pressure allowed is around 250-300 mmWC and the heat recovery unit should have a pressure drop lower than that. The choice of convective waste heat recovery systems with adequate heat transfer areas are known to provide reliable service

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100% Load 11.84 kgs/Sec 3700C90% Load 10.80 kgs/Sec 3500C70% Load 9.08 kgs/Sec 3300C60% Load 7.50 kgs/Sec 3250C

If the normal load is 60%, the flue gas parameters for waste heat recovery unit would be 320oC inlet temperature, 180oC outlet temperature and 27180kgs/Hour gas flow.

At 90% loading, however, values would be 355oC and 32,400 kgs/Hour, respectively

Table 9.4 Typical flue gas temperature and flow pattern in a 5-mw dg set at various loads

The configuration of the heat recovery system and the choice of steam parameters can be judiciously selected with reference to the specific industry (site) requirements. Much good work has taken place in the Indian Industry. Waste heat recovery and one interesting configuration deployed is the installation of the waste heat boiler in the flue gas path along with a vapour absorption chiller, to produce 80C chilled water working on steam from waste heat.

Favourable incentives offered by the Government of India for energy efficient equipment and technologies (100% depreciation at the end of first year), make the waste heat recovery option viable. The payback period is only about 2 years

9.4 Energy Performance Assessment of DG SetsRoutine energy efficiency assessment of DG sets on the shop floor involves the following typical steps:• Ensure reliability of all instruments used for trial.• Collect technical literature, characteristics, and specifications of the plant.• Conduct a 2 hour trial on the DG set, ensuring a steady load, wherein the following measurements are logged • at 15 minute intervals.

Fuel consumption (by dip level or by flow meter) �Amps, volts, PF, kW, kWh �Intake air temperature, Relative Humidity (RH) �Intake cooling water temperature �Cylinder-wise exhaust temperature (as an indication of engine loading) �Turbocharger RPM (as an indication of loading on engine) �Charge air pressure (as an indication of engine loading) �Cooling water temperature before and after the charge air cooler (as an indication of cooler performance) �Stack gas temperature before and after turbocharger (as an indication of turbocharger performance). �

The fuel oil/diesel analysis is referred to from an oil company data.• Analysis: The trial data is to be analysed with respect to:•

Average alternator loading. �Average engine loading. �Percentage loading on the alternator. �Percentage loading on the engine. �Specific power generation kWh/litre. �Comments on the turbocharger performance based on the RPM and gas temperature difference. �Comments on charge air cooler performance. �Comments on load distribution among various cylinders (based on exhaust temperature, the temperature to �be ± 5% of mean and high/low values indicate disturbed condition).Comments on housekeeping issues like drip leakages, insulation, vibrations, etc. �

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A format as shown in Table 9.5 is useful for monitoring performance

DG Set No.

Electricity Generating

Capacity(Site), kW

Derated Electricity Generating

Capacity, kW

Type of Fuel

used

Average Load as % of Derated Capacity

Specific FuelCons.

Lit/kWh

Specific Lube Oil cons.

Lit/kWh

1. 480 300 LDO 89 0.335 0.0072. 480 300 LDO 110 0.334 0.0243. 292 230 LDO 84 0.356 0.0064. 200 160 HSD 89 0.325 0.0035. 200 160 HSD 106 0.338 0.0036. 200 160 HSD7. 292 230 LDO 79 0.339 0.0068. 292 230 LDO 81 0.362 0.0059. 292 230 LDO 94 0.342 0.00310. 292 230 LDO 88 0.335 0.006

Table 9.5 Typical format for DG set monitoring

9.5 Energy Saving Measures for DG SetsEnsure steady load conditions on the DG set, and provide cold, dust free air at the intake (use of air washers for • large sets, in case of dry, hot weather, can be considered).Improve air filtration.• Ensure fuel oil storage, handling and preparation as per manufacturers' guidelines/oil company data.• Consider fuel oil additives in case they benefit fuel oil properties for DG set usage.• Calibrate fuel injection pumps frequently.• Ensure compliance with the maintenance checklist.• Ensure steady load conditions, avoiding fluctuations, imbalance in phases, harmonic loads.• In case of a base load operation, consider the waste heat recovery system adoption for the steam generation or • refrigeration chiller unit incorporation. Even the Jacket Cooling Water is amenable for heat recovery, vapour absorption system adoption.In terms of fuel cost economy, consider the partial use of biomass gas for generation. Ensure tar removal from • the gas for improving availability of the engine in the long run.Consider parallel operation among the DG sets for improved loading and fuel economy thereof.• Carry out regular field trials to monitor DG set performance, and maintenance planning as per requirements.•

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SummaryThe CI engine works in two modes such as two stroke and four stroke. In two stroke engines there is lot of • wastage of fuel since exhaust and fuel inlets are open simultaneously for a small duration. Four stroke engines are more economical in large sizes and cause less pollution.The 4 stroke operations in a diesel engine are: induction stroke, compression stroke, ignition and power stroke • and exhaust strokeA DG set is selected on the basis of the connected load, load factor, largest single motor to be started, and many • more considerations.Diesel engine power plants are most frequently used in small power (captive non-utility) systems. The main reason • for their extensive use is the higher efficiency of the diesel engines compared to gas turbines and small steam turbines in the output range considered. In applications requiring low captive power, without much requirement of process steam, the ideal method of power generation would be by installing diesel generator plants.Diesel engine developments have been steady and impressive. Specific fuel consumption has come down from • a value of 220 g/kWh in the 1970s to a value of around 160 g/kWh in the present times.The normal accepted definition of the high-speed engine is 1500 rpm.• From the point of view of space, operation, maintenance and initial capital investment, it is certainly economical • to go in for one large DG set than two or more DG sets in parallel.Two or more DG sets running in parallel can be an advantage when only the short-fall in power depending upon • the extent of power cut prevailing - needs to filled up. Parts of the exhaust gases are used to drive a small turbocharger pump which helps in pushing and atomising • the fuel. Hence it helps in improving efficiency in the DG set.DG set performance is assessed in the ratio of energy produced in kWH per litre of diesel (Fuel) consumed.• Heat recovery from exhaust gases gives one of the best opportunities in energy savings• Energy Saving Measures for DG Sets include to ensure steady load conditions on the DG set, and provide cold, • dust free air at the intake (use of air washers for large sets, in case of dry, hot weather, can be considered).Improve air filtration. Also to ensure fuel oil storage, handling and preparation as per manufacturers' guidelines/oil company data.

ReferencesMahon, L. L. J., 1992. • Diesel Generator Handbook, Newnes publication.Brady, R. N. & Dagel. J. F., • Diesel Engine and Fuel System Repair, 5th ed., Prentice Hall Publication.DG Set System• [Pdf] Available at: < www.beeindia.in/energy_managers_auditors/documents/.../3Ch9.pdf‎‎‎> [Accessed 5 July 2013].Assembly of Diesel Generator Set • [Pdf] Available at: < www.dcmsme.gov.in/publications/pmryprof/electrical/ch2.pdf‎‎> [Accessed 5 July 2013].2008. • Diesel Engines: Fuel System Design [Video online] Available at: < https://www.youtube.com/watch?v=Gh_mvWESGHw> [Accessed 5 July 2013].2012. • Diesel Electric Set-Non Functional Mock up [Video online] Available at: < https://www.youtube.com/watch?v=NwTKhe9iztE> [Accessed 5 July 2013].

Recommended ReadingAbdulqadar, M., 2006. • Diesel Generator Auxillary Systems and Instruments, Lulu.com Publication.Kaltschmitt, M., Streicher, W. & Wiese, A., 2009. • Renewable energy: technology, economics, and environment. Springer publication.Wharton, J., 2006. • Diesel Engines, Butterworth-Heinemann.

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Self AssessmentTwo stroke and four stroke are classifications of ________.1.

DG seta. CI set b. DM set c. CF setd.

The 4 stroke operations in a diesel engine are: induction stroke, compression stroke, ignition , power stroke 2. and __________.

drain strokea. refresh strokeb. exhaust strokec. master stroked.

The engine power rating should be _____more than the power demand for the end use.3. 20–30 %a. 10–20 %b. 50–40 %c. 30–40 %d.

Efficiency on adopting Diesel Power Plants is ____________.4. 15–10%a. 50–60%b. 24–42%c. 43–45%d.

Which of the following statements is true?5. A water-cooled DG set is not advisable than an air cooled seta. An air cooled DG set is better than a water- cooled setb. A water-cooled DG set is better than an air cooled setc. An air cooled DG set is not advisable than a water cooled setd.

DG set performance is assessed in the ratio of ____________.6. energy produced in kWH per litre of diesel (Fuel) consumed.a. energy produced in kV per litre of diesel (Fuel) consumed.b. energy produced in kWA per litre of diesel (Fuel) consumed.c. energy produced in WH per litre of diesel (Fuel) consumed.d.

________from exhaust gases gives one of the best opportunities in energy savings.7. Warmth recoverya. Heat recoveryb. Water recoveryc. Air recoveryd.

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In two stroke engines there is lot of wastage of fuel since _________.8. exhaust and fuel inlets are closeda. exhaust and fuel inlets are not present b. exhaust and fuel inlets are blockedc. exhaust and fuel inlets are opend.

_________ in the gas path caused by an additional pressure drop in the waste heat recovery unit is another key 9. factor.

Back pressurea. Front pressureb. Upward pressure c. Downward pressured.

The number of hours of operation of the DG set has an influence on the ______ performance of the waste heat 10. recovery unit.

hydroa. tidalb. windc. thermald.

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Chapter X

Energy Efficient Technologies in Electrical Systems

Aim

The aim of this chapter is to:

elucidate maximum demand controllers •

explicate soft starters with energy savers and variable speed drives•

explain energy efficient transformers•

Objectives

The objectives of the chapter are to:

explain energy efficient technologies in electrical systems•

define electronic ballast and occupancy sensors•

explicate energy saving potential of each technology•

Learning outcome

At the end of this chapter, you will be able to:

identify factors affecting energy efficient transformers•

understand the concept of energy efficient lighting controls•

define energy efficient motors•

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10.1 Maximum Demand ControllersHigh-tension (HT) consumers have to pay a maximum demand charge in addition to the usual charge for the • number of units consumed. This charge is usually based on the highest amount of power used during some period (say 30 minutes) during the metering month.The maximum demand charge often represents a large proportion of the total bill and may be based on only one • isolated 30 minute episode of high power use. Considerable savings can be realised by monitoring power use and turning off or reducing non-essential loads during such periods of high power use.Maximum Demand Controller (See Fig.10.1) is a device designed to meet the need of industries conscious of • the value of load management. An alarm is sounded when demand approaches a preset value. If corrective action is not taken, the controller switches off non-essential loads in a logical sequence.This sequence is predetermined by the user and is programmed jointly by the user and the supplier of the device. • The plant equipments selected for load management are stopped and restarted as per the desired load profile. The demand control scheme is implemented by using suitable control contactors. Audio and visual annunciations could also be used.

Fig. 10.1 Maximum demand controller

10.2 Automatic Power Factor ControllersVarious types of automatic power factor controls are available with relay / microprocessor logic. Two of the most common controls are: Voltage Control and kVAr Control

10.2.1 Voltage Control

Voltage alone can be used as a source of intelligence when the switched capacitors are applied at a point where • the circuit voltage decreases as circuit load increases. Generally, where they are applied, the voltage should decrease as the circuit load increases and the drop in voltage should be around 4 - 5 % with increasing load.Voltage is the most common type of intelligence used in substation applications when maintaining a particular • voltage is of prime importance. This type of control is independent of the load cycle. During light load time and low source voltage, this may give a leading PF at the substation, which is to be taken note of.

10.2.2 Kilovar Control

Kilovar sensitive controls (see Fig. 10.2) are used at locations where the voltage level is closely regulated and • not available as a control variable. The capacitors can be switched to respond to a decreasing power factor as a result of the change in system loading.

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This type of control can also be used to avoid a penalty on the low power factor by adding capacitors in steps • as the system power factor begins to lag behind the desired value. Kilovar control requires two inputs - current and voltage from the incoming feeder, which are fed to the PF correction mechanism, either the microprocessor or the relay.

Fig.10.2 Reactive power control relay

10.2.3 Automatic Power Factor Control Relay

It controls the power factor of the installation by giving signals to switch on or off power factor correction • capacitors. Relay is the brain of the control circuit and needs contactors of the appropriate rating for switching on/off the capacitors on/off.There is a built-in power factor transducer, which measures the power factor of the installation and converts it • to a DC voltage of appropriate polarity. This is compared with a reference voltage, which can be set by means of a knob calibrated in terms of the power factor.When the power factor falls below the setting, the capacitors are switched on in sequence. The relays are provided • with the First in First out (FIFO) and First in Last Out (FILO) sequences.The capacitors controlled by the relay must be of the same rating and they are switched on/off in linear sequence. • To prevent over correction hunting, a dead band is provided. This setting determines the range of phase angle over which the relay does not respond; the relay acts only when the PF goes beyond this range.When the load is low, the effect of the capacitors is more pronounced and may lead to hunting. Under current • blocking (low current cut out) shuts off the relay, switching off all capacitors one by one in a sequence, when the load current is below setting.Special timing sequences ensure that the capacitors are fully discharged before they are switched in. This avoids • a dangerous over voltage transient. The solid state indicating lamps (LEDS) display various functions that the operator should know and also indicate each capacitor switching stage.

10.2.4 Intelligent Power Factor Controller (IPFC)This controller determines the rating of the capacitance connected at each step during the first hour of its operation and stores them in memory. Based on this measurement, the IPFC switches on the most appropriate steps, thus eliminating the hunting problems normally associated with capacitor switching.

10.3 Energy Efficient MotorsMinimising Watt Loss in MotorsImprovements in motor efficiency can be achieved without compromising motor performance - at a higher cost - within the limits of existing design and manufacturing technology.

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Fig. 10.3 Energy efficient motors

From Table 10.1, it can be seen that any improvement in motor efficiency must result from reducing the Watt • losses. In terms of the existing state of electric motor technology, a reduction in watt losses can be achieved in various ways.

Watts loss area efficiency improvement

1. Iron Use of thinner gauge, lower loss core steel reduces eddy current losses. Longer core adds more steel to the design, which reduces losses due to lower operating flux densities.

2. Stator I2R Use of more copper and larger conductors increases cross sectional area of stator windings. This lowers resistance (R) of the windingsand reduces losses due to current flow (I).

3. Rotor I2R Use of larger rotor conductor bars increases size of cross section, lowering conductor resistance (R) and losses due to current flow (I).

4. Friction & Windage Use of low loss fan design reduces losses due to air movement.

5. Stray Load Loss Use of optimised design and strict quality control procedures minimizes stray load losses.

Table 10.1 Watt loss area and efficiency improvement

All of these changes to reduce motor losses are possible with the existing motor design and manufacturing • technology. They would, however, require additional materials and/or the use of higher quality materials and improved manufacturing processes resulting in increased motor cost.

Simply Stated: REDUCED LOSSES = IMPROVED EFFICIENCYThus energy-efficient electric motors reduce energy losses through an improved design, better material, and • improved manufacturing techniques. Replacing a motor may be justifiable solely on the electricity cost savings derived from an energy-efficient replacement.This is true if the motor runs continuously, as power rates are high, the motor is oversized for the application, • or its nominal efficiency has been reduced by damage or previous rewinds. Efficiency comparison for standard and high efficiency motors is shown in Figure 10.4.

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STANDARD vs HIGH EFFICIENCY MOTORS(Typical 3-Phase induction Motor)

100

90

80

70

High Effi ciency Motors

Standard Motors

Fig. 10.4 Efficiency range for standard and high efficiency motors

10.3.1 The Technical Aspects of Energy Efficient Motors

Energy-efficient motors last longer, and may require less maintenance. At lower temperatures, bearing grease • lasts longer; the required time between re-greasing increases. Lower temperatures translate to long lasting insulation. Generally, motor life doubles for each 100 C reduction in operating temperature.Select energy-efficient motors with a 1.15 service factor, and design for operation at 85% of the rated motor • load.Electrical power problems, especially poor incoming power quality can affect the operation of energy-efficient • motors.Speed control is crucial in some applications. In polyphase induction motors, slip is a measure of motor winding • losses. The lower the slip, the higher the efficiency. Less slippage in energy efficient motors results in speeds about 1% faster than in standard counterparts.Starting torque for efficient motors may be lower than for standard motors. Facility managers should be careful • when applying efficient motors to high torque applications.

10.4 Soft StarterWhen starting, an AC Induction motor develops more torque than is required at full speed. This stress is transferred • to the mechanical transmission system resulting in excessive wear and the premature failure of chains, belts, gears, mechanical seals, etc. Additionally, rapid acceleration also has a massive impact on electricity supply charges with high in-rush • currents drawing +600% of the normal run current.The use of Star Delta only provides a partial solution to the problem. Should the motor slow down during the • transition period, the high peaks can be repeated and can even exceed the direct online current.The soft starter (see figure 10.5) provides a reliable and economical solution to these problems by delivering a • controlled release of power to the motor, thereby providing smooth, stepless acceleration and deceleration. The motor life will be extended as the damage to windings and bearings is reduced.

STANDARD vs HIGH EFFICIENCY MOTORS(Typical 3-Phase induction Motor)

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Fig. 10.5 Soft Starter

Soft Start and Soft Stop are built into 3 phase units, providing controlled starting and stopping with a selection • of ramp times and current limit settings to suit all applications (see Figure 10.6).

Fig. 10.6 Soft Starter: Starting current, Stress profile during starting

Advantages of Soft Start• Less mechanical stress �Improved power factor �Lower maximum demand �Less mechanical maintenance �

10.5 Variable Speed Drives10.5.1 Speed Control of Induction Motors

The induction motor is the workhorse of the industry. It is cheap rugged and provides high power to weight • ratio. On account of high cost-implications and limitations of the D.C. System, induction motors are preferred for variable speed applications, the speed of which can be varied by changing the supply frequency.The speed can also be varied through a number of other means, including, varying the input voltage, varying the • resistance of the rotor circuit, using multi- speed windings, using Scherbius or Kramer drives, using mechanical means such as gears and pulleys and eddy-current or fluid coupling, or by using rotary or static voltage and frequency converters.

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10.5.2 The Variable Frequency Drive

The VFD operates on a simple principle. The rotational speed of an AC induction motor depends on the number • of poles in that stator and the frequency of the applied AC power. Although the number of poles in an induction motor cannot be altered easily, variable speed can be achieved through a variation in the frequency.The VFD rectifies the standard 50 cycle AC line power to DC, then synthesises the DC to a variable frequency • AC output. Motors connected to VFD provide variable speed mechanical output with high efficiency. These devices are capable of up to a 9:1 speed reduction ratio (11 percent of full speed), and a 3:1 speed increase (300 percent of full speed).In recent years, the technology of AC variable frequency drives (VFD) has evolved into highly sophisticated • digital microprocessor control, along with high switching frequency IGBTs (Insulated Gate Bi Polar Transistors) power devices.This has led to significantly advanced capabilities from the ease of programmability to expanded diagnostics. • The two most significant benefits from the evolution in technology have been those of cost and reliability, in addition to the significant reduction in physical size.

10.5.3 Variable Torque Vs. Constant Torque

Variable speed drives, and the loads that are applied to, can generally be divided into two groups: constant • torque and variable torque. The energy savings potential of variable torque applications is much greater than that of constant torque applications.Constant torque loads include vibrating conveyors, punch presses, rock crushers, machine tools, and other • applications where the drive follows a constant V/Hz ratio.Variable torque loads include centrifugal pumps and fans, which make up the majority of HVAC applications.•

10.5.4 Why Variable Torque Loads Offer Greatest Energy Savings

In variable torque applications, the torque required varies with the square of the speed, and the horsepower • required varies with the cube of the speed, resulting in a large reduction of horsepower for even a small reduction in speed.The motor will consume only 12.5% as much energy at 50% speed than it will at 100% speed. This is referred to • as the Affinity Laws, which define the relationships between speed, flow, torque, and horsepower. The following laws illustrate these relationships:

Flow is proportional to speed �Head is proportional to (speed)2 �Torque is proportional to (speed)2 �Power is proportional to (speed)3 �

10.5.5 Tighter Process Control with Variable Speed Drives

No other AC motor control method compares to variable speed drives when it comes to accurate process control. • Full-voltage (across the line) starters can only run the motor at full speed, and soft starts and reduced voltage soft starters can only gradually ramp the motor up to full speed, and back down to shutdown. Variable speed drives, on the other hand, can be programmed to run the motor at a precise speed, to stop at a precise position, or to apply a specific amount of torque.In fact, modern AC variable speed drives are very close to the DC drive in terms of fast torque response and • speed accuracy. However, AC motors are much more reliable and affordable than DC motors, making them far more prevalent.Most drives used in the field utilise the Volts/Hertz type control, which means they provide open-loop operation. • These drives are unable to retrieve feedback from the process, but are sufficient for the majority of variable speed drive applications.

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Many open-loop variable speed drives do offer slip compensation though, which enables the drive to measure • its output current and estimate the difference in actual speed and the set point (the programmed input value). The drive then automatically adjusts itself towards the set point based on this estimation.Most variable torque drives have Proportional Integral Differential (PID) capability for fan and pump applications, • which allow the drive to hold the set point based on actual feedback from the process, rather than relying on estimation.A transducer or transmitter is used to detect process variables such as pressure levels, liquid flow rate, air flow • rate, or liquid level. Then the signal is sent to a PLC (Programmable Logic Controller), which communicates the feedback from the process to the drive. The variable speed drive uses this continual feedback to adjust itself to hold the set point.High levels of accuracy for other applications can also be achieved through drives that offer the closed-loop • operation. Closed-loop operation can be accomplished with either a field-oriented vector drive, or a sensorless vector drive.The field-oriented vector drive obtains process feedback from an encoder, which measures and transmits to • drive the speed and/or rate of the process, such as a conveyor, machine tool, or extruder. The drive then adjusts itself accordingly to sustain the programmed speed, rate, torque, and/or position.

10.5.6 Extended Equipment Life and Reduced Maintenance

Single-speed starting methods start motors abruptly, subjecting the motor to a high starting torque and to current • surges that are up to 10 times the full-load current. Variable speed drives, on the other hand, gradually ramp the motor up to the operating speed to lessen mechanical and electrical stress, reducing maintenance and repair costs, and extending the life of the motor and the driven equipment.Soft starts, or reduced-voltage soft starters (RVSS), are also able to step a motor up gradually, but drives can be • programmed to ramp up the motor much more gradually and smoothly, and can operate the motor at less than full speed to decrease wear and tear.Variable speed drives can also run a motor in specialised patterns to further minimise mechanical and electrical • stress. For example, a S-curve pattern can be applied to a conveyor application for smoother control, which reduces the backlash that can occur when a conveyor is accelerating or decelerating.Typical full-load efficiencies are 95% and higher. High power units are still more efficient. The efficiency of • VSDs generally decreases with speed but since the torque requirement also decreases with speed for many VSD applications, the absolute loss is often not very significant.The power factor of a VSD drops drastically with speed, but at low power requirement the absolute kVAr • requirement is low, so the loss is also generally not significant. In a suitable operating environment, frequency controllers are relatively reliable and need little maintenance.A disadvantage of static converters is the generation of harmonics in the supply, which reduces motor efficiency • and reduces motor output. In some cases it may necessitate using a motor with a higher rating.

10.5.7 Eddy Current Drives

This method employs an eddy-current clutch to vary the output speed. The clutch consists of a primary member • coupled to the shaft of the motor and a freely revolving secondary member coupled to the load shaft.The secondary member is separately excited using a DC field winding. The motor starts with the load at rest • and a DC excitation is provided to the secondary member, which induces eddy-currents in the primary member. The interaction of the fluxes produced by the two currents gives rise to a torque at the load shaft.By varying the DC excitation, the output speed can be varied to match the load requirements. The major • disadvantage of this system is relatively poor efficiency particularly at low speeds. (See Figure 10.7).

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Fig. 10.7 Eddy current drive10.5.8 Slip Power Recovery SystemsSlip power recovery is a more efficient alternative speed control mechanism for use with slip-ring motors. In essence, a slip power recovery system varies the rotor voltage to control speed, but instead of dissipating power through resistors, the excess power is collected from the slip rings and returned as mechanical power to the shaft or as electrical power back to the supply line. Because of the relatively sophisticated equipment needed, slip power recovery tends to be economical only in relatively high power applications and where the motor speed range is 1:5 or less.

10.5.9 Fluid CouplingFluid coupling is one way of applying varying speeds to the driven equipment, without changing the speed of the motor.

10.5.10 Construction

Fluid couplings (see figure 10.8) work on the hydrodynamic principle. Inside every fluid coupling are two basic • elements the impeller and the runner and together they constitute the working circuit. One can imagine the impeller as a centrifugal pump and the runner as a turbine.The impeller and the rotor are bowl shaped and have a large number of radial vanes. They are suitably enclosed • in a casing, facing each other with an air gap. The impeller is connected to the prime mover while the rotor has a shaft bolted to it. This shaft is further connected to the driven equipment through a suitable arrangement.Thin mineral oil of low viscosity and good-lubricating qualities is filled in the fluid coupling from the filling • plug provided on its body. A fusible plug is provided on the fluid coupling which blows off and drains out oil from the coupling in case of sustained overloading.

Fig. 10.8 Fluid coupling

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10.5.11 Operating Principle

There is no mechanical inter-connection between the impeller and the rotor and power is transmitted by the • virtue of the fluid filled in the coupling. When the impeller is rotated by the prime mover, the fluid flows out radially and then axially under the action of the centrifugal force.It then crosses the air gap to the runner and is directed towards the bowl axis and back to the impeller. To enable • the fluid to flow from the impeller to the rotor, it is essential that there is a difference in head between the two and thus it is essential that there is a difference in RPM known as slip between the two.Slip is an important and inherent characteristic of a fluid coupling resulting in several desired advantages. As • the slip increases, more and more fluid can be transferred.However when the rotor is at a standstill, the maximum fluid is transmitted from the impeller to the rotor and • the maximum torque is transmitted from the coupling. This maximum torque is the limiting torque. The fluid coupling also acts as a torque limiter.

10.5.12 Characteristics

Fluid coupling has a centrifugal characteristic during starting, thus enabling a no-load start up of the prime • mover, which is of great importance. The slipping characteristic of the fluid coupling provides a wide range of choice of power transmission characteristics.By varying the quantity of oil filled in the fluid coupling, the normal torque transmitting capacity can be varied. • The maximum torque or limiting torque of the fluid coupling can also be set to a pre-determined safe value by adjusting the oil filling. The fluid coupling has the same characteristics in both directions of rotation.

10.6 Energy Efficient TransformersMost energy loss in the dry-type transformers occurs through heat or vibration from the core. The new high-• efficiency transformers minimise these losses. The conventional transformer is made up of a silicon alloyed iron (grain oriented) core.The iron loss of any transformer depends on the type of core used in the transformer. However the latest • technology is to use amorphous material a metallic glass alloy for the core (see Figure 10.9).The expected reduction in energy loss over conventional (Si Fe core) transformers is roughly around 70%, which • is quite significant. By using an amorphous core with unique physical and magnetic properties, these new types of transformers have increased efficiencies even at low loads - 98.5% efficiency at 35% load.Electrical distribution transformers made with amorphous metal cores provide excellent opportunity to conserve • energy right from the installation. Though these transformers are a little costlier than conventional iron core transformers, the overall benefit towards energy savings compensate for the higher initial investment. At present amorphous metal core transformers are available up to 1600 kVA.

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Fig. 10.9 1600 kVA amorphous core transformer

10.7 Electronic BallastVarious information regarding the elctronic ballast is as follows:

10.7.1 Role of Ballast

In an electric circuit, the ballast acts as a stabiliser. The fluorescent lamp is an electric discharge lamp. The two • electrodes are separated inside a tube with no apparent connection between them.When sufficient voltage is impressed on these electrodes, electrons are driven from one electrode and attracted • to the other. Current flow takes place through an atmosphere of low-pressure mercury vapour.Since the fluorescent lamps cannot produce light by direct connection to the power source, they need an ancillary • circuit and device to get started and remain illuminated. The auxillary circuit housed in a casing is known as the ballast.

10.7.2 Conventional vs. Electronic Ballasts

Conventional ballasts make use of the kick caused by a sudden physical disruption of the current in an inductive • circuit to produce the high voltage required for starting the lamp and then rely on a reactive voltage drop in the ballast to reduce the voltage applied across the lamp.On account of the mechanical switch (starter) and low resistance of the filament when cold, the uncontrolled • filament current generally tends to go beyond the limits specified by Indian standard specifications. With high values of current and flux densities, the operational losses and temperature rise are on the higher side in the conventional choke.

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The high frequency electronic ballast overcomes the above drawbacks. The basic functions of electronic ballast • are:

To ignite the lamp �To stabilise the gas discharge �To supply power to the lamp �

The electronic ballasts (see fig. 10.10) make use of modern power semiconductor devices for their operation. • The circuit components form a tuned circuit to deliver power to the lamp at a high resonant frequency (in the vicinity of 25 kHz) and voltage is regulated through an in-built feedback mechanism. It is now well established that the fluorescent lamp efficiency in the kHz range is higher than those attainable at low frequencies.At lower frequencies (50 or 60 Hz), the electron density in the lamp is proportional to the instantaneous value • of the current because the ionisation state in the tube is able to follow the instantaneous variations in the current. At higher frequencies (kHz range), the ionisation state cannot follow the instantaneous variations of the current and hence the ionisation density is approximately constant, proportional to the RMS (Root Mean Square) value of the current. Another significant benefit resulting from this phenomenon is the absence of the stroboscopic effect, thereby significantly improving the quality of light output.One of the biggest advantages of electronic ballast is the enormous energy savings it provides. This is achieved • in two ways.The first is its amazingly low internal core loss, quite unlike old fashioned magnetic ballasts.•

The second is increased light output due to the excitation of the lamp phosphors with high frequency. �

If the period of frequency of excitation is smaller than the light retention time constant for the gas in the lamp, • the gas will stay ionised and, therefore, produce light continuously. This phenomenon along with the continued persistence of the phosphors at high frequency will improve the light output from 8-12 percent. This is possible only with the high frequency electronic ballast.

Fig. 10.10 Electronic ballasts

10.8 Energy Efficient Lighting Controls10.8.1 Occupancy SensorsOccupancy-linked control can be achieved using infra-red, acoustic, ultrasonic or microwave sensors, which detect either movement or noise in room spaces. These sensors switch lighting on when occupancy is detected, and off again after a set time period, when no occupancy movements are detected.They are designed to override manual switches and to prevent a situation where lighting is left on in unoccupied spaces. With this type of a system it is important to incorporate a built-in time delay, since occupants often remain still or quiet for short periods and do not appreciate being plunged into darkness if not constantly moving around.

10.8.2 Timed Based ControlTimed-turn off switches are the least expensive type of automatic lighting control. In some cases, their low cost and ease of installation makes it desirable to use them where more efficient controls would be too expensive (see fig. 10.11).

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Fig. 10.11 Timed-turn off switch

Types and FeaturesThe oldest and most common type of timed-turn off switch is the “dial timer,” a spring-wound mechanical �timer that is set by twisting the knob to the desired time. Typical units of this type are vulnerable to damage because the shaft is weak and the knob is not securely attached to the shaft.Some spring-wound units make an annoying ticking sound as they operate. Newer types of timed-turn off �switches are completely electronic and silent. Electronic switches can be made much more rugged than the spring-wound dial timer. These units typically have a spring-loaded toggle switch that turns on the circuit for a preset time interval.Some electronic models provide a choice of time intervals, which you select by adjusting a knob located �behind the faceplate. Most models allow occupants to turn off the lights manually. Some models allow occupants to keep the lights on, overriding the timer.Timed-turn off switches are available with a wide range of time spans. The choice of the time span is a �compromise. Shorter time spans waste less energy but increase the probability that the lights will turn off while someone is in the space.Dial timers allow the occupant to set the time span, but this is not likely to be done with a view toward �optimising efficiency. For most applications, the best choice is an electronic unit that allows the engineering staff to set a fixed time interval behind the cover plate.

10.8.3 Daylight Linked Control

Photoelectric cells can be used either simply to switch lighting on and off, or for dimming. They may be mounted • either externally or internally. It is however important to incorporate time delays into the control system to avoid repeated rapid switching caused, for example, by fast moving clouds.By using an internally mounted photoelectric dimming control system, it is possible to ensure that the sum of • daylight and electric lighting always reaches the design level by sensing the total light in the controlled area and adjusting the output of the electric lighting accordingly.

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If daylight alone is able to meet the design requirements, then the electric lighting can be turned off. The energy • saving potential of dimming control is greater than a simple photoelectric switching system. Dimming control is also more likely to be acceptable to room occupants

10.8.4 Localised SwitchingLocalised switching should be used in applications which contain large spaces. Local switches give individual occupants control over their visual environment and also facilitate energy savings. By using localised switching it is possible to turn off artificial lighting in specific areas, while still operating it in other areas where it is required, a situation which is impossible if the lighting for an entire space is controlled from a single switch.

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SummaryThe new technologies available for improving energy efficiency are the maximum demand controller, automatic • power factor controller, energy efficient motors, occupancy sensors etc.Automatic Power Factor connects and disconnects the part of the capacitor bank depending on the variation of • the power factor. If required, the power factor is set at 0.99. Whenever the power factor tends to go below this, it will connect additional capacitors. Alternatively, whenever the power factor tends to go above this, it will disconnect some of the capacitors so that the power factor remains around the set value.Occupancy sensors are used to sense the presence and absence of human beings in a room and based on this, • electrical appliances in the room such as AC, lights, etc. can be switched On or Off respectively. Broadly, four types of sensors are used. They are infra-red, acoustic, ultra-sonic and microwave type.Automatic demand controllers are microprocessor based instruments which can track the average power • consumption and predict the maximum demand that is likely to occur. Based on the setting of the demand they can cut in or cut out the non-priority load.The soft starter provides a reliable and economical solution to these problems by delivering a controlled release • of power to the motor, thereby providing smooth, stepless acceleration and deceleration. The motor life will be extended as the damage to windings and bearings is reduced.The VFD operates on a simple principle. The rotational speed of an AC induction motor depends on the number • of poles in that stator and the frequency of the applied AC power. Although the number of poles in an induction motor cannot be altered easily, variable speed can be achieved through a variation in the frequency. Variable speed drives, and the loads that are applied to, can generally be divided into two groups: constant • torque and variable torque. The energy savings potential of variable torque applications is much greater than that of constant torque applications. Slip power recovery is a more efficient alternative speed control mechanism for use with slip-ring motors.• The new product in the market for improvement in energy efficiency is as electronic ballast. Electronic ballasts, • there are timer based controls available for street light control. Also timer based voltage control during the night time say 12 midnight to 6 am results in a lot of energy savings • in street lighting. A daylight sensor to switch off street lights is also an effective tool for energy savings.

ReferencesGilbert, M., 2004. • RenewableandEfficientElectricPowerSystems, Wiley-IEEE Press publication.Randolph, J. & Gilbert, M. • Energy for Sustainability: Technology, Planning, Policy, 1st ed., Island Press Publication.EnergyEfficientTechnologiesinElectricalSystems• [Pdf] Available at: <beeindia.in/energy_managers_auditors/documents/guide.../3Ch10.pdf‎‎‎‎> [Accessed 5 July 2013].Chapter6EnergyEfficientElectricalSystems• [Pdf] Available at: <www.teriin.org/ResUpdate/reep/ch_6.pdf‎‎‎> [Accessed 5 July 2013].2008. • Energy-efficienttechnologyfortheentirebuilding [Video online] Available at: <https://www.youtube.com/watch?v=Sn3aRBjAqDw> [Accessed 5 July 2013].2008. • Energy-Efficient Building Systems [Video online] Available at: <https://www.youtube.com/watch?v=1mTbrfhBDZU> [Accessed 5 July 2013].

Recommended ReadingBertoldi, P. & Parasiliti F., • EnergyEfficiencyinMotorDrivenSystems, Springer.Meier, A., 2006. • Electric Power Systems: A Conceptual Introduction (Wiley Survival Guides in Engineering and Science). Wiley-IEEE Press publication.Khartchenko, N., 1997. • Advanced Energy Systems (Energy Technology Series), 1st ed., Taylor & Francis Publication.

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Self AssessmentMaximum demand controller is used to ___________. 1.

switch off essential loads in a logical sequencea. exceed the demand of the plantb. switch off non-essential loads in a logical sequencec. control the power factor of the plantd.

_________ controls the power factor of the installation by giving signals to switch on or off power factor 2. correction capacitors.

KILOVARa. Automatic power factor control relayb. Intelligent power factor controllerc. Maximum demand controllerd.

Capacitors with automatic power factor controller when installed in a plant:3. Reduces active power drawn from grida. Reduces the reactive power drawn from gridb. Reduces the voltage of the plantc. Increases the load current of the plantd.

__________ determines the rating of capacitance connected in each step during the first hour of its operation 4. and stores them in memory.

Maximum demand controllera. Intelligent power factor controllerb. Automatic power factor controllerc. KILOVARd.

Eddy current drive can be a retrofit for ________.5. constant speed system requirementa. single speed system requirementb. dual speed system requirement onlyc. variable speed system requirementd.

Variable speed cannot be obtained with ____. 6. DC motors controllera. AC motor controllerb. soft starter controllerc. AC and DC controllersd.

Energy savings potential of variable torque applications compared to constant torque application is:7. Highera. Lowerb. Equalc. Similard.

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As an energy efficient application, slip power recovery system fits well for ____.8. Squirrel cage and slip ring motorsa. DC motorb. Slip ring motors onlyc. AC motord.

The basic functions of electronic ballast excludes one of the following:9. To ignite the lampa. To stabilize the gas dischargeb. To reduce lumen output of the lampc. To supply power to the lampd.

Application of occupancy sensors is well suited for ___.10. Day light based controllersa. Night based controllersb. Motor controllersc. Movement or noise detector in room spaced.

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Case Study I

Innovative High-Voltage Technology Supports Wind Power Transfer from Remote Californian Desert

Wind and solar farms have sprung up across the southwest deserts of California to help feed the energy starved population centers on the west coast of the United States. To facilitate transfer of this clean renewable energy, San Diego Gas and Electric (SDG&E) constructed a large 500 kilovolt (kV) switching substation at Ocotillo. Such high-voltage switching substations require relatively small amounts of low- and medium-voltage alternating current (AC) to run vital auxiliary and control applications. Usually the power supplied to auxiliary loads at substations is delivered by either a distribution line or the tertiary winding of the main power transformer, but at this remote site neither solution was available.

SDG&E engineers saw a standard single phase power transformer as the simplest solution to the problem, ABB engineers instead identified a more cost-effective way to tackle the problem. They suggested the gas-insulated SSVT “TIP.” The challenge was that ABB had a solution that was only rated up to 420 kV with outputs up to 600V, whereas the site application required a 525 kV product and a medium-voltage (7.2 kV) output at 333 kilovolt-amperes (kVA).

The BIL of the existing ABB's TIP 420kV matched the transformer requirements for Ocotillo substation, but the unit required re-qualifying for 525kV by using bigger cores and tank, and designing the bushing for medium-voltage output to ensure efficient transfer of power from the SSVT to the distant control room. After discussions between ABB’s factory in Italy, where its gas-insulated SSVTs are developed, and SDG&E, the right-sized SSVT was identified and a modification proposed. The new optimized solution proved to be compact, cost effective and energy efficient.

After customer design reviews, two units were manufactured to meet the critical schedule set by SDG&E. The units successfully passed design and witness tests by SDG&E, together with the stringent seismic shock tests required for the region. In less than seven months ABB was able to design, manufacture, test and deliver an upgraded product on time. The units have been installed and successfully energized on site to the customer’s satisfaction.

(Source: http://www.abb.com/cawp/seitp202/2ae2f7d66d7d3663c1257b780041823f.aspx)

Questions1. Wind and Solar farms have sprung up across the southwest deserts of California for what purpose?AnswerWind and solar farms have sprung up across the southwest deserts of California to help feed the energy starved population centers on the west coast of the United States.

2. ABB was able to design , manufacture, test and deliver an upgraded product on time within how many months?AnswerIn less than seven months ABB was able to design, manufacture, test and deliver an upgraded product on time. The units have been installed and successfully energized on site to the customer’s satisfaction.

3. What was the challenge for ABB?AnswerThe challenge was that ABB had a solution that was only rated up to 420 kV with outputs up to 600V, whereas the site application required a 525 kV product and a medium-voltage (7.2 kV) output at 333 kilovolt-amperes (kVA).

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Case Study II

Emerging Energy-Efficient Technologies in Industry

Increasingly, industry is confronted with the challenge of moving toward a cleaner, more sustainable path of production and consumption, while increasing global competitiveness. Technology will be essential for meeting these challenges. At some point, businesses are faced with investment in new capital stock. At this decision point, new and emerging technologies compete for capital investment alongside more established or mature technologies. Understanding the dynamics of the decision-making process is important to perceive what drives technology change and the overall effect on industrial energy use. From a policy-making perspective, the better we understand technology developments the more effective we will be in utilizing our future research dollars and in undertaking sound strategy development.

This report focuses on the long-term potential for energy-efficiency improvement in industry. In 2002, the industrial sector consumed 33% of the primary energy and was responsible for 30% of the energy-related greenhouse gas (GHG) emissions in the U.S. Due to the extremely diverse character of the industrial sector, it is not possible to provide an all-encompassing discussion of technology trends and potentials. Instead we focus on a number of key technology areas that illustrate the significant potential energy savings available to industry, given a sustained state, federal and private R&D effort. These include: near net shape casting, membranes, gasification, motor systems, and advanced cogeneration. The discussion of each of these technologies provides a detailed assessment of the potential for future contributions to energy efficiency improvement, economics and performance, as well as the potential development path, including promising areas for research, demonstration or other support. Some of these technologies have particular applications for a specific industry (e.g. near net shape casting in the metal producing sectors and black liquor gasification in the pulp and paper industry), while others can be found in many industries (e.g. advanced motor systems, membranes and advanced cogeneration applications).

The results demonstrate that the United States is not running out of technologies to improve energy efficiency and economic and environmental performance, and will not run out in the foreseeable future. The five technology areas alone can potentially result in total primary energy savings of just over 2,600 TBtu by 2025, or nearly 6.5% of total industrial energy use by 2025. The savings are additional to energy savings found in the AEO 2004 reference case forecasts. The technical potential of these technologies in the long term is roughly three times larger, while additional technologies beyond the five covered in this report are currently available or under development.

(Source: http://industrial-energy.lbl.gov/node/131)

Questions1. How much percentage of primary energy was consumed by the industrial sector in 2002?2. How much percentage of the energy-related greenhouse gas (GHG) emissions in the U.S was consumed by the industrial sector in 2002?3. What are the key technology areas that illustrate the significant potential energy savings available to industry, given a sustained state, federal and private R&D effort?

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Case Study III

Rowan University, a growing 10,000-student state university in southern New Jersey, was seeking a reliable and cost-effective alternative to purchasing power from the electric grid. Rising energy costs, aging equipment and an extensive expansion project led the university to develop a comprehensive Master Plan with a strong energy component to address the anticipated 50% increase in student population by 2010. It was decided to set up a cogeneration plant.

The new co-generation plant was expected to save the university $1 million annually when compared to the cost of purchasing all its electricity from the grid. High-temperature flue gases are used to produce steam for dual purposes. Primarily, the steam drives a 2,300-ton York centrifugal chiller for chilled water application. Secondarily, the steam piped through the district steam loop provides hot water, heating and laboratory usage.

The inherent variable speed technology makes the turbine drive the most efficient cooling technology for CHP as it operates throughout the cooling season and is ideally paired with gas turbine systems.

Both the electric chillers and steam turbine chiller are derived from the same basic design.

Complementing this technology, traditional electric chillers provide an additional 2,000 tons of chilled water capacity. The hybrid chiller plant allows Rowan to take advantage of favourable off-peak rates for both natural gas and electric energy and helps guarantee redundancy to ensure continuous operation in the wake of power outages.

By generating its own power, Rowan enabled the electric utility to reduce its greenhouse emissions by 8,000 tons of CO2. This is the equivalent of planting nearly 1.1 million trees, or taking 1,139 cars off the road, and constitutes 30 percent of the university’s greenhouse gas reduction target as a member of the New Jersey Higher Education Partnership for Sustainability.

Questions

What is the principle adopted here to achieve energy efficiency?1. How has the energy efficiency plan helped Rowan University?2. What are the environmental benefits of the energy efficiency plan?3.

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McDowall, R., 2007. • Fundamentals of HVAC systems. Academic Press Publication.Meade, N. G.,1908. • Electric motors: their installation, control, operation and maintenance, McGraw publishing company.Meier, A., 2006. • Electric Power Systems: A Conceptual Introduction (Wiley Survival Guides in Engineering and Science). Wiley-IEEE Press publication.Nourbakhsh, A., Jaumotte, A., Hirsch, C. & Parizi, H. B., 2007. • Turbopumps and Pumping Systems, 1st ed., Springer Publication.Patterson, E. G., 2001. Lighting Systems: Advanced Course, Thomson Learning.• Ramli, Y., 2010. • Introduction to Compressed Air Systems.Rosaler, R. C., 1998. • HVAC maintenance and operations handbook. McGraw-Hill Professional publication.Rustebakke, H. M., 1983. • Electric Utility Systems and Practices, 4rth ed., Wiley-Interscience Publication.Smith, R. E., • Electricity for Refrigeration, 8th ed., CengageBrain.com.Srinivasulu, P. & Vaidyanathan, C. V., 1977. • Handbook of machine foundations. Tata McGraw-Hill Publication.Stanford, H. W., 2003. HVAC • Water Chillers and Cooling Towers: Fundamentals, Application, and Operation (Dekker Mechanical Engineering),1st ed., CRC Press publication.Talbott, E. M., 1993. • Compressed air systems: a guidebook on energy and cost savings, 2nd ed. The Fairmont Press, Inc.Wharton, J., 2006. • Diesel Engines, Butterworth-Heinemann.

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Self Assessment Answers

Chapter Ic1. d2. a3. b4. c5. a6. b7. d8. c9. a10.

Chapter IIa1. c2. d3. a4. b5. d6. d7. b8. a9. c10.

Chapter IIIa1. b2. c3. d4. d5. a6. c7. b8. b9. c10.

Chapter IVa1. d2. b3. c4. d5. c6. a7. b8. d9. b10.

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Chapter Va1. c2. b3. a4. a5. b6. a7. d8. c9. b10.

Chapter VIc1. b2. a3. b4. d5. b6. c7. d8. a9. c10.

Chpater VIId1. d2. a3. a4. b5. b6. d7. c8. d9. a10.

Chapter VIIId1. d2. c3. b4. c5. b6. c7. c8. c9. c10.

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Chapter IXa1. c2. b3. d4. c5. a6. b7. d8. a9. d10.

Chapter Xc1. b2. b3. b4. d5. c6. a7. c8. c9. d10.