audel hvac fundamentals volume 2 · 2013-07-17 · hvac fundamentals volume 2 heating system...

Audelª

HVAC FundamentalsVolume 2

Heating SystemComponents, Gas and OilBurners, and Automatic

Controls

All New 4th Edition

James E. Brumbaugh

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www.wiley.com

Vice President and Executive Group Publisher: Richard SwadleyVice President and Executive Publisher: Robert IpsenVice President and Publisher: Joseph B. WikertExecutive Editor: Carol A. LongAcquisitions Editor: Katie FeltmanEditorial Manager: Kathryn A. MalmSenior Production Manager: Fred BernardiDevelopment Editor: Kenyon BrownProduction Editor: Vincent KunkemuellerText Design & Composition: TechBooks

Copyright 2004 by Wiley Publishing, Inc., Indianapolis, Indiana. All rights reserved.

Published simultaneously in Canada

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Printed in the United States of America

10 9 8 7 6 5 4 3 2 1

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eISBN: 0-7645-7436-1

For Laura, my friend, my daughter.

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v

Contents

Introduction xv

About the Author xvi

Chapter 1 Oil Burners 1Gun-Type Oil Burners 3

Construction Details 3Operating Principles 10

Flame-Retention Head Burners 16High-Static Oil Burners 16Rotary Oil Burners 16Vaporizing (Pot-Type) Oil Burners 18Combination Oil and Gas Burners 18Fuel Pump 19

Single-Stage Fuel Pump 21Two-Stage Pump 25Fuel Pump Capacity 26Fuel Pump Service and Maintenance 26Priming Fuel Pumps 29Adjusting Fuel Pump Pressure 29Troubleshooting Fuel Pumps 31

Fuel Supply Tank and Line 32Oil Burner Nozzles 32Electrodes 36

Troubleshooting Electrodes 37Servicing Electrodes 37

Oil Burner Air System 37Primary Safety Control Service 40Installing an Oil Burner 40Starting an Oil Burner 42Air Delivery and Blower Adjustment 43Combustion Testing and Adjustments 43Troubleshooting Oil Burners 48

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vi Contents

Chapter 2 Gas Burners 57Operating Principles 57Electrical Circuits 61Automatic Controls 61Types of Gas Burners 63Integral-Type Gas Burners 65Gas Conversion Burners 66Gas Conversion Burner Combustion

Chambers 67Gas Piping for Conversion Burners 68Venting and Ventilation 71Safety Precautions 72Troubleshooting Gas Burners 73

Chapter 3 Coal Firing Methods 77Coal-Firing Draft Requirements 77Firing Anthracite Coal 78Firing Bituminous Coal 80Firing Semibituminous Coal 81Stoker Firing 81Stoker Construction 84Stoker Automatic Controls 86Stoker Operating Instructions 90Coal Selection 91Starting the Fire 91Natural Stack Draft 91Manual Air Adjustment 92Automatic Air Control 92Changing Coal Feeds 92Motor Overload Protection 92Transmission Overload

Protection 93Removal of Obstruction 93Lubrication 93Summer Service 93How to Remove Clinkers 94How to Adjust Coal Feed 94

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Contents vii

How to Adjust Air Supply 94Troubleshooting Coal Stokers 94

Chapter 4 Thermostats and Humidistats 99Automatic Control Systems 99Temperature Control Circuits 100Thermostats 100

Thermostat Components 105Thermostat Terminal Identification 109Thermostat Anticipators 109Types of Thermostats 119Room Thermostats 119Programmable Thermostats 125Insertion Thermostats 125Immersion Thermostats 126Cylinder Thermostats 127Boiler Thermostats 129Remote-Bulb Thermostats 129Proportional Thermostats 132Outdoor Thermostats 132

Troubleshooting Thermostats 134Humidistats 134

Location of Room Humidistats 140Troubleshooting Humidistats 142

Chapter 5 Gas and Oil Controls 145Gas Controls 145

Gas Control Circuits 146Gas Burner Primary Control 146

Servicing a Gas Burner Primary Control 151Gas Valves 153

Solenoid Gas Valves 153Solenoid Coils 158Direct-Acting Heat Motor Valves 163Diaphragm Valves 164

Pressure Regulators 166Pressure Switches 170

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viii Contents

Automatic Pilot Safety Valve 174Thermopilot Valves 178Thermocouples 181

Troubleshooting Thermocouples 183Thermopiles (Pilot Generators) 184Pilot-Operated Diaphragm Valves 185Combination Gas Valves 187

Standing Pilot Combination Gas Valves 187

Continuous Pilot Dual Automatic Gas Valve 191

Universal Electronic Ignition Combination Gas Valve 194

Pilot Burners 194Installing a Pilot Burner 198Replacing the Pilot Burner Orifice 200Lighting the Pilot 201Pilot Flame Adjustment 202Main Burner Ignition 202

Pilot-Pressure Switch 203Electronic Ignition Modules 203

Intermittent Pilot Ignition Module 204Direct-Spark Ignition Module 207

Hot-Surface Ignition Module 208Igniters 211Flame Sensors 214

Mercury Flame Sensors 216Oil Controls 217Oil Valves 217Oil Burner Primary Control 219Cadmium Cell Primary Controls 220Stack Detector Primary Control 223Combination Primary Control and

Aquastat 227Troubleshooting the Oil Burner Primary

Control 231

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Chapter 6 Other Automatic Controls 233Fan Controls 233

Fan Control 233Air Switch 236Fan Relays 237Fan Center 239Fan Manager 241Fan Timer Switch 241Fan Safety Cutoff Switch 242

Limit Controls 244Limit Control 244Secondary High-Limit Switch 248Combination Fan and Limit Control 251

Switching Relays 256Impedance Relays 259Heating Relays/Time-Delay Relays 261Potential Relay 263Pressure Switches 265Sail Switches 266Other Switches and Relays 268Sequence Controllers 269Contactors 275

Troubleshooting Contactors 277Cleaning Contactors 280Replacing Contactors 280

Motor Starter 281Overload Relay Heater 281Inherent Protector 282Pilot Duty Motor Protector 283Capacitors 284

Troubleshooting Capacitors 287Replacing Capacitors 287

High-Pressure Cutout Switch 288Low-Pressure Cutout Switch 289Transformers 290

Sizing Transformers 291

Contents ix

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x Contents

Installing Transformers 291Control Panels 293

Chapter 7 Ducts and Duct Systems 295Codes and Standards 295Types of Duct Systems 295Perimeter Duct Systems 296Extended Plenum Systems 297Crawl-Space Plenum Systems 297Duct Materials 298Duct System Components 299Supply Air Registers, Grilles, and

Diffusers 301Return Air and Exhaust Air Inlets 302Duct Run Fittings 303Air Supply and Venting 305Duct Dampers 305Damper Motors and Actuators 313Installing Damper Motors 316Troubleshooting Damper Motors 320Blowers (or Fans) for Duct

Systems 321Designing a Duct System 322Duct System Calculations 323Duct Heat Loss and Gain 324Air Leakage 325Duct Insulation 325Equal Friction Method 326Balancing an Air Distribution System 331Duct Maintenance 331Roof Plenum Units 332Mobile Home Duct Systems 333Proprietary Air Distribution Systems 336Duct Furnaces 338Electric Duct Heaters 347

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Chapter 8 Pipes, Pipe Fittings, and Piping Details 355Types of Pipe Materials 355

Wrought-Iron Pipe 356Wrought-Steel Pipe 363Galvanized Pipe 363Copper and Brass Pipes and Tubing 363Plastic Tubing 367Synthetic Rubber Hose 369Composite Tubing 369

Pipe Fittings 369Classification of Pipe Fittings 370Extension or Joining Fittings 370Reducing or Enlarging Fittings 378Directional Fittings 380Branching Fittings 380Shutoff or Closing Fittings 382Union or Makeup Fittings 382Flanges 382Pipe Expansion 382

Valves 384Pipe Threads 384Pipe Sizing 384Sizing Steam Pipes 385Sizing Hot-Water (Hydronic)

Pipes/Tubing 393Pipe Fitting Measurements 396Calculating Offsets 397

First Method 400Second Method 401Third Method 401Fourth Method 403

Pipe Supports 403Joint Compound 403Pipe Fitting Wrenches 406Pipe Vise 409

Contents xi

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xii Contents

Installation Methods 410Pipe Cutting 410Pipe Threading 412Pipe Reaming 414Pipe Cleaning 414Pipe Tapping 414Pipe Bending 415Assembling and Make–Up 415Nonferrous Pipes, Tubing, and Fittings 420Soldering Pipe 420Brazing Pipes 424Braze Welding Pipe 425Welding Pipe 425Gas Piping 429Insulating Pipes 429Piping Details 430

Connecting Risers to Mains 431Connections to Radiators or

Convectors 431Lift Fittings 431Drips 432Dirt Pockets 434Siphons 434Hartford Connections 434Making Up Coils 434Relieving Pipe Stress 436Swivels and Offsets 439

Eliminating Water Pockets 440Pressure Tests 444

Chapter 9 Valves and Valve Installation 445Valve Components and Terminology 445

Valve Materials 451Globe and Angle Valves 454Gate Valves 456Check Valves 458

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Stop Valves 463Butterfly Valves 465Two-Way Valves 467

Three-Way Valves 469Y Valves 469Valve Selection 469Troubleshooting Valves 472

Valve Stuffing-Box Leakage 474Valve Seat Leakage 474Damaged Valve Stems 475

Automatic Valves and Valve Operators 475Valve Pipe Connections 487Valve Installing Pointers 489Soldering, Brazing, and Welding Valves

to Pipes 492Soldering or Silver-Brazing Procedure 494Butt-Welding Procedure 495Socket-Welding Procedure 496

Chapter 10 Steam and Hydronic Line Controls 497Steam and Hydronic System Pumps 497

Condensate Pumps 497Circulators (Water-Circulating Pumps) 505Circulator Selection 511

Steam Traps 518Sizing Steam Traps 519Steam Trap Maintenance 520

Automatic Heat-Up 520Installing Steam Traps 522Float Traps 523Thermostatic Traps 524Balanced-Pressure Thermostatic

Steam Traps 525Maintenance 526

Float and Thermostatic Traps 526Thermodynamic Steam Traps 529

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xiv Contents

Bucket Traps 530Flash Traps 534Impulse Traps 534Tilting Traps 536Lifting Traps 537Boiler Return Traps 537Expansion Tanks 540

Closed Steel Expansion Tanks 541Diaphragm Expansion Tanks 543Sizing Expansion Tanks 543Troubleshooting Expansion Tanks 544

Air Eliminators 545Pipeline Valves and Controls 547

Temperature Regulators 548Electric Control Valves (Regulators) 548Water-Tempering Valves 550Hot-Water Heating Control 554Flow Control Valve 558Electric Zone Valve 559Balancing Valves, Valve Adapters,

and Filters 561Manifolds 564Pipeline Strainers 565

Appendix A Professional & Trade Associations 567

Appendix B Manufacturers 579

Appendix C Data Tables 591

Appendix D Conversion Tables 629

Index 639

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xv

IntroductionThe purpose of this series is to provide the layman with an introduc-tion to the fundamentals of installing, servicing, troubleshooting,and repairing the various types of equipment used in residential andlight-commercial heating, ventilating, and air conditioning (HVAC)systems. Consequently, it was written not only for the HVAC tech-nician and others with the required experience and skills to do thistype of work but also for the homeowner interested in maintainingan efficient and trouble-free HVAC system. A special effort wasmade to remain consistent with the terminology, definitions, andpractices of the various professional and trade associations involvedin the heating, ventilating, and air conditioning fields.

Volume 1 begins with a description of the principles of thermaldynamics and ventilation, and proceeds from there to a generaldescription of the various heating systems used in residences andlight-commercial structures. Volume 2 contains descriptions of theworking principles of various types of equipment and other compo-nents used in these systems. Following a similar format, Volume 3includes detailed instructions for installing, servicing, and repairingthese different types of equipment and components.

The author wishes to acknowledge the cooperation of the manyorganizations and manufacturers for their assistance in supplyingvaluable data in the preparation of this series. Every effort wasmade to give appropriate credit and courtesy lines for materials andillustrations used in each volume.

Special thanks is due to Greg Gyorda and Paul Blanchard (WattsIndustries, Inc.), Christi Drum (Lennox Industries, Inc.), DaveCheswald and Keith Nelson (Yukon/Eagle), Bob Rathke (ITT Bell &Gossett), John Spuller (ITT Hoffman Specialty), Matt Kleszezynski(Hydrotherm), and Stephanie DePugh (Thermo Pride).

Last, but certainly not least, I would like to thank Katie Feltman,Kathryn Malm, Carol Long, Ken Brown, and Vincent Kunkemueller,my editors at John Wiley & Sons, whose constant support andencouragement made this project possible.

James E. Brumbaugh

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About the AuthorJames E. Brumbaugh is a technical writer with many years of expe-rience working in the HVAC and building construction industries.He is the author of the Welders Guide, The Complete RoofingGuide, and The Complete Siding Guide.

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Chapter 1Oil BurnersAn oil burner is a mechanical device used to prepare the oil forburning in heating appliances such as boilers, furnaces, and waterheaters. The term oil burner is somewhat of a misnomer becausethis device does not actually burn the oil. It combines the fuel oilwith the proper amount of air for combustion and delivers it to thepoint of ignition, usually in the form of a spray.

The fuel oil is prepared for combustion either by vaporization orby atomization. These two methods of fuel oil preparation are usedin the three basic types of oil burners employed in commercial,industrial, and residential heating. The following are the three basictypes of oil burners:

1. Gun-type (atomizing) oil burners.2. Vaporizing (pot-type) oil burners.3. Rotary oil burners.

Gun-type atomizing oil burners are available as either low-pressureor high-pressure types (see Figures 1-1, 1-2, and 1-3). Both are usedin residential heating applications with the latter being by far themore popular of the two. The remainder of this chapter is devoted toa description of the gun-type high-pressure atomizing oil burnersused in residential and light commercial oil heating systems.

1

Figure 1-1 Basic shape of a gun-type oil burner.(Courtesy Stewart-Warner Corp.)

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The advantage of the vaporizing (pot-type) oil burner is its lowoperating cost. It is the least expensive to use, but it has limitedheating applications. It is currently used only in small structureslocated in milder climates. Vaporizing burners can be divided intothe three following types:

1. Natural-draft pot burners.2. Forced-draft pot burners.3. Sleeve burners.

Rotary oil burners are commonly used in the heating systems ofcommercial or industrial buildings, although they can and have beenused for residential heating applications (see Figures 1-4 and 1-5).The following types of rotary oil burners are available for heatingpurposes:

2 Chapter 1

DRAFT TUBE

PUMP STRAINER AND VALVE

UNIT

COUNTER-BALANCEDDRAFT SHUTTER

LEGS FOR PEDESTAL-MOUNTEDATTACHMENT

FAN HOUSING

MOTOR

BUILT-INMOTOR

PROTECTOR

PYPEX FIRE INSPECTION HOLES

OIL FEED TUBEFROM PUMP TO

IGNITION ANDFIRING ASSEMBLY PLATE

TRANSFORMER

Figure 1-2 Principal components of an S.T. Johnson gun-type oilburner. (Courtesy S.T. Johnson Company)


• Vertical rotary burners• Horizontal rotary burners• Wall-flame rotary burners

Gun-Type Oil BurnersGun-type, high-pressure atomizing oil burners are sometimescalled sprayers or atomizing burners because they spray the fueloil instead of vaporizing it. They are also referred to as gun orpressure oil burners because the oil is forced under pressurethrough a special gun-like atomizing nozzle. The liquid fuel isbroken up into minute liquid particles or globules to form thespray.

Construction DetailsThe principal components and parts of a gun-type, high-pressureatomizing oil burner used in residential and light commercial oilheating systems are illustrated in Figures 1-6 and 1-7. The

Oil Burners 3

MOTORFAN

FUEL LINE

STRAINER

PUMPTRANSFORMER AIR TUBE

REFRACTORY FIREPOT

Figure 1-3 Gun-type oil burner firing into furnace combustionchamber. (Courtesy U.S. Department of Agriculture)


construction details of gun-type oil burners will vary somewhatin different makes and models, but the overall design of theseburners is now nearly standardized. The components and parts ofa typical gun-type oil burner can be divided into the followingcategories:

4 Chapter 1

Figure 1-4 Cutaway view of a vertical rotary burner of the vaporizingor wall-flame type. (Courtesy Integrated Publishing)


1. Burner control.2. Primary safety control.3. Gun assembly.4. Ignition transformer.5. Burner motor and coupling.6. Fuel pump.7. Combustion air blower.

Burner ControlThe burner control is the operational control center of the burner.As shown in Figures 1-6 and 1-7, it is located on the right side of theburner assembly directly above the combustion air blower housing.It operates in conjunction with the primary control and a bimetallic

Oil Burners 5

Figure 1-5 Horizontal rotary burner. (Courtesy Integrated Publishing)

FUEL TUBE TIP(STATIONARY)

AUTOMIZING CUP(ROTATING)

PRIMARYAIROIL SWIRLS IN

COUNTERCLOCKWISEDIRECTION

AIR SWIRLS INCLOCKWISEDIRECTION

FAN

FUELTUBE

HOLLOWMAIN

SHAFT

BAFFLEPLATE

NOZZLEPROTECTOR

ANGULAR-VANEAIR NOZZLE

OILFILM


temperature sensor. When the room thermostat calls for heat andthe ignition cycle begins, the burner control will start the burneronly when the cad cell detects (proves) a flame. The burner controlshuts off the burner if the cad cell fails to prove the flame or if thebimetallic sensor detects a temperature too high for safe operation.

Primary Safety ControlThe primary safety control is an automatic safety device designedto stop the flow of fuel oil at the burner should ignition or flamefailure occur. Modern oil-fired furnaces and boilers use a cad cell asthe primary control to prove the flame; older ones were equippedwith a stack detector primary control. The former is mountedinside the burner behind the access door (see Figure 1-8), and thelatter is located in the stack.

Gun AssemblyThe oil burner gun assembly consists of a burner nozzle, the elec-trodes, and a tube connecting the electrodes to the fuel pump (seeFigure 1-9). The burner nozzle changes the fuel oil into a form thatcan be burned in the combustion chamber. It accomplishes this byforcing the oil under pressure through a small hole at the end of thenozzle. The atomized fuel oil is ignited by spark from the electrodes.

6 Chapter 1

Figure 1-6 Typical gun-type oil burner (side view).(Courtesy Lennox Industries Inc.)

RETAININGCLIP

AIR TUBEASSY FORFB HEADS

FLANGEGASKET FLANGE

AIR TUBESCREWS HOLE PLUG ELECTRONIC IGNITION

TRANSFORMER

MAINHOUSING

ASSY

ESCUTCHEONPLATE

CONNECTORTUBE

ELECTRODEASSY

NOZZLE LINEELECTRODE HEAD

ASSY

NOZZLEADAPTER

HEADINSULATOR

FB-HEAD


Ignition TransformerA step-up ignition transformer located on top of the burner assem-bly produces the voltage used by the electrodes to ignite the fuel oil.This type of transformer is designed to increase the voltage of ahigh-voltage (110 VAC) circuit to the ultrahigh 14,000 voltsrequired to ignite the fuel oil.

Burner Motor and CouplingAs shown in Figure 1-5, the burner motor is located on the right sideof the oil burner assembly. The drive shaft of the burner motor isconnected to both the fuel pump and the combustion air blower by acoupling that functions as the drive shaft for both of these units. Aburner motor is also sometimes called an oil pump motor or a pumpmotor because it is connected to and drives the fuel (oil) pump.

Oil Burners 7

Figure 1-7 Typical gun-type oil burner (front view).(Courtesy Lennox Industries Inc.)

10

345678

BLOWERWHEEL

INLETAIR SCOOPCOMBUSTION AIR

MOTORCOUPLING

OIL PUMP

SPLINEDNUT

REAR ACCESSDOOR ASSY

VIEW PORT

REAR ACCESSDOOR GASKET

ELECTRONIC IGNITIONTRANSFORMER

BURNERCONTROL RESET

BUTTON

AIRADJUSTMENT

DIAL


Fuel PumpThe fuel pump (also called an oil pump or a fuel unit) is used todraw fuel oil from the storage tank and deliver it under high pres-sure (100 to 140 psi) to the nozzle assembly (see Figure 1-11). It isdriven by the burner motor and coupling and is located on the leftside of the oil burner.

Combustion Air BlowerThe combustion air blower is also driven by the burner motor andcoupling. It is located between the burner motor and the fuel pump.Its function is to introduce the required amount of air for the

8 Chapter 1

BURNER CONTROL

CAD CELLFLAME DETECTORLOCATED INSIDEBURNER ASSEMBLY

FUEL PUMP

COMBUSTION AIRBLOWER ASSEMBLY

Figure 1-8 Locations of burner control and cadmium cell primarysafety control.


Oil Burners 9

Figure 1-9 Oil burner with transformer removed revealing the gunassembly. (Courtesy Wayne Home Equipment Co., Inc.)

NOZZLE ADAPTER

IGNITION POINTS BAFFLE PLATE

PORCELAIN INSULATORS

ELECTRODE SPRINGS

OIL-TIGHT FITTINGS


combustion process. The amount of air can be manually adjustedby an air adjustment gauge located between the blower wheel andthe inlet air scoop (see Figure 1-7). Depending on the oil burnermanufacturer, a combustion air blower is also sometimes called ablower wheel, a burner motor fan, or an induction blower. Do notconfuse the combustion air blower with the furnace indoor blower.The former delivers air to the oil burner for combustion. The latterdelivers the heated air to the rooms and spaces inside the structure.

Operating PrinciplesThe operation of a gun-type, high-pressure atomizing oil burnercan be traced in Figure 1-12. The fuel oil is drawn through astrainer from the supply tank by the fuel pump and is forced underpressure past the pressure relief cutoff valve via the oil line where iteventually passes through the fine mesh strainer and into the noz-zle. The amount of pressure required to pump the fuel oil through

10 Chapter 1

Figure 1-10 Gun assembly details. (Courtesy Lennox Industries Inc.)

GUNASSEMBLY

CONNECTORTUBE

OIL PUMP


the line depends on the size and capacity of the oil burner and thepurpose for which it is used. For example, residential oil burnersrequire 80 to 125 psi, whereas commercial and industrial oil burn-ers operate on 100 to 300 psi.

As the fuel oil passes through the nozzle, it is broken up andsprayed in a very fine mist. The air supply is drawn in through theinlet air scoop opening (see Figure 1-5) and forced through thedraft tube portion of the casing by the combustion air blower. Thisair mixes with the oil spray after passing through a set of vanes,

Oil Burners 11

Figure 1-11 Typical fuel pump. (Courtesy Lennox Industries Inc.)

RETURN PORT(1⁄4" [6 MM])

INLET PORTSHOWN (1⁄4" [6 MM])

PRESSURE GAUGEPORT (1⁄8" [3 MM])

NOZZLE PORT(1⁄8" [3 MM])

INLET PORT(1⁄4" [6 MM])

BLEED PORT


called a turbulator. The turbulator gives a twisting motion to the airstream just before it strikes the oil spray, producing a more thor-ough mixture of the oil and air (see Figure 1-13).

Ignition of the oil spray is provided by a transformer thatchanges the house lighting current and feeds it to the electrodes toprovide a spark at the beginning of each operating period.

12 Chapter 1

Figure 1-12 Schematic of a gun-type oil burner.

TURBULATOR AIR CONE ELECTRODE

JET LINEDRAFT TUBE

Figure 1-13 Details of draft tube illustrating the location of theturbulator, air cone, and electrode.

AIRADJUSTMENT COLLAR

FAN

SO-CALLED ONE-PIPE

STRAINER

OIL SUPPLYLINE FROMTANK

PUMP BYPASSAROUND PUMP

PRESSURERELIEF VALVE

CUTOFF VALVE

ELECTRIC MOTOR

OIL LINE

ROTARYTURBULATOR VANES

NOZZLESTRAINER

IGNITIONELECTRODES

AIRENTRANCE

CASE ORHOUSING

ELECTRODES BRACKET

NOZZLE TUBE NOZZLE


The starting cycle of the oil burner is initiated by the closing ofthe motor circuit. When the motor circuit is closed (automaticallyby room temperature control), the motor starts turning the fan andthe pump. At the same time, the ignition transformer produces aspark at the electrodes ready to light the oil and air mixture.

The action of the pump draws the fuel oil from the tank throughthe strainer on the fuel line. Its flow is controlled by an oil cutoffvalve, which prevents oil passing to the nozzle unless the pressure ishigh enough to spray the oil (approximately 60 lbs of pressure).Because the pump in the oil burner pumps oil much faster than itcan be discharged through the nozzle at that pressure (i.e., 60 lbs ofpressure), the oil pressure continues to rise very fast between thepump and the nozzle. When the pressure begins to rise above thenormal operating pressure (100 lbs), a pressure relief valve opensand allows the excess oil to flow through the bypass line to theinlet, as in the so-called one-pipe system, or to flow through a sec-ond or return line to the supply tank. The pressure relief valve ineither system maintains the oil at the correct operating pressure.

When the oil burner is turned off (i.e., when the burner motorstops), the oil pressure quickly drops below the operating pressure,and a pressure relief valve closes. The flame continues until thepressure drops below the setting of the cutoff valve.

The cutoff and pressure relief (regulating) valves may be eithertwo separate units or combined into one unit. Figure 1-14 showsthe essentials of the two-unit arrangement. These are, as shown,simply elementary schematics designed to illustrate basic operatingprinciples. The cutoff needle valve is shown with a spring inside thebellows, and the pressure relief (mushroom) valve is shown withexposed spring. In the cutoff valve arrangement, the spring actsagainst oil pressure on the head of the bellows (tending to collapseit); in the pressure relief valve, the spring acts against the oil pres-sure, which acts on the lower face of the mushroom valve (tendingto open it).

When the pump starts and the pressure in the line rises to about60 lbs (depending on the spring setting), this pressure acting on thehead of the bellows overcomes the resistance of the spring, causingthe cutoff valve to open. Since the pump supplies more oil than thenozzle can discharge, the pressure quickly rises to 100 lbs, over-coming the resistance of the relief valve spring and causing thevalve to open. This allows excess oil to bypass or return to the tank.

The relief valve will open high enough to maintain the workingpressure constant at 100 lbs. When the oil burner is turned off, theoil pressure quickly drops, and the pressure relief valve closes.

Oil Burners 13


However, oil will continue to discharge from the nozzle until thepressure drops below the cutoff valve setting when the cutoff valvecloses and stops the nozzle discharge.

A passage to the return line is provided by a small slot cut in theseat of the mushroom valve. This causes any remaining pressuretrapped in the line by the closing of the cutoff valve to be equalized.

Frequently the cutoff valve and pressure relief valve are com-bined in a compact cylindrical casing (see Figure 1-15). Here thetwo valves are attached to a common stem with a flange, whichcomes in contact with a stop when moved upward by the pressureof the valve actuating the spring.

The position of the stop limits the valve movements to propermaximum lift. A piston, free to move in the cylindrical casing, hasan opening in its head that forms the valve seat for the pressure relief

14 Chapter 1

Figure 1-14 Schematic sectional view of separate unit cutoff valveand pressure relief valve showing strainer, pump, and piping.

INLET

STRAINER

CUTOFF VALVE

ADJUSTING SCREW

CLOSING SPRING

BELLOWS

NOZZLE

SPRAY

ADJUSTING SCREW

PRESSURE-RELIEF VALVE

CLOSING SPRING

RETURN TO TANK "TWO-PIPE"

PUMP

RETU

RN T

O IN

LET

LIN

E "O

NE-

PIPE

"


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