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supply and distribution Revised: 6 Sep 06 University of Leeds University of Leeds 20 September 2006 20 September 2006

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supply and distribution

University of Leeds 20 September 2006

Revised: 6 Sep 06

Recap of Morning IssuesAssess opportunities from a sustainable, whole buildings design approach to maintain superior recruitment and retention of scientists. Conduct design charrettes; use forum to increase communication and to facilitate iterative process. Involve all design disciplines and stakeholders early in project: resolve energy-efficiency and sustainability as high priorities. Focus on right-sized ventilation systems: air conditioning, both heating and cooling; air distribution; HVAC control strategies. Evaluate energy recovery techniques.University of Leeds 20 September 2006 2

Afternoon AgendaSupply and Distribution Systems. Exhaust Systems and Devices. Controls and Commissioning. Lighting.

University of Leeds 20 September 2006


Supply & Distribution IssuesCentral plant sizing and primary device selection. Optimizing air handling unit (AHU) performance. Air distribution and control strategies, including VAV design. Energy recovery techniques. Evaporative cooling: can be applied in many climates.

University of Leeds 20 September 2006


Basic Cooling System Diagram

Energy recovery system

Cooling System with Chiller

Central Plant Design

Energy-Efficient Central Plant DesignRight-size chillers and boilers. Evaluate the plants part-load efficiency. Pursue modular plant design; consider units of unequal size. Consider all-variable speed cooling systems including chillers, condenser pumps, and tower fans. Apply variable speed devices in water supply systems for heating and cooling, use 2 way valves. Evaluate raising chilled water temperature as high as possible; converse for heating. Employ a tower side economizer for free cooling. Consider a Combined Heating and Power (CHP) system.

University of Leeds 20 September 2006


Right-SizingIn the past, mechanical plants have been sized for 100% of peak load at 99% climatic tolerance with an additional 20-50% "startup factor" and another 20-30% "safety factor." Those days are over! Apply realistic diversity factors. "Right-sizing" a labs mechanical plant reduces first costs and operational energy costs. It can reduce energy waste by 10-50% compared to conventional design practices.

University of Leeds 20 September 2006


Chiller Part-Load EfficiencyChillers can consume 25-40% of a lab's annual energy budget. Chillers come in different configurations with varying part-load efficiencies. Reciprocating Rotary-screw Centrifugal

University of Leeds 20 September 2006


Modular Plant DesignA modular design approach will match loads more effectively due to their adjustability. Evaluate plant units (chillers or boilers) of unequal size instead of two units of equal size; allows more flexibility in matching load. For example: 2 chillers, one at 66% load size and one at 33%.

Understand relative merits of different plant unit designs. For example, consider a rotary-screw compressor and a centrifugal compressor instead of two centrifugal compressors.

University of Leeds 20 September 2006


Optimize Boiler PlantUse modular boilers to match variable loads. Consider condensing and non-condensing boilers. Size heating systems for large temperature drop (30 degrees or more) and low return temperature. Low return water temperature is especially important for condensing boilers. Use variable speed heating water supply and 2-way control valves.11

University of Leeds 20 September 2006

Modular Chiller PlantModular approach helps designers match loads efficiently. New, variable speed chillers offer high part-load efficiencies.

University of Leeds 20 September 2006


All-Variable Speed Chiller PlantsPrime-mover chiller Each type can employ variable speed. Condenser Pumps Respond to changes in load by adjusting pumping rate. Cooling tower fans Adjust to changing ambient temperatures to optimize cooling effect.

University of Leeds 20 September 2006


Variable Speed Drives1 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0 Chiller without VSD Chiller with VSD

Chiller Efficiency (kW/ton)







Percent LoadData provided by York International Corporation for chillers running at 42 deg F CHWS and 65 deg F CWS.

Use a Variable Speed Drive (VSD) on each pump

Energy recovery system

Cooling System with Chiller

Comparison of Chilled Water Plant to Best PracticeCentral Plant Performance0.9 0.8 0.7

Cooling Tower CW Pumps CHW Pumps Chiller


0.6 0.5 0.4 0.3 0.2 0.1 0.0

MDL Plant

Best Practice


Set system targets in kW/ton

Chilled Water TemperaturesEvaluate chilled water supply at a temperature as warm as possible; include fan, pump, and tower energy consumption in your evaluations. Investigate raising maximum space humidity limit; it often drives chilled water supply temperature. Consider adding a medium temperature chiller for sensible and process loads when low temperature chilled water is required. Use a process cooling water system to remove loads from the space. Oversize evaporators, condensers, and cooling towers. Lower condenser water temperature.

University of Leeds 20 September 2006


Low vs. Medium Temperature Chillers1 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0 200 300 400 500 600 700 800 900 1000

Efficiency (kW/ton)

1,000 Ton Chiller operating at 42 F CHWS Temp and 70 F CWS Temp

1,000 Ton Chiller operating at 60 F CHWS Temp and 70 F CWS Temp

TonsData provided by York International Corporation.

System Efficiencies Under Different Conditions 85F Condenser Water Water F 60 55 50 48 46 45 44 42 40 kW/Ton 0.70 0.75 0.81 0.83 0.85 0.86 0.88 0.90 0.92 Tons 128.9 118.5 108.3 104.5 100.7 98.8 96.8 93.2 89.7 75F Condenser Water kW/Ton 0.59 0.64 0.70 0.72 0.74 0.75 0.77 0.79 0.82 Tons 135.5 124.4 113.8 109.7 105.7 103.8 101.8 98.0 94.2

Consider Free CoolingIn some climates, free cooling can be obtained by running a cold cooling tower and installing a plate and frame heat exchanger. Consider in locations where the wet bulb temperature is below the cooling water supply temperature for a large number of hours.

Energy recovery system

University of Leeds 20 September 2006


Energy recovery system

Combined Heating and Power SystemsCentralized heating and cooling plants for laboratories are ideal applications for combined heating and power (CHP) systems. CHP, or cogeneration plants, need to maximize use of the thermal output. Heat is used for space heating, process heating, dehumidification, cooling, and re-heat.

CHP can be designed to provide high reliability power.

University of Leeds 20 September 2006


Air Distribution and Controls

Principles of Air DistributionPromote low pressure-drop design. Use low face-velocity and low pressure-drop coils and filters. Provide efficient duct routing to minimize fittings and resistance to flow. Oversize passive system components such as piping and ductwork Avoid silencers, if possible.

University of Leeds 20 September 2006


Principles of Air Distribution, continuedSpecify a system efficiency in W/cfm. Eliminate or reduce the use of re-heat coils. Provide multiple AHUs for variable requirements. Provide separate air handling units for office and lab spaces.University of Leeds 20 September 2006 26

Low Pressure-Drop Design GuidelinesComponentAir handler face velocity Air Handler Heat Recovery Device VAV Control Devices

Standard500 2.5 in. w.g. 1.00 in. w.g. Constant Volume, N/A

Good400 1.5 in. w.g. 0.60 in. w.g. Flow Measurement Devices, 0.60 - 0.30 in. w.g. 0.30 in. w.g. 2.25 in. w.g. 0.7 w.g. full design flow through fan and stack only, VAV System with bypass 0.25 w.g. 6.2 w.g. 1.2

Better300 0.75 in.w.g. 0.35 in. w.g. Pressure Differential Measurement and Control, 0.10 in. w.g. 0.05 in. w.g. 1.2 in. w.g. 0.75 w.g. averaging half the design flow, VAV System with multiple stacks 0.0 w.g. 3.2 w.g. 0.6

Zone Temperature Control Coils Total Supply and Return Ductwork Exhaust Stack CFM and

0.5 in. w.g. 4.0 in. w.g. 0.7 w.g. full design flow through entire exhaust system, Constant Volume 1.0 w.g. 9.7 w.g. 1.8

Noise Control (Silencers) Total Approximate W / CFM

Source: J. Weale, P. Rumsey, D. Sartor, L. E. Lock, Laboratory Low-Pressure Drop Design, ASHRAE Journal, August 2002.University of Leeds 20 September 2006 27

Just Say No to ReheatSimultaneous heating and cooling can be much more problematic in a lab where the variations of internal loads can be enormous. When reheat is employed, a single zone requiring cooling can create artificial heating and cooling loads throughout the building. Some possible solutions are: Put cooling coils or cooling fan coils in each zone. Use a dual duct system with cool duct and warm duct.

University of Leeds 20 September 2006


Constant Volume (CV) SupplyTraditional, constant volume (CV) supply systems are relatively simple in operation and maintenance. However, a constant volume (CV) system has significant drawbacks, as well:Very energy inefficient, particularly in buildings with high diversity and without cooling coils or dual ducts in each zone. Does not readily lend itself to change.

University of Leeds 20 September 2006


Variable Air Volume (VAV)Variable air volume (VAV) systems have several advantages over CV systems:Provide only as much air as needed to meet requirements. Take advantage of diversity in hood use and internal heat gains. Are energy efficiency.

VAV systems for labs must control supply and exhaust air together.

University of Leeds 20 September 2006


VAV AdvantagesVAV systems have other advantages:Monitors and controls pressurization in laboratories for increased safety. Provides constant face velocity that improves fume hood safety. Adapts to adding or moving fume hoods during remodeling more readily than CV.

University of Leeds 20 September 2006


Air Handling Units

Air Handling Units (AHU)Reduce AHU pressure dropLower face velocity using larger area coils, filter elements, and AHU housings. Reduced pressure drop results in smaller, quieter fans and may eliminate the need for silencers.

Incorporate the most efficient fan systemCentrifugal fan efficiencies range from 50-70% depending upon blade configuration, and vane-axial fan efficiencies range from 80-90%. Specify the highest efficiency motors available (NEMA Premium). Keep in mind that all fans add heat to the air stream, usually between 2 and 4 F Consider system effects (AHU inlet and outlet ductwork configurations).

University of Leeds 20 September 2006


Selecting FansUse the most efficient fan for the application. Consider fan curves, especially for lowspeed operation. Refer to ASHRAE HVAC Systems and Equipment Chapter on Fans.

University of Leeds 20 September 2006


Adapted from J. Trost, Efficient Buildings 2.

Economizer CyclesAppreciate that a 100% outside air (O.A.) lab is always in economizer mode. Consider the use of economizers in all applications with non-lab spaces such as offices, meeting rooms or librariesapplications where cooling is required & outside air temperature is below space temperature. Specify enthalpy controls.

University of Leeds 20 September 2006


Direct Evaporative CoolingMoisture evaporated in a low humidity air stream lowers the dry-bulb temperature, enthalpy (total energy content) remains constant. Consider direct evaporative cooling in dry climates, particularly when humidification is required. Use in exhaust air stream with energy recovery system.

University of Leeds 20 September 2006


Energy Recovery

Energy RecoveryFactors that improve energy recovery economics include:Colder climates (e.g. more than 3,000 heating degreedays) High exhaust rates Long service life High utility rates Low discount (or interest) rate

Consider impact of increase pressure drop due to energy recovery devices. Evaporative cooling in exhaust stream can increase cooling energy recovery without adding moisture to supply air.University of Leeds 20 September 2006 39

Energy Recovery,continued

Run-around systemsSimple piping loop connecting a coil in the exhaust plenum with a coil in the make-up air plenum or AHU. Supply and exhaust ducts do not need to be adjacent.

Energy recovery system

Heat pipesTransfer energy using phase change of a fluid within a pipe. Requires adjacent supply and exhaust ducts. Bypass on supply side recommended to prevent unwanted recovery.University of Leeds 20 September 2006 40

Energy recovery system

Energy Recovery,


Fixed-plate air-to-air heat exchangerTypically, coated, air-to-air aluminum heat exchangers. May have to be quite large to perform effectively. Requires adjacent supply and exhaust ducts.

Desiccant energy wheelsRecovers latent and sensible heat. Rotating energy recovery wheels have not been recommended in some situations because of the potential carryover of contaminants from the exhaust to the supply air stream. Purge sectors and good seals minimize cross contamination. Recently, a 3A (Angstrom) molecular sieve, desiccant-based heat wheel technology has been developed that will not absorb large molecules. Requires adjacent supply and exhaust ducts.University of Leeds 20 September 2006 42

End of Session