vav presentation

supply and distribution

Revised: 6 Sep 06University of Leeds University of Leeds –– 20 September 200620 September 2006

University of Leeds – 20 September 20062

Recap of Morning Issues…

Assess 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.


Afternoon Agenda…

Supply and Distribution Systems.

Exhaust Systems and Devices.

Controls and Commissioning.

Lighting.


Supply & Distribution Issues

Central 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.

Basic Cooling System Diagram

Cooling System with Chiller

Energy recovery system

Central Plant Design


Energy-Efficient Central Plant Design

“Right-size” chillers and boilers.Evaluate the plant’s 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.


“Right-Sizing”

In the past, mechanical plants have been sized for 100% of peak load at 99% climatic tolerance with an additional 20-50% "start-up factor" and another 20-30% "safety factor." Those days are over! – Apply realistic diversity factors.– "Right-sizing" a lab’s mechanical plant reduces

first costs and operational energy costs. It can reduce energy waste by 10-50% compared to conventional design practices.


Chiller Part-Load Efficiency

Chillers 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


Modular Plant Design

A 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.


Optimize Boiler Plant

Use 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.


Modular Chiller Plant

Modular approach helps designers match loads efficiently.

New, variable speed chillers offer high part-load efficiencies.


All-Variable Speed Chiller Plants

Prime-mover chillerEach type can employ variable speed.Condenser PumpsRespond to changes in load by adjusting pumping rate.Cooling tower fans Adjust to changing ambient temperatures to optimize cooling effect.

Variable Speed Drives

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Chiller without VSD

Chiller with VSD

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

Cooling System with Chiller

Use a Variable Speed Drive (VSD) on each pump


Comparison of Chilled Water Plant to Best Practice

Central Plant Performance

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MDL Plant Best Practice Difference

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Cooling Tower

CW Pumps

CHW Pumps

Chiller

Set system targets in kW/ton


Chilled Water Temperatures

Evaluate 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.

Low vs. Medium Temperature Chillers

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

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

Data provided by York International Corporation.

System Efficiencies Under Different Conditions

85°F Condenser Water

75°F Condenser Water

Water °F kW/Ton Tons kW/Ton Tons

60 0.70 128.9 0.59 135.555 0.75 118.5 0.64 124.450 0.81 108.3 0.70 113.848 0.83 104.5 0.72 109.746 0.85 100.7 0.74 105.745 0.86 98.8 0.75 103.844 0.88 96.8 0.77 101.842 0.90 93.2 0.79 98.040 0.92 89.7 0.82 94.2



Consider “Free” Cooling

In 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.


Combined Heating and Power Systems

Centralized 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.

Air Distribution and Controls


Principles of Air Distribution

Promote 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.


Principles of Air Distribution, continued

Specify 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.


Low Pressure-Drop Design GuidelinesComponent Standard Good BetterAir handler face velocity 500 400 300

Air Handler 2.5 in. w.g. 1.5 in. w.g. 0.75 in.w.g.

Heat Recovery Device 1.00 in. w.g. 0.60 in. w.g. 0.35 in. w.g.

VAV Control Devices Constant Volume, N/A Flow Measurement Devices, 0.60 - 0.30 in. w.g.

Pressure Differential Measurement and Control, 0.10 in. w.g.

Zone Temperature Control Coils

0.5 in. w.g. 0.30 in. w.g. 0.05 in. w.g.

Total Supply and Return Ductwork

4.0 in. w.g. 2.25 in. w.g. 1.2 in. w.g.

Exhaust Stack CFM and

0.7” w.g. full design flow through entire exhaust system, Constant Volume

0.7” w.g. full design flow through fan and stack only, VAV System with bypass

0.75” w.g. averaging half the design flow, VAV System with multiple stacks

Noise Control (Silencers)

1.0” w.g. 0.25” w.g. 0.0” w.g.

Total 9.7” w.g. 6.2” w.g. 3.2” w.g.

Approximate W / CFM 1.8 1.2 0.6

Source: J. Weale, P. Rumsey, D. Sartor, L. E. Lock, “Laboratory Low-Pressure Drop Design,” ASHRAE Journal, August 2002.


“Just Say No to Reheat”

Simultaneous 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.


Constant Volume (CV) Supply

Traditional, 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.


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.


VAV Advantages

VAV 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.

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 4º FConsider “system effects” (AHU inlet and outlet ductwork configurations).


Selecting Fans

Use the most efficient fan for the application.Consider fan curves, especially for low-speed operation.Refer to ASHRAE HVAC Systems and Equipment Chapter on Fans.

Adapted from J. Trost,Efficient Buildings 2.


Economizer Cycles

Appreciate 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 libraries—applications where cooling is required & outside air temperature is below space temperature.Specify enthalpy controls.


Direct Evaporative Cooling

Moisture 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.

Energy Recovery


Energy RecoveryFactors that improve energy recovery economics include:

Colder climates (e.g. more than 3,000 heating degree-days) High exhaust ratesLong service lifeHigh utility ratesLow 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.



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.

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.


Energy Recovery, continued

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.

End of Session

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