vav application considerations

8/3/2019 VAV Application Considerations

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ApplicationConsiderations

VAV-PRC008-EN AC 1

Introduction AC 1

Systems AC 2 4

Parallel vs. Series AC 5

Low-Temperature Air AC 6 7

Energy Savings and System Control AC 8

Agency Certifications AC 9

Control Types AC 10 12

Flow Measurement and Control AC 13 14

Reheat Options AC 15

Insulation AC 16

Acoustics AC 17 18

Duct Design AC 19

Selection Program AC 20

Best Practices AC 21

Unit Conversions AC 22

Additional References AC 23

Table ofContents

IntroductionThe VariTrane line of variable-air-volume (VAV) products has been anindustry leader in performance and

quality for many years. The VariTraneline includes single-duct VAV units,dual-duct VAV units, fan-powered VAVunits (series, parallel, and low-heightseries and parallel), direct digitalcontrols, pneumatic controls, analog-electronic controls, direct digital controlretrofit kits and diffusers. Thisapplication section will focus on VAVunits.


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VAV-PRC008-ENAC 2

Systems

VAV SystemsThere are two primary types of VAV systemssingle-duct and dual-duct.

Single-Duct Systems

Single-duct systems include one supply fan and a single supply duct, which isattached to each zone. The supply fan delivers cooled air to the VAV zones in variablevolumes, depending upon the cooling requirements. The supply fan is usuallydesigned to modulate airflow delivered to the VAV zones.

Many VAV zones require heating as well as cooling. The supply air-handling unitprovides either no heat (cooling only), morning warm-up heat or occupied(changeover) heat. In addition, heat may be provided at any individual VAV zone(within the zone or within the VAV terminal) by reheating cool air provided by thecentral air handler.

No HeatCentral Cooling OnlyIn some systems, the central air handler provides onlycooling and ventilation during zone occupied periods. The supply air is maintained ata constant temperature and the supply airflow is modulated to match the VAVairflow rate with the zone cooling requirements.

Central HeatCentral Heat for Morning Warm-upMany buildings cool down during the night. Tobe at a comfortable temperature in the morning when the building is againoccupied, heat must be added to the spaces. Heat provided by the central airhandler for morning warm-up is supplied at constant air volume to the zones, priorto the time of occupancy. During the morning warm-up period, the VAV terminalunits must open to allow heated air to flow into the zones. In most instances verylittle additional heat is needed once the building is occupied.

Variable-Air-Volume (VAV) System

RAEA

A PAsupplyfan

coolingcoil

variable-speed drive

thermostat

VAV

box

SA

Central Occupied Heating-ChangeoverSomebuildings use the same air handler to provide bothoccupied cooling and occupied heating. This iscommonly referred to as a changeover system. Thesystem changes between heating and coolingdepending on the need of the zones on the system.

In a changeover system, the operation of the VAVterminal units must also change over, opening toprovide heat in the heating mode and opening toprovide cooling in the cooling mode. Trane's mainproduct in this type of application is called VariTrac.VariTrane products can also be used in thesesystems. (These types of systems are beyond thescope of this manual and are discussed in detail inthe VariTrac II Manual, VAV-PRC003-EN.)

Terminal HeatRemote HeatIn some zones of a single-duct VAVsystem, perimeter heating equipment, remote fromthe terminal unit, is used to add heat to the zone

VSD

supplyfan

Single-Fan, Dual-Duct VAV System

central air handlercooling

coil

coilheating

105F (40.6C)

ual-ductAV

terminalunits

A

A40F

(4.4C)

(23.9C)75F

(12.8C)55F

A

when the cooling load is lower than theminimum cooling capacity of the VAVterminal unit. Heat is added directly tothe zone while cool supply air continues

to enter the zone at a minimum rate forzone ventilation.

Terminal ReheatIn some zones of asingle-duct VAV system, a minimumflow of cool supply air is reheated at theterminal unit before entering the zone.Terminal reheat can be provided byelectrical resistance heaters or by hotwater coils.

Parallel Fan-Powered HeatIn somezones of a single-duct VAV system, coolsupply air at minimum flow is mixedwith warm plenum air before enteringthe zone at a constant flow rate. A fan inthe terminal unit, in parallel with thecentral fan, draws air from the plenumwhenever the zone requires heat.

Series Fan-Powered HeatIn somezones of a single-duct VAV system, theairflow to the zone is held constant,during both heating and cooling, by aterminal unit fan that is in series with thecentral fan. The terminal unit fan runscontinuously. When the zone requiresheat, cool supply air at minimum flow ismixed with warm, return plenum airbefore entering the zone.

Dual-Duct SystemsDual-duct systems have either one or

two supply fans and two duct systems.One duct system carries heated air andthe other duct system carries cooled air.Heated air and cooled air are modulatedand/or mixed at each zone in the properproportions to control zone temperature.Terminal reheat is not required in a dual-duct system.


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VAV-PRC008-EN AC 3

Systems

VariTrane VAV Terminal UnitsThe function of the VariTrane terminalunit in a VAV control zone is to vary thevolumetric airflow rate to the zone.

VariTrane units are available with eithermicroprocessor-based DDC controls orpneumatic or analog electronic controls.Factory-installed controls are availablewith all types of terminal units.

VariTrane VAVTerminal Unit TypesSingle-DuctSingle-duct terminal unitscontrol thevolumetric flow of supply air to thespace to maintain the zone temperatureat setpoint. These units are generallyapplied in cooling-only VAV zones that

require no heat during occupied hours. Iflocal zone heat is necessary it can beprovided either remotely (for example,perimeter heat) or by terminal reheat(either electric or hot water coils).

Single-Duct Unit with Hot Water Coil

Parallel Fan-Powered Unit withHot Water Coil

Parallel Fan-Powered Unit withElectric Coil

Single-Duct Cooling Only Unit

Parallel Fan-Powered UnitCooling Only

Dual-DuctDual-duct terminal units are used in aspecial type of air distribution systemwhere the main system has both warm

air and cold air separately ducted toeach terminal unit. The flow of bothwarm air and cool air is modulated,delivering air to the VAV zone atvariable air volumes as well as variabletemperatures. Since full capacityoccupied heating is always available,control of additional local heat isnot provided.

Dual-Duct Terminal Unit

Parallel Fan-PoweredParallel fan-powered unitsarecommonly used in VAV zones which

require some degree of heat duringoccupied hourswhen the primarysupply air is cool. The terminal unit fan isin parallel with the central unit fan; noprimary air from the central fan passesthrough the terminal unit fan. Theterminal unit fan draws air from thespace return plenum.

When no heat is needed, the localparallel fan is off and a backdraftdamper on the fans discharge is closedto prevent cool air entry into the return

plenum. When cool airflow to the VAVzone is at a minimum and the zonetemperature drops below setpoint, thelocal parallel fan is turned on and thebackdraft damper opens. A constantvolume of air is delivered to the zonebecause the fan delivers a constantvolume of warm plenum air which ismixed with cool primary air at aminimum flow. Remote heat orterminal reheat can provide additionallocal heating.


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VAV-PRC008-ENAC 4

Systems

Series Fan-PoweredSeries fan-powered terminal units areused commonly in VAV zones that notonly require heat during occupied

hours, but also desire constant airvolume delivery. The terminal unit fanis in series with the central fan. Primaryair from the central fan always passesthrough the terminal unit fan.

The local series fan within the terminalunit operates whenever the unit is inthe occupied mode. The volume of airdelivered to the VAV zone is constant,but the temperature of the delivered airvaries. As the zone requires lesscooling, the primary air damper closes.As the primary air damper closes, theair mixture supplied to the zonecontains less cool air and more warm

plenum air. Remote heat or terminalreheat can provide additional localheating.

Series fan-powered terminal units arealso useful in low supply airtemperature systems, since theterminal unit fan can be sized so thatwarm plenum air is always mixed withlow temperature supply air. This raisesthe supply air temperature to anacceptable distribution level andreduces condensation potential.

Series Fan-Powered Unit Cooling Only

Low-Height Fan-PoweredLow-height fan-powered terminal unitsare a slightly modified version of a fan-powered terminal unit. As its name

suggests, the low-height fan-poweredunit has a shorter height dimension toaccommodate applications whereceiling space is limited. To reduce theheight, shorter terminal unit fans areintegrated into the standard heightseries or parallel terminal unit. Theresult is a unit with a maximum heightof 11.0"11.5".

For low-height units with the smallerfan sizes (sizes 08SQ and 09SQ), asingle low-profile fan is used. Low-height units with the largest fan size(size 10SQ) use two low-profile fans.Each fan operates off a separate motor.

The fans still remain in series orparallel with the primary systemcentral fan. Low acoustic levels aremuch more challenging in these lowceiling space applications, due to thereduced radiated ceiling pleunumeffect.

The operation of the low-heightterminal unit is exactly the same asthat of a series or parallel terminal unit,as are the options for high-efficiencyECMs, insulation options, etc. As withthe other fan-powered terminal units,additional local heating can beprovided by remote heat or terminal

reheat.

Series Fan-Powered Unit with

Hot Water Coil


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VAV-PRC008-EN AC 5

Parallel vs.Series

AIR

AIRVALVE

PRIMARY

FANAIR

PLENUM

AIRFLOW

FAN

AIRFLOW

PRIMARYAIR

AIRVALVE

AIR

PLENUM

PARALLEL FAN-POWERED TERMINAL SERIES FAN-POWERED TERMINAL

Fan-Powered versus Single-Duct VAV Terminal UnitsIn many climates, fan-poweredsystems are a lower operating costalternative than single-duct systems.The energy inefficiencies inherent inreheating cold primary air can beeliminated with a key designcharacteristic of fan-powered terminalunits, plenum air heating. Heating withwarmer plenum air allows for recoveryof heat from lighting and other heatsources in the building.

Comparison of Parallel andSeries ModelsOnce it has been determined that afan-powered system is to be specified,

the designer must decide betweenparallel and series configurations. Eachmodel carries its own characteristics ofdelivered airflow, energy consumption,and acoustics. For the end user, thedesigner might consider three goals: acomfortable and productive tenant

environment, acceptable installed cost,and low operating costs.

Parallel and series fan-poweredterminal units offer specific advantagesfor particular applications. Thefollowing table compares the keysimilarities and differences betweenthe models that the designer shouldconsider in performing an engineeringanalysis.

Typical Application ofParallel Units:Parallel intermittent fan-poweredterminal units are very common inperimeter zones or buildings whereloads vary during occupied hours.Core zones, which maintain a more

constant cooling requirement, arebetter suited for variable airflow(single-duct) units. Typical jobscombine parallel fan-powered units(exterior) and single-duct units(interior) to provide an efficient systemwith lowest first cost. Although the

overall NC of parallel systems is lowerthan an equivalent series system, theintermittent fan is sometimes noticedwhen energized. To minimize the

impact of this NC change, an ECM(Electrically Commutated Motor) canbe used which has soft-starttechnology.

Typical Application ofSeries Units:Applications requiring constant airmovement or blending utilize seriesconstant fan-powered terminal units.Conference rooms, laboratories, andlobbies are common applications.Because the series fan also adds to thesystem external static pressure, officebuildings take advantage of this design

feature and down size main airhandling equipment. Finally, seriesterminals are used in low-temperatureair systems to temper cold primary airwith warm plenum air and deliver it tothe zone.

Parallel Series

Fan Operation Intermittent operation during occupied Continuous operation during theand unoccupied modes. occupied modes. Intermittent operation

during unoccupied mode.

Operating Sequence Variable-volume, constant-temperature Constant-volume, variable-temperature

device during cooling. Constant-volume, device at all times. Delivers designvariable-temperature during heating. airflow regardless of the load.

Fan Energization Based on zone temperature deviation Interlocked with central system fan tofrom setpoint. No interlock with deliver required air to the zone in bothcentral system fan required. heating and cooling modes

Terminal Fan Fan runs during heating load. Size for Fan runs continually. Fan sizing shouldOperating and Size design heating load. Typically this is 40 to meet the greater of design cooling or

60% of design primary cooling airflow. heating airflow to the zone.

Air valve Sizing Design cooling airflow. Design cooling airflow.

Minimum Inlet Static Sufficient to overcome unit, heating Sufficient to overcome air valvePressure Required for coil, downstream duct and diffuser pressure loss only.Central Fan Sizing pressure losses.

Acoustics When operating under cooling loads Produces slightly higher backgroundthe terminal fan does not run, offering sound pressure levels in the occupiedsuperior acoustic performance similar to space. This sound level remainssingle-duct VAV. Under heating loads, the constant and is less noticeable thanfan operates intermittently. Impact can be intermittent fan operation with PSCminimized by use of a ECM. motors.


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VAV-PRC008-ENAC 6

Low-Temperature Air SystemBenefits of Low-Temperature AirThe benefits of low-temperature airsystems include reduced first cost,

reduced operating cost and increasedrevenue potential. Since low-temperature air transports moreenergy per cubic foot, smaller fans andducts can be used. An EarthWisesystem takes that a step farther andincludes optimizing the waterside ofthe HVAC system as well with low flowrates through the chilled water andcondenser loops.

Since low-temperature water cantransport more thermal energy pergallon, smaller pumps, pipes, andvalves can be used. Smaller HVACequipment consumes less energy soboth electrical demand andconsumption are lowered, reducingoperating costs. The amount ofrevenue generated by a commercialbuilding is related to the amount andquality of rental floor space. Theamount of rental floor space isincreased in a low-temperature airsystem, since air handlers, riser ducts,and equipment rooms are smaller.Since smaller ducts reduce therequired ceiling plenum, additionalfloors may be included withoutincreasing building height.

The concept of the EarthWise systemis to deliver superior comfort and beless expensive to install and operate.The method to do this involves bothwaterside optimization and airsideoptimization. The waterside isoptimized using techniques of lowwater flow through the evaporator andcondenser of the chiller as well asusing chiller-tower optimization controlstrategies. For more information on thewaterside of the EarthWise system,contact your local Trane representativeor visit www.trane.com.

Low Temperature Air System Layout

Airside savings are obtained using acombination of lower air temperatureand intelligent control strategies. Theability of the VAV unit to communicateinformation is vital to systemcoordination.

System OperationA low-temperature air system could bedone with chilled water or directexpansion equipment. A chilled watersystem includes a chiller plant, VAV airhandlers, and series or parallel fan-powered VAV terminal units. The VAV

air handlers use cold water, typicallyaround 40F (4.4C), from the chillerplant, to cool the supply air to 4550F(7.210C). The volume of supply air isdetermined by the airflow needs of theVAV terminal units. A direct-expansionsystem would include a VAV airhandler or rooftop with series orparallel fan-powered VAV terminalunits. The supply air would be cooledto 4852F (8.911.1C).

The VAV terminal units include aparallel or series fan with the central airhandler or rooftop fan. The terminalunit fan operates continuously, mixing

45-50F (7.210C) supply air with warmplenum air, to provide 5055F(1012.8C) cooling air to the occupiedspace at design conditions. As thecooling load in the space decreases,the VAV terminal air valve closes toreduce the flow of cold supply air andincrease the flow of warm plenum airin the case of series terminal units. Thetemperature of air supplied to thespace rises, but the volume flow rate tothe space is constant for the seriesunit.

Considerations for VAV productsTo achieve the maximum benefit fromthe low-temperature air system,several VAV considerations must beaddressed.

InsulationThe units must be insulated to ensurethat no condensation occurs on theunits. How much insulation is needed?Trane has tested its insulation with thegoal of developing a thermalresistance ratio for each type ofinsulation. The thermal resistance (TR)

ratio is discussed on page AC 16. TheTR ratio can be used, along with theproperties of the insulation and thesystem operating conditions todetermine the necessary insulationthickness required.

In the low-temperature air system withfan-powered units, the ducts anddiffusers downstream from theterminal unit handle air that is 55F(12.8C) or warmer. Therefore,condensation considerations are nodifferent from conventional systems.Linear slot diffusers are recommendedto take advantage of the Coanda effect

described in the Diffusers section laterin the catalog.

Terminal unit surfaces that aretraditionally not insulatedelectric andhot water reheat coils and the primaryair inlet for exampleshould bethoroughly field-insulated.

Low-TemperatureAir

VariableVolumeExhaust

Fan

PreheatCoil

Variable VolumeSupply Fan

Series or ParallelFan-powered Unit

CoolingCoil

Heating Coil

Zone 1

Zone 25548


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VAV-PRC008-EN AC 7

Low-TemperatureAir

Fan Airflow + Valve Minimum

Fan Airflow + Valve Design( )x SP1 = SP2

LeakageWhen the terminal unit fan is off, theair valve will close, and not leak. Ductsupstream of the terminal unit must

also be thoroughly insulated andconstructed for very low leakage.

Duct and terminal unit insulation canbe internal or external. Keep in mindthat internal insulation has hiddenthermal leaks at joints and seams.These areas must be located andinsulated externally to avoidcondensation. External Insulation, onthe other hand, allows a complete,uniform thermal seal.

Minimum settings and IAQIndoor air quality is usually best whena specific quantity of outsideventilation air reaches each building

occupant. Maintaining a minimumventilation rate is a challenge in anyVAV system because the amount ofsupply air that reaches a particularspace decreases as the cooling loaddecreases. To insure that a minimumamount of supply air reaches the spaceat all times, a minimum flow setting onthe terminal unit is used. In low-temperature air systems, when thespace needs heating, this minimumflow setting results in increasedheating load. Therefore, it is importantto include the additional load imposedby the cold supply air when calculating

heating loads. Reheat may be requiredsince the ventilation values areabsolute requirements and notpercentage of total airflowrequirements.

EarthWise or Low-Temperature AirDistribution Design Considerationswith Parallel Fan-powered TerminalUnitsThe parallel fan-powered unit needs tobe set up to run continuously ratherthan intermittently. Since it is inparallel, the airflow required by the fanis less than a comparable series unit.This results in energy savings. Running

the parallel fan continuously will takesome minor control changes. It will,however, create a better acousticalinstallation.

The parallel fan should be largeenough to temper the design coolingairflow at 4550F to 5055F (7.210Cto 1012.8C). For instance, if thedesign cooling airflow is 1000 cfm at55F (472 L/s at 12.8C), you will need781 cfm of 48F (368 L/s of 8.9C)supply air and 219 cfm of 80F (103 L/sof 26.7C) plenum air. The parallel fancan be sized for the 219 cfm (103 L/s)rather than the total room airflow.

The fan airflow plus the minimumprimary airflow must be checked withthe minimum airflow of the diffusers toinsure that dumping doesnt occur. If

that is a concern, the minimum couldbe adjusted up or the fan airflow couldbe adjusted up.

As the valve closes, the downstreamstatic pressure will decrease becausethe pressure is related to the airflow.The fan will supply more air at thevalve minimum condition than atdesign due to the decreased staticpressure. This should be aconsideration when calculating howmuch airflow would occur at theminimum valve plus fan airflowcondition. The new fan airflow wouldbe found by looking at a fan curve at

the new SP point. The new SP canbe calculated:

The following table can be used todetermine what percentage of the totalairflow should come from the fan totemper the supply air, assuming 80F(26.7C) plenum air.

If anything other than 80F (26.7C),the following equation can be used tocalculate the percentage:

SupplyTemperature =(%*primarytemperature) + (1-%)*plenumtemperature

downstream of the VAV terminal unit,the system designer will have someconcerns related to condensation ondiffusers and other low-pressure

ductwork accessories. For instance, ifthe occupied space must receive 1000cfm of 55F (472 L/s at 12.8C) air tosatisfy to design cooling load, 715 cfmmust be 45F (337 L/s must be at 7.2C)supply air and 285 cfm must be 80F(135 L/s must be 26.7C) plenum air.Therefore, the series fan-poweredterminal must be sized to have the airvalve deliver 715 cfm (337 L/s) ofsupply air at design conditions, but thefan must be sized to deliver 1000 cfm(472 L/s).

Airside System FactorsA couple of system related factors

should be noted as they apply tocondensation. The first is theadvantage the colder primary air hasfrom a humidity standpoint. As notedin the description above, the low-temperature system operates at spacerelative humidity of 3045% while astandard system operates at spacerelative humidity of 5060%. The drierzone air means that the plenum airreturning to the series terminal unit willalso be drier and, therefore, less of a

problem withcondensation.

The second

condensation factor tonote is related tosystems that shut downin the evening. Manypeople believe that

immediately sending low-temperatureprimary air to these boxes that havebeen off for some time will cause ashock to the system and may causecondensation problems at startup. Thesolution to this has been the advent ofgradual pull-down or soft startsystems. In this type of system, theprimary air temperature is higher oninitial startup (typically 55F(12.8C))

and then gradually reduced to thenormal operating point over the next30 to 60 minutes.

Low-Temperature Air DistributionDesign Considerations withSeries Fan-powered TerminalUnits

The VAV terminal unit includes afan that operates continuously.The series fan should be largeenough to insure that the mixtureof cold supply air and warmplenum air is 5055F (1012.8C)at design cooling flow conditions.In these types of systems, it is agood design practice to developthe system based upon 55F(12.8C) air being provided to thespace from the fan-poweredterminal unit. If a lowertemperature air is used

Percentage of Airflow from FanSupply Air

Temp. Primary Air Temperature (deg. F (C))(deg. F (C) 45 (7.2) 46 (7.8) 47 (8.3) 48 (8.8) 49 (9.4) 50 (10)

50 (0) 14% 12% 9% 6% 3% 0%

51 (10.6) 17% 15% 12% 9% 6% 3%52 (11.1) 20% 18% 15% 13% 10% 7%

53 (11.7) 23% 21% 18% 16% 13% 10%54 (12.2) 26% 24% 21% 19% 16% 13%55 (12.8) 29% 26% 24% 22% 19% 17%


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VAV-PRC008-ENAC 8

Energy Savings &System Control

Electrically CommutatedMotor (ECM)

Fan-Pressure OptimizationWith Trane's Integrated ComfortSystem, the information from VAVterminal units can be used for otherenergy-saving strategies. Fan-pressureoptimization is the concept of reducingthe supply fan energy usage based onthe position of the terminal unitdampers.

The control system allows thisscenario. The system polls the VAVunits for the damper position on each

unit. The supply fan is modulated untilthe most wide-open damper isbetween 85% and 95% open. Thecorrect airflow is still being sent to thezones since the controls of the VAVunits are pressure-independent, andthe fan modulates to an optimal speedand duct static pressure which resultsin fan energy savings.

Ventilation ResetThe Ventilation Reset control strategyenables a building ventilation system

The ECM provides an additionalenergy-saving option to the systemdesigner. Some of the advantages ofthe motor include high efficiency, quietoperation, short payback, and easyinstallation. There are several

considerations that need to beaddressed when deciding whether touse these motors or not. The primarybenefit may be seen as increasedefficiency.

Operating HoursThe added cost ofan ECM can be offset more quickly inapplications which require a relativelyhigh number of hours of operation.However, if a space does not requireextensive running time for the unit fan,then it may not be a good candidatefor this type of motor based solely onpayback. Therefore, the decision aboutusing the ECM may be based on other

benefits, depending on the needs ofthe customer.

Airflow FlexibilityThe ECM allows agreater airflow range per fan size. If aspace is going to change uses and loadcomponents frequently, the ability tochange supply airflow with the ECMwithout changing units will be abenefit.

Airflow BalancingThe ability of theECM motor to self-balance to anairflow regardless of pressure can bean asset when trying to air balance a

job. This will help eliminate additional

dampers or changes to downstreamductwork to ensure proper airflow. Formore information, please contact yourlocal Trane sales engineer.

to bring in an appropriate amount ofoutdoor air per ASHRAE Standard62.1. The basis for the strategy ismeasuring airflow at each zone,calculating current system ventilationefficiency using the multiple-zonesystem equations of the standard, andcommunicating a new outdoor airflowsetpoint to the air handler.

This strategy continually monitors thezone ventilation needs and systemoutdoor air intake flow, minimizing theamount of ventilation air and increasingthe energy efficiency of the system. Thisinsures that the right amount of air isbrought in at all times and that properventilation can be documented. Tranehas integrated this control ability intothe VAV controls, air-handler controls,and building controls.

For more detailed information on theseenergy-saving strategies, please referto the Additional References sectionon page AC 23 of the catalog forappropriate material.


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VAV-PRC008-EN AC 9

Agency CertificationsThere are numerous regulations andstandards in the industry that determine

the construction and performanceparameters for VAV terminal units.Some of the more important of thosestandards and regulations are listedbelow, along with a brief description ofwhat each one addresses.

American Society of Heating,Refrigerating and Air-conditioningEngineers (ASHRAE) - 41.1

ASHRAE - 41.2

ASHRAE - 41.3These standards specify methods fortemperature measurement (41.1),laboratory airflow measurement (41.2),

and pressure measurement (41.3). Whilenone of these standards specificallydiscusses VAV air terminals, theydiscuss topics that are aspects ofterminal box systems. Therefore, someengineers will include these standardsin their specifications as a primer onaccepted measurement techniques.

ASHRAE - 62This standard specifies the minimumventilation rates and indoor air qualitythat are acceptable for occupied spaces.

ASHRAE - 111This standard calls out procedures to befollowed for testing and balancing

HVAC systems. It includes descriptionsof the equipment used, proceduresfollowed, and field changes that mustbe made when a system is balanced.

Air Conditioning and RefrigerationInstitute (ARI)

ARI 880 - 1998This standard sets forth classifications,performance testing requirements, andtest results reporting requirements forair terminal units. The standardcontains very detailed procedures thatare to be followed for the testing andcertification program associated withthis standard. This is one of the mostcommonly referenced standards in theVAV terminal unit industry. The ARI-880certification program is designed topolice the accuracy of documentedperformance for terminal units. Thecertification program requires asampling of at least four units be testedannually. The tested units are chosen at

random by ARI and sent to anindependent laboratory for the testing.The performance is tested at onespecific operating condition. Theoperating characteristics tested includedischarge and radiated sound power(for the damper and, in the case of fan-powered boxes, the fan), wide-opendamper pressure drop, and fan motoramp draw. VariTrane terminal unitsare certified according to ARI-880.

ARI 885-98-02This document provides a procedureto estimate sound pressure levels in anoccupied space. The standard accounts

for the amount of sound pressure inthe space due to the VAV air terminal,diffusers and their connecting lowpressure ductwork. While soundgenerated from the central system fanand ductwork may be a significantfactor in determining the soundpressure level in the room, thisstandard does not address thosefactors. It focuses solely on the VAVterminal and items downstream of it.This standard is related to ARI-880 byusing sound power determined usingARI-880 methodology as a startingpoint for the ARI-885 procedure.

Underwriters Laboratory (UL) 1995Underwriters Laboratory is anindependent testing agency thatexamines products and determines if

those products meet safetyrequirements. Equipmentmanufacturers strive to meet ULguidelines and obtain listing andclassifications for their productsbecause customers recognize ULapproval as a measure of a safelydesigned product. VariTrane VAV airterminals are listed per UL-1995,Heating and Cooling Equipment.The terminals are listed as anentire assembly.

National Fire Protection Association(NFPA) 70This standard is also known as theNational Electrical Code (NEC). TheCode gives standards for installation ofwiring and electrical equipment formost types of commercial andresidential buildings. It is often referredto in VAV air terminal specificationswhen fan-powered boxes, electric heator electric controls are included.

NFPA 90AThis standard does not speak directlyto VAV air terminals but does discusscentral system considerationspertaining to a fire and/or smokecondition. The standard discussessafety requirements in design andconstruction that should be followed tokeep the air-handling system from

spreading a fire or smoke. Thestandard specifies practices that areintended to stop fire and smoke fromspreading through a duct system, keepthe fire-resistive properties of certainbuilding structures (fire walls, etc.)intact, and minimize fire ignitionsources and combustible materials.

AgencyCertifications


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VAV-PRC008-ENAC 10

ControlTypes

Control TypesVAV terminal units are available withmany different options. These optionsfall into three main categories of

controls: direct digital (DDC),pneumatic, and analog electronic. Allof these control types can be used toperform the same basic unit controlfunctions, yet differences exist inaccuracy of performance, versatility,installed cost, operating cost, andmaintenance cost.

Disadvantages:

VersatilityThe communicationsprotocol between controllers will bedifferent from one controllermanufacturer to another.

Installed CostDDC controls are themost expensive of the three controltypes.

Operating and Maintenance CostsBuilding personnel must be trained tooperate and maintain the system.

Operating and Maintenance CostsDiagnostic capability for analogelectronics is not available.

Pneumatic control systems usecompressed air through simplemechanical control devices, such asdiaphragms, springs, and levers tochange an output in response to achange in a monitored variable. WithVAV terminal units, the output istypically a primary airflow and themonitored variable is zone temperature.

Benefits:

PerformancePneumatic controls are aproven technology that is effective andhas a long life cycle.

Installed CostWhen a source ofcompressed air exists at the facility,

pneumatics generally have a lowerinstalled cost than other types ofcontrols when only a basic functionalityis required.

Operating and Maintenance CostsPneumatics are still the most familiarcontrol technology to many buildingdesigners and maintenance people.

Large Installed BasePneumaticsystems are very common in existingbuildings. This eliminates the need topurchase the most expensive piece ofequipment in a pneumatic controlsystemthe control air compressor.

Extensions to existing pneumaticsystems are generally very simple andextremely cost-effective.

Disadvantages:

PerformancePneumatic controlsprovide proportional-only control forVAV terminal unit systems. This controlscheme is less accurate than the moreadvanced control schemes. Impropercalibration of pneumatic controls leadsto poor energy utilization.

VersatilityA central pneumatic controlsystem, where each of the control zonescan be monitored and adjusted from a

remote location, is extremely costly toconfigure and to modify.

Operating and Maintenance CostsPneumatics easily drift and requireconstant upkeep and scheduledmaintenance. Diagnostic capability forpneumatics is not available. A maincompressor which is not maintainedand becomes contaminated with oil orwater can pump those contaminantsinto the compressed-air-distributionsystem. This may require costly cleaningof the system and a possiblereplacement of system components.

Direct digital control (DDC)systems became available asadvances in computer technologymade small microprocessorsavailable and affordable. Much of thehardware in DDC systems is similarto analog electronic systems. Theprimary difference is that DDCcontrollers allow system integration,remote monitoring, and adjustment.

The microprocessor is programmedusing software that gives thecontroller a higher level of capabilitythan either the pneumatic or analogelectronic options.

Benefits:

PerformanceDDC controls offer PIcontrol capability. A PI controlscheme is the most accurate andrepeatable control scheme availablein the VAV terminal unit industry.

VersatilityDDC controls acceptssoftware commands to determinehow its outputs will be controlled.

When a control sequence must bemodified, making changes to thesoftware instructions is easier andquicker than changing hardware.

Operating and Maintenance CostsDDC controls can be networkedtogether to provide system-controlstrategies for energy savings.Multiple controllers can be easilymonitored and adjusted from aremote location. DDC controls alsohave system and individualdiagnostic capability.

Analog electronic control systemsbegan to be used in the 1970s and1980s. Cost effective and reliabletransistors, resistors, relays, and triacs(electronic relays) allowed analogelectronics to become a substitute forpneumatic controls. Analog electroniccontrols use varying voltage signals tochange an output in response to amonitored variable.

Benefits:

PerformanceAnalog electroniccontrols are a basic technology thathas good repeatability.

Operating and Maintenance CostsAnalog electronics have minimal driftand therefore require much lessrecalibration than pneumatics.

Ease of UseAnalog electroniccontrols can be modified using tools asbasic as a screwdriver and a voltmeter.Knowledge and availability of apersonal computer is not required.

Disadvantages:PerformanceAnalog electronicsprovide proportional-only control forVAV terminal unit systems. This controlscheme is less accurate than the moreadvanced control schemes.

Installed CostAnalog electronicshave a higher installed cost thanpneumatic controls for systems withbasic functions.


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VAV-PRC008-EN AC 11

ControlTypes

DDC Controls Basic InformationDDC controls have become theindustry standard for VAV terminal unitcontrol systems. DDC systems use

electronic field devices such as a flowtransducer, a primary air modulatingdamper, and an electronic thermostat.These field devices report softwareinstructions of how the outputs arepositioned in relation to the inputs to acontroller. The VariTranesystem uses aprimary air valve and flow transducerfor both DDC systems and analogelectronic systems. However, the DDCzone sensor is different from theanalog electronic thermostat.

DDC controls provide much flexibilityand considerable diagnostic capability.DDC controllers can be connected

together to form a network ofcontrollers. Once the controllers arenetworked, they can be monitored forproper operation from a remotelocation. Commands and overrides canbe sent for groups of controllers at onetime to make system-wide changes.Commands and overrides can be sentto individual units to allow problemdiagnosis, temporary shutdown,startup schedules or other specializedchanges. When integrated into abuilding management system, theoperation of the VAV terminal unitsystem can be modified to do such

things, as coincide with occupancyschedules and reduce energy charges.

DDC control of VAV terminal units is akey element in providing intelligentand responsive building management.Precision control, flexible comfort, andafter hours access are all available withthe VariTraneDDC control system forVAV terminal units.

Key features of the system include:

An advanced unit controller

Flexible system design

User-friendly interaction

Analog Electronic Controls BasicInformationAnalog electronic controls continue tobe useful in specific applications. Theusers of analog electronic controls canbenefit from the analog electronicproduct without the necessary aircompressor capacity for pneumaticapplications or computer-literatepersonnel for DDC applications.

However, as more and more peoplebecome computer literate, DDCcontrols have become the standard fornon-pneumatic VAV terminal unit

controls. The analog electronic controlsystem will control room temperatureby modulating the position of theelectronic air valve in response to zonetemperature changes. VariTraneanalogelectronic controls are only available inpressure-independent operation.Therefore, the flow is proportional tothe deviation from the zone setpoint.The primary airflow through the airvalve is monitored by means of anelectronic pressure transducerconnected to the standard VariTraneflow ring. The thermostat used withthe VariTrane electronic control system

is a thermistor which completes avoltage divider circuit when wired backto the analog control board. Thethermostat is designed to operatespecifically with VariTraneanalogelectronic controls and is notinterchangeable with the VariTraneDDCzone sensor.

Pneumatic Controls Basic InformationPneumatic controls modulate airpressure of a controller to maintainsetpoint. For VAV systems, there aretwo primary types of pneumaticcontrollersthe room thermostat andthe pneumatic volume regulator (PVR).

Room ThermostatsThe most visible controller to thecustomer is the room thermostat.Pneumatic room thermostats can beclassified by two characteristics: thetubing connection(s) to the thermostatand the action of the thermostat outputin response to a change in the input.

Room thermostats are available inmodels that require a one-pipe or atwo-pipe configuration. The name isderived from the number of tubes thatmust run to the thermostat location.The difference is really in theconstruction of the thermostats. The

two-pipe thermostats have a constantpressure supply connected via an airtube to the thermostat supply air port.The supply air travels through thethermostats relays, levers, diaphragm,and bleed port to produce an output.The output line is connected to theoutput port of the thermostat andextends to the controlled device. The

one-pipe thermostat has, as its namesuggests, only one air line connection.The thermostat works by opening andclosing an air bleed valve. This will

either decrease or increase thepressure on the controlled device,which is connected to the same linethat runs to the thermostat.

Room thermostats also can beclassified by their reaction to a changein temperature. Room thermostatsclassified this way are denoted aseither direct-acting or reverse-acting.Direct-acting thermostats will increasetheir output pressure as thetemperature the thermostat measuresincreases.

On the contrary, reverse-actingthermostats will decrease their outputpressure as the temperature thethermostat measures increases.

Direct-Acting Thermostat Response

OupPe

e

Input Temperatmperature

Reverse-Acting Thermostat Response

OupPe

e

Inputnput Temperatmperature


12/24


VAV-PRC008-ENAC 12

ControlTypes

Pneumatic Volume RegulatorsThese controllers accept the roomthermostat signal and modulate theVAV terminal unit primary air damper.

The primary air damper is controlledfor an airflow setpoint that isdetermined by the room thermostat.The thermostat increases the PVRsairflow setting when the temperaturein the space is warm. On the otherhand, the thermostat decreases thePVRs airflow setting when thetemperature in the space is cold.

Currently, VariTrane offers two modelsof pneumatic volume regulators in itscontrols offeringthe 3011 regulator(used in most applications) and the3501 model (used in dual-ductconstant- volume applications). The

primary difference is the 3501 PVRsability to change the velocity pressurelinearly with a change in thermostatpressure, which results in improvedstability at low flows. In contrast, the3011 PVR resets the velocity pressurewith a change in thermostat pressure.

Reset Control of Minimum andMaximum FlowThe 3011 PVR and3501 use fixed reset control ofminimum and maximum flow settings.The primary benefit of fixed reset in apneumatic volume regulator is stableflow control without excessive dampermovement.

Fixed ResetA fixed reset controlleroperates over a thermostat signalchange of 5 psi between minimum andmaximum flow, regardless of thedifferential pressure flow sensor signal.The thermostat is usually set for a gainof 2.5; i.e. it produces a 2.5 psi output

change per degree of spacetemperature change. This controlstrategy provides stable flow controlwith the primary air valve throttling

between minimum and maximumflow over a 2F space temperaturechange.

Example 1: Air valve with a 6" inlet,Pneumatic thermostat gain = 2.5 psi/degree:

Minimum Flow = 0 cfm, 0.0 in. wgflow signal

Maximum Flow = 680 cfm, 2.0 in. wgflow signal

2.0 in. wg signal range

The damper will modulate from zero tomaximum position over a 2Ftemperature change.

Bleed Port to AtmosphereBleeding air to the atmosphere is anormal operation for a volumeregulator. The 3011 volume regulatoraddresses this function with adedicated bleed port. When air is bledthrough the flow sensor, the differentialpressure signal from the sensor isaffected. As a result, the flow sensorsignal can be radically altered if thevolume regulator is bleeding air, andmay cause excessive dampermovement.

CalibrationThe minimum andmaximum settings are independent ofeach other and need to be set onlyonce during calibration.

Signal Configuration FlexibilityBoth can be configured to work withboth normally-open and normally-closed pneumatic air valves, and bothdirect-acting and reverse-actingthermostats.

Pneumatic VolumeRegulators

PVR 3501

PVR 3011


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VAV-PRC008-EN AC 13

Air Flow

TotalPressure

StaticPressure

WakePressure

FlowMeasurementand Control

Flow Measurement andControlOne of the most importantcharacteristics of a VAV terminal unit isits ability to accurately sense andcontrol airflow. The VariTrane terminalunit was developed with exactly thatgoal in mind. The patented, multiple-point, averaging flow ring measuresthe velocity of the air at the unitprimary air inlet. The differentialpressure signal output of the flow ringprovides the terminal unit controller ameasurement of the primary airflowthrough the inlet. The terminal unitcontroller then opens or closes theinlet damper to maintain the controllerairflow setpoint

perpendicular to the airflow. The low-pressure taps on the VariTrane flowring measure a pressure that is parallelto the direction of flow but in the

opposite direction of the flow. Thiswake pressure that the downstreamring measures is lower than the actualduct static pressure. The differencebetween the wake pressure and thestatic pressure can be accounted for sothat the above relationship betweenflow and differential pressure remainvalid. The difference also helps create alarger pressure differential than thevelocity pressure. Since the pressuresbeing measured in VAV terminal boxapplications are small, this largerdifferential allows transducers andcontrollers to measure and control at

lower flow settings than wouldotherwise be possible.

The average velocity of air travelingthrough the inlet is expressed inthe equation:

Where:FPM = Velocity of air in feet per

minute1096.5 = A constantVP = The velocity pressure of the

air expressed in inches ofwater

DENS = The density of the airexpressed in pounds percubic foot

Often, the density is assumed to be aconstant for dry air at standardconditions (68F (20C)) and sea levelpressure of 14.7 psi (101.4 kPa)). Theseconditions yield the followingcommonly used equation:

The velocity pressure is defined as thedifference between the total pressurein the duct and the static pressure inthe duct:

VP = TP - SP (All units are expressed ininches of water)

The amount of air traveling throughthe inlet is related to the area of theinlet and the velocity of the air:

AIRFLOW = AREA (square feet) xAVERAGE VELOCITY (feet per minute)

AccuracyThe multiple, evenly spaced orifices inthe flow ring of the VariTrane terminalunit provide quality measurementaccuracy even if ductwork turns orvariations are present before the unitinlet. For the most accurate readings, a

minimum of 1 diameters, andpreferably 3 diameters, of straight-runductwork is recommended prior to theinlet connection. The straight-runductwork should be of the samediameter as the air valve inletconnection. If these recommendationsare followed, and the air density effectsmentioned below are addressed, theflow ring will measure primary airflowwithin 5% of unit nominal airflow.

Air Density EffectsChanges in air density due to theconditions listed below sometimescreate situations where the standardflow sensing calibration parametersmust be modified. These factors mustbe accounted for to achieve accuracywith the flow sensing ring. Designers,installers, and air balancers should beaware of these factors and know of thenecessary adjustments to correct forthem.

Air Pressure Measurement Orientations

Flow MeasurementMost VAV terminal units contain a

differential pressure airflowmeasurement device, mounted at theprimary air inlet, to provide a signal tothe terminal unit controller. Numerousnames exist for the differentialpressure measurement deviceflowsensor, flow bar, flow ring. Thedifferential pressure measured at theinlet varies according to the volumetricflow rate of primary air enteringthe inlet.

The total pressure and the staticpressure are measurable quantities.The flow measurement device in a VAVterminal unit is designed to measure

velocity pressure. Most flow sensorsconsist of a hollow piece of tubing withorifices in it. The VariTrane air valvecontains a flow ring as its flowmeasuring device. The flow ring is tworound coils of tubing. Evenly spacedorifices in the upstream coil are thehigh-pressure taps that average thetotal pressure of air flowing throughthe air valve. The orifices in thedownstream ring are low-pressure tapsthat average the air pressure in thewake of flow around the tube. Bydefinition, the measurement of staticpressure is to occur at a point

Flow Ring

FPM = 1096.5VP

DENS

FPM = 4005VP


14/24


VAV-PRC008-ENAC 14

FlowMeasurementand Control

ElevationAt high elevations the air isless dense. Therefore, when measuringthe same differential pressure atelevation versus sea level the actual

flow will be greater at elevation than itwould be at sea level. To calculate thedensity at an elevation other thanstandard conditions (mostmanufacturers choose sea level as thepoint for their standard conditions),you must set up a ratio between thedensity and differential pressure atstandard conditions and the densityand differential pressure at thenew elevation.

Since the data from the manufacturer

is published at standard conditions,this equation should be solved for thedifferential pressure at standardconditions and the other quantitiessubstituted to determine the ratio forthe differential pressure measured atthe new conditions.

P Standard Conditions

DENS Standard Conditions

P New Conditions

DENS New Conditions=

Duct Pressure and Air TemperatureVariationsWhile changes in thesefactors certainly affect the density ofair, most operating parameters which

VAV systems need keep these effectsvery small. The impact on accuracy dueto these changes is less than one halfof one percent except in very extremeconditions (extreme conditions aredefined as those systems with staticpressures greater than 5 in. wg(1245 Pa) and primary air temperaturesgreater than 100F (37.8C)). Sincethose types of systems occur soinfrequently, we assume the effects ofduct pressure and air temperaturevariations to be negligible.

Linearity With the increase in DDCcontrols over pneumatic controls, the

issue of linearity is not as great as itonce was. The important aspect of flowmeasurement versus valve position isthe accuracy of the controller indetermining and controlling the flow.Our units are tested for linearity andthat position versus airflow curve isdownloaded and commissioned in thefactory to insure proper control ofthe unit.


15/24


16/24


VAV-PRC008-ENAC 16

Insulation

InsulationInsulation in a VAV terminal unit isused to avoid condensation on theoutside of the unit, to reduce the heat

transfer from the cold primary airentering the unit, and to reduce theunit noise. The VariTrane line offers fourtypes of unit insulation. The type offacing classifies the types of insulation.To enhance IAQ effectiveness, edges ofall insulation types have metalencapsulated edges.

Matte-FacedThis type of insulation is used fortypical applications. It consists of afiberglass core covered by a high-

density skin. The dual-densityconstruction provides good soundattenuation and thermal performance.

Foil-FacedThis type of insulation is used inapplications where there is someconcern regarding airbornecontaminants entering the space, ordirt being trapped in the fibers of theinsulation. The insulation is composedof a fiberglass core laminated to a foilsheet. Foil-faced insulation will providethe same sound attenuationperformance as matte-faced insulation.

Encapsulated Edges

Double-WallThis type of insulation is used inapplications where there is extremeconcern regarding airborne

contaminants entering the space or dirtbeing trapped in the fibers of theinsulation. The insulation is the sameas the matte-faced insulation.However, after the insulation isinstalled, a second solid wall of26-gage steel covers the insulation. Allwire penetrations of this insulation arecovered by a grommet. This type ofinsulation will result in higherdischarge and radiated sound power.

Closed-CellThis type of insulation is used inapplications where IAQ and fibers areof primary concern. The acoustics of

the closed-cell insulation are similar todouble-wall insulation. The thermalproperties are similar to fiberglassinsulation. This insulation containsno fiberglass.


17/24


VAV-PRC008-EN AC 17

Acoustics

Series vs. Parallel Fan-Powered UnitsAcoustical considerations affectwhether a series or parallel fan-powered terminal unit is selected. Bothunits have their advantages.

The parallel unit has the advantage of

the fan being on and contributing tothe sound levels only when heating isneeded. The fans are usually smallerbecause they are sized for 3060% oftotal unit flow. This creates a unit whichis quieter than series units. Thedisadvantage of the parallel unit is thatthe sound is intermittent. This impactcan be minimized by using an ECM,which has slow fan ramp-up speed.

The primary acoustic benefit to theseries fan-powered unit is that the fanruns continuously. Sometimes the unitcan be selected at slightly highersound levels due to the constant

nature of the sound.

The primary acoustic disadvantage theseries unit has compared to the parallelunit is the need to size the unit fan forthe total room airflow. Series unitsrequire a larger, louder fan thanparallel configurations.

Note: Operating parallel units with acontinuously operating fan may beconsidered for some applications. Thisprovides the quietest overall fan-powered system with the benefit ofcontinuous fan operation.

Insulation typesInsulation is a factor to consider whendealing with the acoustics of terminalunits. Most insulation types will provide

similar acoustical results, but there areexceptions. Double-wall and closed-cellfoam insulation will generally increaseyour sound levels because of theincreased reflective surface area that thesolid inner-wall and closed-cellconstruction provides. This increase insound will have to be balanced with theIAQ and cleanability considerations of thdual-wall and closed-cell construction.

Placement of unitsUnit placement in a building can have asignificant impact on the acceptablesound levels. Locating units above non-critical spaces (hallways, closets, and

storerooms) will help to contain radiatedsound from entering the critical occupiedzones.

Unit AttenuationTerminal unit-installed attenuators are anoption available to provide path soundattenuation. Manufacturer-providedattenuators on the discharge of aterminal unit are targeted at reducingdischarge path noise and are typically asimple lined piece of ductwork. It wouldoften be easier and less expensive todesign the downstream ductwork to beslightly longer and require the installingcontractor to include lining in it.

Attenuators on the plenum inlet of fan-powered terminals are targeted atreducing radiated path noise since theplenum opening on a fan-poweredterminal unit is typically the critical pathsound source. Significant reduction inradiated path noise can result from awell-designed inlet attenuator. Theattenuation from these attenuators is dueto simple absorption from the attenuatorlining and occupant line of sight soundpath obstruction. Therefore, longerattenuators and attenuators that requirethe sound to turn multiple corners beforereaching the occupied space provide

superior results, particularly in the lowerfrequency bands.

Octave Band FrequenciesOctave Center Band EdgeBand Frequency Frequencies

1 63 44.688.52 125 88.51773 250 1773544 500 354707

5 1000 70714146 2000 141428307 4000 283056508 8000 565011300

Acoustical best practices:Acoustics with terminal units issometimes more confusing than it

needs to be. As we know, lowervelocities within a unit leads toimproved acoustical performance.Additionally, if the VAV terminal unithas a fan, a lower RPM provides betterAcoustics performance. It is as simpleas thatthere are some catches,however.

We know that lower velocities andlower RPMs in VAV terminal unitsresult in improved acousticalperformance. Additionalconsiderations will be discussed inmore detail throughout this portion ofApplication Considerations that pertain

to unit size and type, appurtenanceaffects (due to insulation, attenuation,etc.) certification, and computermodeling. Lets take a look at the firstconsideration, sizing of units.

Sizing of unitsBefore blindly increasing the size ofunits, we must first understand what issetting the acoustics within the space.In general, over 95% of acoustics inVAV terminal units, which set thesound pressure levels and ultimatelythe NC within the space, is fromradiated sound. This is readily knownfor fan-powered units, but less

commonly known for single- and dual-duct units. Radiated sound emanatesfrom the unit and enters the occupiedspace via means other than throughthe supply ductwork. The most typicalpath is through the plenum space, thenthrough the ceiling, then into theoccupied space. While dischargesound should never be ignored,radiated sound is the most dominantand usually the most critical soundsource.

When increasing air valve sizes, BECAREFUL. Oversizing an air valve canadversely impact the ability to

modulate and properly controltemperature in the space. In extremelyoversized situations, the air valve willoperate like a two-position controlleddevice, with air either being on, oroff, and not really much in between.The best way to avoid this is tounderstand that the minimum for mostair valves is 300 FPM. This is a functionof the flow sensing device (see wakepressures pp. AC 13) and the ability ofthe pressure transducer and controllerto properly read and report flow. Thisis not manufacturer specific, as physicsapplies to all. Therefore, when sizing

air valves, regardless of the maxcooling velocity the minimum velocityfor proper pressure independent flowis 300 FPM.

Modulation capability and range isvital for proper operation of VAVsystems. With grossly oversized units,the unit will act as a constant volumesystem eliminating the energy savingand individual zone control advantagesof VAV systems. A good rule of thumbis to size cooling airflow for around2000 FPM. VAV systems only operateat full flow when there is a maximumcall for cooling in the zone. Thegreatest portion of the time, an airvalve will be operating at partial flows.

When sizing fan-powered units, the fanairflow range can be determined bylooking at the fan-curve. Becauseparallel and series fan-powered unitsoperate at a constant fan flow,selections can be made all the way tothe lowest flow ranges of the fancurve. A good balance of performanceand cost is to select fans at 70-80% ofmaximum fan flow.


18/24


VAV-PRC008-ENAC 18

Acoustics

Attenuators that are simple cups atthe plenum inlet(s) have been shownin Tranes acoustical mock-up toprovide no measurable reduction in

sound pressure in the critical octavebands which set the occupied spacenoise criteria.

Certification and TestingTerminal units should be submittedbased on the same criteria. There areseveral ways to ensure this bycertification and testing.

Raw unit sound data can be goodmeasurement criteria for evaluation. Inusing this as a basis for comparison,the designer needs to make sure thatthe information is based on the ARIStandard 880 that gives the procedurefor testing.

Specifying NC or RC sound levels is apossible comparison, but the designerneeds to be sure the comparison is fair.Two options are to specify theattenuation effect on which you wouldlike the units to be evaluated or tospecify that ARI Standard 885-98transfer functions be used. Theimportance of ARI Standard 885-98 isthat it is the first ARI Standard thatspecifies exact transfer functions to beused for evaluation. Previous versionsof the standard gave guidelines, butthe manufacturers could choose their

own set of factors.

By using NC sound levels, it is possibleto express acceptable sound levels forvarious types of buildings orenvironments. A few examples are:

Concert Hall NC-22Hospital Room NC-30School Room NC-35General Office NC-40Cafeteria NC-45Factory NC-65

Path AttenuationSound is generated by a terminal unitcan reach the occupied space alongseveral paths. The terminal unitgenerated sound will lose energyi.e.,the energy is absorbed by pathobstaclesas it travels to the occupiedspace. This acoustical energydissipation as it travels to the occupiedspace is called path attenuation. Theamount of energy lost along aparticular path can be quantified andpredicted using the procedure outlinedin ARI-885. Each path must beconsidered when determiningacceptable sound power generated bya terminal unit.

The term transfer function is oftenused to describe the entire pathattenuation value for each octave band(i.e., the sum of all components of aparticular path).

Examples of path attenuation include

locating the terminal unit away fromthe occupied space, increasing the STC(sound transmission classification) ofthe ceiling tile used, internally liningductwork, drywall lagging the ceilingtiles or enclosing the terminal unit indrywall. All of these choices have costsassociated with them that must beweighed against the benefits. Some ofthese alternatives can be acousticallyevaluated from application dataprovided in ARI-885. Others mayrequire professional analysis from anacoustical consultant.

Computer ModelingComputer modeling of acousticalpaths is available to help estimatesound levels and determine problem

sources. The software used by Trane forcomputer modeling is called TraneAcoustics Program ( TAP).

TAP can analyze different roomconfigurations and materials to quicklydetermine the estimated total soundlevels (radiated and discharged) in aspace.

The Trane Official Product SelectionSystem (TOPSS) can also be used todetermine sound levels of terminalunits. You can base selections on amaximum sound level and enter yourown attenuation factors (defaultsbased on ARI-885 are also available).

Other ResourcesPlease refer to "Additional References"(page 29) of the Applications section tosee a list of publications to help withthe basics of acoustical theory andmodeling. You can also contact yourlocal Trane salesperson to discussthe issue.


19/24


VAV-PRC008-EN AC 19

Duct Design

Duct DesignDesigning cost-effective VAV ductsystems is challenging. Some ductdesign methods result in better

pressure balance than others do. Ductshape and duct material can influenceduct system design and cost. Inaddition, duct layout is properlydesigned for optimal duct installationand operation.

Design MethodsThe two most widely used supply ductdesign methodsequal friction andstatic regainare discussed below.

Equal Friction Using this method,ducts are sized at design flow to haveroughly the same static pressure dropfor every 100 feet of duct. Static

pressures throughout the duct systemcan be balanced at design flow usingbalancing dampers, but are no longerbalanced at part load flows. For thisreason, equal friction duct designs arebetter suited for constant volumesystems than for VAV systems. If theequal friction method is used for theVAV supply duct design, the terminalunits usually require pressure-independent (PI) control capability toavoid excessive flow rates when ductpressures are high.

In VAV systems, the ducts locateddownstream of the terminal unit are

usually sized for equal friction. Theadvantage of this design method is itssimplicity. Often, calculations can bemade using simple tables and ductcalculators. Drawbacks includeincreased higher total pressure dropsand higher operating costs.

Static Regain In the static regainmethod, ducts are sized to maintainconstant static pressure in eachsection, which is achieved by balancingthe total and velocity pressure drops ofeach section. In other words, staticpressure is regained by the loss ofvelocity pressure. Since the static

pressures throughout the duct systemare roughly balanced at design andpart load flow, static regain ductdesigns can be used successfully foreither constant volume or VAVsystems. When the static regainmethod is used for VAV systems, thesystem is roughly pressure balancedat design.

Advantages of the static regain methodinclude reduced total pressure drops,lower operating costs, and balancedpressures over a wide range of flows.

The drawback of this design is thetime-consuming, iterative calculationprocedure and for large systems, it isessential to have a duct designcomputer program.

Duct Design ProgramTrane has developed a computerprogram, VariTrane Duct Designer,to aid in the duct design process. Thisprogram is used to calculate duct sizes,

fitting sizes, terminal unit sizes, andpressure drops according to the equalfriction or static regain method.

The duct design program can be easilyincorporated into the selection of VAVterminal units. The inputs and outputsfor the program enable VariTrane unitsto be selected based on the conditionsyou require. This makes selecting andscheduling units much easier. Contactthe local sales office or the TraneC.D.S. department for more detailson this program.


20/24


VAV-PRC008-ENAC 20

SelectionProgram

Selection ProgramThe advent of personal computers hasserved to automate many processesthat were previously repetitive and

time-consuming. One of those tasks isthe proper scheduling, sizing, andselection of VAV terminal units. Tranehas developed a computer program toperform these tasks. The software iscalled the Trane Official ProductSelection System (TOPSS).

The TOPSS program will take the inputspecifications and output the properlysized VariTrane VAV terminal unit alongwith the specific performance for thatsize unit.

With TOPSS, the user can integrateselections of single-duct, dual-duct,

and fan-powered VAV boxes with otherTrane products allowing you to selectall your Trane equipment with onesoftware program.

The program has several requiredfields, denoted by red shading in theTOPSS screen, and many otheroptional fields to meet the givencriteria. Required values for selectionsinclude the maximum and minimumairflows, the control type, and unitmodel. When selecting models withreheat, information regarding theheating coil is needed for selection. Inaddition, the user is given the option to

look at all the information for oneselection on one screen or as aschedule with the other VAV units onthe job.

Also, TOPSS will calculate sound-power data for a selected terminal unit.The user can enter a maximumindividual sound level for each octaveband or a maximum NC value. Theprogram will calculate acoustical datasubject to default or user suppliedsound attenuation data.

The program has many time-savingfeatures such as:

Copy/Paste from spreadsheets likeMicrosoft Excel

Easily arranged fields to match yourschedule

Time-saving templates to store defaultsettings

Several output report options includingschedules

The user can also export the ScheduleView to Excel to modify and put into aCAD drawing as a schedule.

Specific details regarding the program, its operation, and how to obtain a copy of itare available from your local Trane sales office.

VariTrane Quick SelectThe VariTrane Quick Select is a tool used by consulting and contracting firms forspecifying and choosing VariTrane VAV terminal units. The tool has basic informationregarding dimensions, pressure drops, acoustics, electric and hot water reheat, and

fan data. For more information, please contact your local Trane sales office.

Sample screen image from TOPSS Selection Program


21/24


VAV-PRC008-EN AC 21

BestPractices

Common MistakesSome of the most common system orinstallation errors are discussed below.

Reducers at Unit InletThis problem is a very common issuethat is seen in applications of VariTraneproducts. It is often mistaken by thosein the field as an unacceptably largestatic pressure drop through the unit. Itis also sometimes mistaken as amalfunctioning flow ring, pressuretransducer (if DDC or analog electroniccontrols are present) or PVR (ifpneumatic controls are present).

This problem is sometimesunknowingly encountered because ofthe capability of the VariTrane unit toallow greater airflow for a specific size

duct than other terminal units. Forexample, a project engineer specifiesan 8" (203 mm) round take off from themain duct trunk to the VAV terminalunit. The person supplying the VAVterminal unit checks the requiredairflow and finds that a VariTrane unitwith a 6" (152 mm) inlet will providethe specified terminal unitperformance. The terminal unitsupplier submits, receives approval,and orders the 6" (152 mm) inlet unit.While this is happening, the installingcontractor has run the connecting ductfrom the main trunk to the terminalunit in the specified 8" (152 mm) round.The unit arrives at the job site, and theinstaller notices that the 8" (203 mm)duct and the 6" (152 mm) terminal unitinlet do not match. To get the unitinstalled, an 8- to 6-inch reducer isplaced at the inlet to the terminal unitair valve.

The reducer will cause a phenomenoncalled flow separation at the unit inlet.Fluid dynamics analysis can present adetailed technical explanation of flow

separation, but the characteristicsimportant to this discussion are theproduction of pressure loss andturbulence. The reducer will have asignificant static pressure dropassociated with it since the air velocityis increased (i.e., static pressure isgiven up for increased velocitypressure). The pressure loss issometimes mistaken as a loss due tothe function of the terminal unit. Theturbulence is at its greatest justdownstream of the reducer.Unfortunately, this is the location of theflow ring at the air-valve inlet. The

reducer will cause the flow ring to givean inaccurate and inconsistent readingbecause of the turbulent air.

The solutions to this situation are:

Locate the reducer upstream of theterminal unit at least three ductdiameters to eliminate flow separationand turbulence at the unit inlet and toimprove the airflow measurementaccuracy.

Consider proper sizing of the terminalunit in the duct design and account forthe pressure loss of the reducer in thecentral fan selection if a reducer is

required. Be cautious of oversizing aVAV terminal. It is good practice tomake sure that the inlet duct velocity atthe minimum airflow setting is nolower than 500 feet per minute.

Improper Use of Flexible DuctworkWhile flexible ductwork has manybenefits, improper use can causenumerous problems in a VAV system.Flexible ductwork causes turbulentairflow and relatively large static

pressure drops. Flexible ductwork at aprimary damper inlet (i.e., the flowsensor location) may cause flowaccuracy and repeatability problems

due to turbulence. The use of flexibleductwork should be primarily limited tothe downstream side of the terminalunits in a VAV system. Use of flexibleductwork upstream of terminal unitsshould be kept to an absoluteminimum. All runs of flexible ductworkshould be kept as short as possible.While most know these guidelines, theease of installation which flexibleductwork provides is always anenticement to push the limits of whatare acceptable practices.

Static Pressure Measurement ErrorsImproper measurement techniques for

static pressure can lead many tomistakenly believe that the terminalunit is causing a large pressure drop inthe system. The chief error made hereis taking a static pressuremeasurement in turbulent locationssuch as flexible ductwork or neartransitions. This produces invalid staticpressure readings. Another errorcommonly made is trying to read thestatic pressure at the same point as theflow sensing device. The inlets to VAVterminal units produce turbulence andwill give poor readings. Flow sensorswith their multiple-point averaging

capability are best equipped to dealwith this type of flow, while a single-point static probe is not. Anothercommon error is the incorrectorientation of the static pressure probe.The static pressure is correctlymeasured when the probe is orientedperpendicular to the direction ofairflow. The probe, or a part of it,should never be facing the direction ofairflow, because the total pressure willinfluence the reading of the probe.


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UnitConversions

Conversions of Velocity, Pressure, and Flow Rate

To convert From To Multiply by

Velocity Ft/min M/s 0.00508Velocity M/s Ft/min 196.850

Pressure Psi Pa 6894.76Pressure Ft of water Pa 2988.98Pressure In. of water Pa 249.082Pressure Pa Psi 0.000145038Pressure Pa Ft of water 0.000334562Pressure Pa In. of water 0.00401474

Flow Rate Cfm L/s 0.4719Flow Rate Cfm m3 /s 0.000471947Flow Rate Gpm L/s 0.0630902Flow Rate m3 /s Cfm 2118.88Flow Rate L/s Cfm 2.1191Flow Rate L/s Gpm 15.8503

Conversions of Length and Area

To convert From To Multiply by

Length In. m 0.0254

Length Ft m 0.3048Length m In. 39.3701Length m Ft 3.28084

Area In.2 m2 0.00064516Area Ft2 m2 0.092903Area m2 In.2 1550Area m2 Ft2 10.7639


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AdditionalReferences

VAV System and ProductReferencesVAV Systems Air Conditioning Clinic

This clinic is designed to explain thesystem components, the systemconfigurations, many of the VAVsystem options and applications. Agreat resource for VAV systemunderstanding.

Literature # TRG-TRC014-EN

Indoor Air Quality A guide tounderstanding ASHRAE Standard62-2001The guide helps to explain theASHRAE Standard as well as thefundamentals of good indoor airquality. A great resource forunderstanding the standard andways of designing VAV systemsaround that standard.

Literature # ISS-APG001-EN

Managing Outdoor Air TraqComfort SystemsThis brochure is a good, quickreference of the issues of managingoutdoor air for a VAV system.

Literature # CLCH-S-26

Ventilation and Fan PressureOptimization for VAV SystemsAn engineering bulletin designed tohow a Trane Integrate Comfort

system can effectively control buildingventilation and supply fan pressurefor increased comfort and IAQ whilekeeping energy costs to the lowestpossible.

Literature # SYS-EB-2

Trane DDC/VAV Systems ApplicationsEngineering ManualThis manual gives detaileddescriptions of the Trane DDC/VAV

system. Topics include systemcomponents, how the system interactsand specific inputs and outputs ofthe system.

Literature # ICS-AM-6

Acoustics in Air ConditioningApplications Engineering ManualThis manual describes the basicfundamentals, behavior,measurement, and control of sound, alldirected at the design of quiet systems.

Literature # FND-AM-5

VariTrac CatalogThe catalog will help explain featuresand benefits of VariTrac, how theVariTrac product works, applications forthe product, and selection procedures.

Literature # VAV-PRC003-EN

ASHRAE Handbook of Fundamentals

ASHRAE Handbook of HVAC Systemsand Equipment

ASHRAE Handbook of HVACApplications

ASHRAE Handbook of Refrigeration

Web sites:www.ashrae.org

www.ari.orgwww.trane.com


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Documents