20285100 Air Conditioning Principles and Concepts

8/2/2019 20285100 Air Conditioning Principles and Concepts

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AIRCONDITIONINGPrinciplesandConcepts

AirconditioningistheprocesswherebytheconditionofAir,asdefinedbyitstemperatureandmoisturecontent,ischanged.Inpracticeotherfactorsmustalsobetakenintoaccountespeciallycleanliness;odor;velocity&distributionpattern.PrinciplesofAir-Conditioning:HumancomfortInevitably'comfort'isaverysubjectivematter.TheEngineeraimstoensure'comfort'formostpeoplefoundfromstatisticalsurveys.Mostpeople(90%)arecomfortablewhentheairtemperatureisbetween18-22Candthe%satisbetween40-65%.Thiszonecanbeshownonthepsychometricchart.Andisknownasthecomfortzone.


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Outsideairisquitelikelytobeatadifferentconditionfromtherequiredcomfortzonecondition.Inordertobringitsconditiontowithinthecomfortzonewemayneedtodooneormoreofthefollowing:-heatit;coolit;dehumidifyit;humidifyit;ormixit.

DryairmassflowInordertousethepsychometricchartforair-conditioningworkweneedtofind&usedryairmassflows.However,inpracticeair-flowsarefrequentlymeasuredintermsofvolumeflow.Inordertofinddryairmassflowweneedtousethespecificvolumeoftheair.Specificvolume=volume/massThespecificvolumeoftheairisgivenfromthePsychometricchartinm/kgofdryair,thereforetheMassflowwillbeintermsofdryairmassflow.Obviouslytheconditionoftheairmustbeknown(Typicallyd.b.temp.&%sat)inordertofindthespecificVolume.

AirheatingTheheatingprocesscanbeillustratedonthepsychometricchartthus:


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Cooling/DehumidificationInthecaseofcooling,themixturewillfirstlybesensiblycooledtothepointofsaturation(calledthedewpoint)thenliquidwaterwillprecipitateifwecoolfurther.Becausemoistureisremoveddehumidificationisachieved.Thecooling/dehumidificationprocesscanbeillustratedonthepsychometricchartthus:


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Humidification

Theprocessofhumidificationallowstheairtomixwithextrawater.Asufficientcontacttimebetweentheairandwaterwillnormallyresultintheairreaching100%Saturation.Theprocessisveryclosetotheevaporationfromawetbulb.ItthereforefollowsalineofconstantwetbulbTemperature.

MixingOften,insteadofexhausting'stale'aircompletelysomeofitisfiltered,deodorizedandmixedwithfreshincomingair.Thisconservesenergyandnarrowstheoperatingconditionsfortheair-conditioningsystem.


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HeatTransferHeatisaformofenergy.Everyobjectonearthhassomeheatenergy.Thelessheatanobjecthas,thecolderwesayitis.Coolingistheprocessoftransferringheatfromoneobjecttoanother.Whenanair-conditioningsystemcools,itisactuallyremovingheatandtransferringitsomewhereelse.ThiscanbedemonstratedbyturningonaSpotCoolerandplacingonehandinfrontofthecoldairnozzleandtheotheroverthewarmairexhaust.Youwillfeeltheactionofthetransferofheat.

SensibleandLatentHeatTherearetwoformsofheatenergy:sensibleheatandlatentheat.Sensibleheatistheformofheatenergywhichismostcommonlyunderstoodbecauseitissensedbytouchormeasureddirectlywithathermometer.Whenweatherreporterssayitwillbe90degrees,theyarereferringtosensibleheat.Latentheatcannotbesensedbytouchormeasuredwithathermometer.Latentheatcausesanobjecttochangeitsproperties.Forexample,whenenoughlatentheatisremovedfromwatervapor(steamorhumidity),itcondensesintowater(liquid).Ifenoughlatentheatisremovedfromwater(liquid),itwilleventuallyfreeze.Thisprocessisreversedwhenlatentheatisadded.

ChangeofStateAnobjectthatchangesfromasolidtoaliquidorliquidtovaporisreferredtoasachangeofstate.Whenanobjectchangesstate,ittransfersheatrapidly.HumidityMoistureintheairiscalledhumidity.Theabilityofairtoholdmoisturedirectlyrelatestoitstemperature.Thewarmerairis,themoremoistureitiscapableofholding.Relativehumidityisthepercentageofmoistureintheaircomparedtotheamountofmoistureitcanhold.Amoisture

contentof70Fairwith50%relativehumidityislowerthan80Fairwith50%relativehumidity.


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Whenthehumidityislow,sweatevaporatesfromyourbodymorequickly.Thisallowsyoutocoolofffaster.Highhumidityconditionsdonotallowsweattoevaporateaswellbecausetheairisatitsmaximumcapacity.Humidityisalsoaformoflatentheat.Whenaircontainsmorehumidity,ithasmorelatentheat.

REFRIGERANTRefrigerantsaresubstancesusedbyairconditionerstotransferheatandcreateacoolingeffect.Air-conditioningsystemsusespeciallyformulatedrefrigerantsdesignedtochangestateatspecifictemperaturesprovidingoptimumcooling.PortablesusearefrigerantcalledR-22orHCFC-22.HCFCstandsforhydrochlorofluorocarbon.Thisiscurrentlythemostcommonrefrigerantusedbyair-conditioningsystems.

REFRIGERANTPHASE-OUTManyofthecurrentformsofrefrigerantsusedtodayarebeingphasedoutbasedonconcernfordepletionoftheozonelayer.PortablesuseR-22,whichhasbeendeemedacceptableforusebytheEPAuntiltheyear2010.Bythattime,anozone-friendlyrefrigerantthatcanbeeasilysubstitutedforR-22willbereadilyavailable.


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PSYCHOMETRICCHART

PsychometricChart

Theprinciplesofpsychometricchartapplytoanyphysicalsystemconsistingofgas-vapormixtures.Themostcommonsystemofinterest,however,aremixturesofwatervaporandairbecauseofitsapplicationinheating,ventilating,andairconditioningandmeteorology.PsychometricratioThepsychometricratioisanimportantpropertyintheareaofpsychometricsasitrelatestheabsolutehumidityandsaturationhumiditytothedifferencebetweenthedrybulbtemperatureandtheadiabaticsaturationtemperature.Mixturesofairandwatervaporarethemostcommonsystemsencounteredinpsychometric.Thepsychometricratioofair-watervapormixturesisapproximatelyunitywhichimpliesthatthedifferencebetweentheadiabatic


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saturationtemperatureandwetbulbtemperatureofair-watervapormixturesissmall.Thispropertyofair-watervaporsystemssimplifiesdryingandcoolingcalculationsoftenperformedusingpsychometricrelationships.Apsychometricchartisagraphofthephysicalpropertiesofmoistairataconstantpressure(oftenequatedtoanelevationrelativetosealevel).Thechartgraphicallyexpresseshowvariouspropertiesrelatetoeachother,andisthusagraphicalequationofstate.Thethermophysicalpropertiesfoundonmostpsychometricchartsare:Dry-bulbtemperature(DBT)isthatofanairsample,asdeterminedbyanordinarythermometer,thethermometer'sbulbbeingdry.Itistypicallytheabscissa,orhorizontalaxisofthegraph.TheSIunitsfortemperatureareCelsius;otherunitsareFahrenheit.Wet-bulbtemperature(WBT)isthatofanairsampleafterithaspassedthroughaconstant-pressure,ideal,adiabaticsaturationprocess,thatis,aftertheairhaspassedoveralargesurfaceofliquidwaterinaninsulatedchannel.Inpractice,thisisthereadingofathermometerwhosesensingbulbiscoveredwithawetsockevaporatingintoarapidstreamofthesampleair.TheWBTisthesameastheDBTwhentheairsampleissaturatedwithwater.TheslopeofthelineofconstantWBTreflectstheheatofvaporizationofthewaterrequiredtosaturatetheairofagivenrelativehumidity.Dewpointtemperature(DPT)isthattemperatureatwhichamoistairsampleatthesamepressurewouldreachwatervaporsaturation.Atthissaturationpoint,watervaporwouldbegintocondenseintoliquidwaterfogor(ifbelowfreezing)solidhoarfrost,asheatisremoved.Thedewpointtemperatureismeasuredeasilyandprovidesusefulinformation,butisnormallynotconsideredanindependentproperty.Itduplicatesinformationavailableviaotherhumiditypropertiesandthesaturationc

urve.RelativeHumidity(RH)istheratioofthemolefractionofwatervaportothemolefractionofsaturatedmoistairatthesametemperatureandpressure.RHisdimensionless,andisusuallyexpressedasapercentage.LinesofconstantRH


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reflectthephysicsofairandwater:theyaredeterminedviaexperimentalmeasurement.Note:thenotionthatair"holds"moisture,orthatmoisturedissolvesindryairandsaturatesthesolutionatsomeproportion,isanerroneous(albeitwidespread)conceptHumidityRatio(alsoknownasMoistureContent,MixingRatio,orSpecificHumidity)istheproportionofmassofwatervaporperunitmassofdryairatthegivenconditions(DBT,WBT,DPT,RH,etc.).Itistypicallytheordinateorverticalaxisofthegraph.ForagivenDBTtherewillbeaparticularhumidityratioforwhichtheairsampleisat100%relativehumidity:therelationshipreflectsthephysicsofwaterandairandmustbemeasured.HumidityRatioisdimensionless,butissometimesexpressedasgramsofwaterperkilogramofdryairorgrainsofwaterperpoundofair.SpecificEnthalpysymbolizedbyh,alsocalledheatcontentperunitmass,isthesumoftheinternal(heat)energyofthemoistairinquestion,includingtheheatoftheairandwatervaporwithin.Intheapproximationofidealgasses,linesofconstantenthalpyareparalleltolinesofconstantWBT.Enthalpyisgivenin(SI)JoulesperkilogramofairorBTUperpoundofair.SpecificVolume,alsocalledInverseDensity,isthevolumeperunitmassoftheairsample.TheSIunitsarecubicmetersperkilogramofair;otherunitsarecubicfeetperpoundofdryair.Theversatilityofthepsychometricchartliesinthefactthatbyknowingthreeindependentpropertiesofsomemoistair(oneofwhichisthepressure),theotherpropertiescanbedetermined.Changesinstate,suchaswhentwoairstreamsmix,canbemodeledeasilyandsomewhatgraphicallyusingthecorrectpsychometricchartforthelocation'sairpressureorelevationrelativetosealevel.Forlocationsatorbelow2000ft(600m),acommonassumptionistousethesealevelpsychomet

ricchart.


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HowtoreadthechartThemostcommonchartusedbypractitionersandstudentsalikeisthe"-t"(omega-t)chartinwhichtheDryBulbTemperature(DBT)appearshorizontallyastheabscissaandthehumidityratios()appearastheordinates.Inordertouseaparticularchart,foragivenairpressureorelevation,atleasttwoofthesixindependentpropertiesmustbeknown(DBT,WBT,RH,HumidityRatio,SpecificEnthalpy,andSpecificVolume).Thisgivesriseto15possiblecombinations.

DBT:ThiscanbedeterminedfromtheabscissaDPT:Followthehorizontallinefromthepointwherethelinefromthehorizontalaxisarrivesat100%RH,alsoknownasthesaturationcurve.WBT:LineinclinedtothehorizontalandintersectssaturationcurveatDBTpoint.RH:Hyperboliclinesdrawnasymptoticallywithrespecttothesaturationcurvewhichcorrespondsto100%RH.HumidityRatio:MarkedonOrdinateaxis.SpecificEnthalpy:linesofequalvalues,orhashmarksfor,slopefromtheupperlefttothelowerright.SpecificVolume:Equallyspacedparallelfamilyoflines.


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REFRIGERATIONCYCLERefrigerantRefrigerantsaresubstancesusedbyairconditionerstotransferheatandcreateacoolingeffect.Air-conditioningsystemsusespeciallyformulatedrefrigerantsdesignedtochangestateatspecifictemperaturesprovidingoptimumcooling.PortablesusearefrigerantcalledR-22orHCFC-22.HCFCstandsforhydrochlorofluorocarbon.Thisiscurrentlythemostcommonrefrigerantusedbyair-conditioningsystems.RefrigerantPhase-OutManyofthecurrentformsofrefrigerantsusedtodayarebeingphasedoutbasedonconcernfordepletionoftheozonelayer.PortablesuseR-22,whichhasbeendeemedacceptableforusebytheEPAuntiltheyear2010.Bythattime,anozone-friendlyrefrigerantthatcanbeeasilysubstitutedforR-22willbereadilyavailable.Intherefrigerationcycle,aheatpumptransfersheatfromalowertemperatureheatsourceintoahighertemperatureheatsink.Heatwouldnaturallyflowintheoppositedirection.Thisisthemostcommontypeofairconditioning.Arefrigeratorworksinmuchthesameway,asitpumpstheheatoutoftheinteriorintotheroominwhichitstands.ThiscycletakesadvantageoftheuniversalgaslawPV=nRT,wherePispressure,Visvolume,Ristheuniversalgasconstant,Tistemperature,andnisthenumberofmolesofgas(1mole=6.0221023molecules).Intherefrigerator,thecycleiscontinuous.Inthefollowingexample,providedthattherefrigerantbeingusedispureammonia,whichboilsat-27degreesF.Thisiswhathappenstokeeptherefrigeratorcool:Thecompressorcompressestheammoniagas.Thecompressedgasheatsupasitispressurized(orange).Thecoilsonthebackoftherefrigeratorletthehotammoniagasdissipateitsheat.Theammoniagascondensesintoammonialiquid(darkblue)athighpressure.Thehigh-pressureammonialiquidflowsthroughtheexpansionvalve.Expansionvalvecan

beconsideredasasmallhole.Ononesideofthehole


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ishigh-pressureammonialiquid.Ontheothersideoftheholeisalowpressurearea(becausethecompressorissuckinggasoutofthatside).Theliquidammoniaimmediatelyboilsandvaporizes(lightblue),itstemperaturedroppingto-27F.Thismakestheinsideoftherefrigeratorcold.Thecoldammoniagasissuckedupbythecompressor,andthecyclerepeats.

TheRefrigerationCycleA=InsidetherefrigeratorB=CompressorC=ExpansionValve


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HeatpumpandrefrigerationcycleThermodynamicheatpumpandrefrigerationcyclesarethemodelsforheatpumpsandrefrigerators.Thedifferencebetweenthetwoisthatheatpumpsareintendedtokeepaplacewarmandrefrigeratorsdesignedtocoolit.Technicallyarefrigeratorcycleisalsoaheatpumpcycle.Aheatpumpiswhenheatisremovedfromalow-temperaturespaceorsourceandrejectedtoahigh-temperaturesinkwiththehelpofexternalmechanicalwork.Theinverseoftheheatpumpcycleisthethermodynamicpowercycle.Inthepowercycle,heatissuppliedfromahigh-temperaturesourcetotheheatengine,partoftheheatbeingusedtoproducemechanicalworkandtherestbeingrejectedtoalow-temperaturesink.Thissatisfiesthesecondlawofthermodynamics.Aheatpumpdescribesthechangesthattakeplaceintherefrigerantasitalternatelyabsorbsandrejectsheatasitcirculatesthrougharefrigerator.ItisalsoappliedtoHVACRwork,whendescribingthe"process"ofrefrigerantflowthroughanHVACRunit,whetheritisapackagedorsplitsystem.Heatnaturallyflowsfromhottocold.Workisappliedtocoolalivingspaceorstoragevolumebypumpingheatfromalowertemperatureheatsourceintoahighertemperatureheatsink.Insulationisusedtoreducetheworkandenergyrequiredtoachieveandmaintainalowertemperatureinthecooledspace.TheoperatingprincipleoftherefrigerationcyclewasdescribedmathematicallybySadiCarnotin1824asaheatengine.Themostcommontypesofheatpumpsystemsusethereverse-Rankinevaporcompressionrefrigerationcyclealthoughabsorptionheatpumpsareusedinaminorityofapplications.Heatpumpcanbeclassifiedas:Vaporcycle,Gascycle,andStirlingcycle


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Vaporcyclerefrigerationcanbeclassifiedas:VaporcompressionrefrigerationGasabsorptionrefrigerationVapor-compressioncycleThevapor-compressioncycleisusedinmosthouseholdrefrigeratorsaswellasinmanylargecommercialandindustrialrefrigerationsystems.ThefollowingFigureprovidesaschematicdiagramofthecomponentsofatypicalvapor-compressionrefrigerationsystem.Thermodynamicsofthecyclecanbeanalyzedonadiagram.Inthiscycle,acirculatingrefrigerantsuchasFreonentersthecompressorasavapor.Frompoint1topoint2,thevaporiscompressedatconstantentropyandexitsthecompressorsuperheated.Frompoint2topoint3andontopoint4,thesuperheatedvaportravelsthroughthecondenserwhichfirstcoolsandremovesthesuperheatandthencondensesthevaporintoaliquidbyremovingadditionalheatatconstantpressureandtemperature.Betweenpoints4and5,theliquidrefrigerantgoesthroughtheexpansionvalve(alsocalledathrottlevalve)whereitspressureabruptlydecreases,causingflashevaporationandauto-refrigerationof,typically,lessthanhalfoftheliquid.


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Thatresultsinamixtureofliquidandvaporatalowertemperatureandpressureasshownatpoint5.Thecoldliquid-vapormixturethentravelsthroughtheevaporatorcoilortubesandiscompletelyvaporizedbycoolingthewarmair(fromthespacebeingrefrigerated)beingblownbyafanacrosstheevaporatorcoilortubes.Theresultingrefrigerantvaporreturnstothecompressorinletatpoint1tocompletethethermodynamiccycle.Theabovediscussionisbasedontheidealvapor-compressionrefrigerationcycle,anddoesnottakeintoaccountreal-worldeffectslikefrictionalpressuredropinthesystem,slightthermodynamicirreversibilityduringthecompressionoftherefrigerantvapor,ornon-idealgasbehavior(ifany).


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VaporabsorptioncycleIntheearlyyearsofthetwentiethcentury,thevaporabsorptioncycleusingwater-ammoniasystemswaspopularandwidelyusedbut,afterthedevelopmentofthevaporcompressioncycle,itlostmuchofitsimportancebecauseofitslowcoefficientofperformance(aboutonefifthofthatofthevaporcompressioncycle).Nowadays,thevaporabsorptioncycleisusedonlywherewasteheatisavailableorwhereheatisderivedfromsolarcollectors.Theabsorptioncycleissimilartothecompressioncycle,exceptforthemethodofraisingthepressureoftherefrigerantvapor.Intheabsorptionsystem,thecompressorisreplacedbyanabsorberwhichdissolvestherefrigerantinasuitableliquid,aliquidpumpwhichraisesthepressureandageneratorwhich,onheataddition,drivesofftherefrigerantvaporfromthehigh-pressureliquid.Someworkisrequiredbytheliquidpumpbut,foragivenquantityofrefrigerant,itismuchsmallerthanneededbythecompressorinthevaporcompressioncycle.Inanabsorptionrefrigerator,asuitablecombinationofrefrigerantandabsorbentisused.Themostcommoncombinationsareammonia(refrigerant)andwater(absorbent),andwater(refrigerant)andlithiumbromide(absorbent).

GascycleWhentheworkingfluidisagasthatiscompressedandexpandedbutdoesn'tchangephase,therefrigerationcycleiscalledagascycle.Airismostoftenthisworkingfluid.Asthereisnocondensationandevaporationintendedinagascycle,componentscorrespondingtothecondenserandevaporatorinavaporcompressioncyclearethehotandcoldgas-to-gasheatexchangersingascycles.ThegascycleislessefficientthanthevaporcompressioncyclebecausethegascycleworksonthereverseBraytoncycleinsteadofthereverseRankinecycl

e.Assuchtheworkingfluiddoesnotreceiveandrejectheatatconstanttemperature.Inthegascycle,therefrigerationeffectisequaltotheproductofthespecificheatofthegasandtheriseintemperatureofthegasinthelowtemperatureside.


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AIRCYCLEAirisbynaturethesafestandcheapestrefrigerant.Environmentalconcernsaboutozonedepletion,globalwarmingandincreasinglystringentlegislationhaverenewedinterestinalternativerefrigerationtechnologies.Aircyclesystemshavespecificadvantagesthatapplytoallpotentialapplications:Theworkingfluid(air)isfree,environmentallybenign,totallysafeandnontoxic.Aircycleequipmentisextremelyreliable,therebyreducingmaintenancecostsandsystemdown-time.Theperformanceofanaircycleunitdoesnotdeteriorateasmuchasthatofavapor-compressionunitwhenoperatingawayfromitsdesignpoint.Whenoperatinginarefrigerationcycle,anaircycleunitcanalsoproduceheatatausefultemperature.Theuseofairasarefrigerantisbasedontheprinciplethatwhenagasexpandsisentropicallyfromagiventemperature,itsfinaltemperatureatthenewpressureismuchlower.Theresultingcoldgas,inthiscaseair,canthenbeusedasarefrigerant,eitherdirectlyinanopensystem,orindirectlybymeansofaheatexchangerinaclosedsystem.Theefficiencyofsuchsystemsislimitedtoagreatextentbytheefficienciesofcompressionandexpansion,aswellasthoseoftheheatexchangersemployed.Originally,slowspeedreciprocatingcompressorsandexpanderswereused.Thepoorefficiencyandreliabilityofsuchmachineryweremajorfactorsinthereplacementofsuchsystemswithvaporcompressionequipment.However,thedevelopmentofrotarycompressorsandexpandersgreatlyimprovedtheisentropicefficiencyandreliabilityoftheaircycle.Advancesinturbinetechnology,togetherwiththedevelopmentofairbearingsandceramiccomponentofferfurtherefficiency.Combiningthiswithnewlyavailablecompactheatexchangerswithgreatlyimprovedheattransfercharacteristicsmakescompetitionwithmanyexistingvaporcompression,andcert

ainlyliquidnitrogensystems,quitefeasible.


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EnvironmentalcontrolinbuildingsUntilrecentlytheuseofaircyclehasbeenlargelyrestrictedtoaircraftcabinairconditioningsystems.Arecenttrialhasdemonstratedtheadvantagesthataircycletechnologiescanoffertopassengertrainairconditioningsystems.Animportantconclusionofthistrialwasthataircycletrainairconditioningsystemswillhaveloweroveralllifecycleownershipcoststhancomparablevapourcompressionsystems.ThesuccessfuldemonstrationoftheseunitsinGermanysICE2.2highspeedtrainsbyNormalair-GarrettLtd.ledtothecompanyreceivingtheEngineeringCouncilsEnvironmentalAwardforEngineersin1996.StudiescarriedoutbytheBuildingsResearchEstablishment(BRE)andfrperchavedemonstratedthataircyclesystemsinbuildingswouldhaveanumberofadvantages.Theseinclude

LaminationoftheneedtouseenvironmentallydamagingCFC,HCFCorotheralternativerefrigerantsinbuildingairconditioningsystemsUseofhighgradeheatrecoveryfromaircyclecoolingsystemsresultinginlowerenergyconsumptionImprovedreliabilityandreducedmaintenancecomparedwithconventionalsystemsMaintenanceofnearfullloadefficiencyatpartloadconditionsNosusceptibilitytorefrigerantleakage


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FoodfreezingsystemCurrentlyfrpercareworkingonthedesign,constructionandinstallationofanaircyclefluidizedbedfreezerforfoodfreezing.Theaircycleplantwilloperatewithairastherefrigerantdeliveringittothefreezerbedat-75C.Fluidizedbedshaveanumberofusefulcharacteristics.Heatandmasstransferratestoandwithinthebedarehighandthereisagooduniformityoftreatmentoftheparticlestoyieldhighqualityindividuallyquickfrozenproducts.Freezingfoodfastercanincreaseturnoveronanexistingfootprint,reducethefreezingcostandproduceahigherqualityoffrozenfood.Freezingfoodwithanaircyclerefrigerationplanthastwoadvantages;Theaircanreplacetoxic,inflammableorenvironmentallyunfriendlyrefrigerantsandreplaceitwithasafeandreplaceablerefrigerantItiscapableofproducingfreezingtemperaturesfarcolderthanvaporcompressionplantforlessenergyconsumption,sizeandcost.Freezingtemperaturesaslowasthoseproducedbycryogenicrefrigerationarepossiblebutwithoutthehighrunningcostsandenergyconsumptioninherentinsuchsystems.


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CFCfreeheatpumps

Theobjectiveoftheprojectistodevelopheatpumpsystems,tobeusedinexistingaswellasnewbuildings,usingairastheenvironmentallybenignworkingfluidtoimprovetheprimaryenergyratioofheatingandcoolingsystems.Toimprovetheefficiencyofaircyclesystemsthe(isentropic)efficiencyoftherotatingequipment(expandersandcompressors)iscrucial.Highefficiencyequipmentisavailableinotherapplicationfieldssuchaspressurizedairsystemsandenergyrecoverysystems.


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COMFORTCOOLINGSYSTEMS

Theneedforheatingandcoolinginbuildings:Theprimerequirementinrespectoftheindoorclimateinabuildingisthatroomtemperatureshouldbeatacomfortablelevel,regardlessoftheweatherconditionsoutside.Inaddition,theindoorairmustbeacceptablyclean,lightingandacousticconditionsmustbegoodetc.Nevertheless,thefirstandforemostconditionforabuildingtobeusableatalliesthattheindoortemperatureinacceptable.AssoonastheambienttemperatureislowerthantheIndoortemperature,heatflowsoutfromthebuildingthroughitsboundarysurfaces(thebuildingenvelope).Atthesametime,thebuildingalsolosesheatthroughairinfiltration,i.e.theinwardleakageofoutdoorairintothebuildingthroughgapsandcavitiesinwalls,roofs,doorsandwindows.Bearinginmindthefactthattheindoortemperatureinmostbuildingsismaintainedatalittleover20C,thismeans,throughoutmostoftheyear,thebuildingislosingheattoitssurroundings.


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Theinternalheatgenerationincommercialpremisesandsomeindustrialbuildings,ontheotherhand,isoftenrelativelygreat.Incombinationwiththefactthatconstructionstandardshavebeendevelopedandimproved,sothatbuildingsarenowadayswellinsulatedandairtight,thismeansthattheheatlossesthroughthebuildingenvelopearesmall.Ifweconsidernewofficebuildings,departmentstores,hospitalsandsimilarbuildingswithinthecommercialpremisesandindustrialsector,wefindthatheatdeficitsusuallyoccuronlyduringthenightandatweekends,whilethereisnearlyalwaysaheadsurplusduringworkinghours.Suchbuildingsrequireonlysimpleheatingsystemstomeetthemodestheatdeficits,asopposedtotheconsiderablymoreextensivesystemsneededinordertodealwiththesubstantialheatsurpluses,andtopreventtheindoortemperaturebecomingunacceptablyhighduringworkinghours.Ingeneralterms,thegreatertheheatsurplus,andthereforethegreaterthecapacityofthecoolingsystem,themoredifficultitistoproduceanindoorclimatethatisgoodinallrespects.Itisthereforealwaysimportanttoattempttodesignthebuildingingeneralsothattherewillbeonlyalowheatsurplus.

ComfortcoolingThesurplusheatthathastoberemovedfrombuildingsinordertomaintaintheindoortemperaturebelowsomepreviouslydeterminedmaximumpermissibletemperatureisreferredtoasthecoolingrequirement.Inotherwords,thecoolingrequirementofthebuildingisexactlythesameasitsheat.


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Theclimatecontrolsysteminbuildinghastomaintainboththethermalclimateandtheairquality.Maintainingthethermalclimateconsistsprimarilyofkeepingthetemperatureoftheindoorairwithingivenlimits.Maintainingtheairqualityconsistsofcontrollingthecleanlinessoftheindoorairbysupplyingasufficientquantityofoutdoorairtoventilatetheinteriorofthebuilding.MaintenanceofairqualitysometimesalsoincludesensuringthatgivenConcentrationsofparticlesand/orgasesarenotexceeded.Thecoolingsystemmustbeabletodealwithvariationsinthecoolingrequirement,whetheroverthedayorovertheyear.Thetwobasictypesofall-aircoolingsystemsaretheconstantairflowsystemandthevariableairflowsystem,Althoughtherearealsocombinationsofthetwomethods.Theneedforcomfortcoolingarises,therefore,whenrequirementsinrespectofthethermalclimatealsoincluderequirementsinrespectofmaximumpermissibleindoortemperatures.Ingeneral,HVAC(Heating,VentilationandAir-Conditioning)systemsusedinorderactivelytocoolbuildingscanbedividedupintothreemaintypes:_all-aircoolingsystems_all-watercoolingsystems_combinedsystems(Withcoolingsuppliedbothbyairandbywater)


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All-aircoolingsystemsThedesignairflowrateinthesesystems,andthusthenecessarysizesofventilationducts,isdeterminedbythedesigncoolingrequirement.Inotherwords,itisthethermalrequirements,andnottheairqualityrequirements,thatdeterminethenecessaryairflowrate.Inexistingbuildings,itisnormallybothdifficultandexpensivetoreplacetheventilationductsystem.Iftheexistingductscannottransportsufficientlylargeairquantitiestomeetthecoolingrequirements,all-water-coolingsystemswillusuallybeinstalledinconnectionwithConversionormodernization.

Constantairvolumesystems(CAVsystems)Insuchsystems,thetemperatureoftheairsuppliedtothebuildingcanvary,buttheairflowrateiskeptconstant.SuchsystemsarereferredtoasConstantAirVolume(CAV)systems.Itistheroomshavingthegreatestcoolingrequirementthatnormallydeterminethesupplyairtemperaturedeliveredbythecentralairconditioningunit:theairmay,ifnecessary,beheatedbeforesuppliedtootherrooms.AlthoughaCAVsystemsuppliesairataconstantflowrate,thefansaresometimespoweredbytwo-speedmotors,runningatthelowerspeedwhenthebuildingcoolingrequirementfalls.Theairflowrateisthenreducedinproportiontothefanspeed.


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Variableairvolumesystems(VAVsystems)Theairflowratetoeachroomisvariedasnecessary,butwithmaintenanceofaconstantsupplytemperature,i.e.thesupplytemperaturedoesnotchangeeveniftheloadchanges.However,thesupplyairtemperatureisnormallychangedinstepwiththetimeofyear,asafunctionoftheambienttemperature.SystemsofthistypearereferredtoasVariableAirVolume(VAV)systems.Theairflowtoeachindividualroomiscontrolledbydampersinsomeformofbox(VAV-box)intheimmediatevicinityofthesupplypointtotheroom,whilethecentralsupplyandexhaustairfansarecontrolledbyvariableinletvanesorbyadjustablespeeddrivecontrolledmotors,usuallyofthefrequency-invertertype.Thecontrolsystemnormallymaintainsaconstantstaticpressureinthesupplyairduct.Theflowratevariesfromamaximum,duringthehottestdays,downtoperhaps20%ofmaximumflowrateduringthecoldestdaysoftheyear,whenthepurposeoftheairisonlytomaintaintheairquality.


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All-watercoolingsystemsSystemsofthistypesupplyall-watercoolingtotheindividualrooms,withtheventilationsystemdesignedpurelytomaintaintheairquality.Systemsofthistypeareoftenchoseninconnectionwithconversionorrenovationprojects.Thereisusuallyspaceabovethefalseceilingstoinstallthewaterpipesneededfordistributionofcoldwaterthroughoutthebuilding.

CombinedsystemsAll-airandall-watercoolingsystemscanbecombinedinmanyways.Onesuchneedforacombinedsystemisifall-aircoolingisused,butthecoolingrequirementissogreatthatanall-aircoolingsystemaloneisnotcapableofdealingwithitsatisfactorily,assuchhighairflowrateswouldberequiredthatdraughtswouldbeunavoidable.Itisalsopossibletocombineall-aircoolingsystemssothatcertainpartsofthebuilding,orcertainrooms,arecooledbyaVAVsystem,whileotherpartsofthebuildingarecooledbyaCAVsystem.


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CoolingsupplydevicesCoolingcanbesuppliedtoaroominanumberofdifferentways.Thefollowingarebriefdescriptionsofhowchilledbeams,coolingpanels,fancoilunitsandinductionunitsoperate.Fancoilunitsandinductionunitsarenormallypositionedbelowwindowsintheoutsidewalls.ChilledbeamsTheseareunitswhich,bynaturalconvectionfromafinnedheatexchanger,cooltheairintheroom.Theymayalsobecombinedwiththesupplyairterminaldeviceinordertoprovidebothfunctionsand,inmanycases,toincreasethecoolingcapacityofthebaffle.Somechilledbeamscanalsoincorporateaheatingfunction.CoolingpanelsCoolingpanelscanbehungfromtheceiling.Coldwaterflowsthroughanaluminiumplate,whichtransfersheatfromtheairtothecoldwater.Thepanelcoolsthewarmroomairandalsocoolstheroomsurfacesbylow-temperatureradiation.Thesepanelsareproducedinanumberofversions,e.g.formountingflatagainsttheceiling,hanging,orforintegrationinafalseceiling.Mostoftheircoolingcapacityisprovidedbyradiation.FancoilunitsTheseareunitsbywhichbothheatingandcoolingcanbesuppliedtoaroom(althoughnotatthesametime).Afancoilunitincorporatesafanwhichcirculatestheroomairthroughtheunit,inwhichtheairiseitherheatedorcooledasrequired.Thetwoheatexchangers(heatingandcooling)aresuppliedwithhotorcoldwaterfromacentralunitinthebuilding.Thistypeofroomcoolerunitcanmeetthehighestcoolingrequirements,butitalsohasthehighestnoiselevel.InductionunitsTheseareunitsbywhichbothheatingandcoolingcanbesuppliedtoaroom.Wheninuse,theventilationairfortheroomissuppliedthroughtheinductionunit.Itflowsthroughanozzlewithhighvelocity,whichthereforehastheeffectofinducingairfromtheroomthroughtheheatingorcoolingheatexch

angers.


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AIRCONDITIONINGThetermairconditioningmostcommonlyreferstothecoolinganddehumidificationofindoorairforthermalcomfort.Inabroadersense,thetermcanrefertoanyformofcooling,heating,ventilationordisinfectionthatmodifiestheconditionofair.[1]Anairconditioner(ACorA/CinNorthAmericanEnglish,airconinBritishandAustralianEnglish)isanappliance,system,ormechanismdesignedtostabilizetheairtemperatureandhumiditywithinanarea(usedforcoolingaswellasheatingdependingontheairpropertiesatagiventime),typicallyusingarefrigerationcyclebutsometimesusingevaporation,mostcommonlyforcomfortcoolinginbuildingsandtransportationvehicles.TheconceptofairconditioningisknowntohavebeenappliedinAncientRome,whereaqueductwaterwascirculatedthroughthewallsofcertainhousestocoolthem.SimilartechniquesinmedievalPersiainvolvedtheuseofcisternsandwindtowerstocoolbuildingsduringthehotseason.Modernairconditioningemergedfromadvancesinchemistryduringthe19thcentury,andthefirstlarge-scaleelectricalairconditioningwasinventedandusedin1902byWillisHavilandCarrier.

Airconditioningapplications:Airconditioningengineersbroadlydivideairconditioningapplicationsintocomfortandprocess.Comfortapplicationsaimtoprovideabuildingindoorenvironmentthatremainsrelativelyconstantinarangepreferredbyhumansdespitechangesinexternalweatherconditionsorininternalheatloads.Thehighestperformancefortasksperformedbypeopleseatedinanofficeisexpectedtooccurat72F(22C)Performanceisexpectedtodegradeabout1%forevery2Fchangeinroomtemperature.[6]Thehighestperformancefortas

ksperformedwhilestandingisexpectedtooccuratslightlylowertemperatures.Thehighestperformancefortasksperformedbylargerpeopleisexpectedtooccuratslightlylowertemperatures.Thehighestperformancefortasksperformedbysmallerpeopleisexpectedtooccuratslightlyhighertemperatures.Although


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generallyaccepted,somedisputethatthermalcomfortenhancesworkerproductivity,asisdescribedintheHawthorneeffect.Comfortairconditioningmakesdeepplanbuildingsfeasible.Withoutairconditioning,buildingsmustbebuiltnarrowerorwithlightwellssothatinnerspacesreceivesufficientoutdoorairvianaturalventilation.Airconditioningalsoallowsbuildingstobetallersincewindspeedincreasessignificantlywithaltitudemakingnaturalventilationimpracticalforverytallbuildings.Comfortapplicationsforvariousbuildingtypesarequitedifferentandmaybecategorizedas:Low-RiseResidentialbuildings,includingsinglefamilyhouses,duplexes,andsmallapartmentbuildingsHigh-RiseResidentialbuildings,suchastalldormitoriesandapartmentblocksCommercialbuildings,whicharebuiltforcommerce,includingoffices,malls,shoppingcenters,restaurants,etc.Institutionalbuildings,whichincludeshospitals,governmental,academic,andsoon.Industrialspaceswherethermalcomfortofworkersisdesired.Inadditiontobuildings,airconditioningcanbeusedforcomfortinawidevarietyoftransportationincludinglandvehicles,trains,ships,aircraft,andspacecraft.

Processapplicationsaimtoprovideasuitableenvironmentforaprocessbeingcarriedout,regardlessofinternalheatandhumidityloadsandexternalweatherconditions.Althoughofteninthecomfortrange,itistheneedsoftheprocessthatdetermineconditions,nothumanpreference.Processapplicationsincludethese:Hospitaloperatingtheatres,inwhichairisfilteredtohighlevelstoreduceinfectionriskandthehumiditycontrolledtolimitpatientdehydration.Althoughtemperaturesareofteninthecomfortrange,somespecialistproceduressu

chasopenheartsurgeryrequirelowtemperatures(about18C,64F)andotherssuchasneonatalrelativelyhightemperatures(about28C,82F).


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Cleanroomsfortheproductionofintegratedcircuits,pharmaceuticals,andthelike,inwhichveryhighlevelsofaircleanlinessandcontroloftemperatureandhumidityarerequiredforthesuccessoftheprocess.Facilitiesforbreedinglaboratoryanimals.Sincemanyanimalsnormallyonlyreproduceinspring,holdingtheminroomsatwhichconditionsmirrorspringallyearcancausethemtoreproduceyearround.Aircraftairconditioning.Althoughnominallyaimedatprovidingcomfortforpassengersandcoolingofequipment,aircraftairconditioningpresentsaspecialprocessbecauseofthelowairpressureoutsidetheaircraft.DataprocessingcentersTextilefactoriesPhysicaltestingfacilitiesPlantsandfarmgrowingareasNuclearfacilitiesChemicalandbiologicallaboratoriesMinesIndustrialenvironmentsFoodcookingandprocessingareasInbothcomfortandprocessapplicationstheobjectivemaybetonotonlycontroltemperature,butalsohumidity,airquality,airmotion,andairmovementfromspacetospace.HumiditycontrolRefrigerationairconditioningequipmentusuallyreducesthehumidityoftheairprocessedbythesystem.Therelativelycold(belowthedewpoint)evaporatorcoilcondenseswatervaporfromtheprocessedair,(muchlikeanicecolddrinkwillcondensewaterontheoutsideofaglass),sendingthewatertoadrainandremovingwatervaporfromthecooledspaceandloweringtherelativehumidity.Sincehumansperspiretoprovidenaturalcoolingbytheevaporationofperspirationfromtheskin,drierair(uptoapoint)improvesthecomfortprovided.Thecomfortairconditionerisdesignedtocreatea40%to60%relativehumidityintheoccupiedspace.Infoodretailingestablishmentslargeopenchillercabinetsactashighlyeffectiveairdehumidifyingunits.


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Someairconditioningunitsdrytheairwithoutcoolingit,andarebetterclassifiedasdehumidifiers.Theyworklikeanormalairconditioner,exceptthataheatexchangerisplacedbetweentheintakeandexhaust.Incombinationwithconvectionfanstheyachieveasimilarlevelofcomfortasanaircoolerinhumidtropicalclimates,butonlyconsumeaboutathirdoftheelectricity.Theyarealsopreferredbythosewhofindthedraftcreatedbyaircoolersdiscomforting.

EnergyuseItshouldbenotedthatinathermodynamicallyclosedsystem,anyenergyinputintothesystemthatisbeingmaintainedatasettemperature(whichisastandardmodeofoperationformodernairconditioners)requiresthattheenergyremovalratefromtheairconditionerincrease.Thisincreasehastheeffectthatforeachunitofenergyinputintothesystem(saytopoweralightbulbintheclosedsystem)requirestheairconditionertoremovethatenergy.Inordertodothattheairconditionermustincreaseitsconsumptionbytheinverseofitsefficiencytimestheinputunitofenergy.Asanexamplepresumethatinsidetheclosedsystema100wattlightbulbisactivated,andtheairconditionerhasanefficiencyof200%.Theairconditionersenergyconsumptionwillincreaseby50wattstocompensateforthis,thusmakingthe100Wlightbulbsutilizeatotalof150Wofenergy.

PortableairconditionersAportableairconditionerorportableA/Cisanairconditioneronwheelsthatcanbeeasilytransportedinsideahomeoroffice.Theyarecurrentlyavailablewithcapacitiesofabout6,000to60,000BTU/h(1800to18000wattsoutput)andwithandwithoutelectricresistanceheaters.Portabl

eairconditionerscomeinthreeforms,split,andhoseandevaporative:Asplitsystemhasanindoorunitonwheelsconnectedtoanoutdoorunitviaflexiblepipes,similartoapermanentlyfixedinstalledunit.HosesystemsAir-to-AirandMonoblockareventedtotheoutsideviaairducts.Afunctionofallcoolingthatuseacompressor,istocreatewaterasitcoolstheair.


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The"monoblock"versioncollectsthewaterinabucketortrayandstopswhenfull.TheAir-to-Airversionre-evaporatesthewateranddischargesitthroughtheductedhoseandcanhenceruncontinuously.Asingleductunitdrawsairoutoftheroomtocoolitscondenser.Thisairisthenreplacedbyhotairfromoutsideorotherrooms,thusreducingefficiency.However,modernunitsrunonapproximately1to3ratioi.e.,toproduce3kWofcoolingthiswilluse1kWofelectricity.Aircooledportableairconditionersarecompressor-basedrefrigerantsystemthatusesairtoexchangeheat,similartoacarortypicalhouseholdairconditioner.Withthistypeofsystemtheairisdehumidifiedasitiscooled.Evaporativeairconditionersdonothaveacompressororcondenser.Instead,liquidwaterispouredinandreleasedasvapor.Becausetheydonothaveacondenserwhichneedscooling,theydonotneedhosesorpipes,allowingthemtobetrulyportable.Asaruleofthumb,400squarefeet(37m)canbecooledper12,000BTU/h(3.5kWoronetonofairconditioning)byarefrigerativeairconditioner.However,otherfactorswillaffectthetotalheatload.Evaporativeairconditionersusemuchlessenergy.

TypesofairconditionerequipmentWindowandthrough-wallunitsManytraditionalairconditionersinhomesorotherbuildingsaresinglerectangularunitsusedtocoolanapartment,ahouseorpartofit,orpartofabuilding.Foranexample,seethephotostotheright.HotelsfrequentlyusePTACsystems,whichcombineheatingintothesameunit.Airconditionerunitsneedtohaveaccesstothespacetheyarecooling(theinside)andaheatsink;normallyoutsideairisusedtocoolthecondensersection.Forthisreason,singleunitairconditioners

areplacedinwindowsorthroughopeningsinawallmadefortheairconditioner;thelattertypeincludesportableairconditioners.Windowandthrough-wallunitshaveventsonboththeinsideandoutside,soinsideairtobecooledcanbeblowninandoutbyafanintheunit,andoutsideaircanalsobeblowninandoutbyanotherfantoactastheheatsink.Thecontrolsareontheinside.


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EvaporationcoolersInverydryclimates,so-called"swampcoolers"arepopularforimprovingcomfortduringhotweather.ThistypeofcooleristhedominantcoolerusedinIranwhichhasthelargestnumberofunitsthananywhereelseintheworld,hencesomereferringto"swampcoolers"asPersiancoolers.Anevaporativecoolerisadevicethatdrawsoutsideairthroughawetpad,suchasalargespongesoakedwithwater.Thesensibleheatoftheincomingair,asmeasuredbyadrybulbthermometer,isreduced.Thetotalheat(sensibleheatpluslatentheat)oftheenteringairisunchanged.Someofthesensibleheatoftheenteringairisconvertedtolatentheatbytheevaporationofwaterinthewetcoolerpads.Iftheenteringairisdryenough,theresultscanbequitecomfortable.Thesecoolerscostlessandaremechanicallysimpletounderstandandmaintain.Thereisarelated,morecomplexprocesscalledabsorptiverefrigerationwhichusesheattoproducecooling.Inoneinstance,athree-stageabsorptivecoolerfirstdehumidifiestheairwithasprayofsalt-waterorbrine.Thebrineosmoticallyabsorbswatervaporfromtheair.Thesecondstagesprayswaterintheair,coolingtheairbyevaporation.Finally,tocontrolthehumidity,theairpassesthroughanotherbrinespray.Thebrineisreconcentratedbydistillation.Thesystemisusedinsomehospitalsbecause,withfiltering,asufficientlyhotregenerativedistillationremovesairborneorganisms.

AbsorptivechillersSomebuildingsusegasturbinestogenerateelectricity.Theexhaustsofthesearehotenoughtodriveanabsorptivechillerthatproducescoldwater.Thecoldwateristhenrunthroughradiatorsinairductsforhydroniccooling.Thedualuseoftheenergy,bothtogenerateelectricityandcooling,

makesthistechnologyattractivewhenregionalutilityandfuelpricesareright.Producingheat,power,andcoolinginonesystemisknownastrigeneration.


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CentralairconditioningCentralairconditioning,commonlyreferredtoascentralair(US)orair-con(UK),isanairconditioningsystemwhichusesductstodistributecooledand/ordehumidifiedairtomorethanoneroom,orusespipestodistributechilledwatertoheatexchangersinmorethanoneroom,andwhichisnotpluggedintoastandardelectricaloutlet.Withatypicalsplitsystem,thecondenserandcompressorarelocatedinanoutdoorunit;theevaporatorismountedintheairhandlingunit(whichisoftenaforcedairfurnace).Withapackagesystem,allcomponentsarelocatedinasingleoutdoorunitthatmaybelocatedonthegroundorroof.Centralairconditioningperformslikearegularairconditionerbuthasseveraladdedbenefits:Whentheairhandlingunitturnson,roomairisdrawninfromvariouspartsofthehousethroughreturn-airducts.Thisairispulledthroughafilterwhereairborneparticlessuchasdustandlintareremoved.Sophisticatedfiltersmayremovemicroscopicpollutantsaswell.Thefilteredairisroutedtoairsupplyductworkthatcarriesitbacktorooms.Whenevertheairconditionerisrunning,thiscyclerepeatscontinually.Becausethecentralairconditioningunitislocatedoutsidethehome,itoffersalowerlevelofnoiseindoorsthanafree-standingairconditioningunit.


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ThermostatsThermostatscontroltheoperationofHVACsystems,turningontheheatingorcoolingsystemstobringthebuildingtothesettemperature.Typicallytheheatingandcoolingsystemshaveseparatecontrolsystems(eventhoughtheymayshareathermostat)sothatthetemperatureisonlycontrolled"one-way".Thatis,inwinter,abuildingthatistoohotwillnotbecooledbythethermostat.Thermostatsmayalsobeincorporatedintofacilityenergymanagementsystemsinwhichthepowerutilitycustomermaycontroltheoverallenergyexpenditure.Inaddition,agrowingnumberofpowerutilitieshavemadeavailableadevicewhich,whenprofessionallyinstalled,willcontrolorlimitthepowertoanHVACsystemduringpeakusetimesinordertoavoidnecessitatingtheuseofrollingblackouts.EquipmentcapacityAirconditionerequipmentpowerintheU.S.isoftendescribedintermsof"tonsofrefrigeration".A"tonofrefrigeration"isdefinedasthecoolingpowerofoneshortton(2000poundsor907kilograms)oficemeltingina24-hourperiod.Thisisequalto12,000BTUperhour,or3517watts(http://physics.nist.gov/Pubs/SP811/appenB9.html).Residential"centralair"systemsareusuallyfrom1to5tons(3to20kW)incapacity.Theuseofelectric/compressiveairconditioningputsamajordemandonthenation'selectricalpowergridinwarmweather,whenmostunitsareoperatingunderheavyload.HealthimplicationsAirconditioninghasnogreaterinfluenceonhealththanheatingthatistosay,verylittlealthoughpoorlymaintainedair-conditioningsystems(especiallylarge,centralizedsystems)canoccasionallypromotethegrowthandspreadofmicroorganisms,suchasLegionellapneumophila,theinfectiousagentresponsibleforLegionnaire'sdisease,orthermophilicactinomycetes.Conversely,airconditioning(includingfiltration,humidification,cooling,disinfection,etc.)

canbeusedtoprovideaclean,safe,hypoallergenicatmosphereinhospitaloperatingroomsandotherenvironmentswhereanappropriateatmosphereiscriticaltopatientsafetyandwell-being.Airconditioningcanhaveapositiveeffectonsufferersofallergiesandasthma.


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Inseriousheatwaves,airconditioningcansavethelivesoftheelderly.Somelocalauthoritiesevensetuppubliccoolingcentersforthebenefitofthosewithoutairconditioningathome.Properlymaintainedair-conditioningsystemsdonotcauseorpromoteillness,despitesuperstitionsthatair-conditioningisunconditionallydangeroustoone'shealth.Aswithheatingsystems,theadvantagesofairconditioninggenerallyfaroutweighthedisadvantages.

Theinternalsectionofthesameunit.

AmodernAmericoolwindowairconditionerinternalsection

ExternalsectionofatypicalACAirConditioningUnits


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HowAirConditionersworkAirconditionersandrefrigeratorsworkthesameway.Insteadofcoolingjustthesmall,insulatedspaceinsideofarefrigerator,anairconditionercoolsaroom,awholehouse,oranentirebusiness.Airconditionersusechemicalsthateasilyconvertfromagastoaliquidandbackagain.Thischemicalisusedtotransferheatfromtheairinsideofahometotheoutsideair.Themachinehasthreemainparts.Theyareacompressor,acondenserandanevaporator.Thecompressorandcondenserareusuallylocatedontheoutsideairportionoftheairconditioner.Theevaporatorislocatedontheinsidethehouse,sometimesaspartofafurnace.Theworkingfluidarrivesatthecompressorasacool,low-pressuregascalledFreon.Thecompressorsqueezesthefluid.Thispacksthemoleculeofthefluidclosertogether.Thecloserthemoleculesaretogether,thehigheritsenergyanditstemperature.Theworkingfluidleavesthecompressorasahot,highpressuregasandflowsintothecondenser.Ifyoulookedattheairconditionerpartoutsideahouse,lookforthepartthathasmetalfinsallaround.Thefinsactjustlikearadiatorinacarandhelptheheatgoaway,ordissipate,morequickly.Whentheworkingfluidleavesthecondenser,itstemperatureismuchcoolerandithaschangedfromagastoaliquidunderhighpressure.Theliquidgoesintotheevaporatorthroughaverytiny,narrowhole.Ontheotherside,theliquid'spressuredrops.Whenitdoesitbeginstoevaporateintoagas.Astheliquidchangestogasandevaporates,itextractsheatfromtheairaroundit.Theheatintheairisneededtoseparatethemoleculesofthefluidfromaliquidtoagas.Theevaporatoralsohasmetalfinstohelpinexchangethethermalenergywiththesurroundingair.


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Bythetimetheworkingfluidleavestheevaporator,itisacool,lowpressuregas.Itthenreturnstothecompressortobeginitstripalloveragain.Connectedtotheevaporatorisafanthatcirculatestheairinsidethehousetoblowacrosstheevaporatorfins.Hotairislighterthancoldair,sothehotairintheroomrisestothetopofaroom.Thereisaventtherewhereairissuckedintotheairconditionerandgoesdownducts.Thehotairisusedtocoolthegasintheevaporator.Astheheatisremovedfromtheair,theairiscooled.Itisthenblownintothehousethroughotherductsusuallyatthefloorlevel.Thiscontinuesoverandoverandoveruntiltheroomreachesthetemperatureyouwanttheroomcooledto.Thethermostatsensesthatthetemperaturehasreachedtherightsettingandturnsofftheairconditioner.Astheroomwarmsup,thethermostatturnstheairconditionerbackonuntiltheroomreachesthetemperature.


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A-ExpansionValveB-CompressorSchematicdiagramofanair-conditioner

WindowACUnitsAwindowairconditionerunitimplementsacompleteairconditionerinasmallspace.Theunitsaremadesmallenoughtofitintoastandardwindowframe.Itcontains:AcompressorAnexpansionvalveAhotcoil(ontheoutside)Achilledcoil(ontheinside)TwofansAcontrolunit.


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Thefansblowairoverthecoilstoimprovetheirabilitytodissipateheat(totheoutsideair)andcold(totheroombeingcooled).

BTUandEERMostairconditionershavetheircapacityratedinBritishthermalunits(BTU).Generallyspeaking,aBTUistheamountofheatrequiredtoraisethetemperatureofonepound(0.45kg)ofwater1degreeFahrenheit(0.56degreesCelsius).Specifically,1BTUequals1,055joules.Inheatingandcoolingterms,1"ton"equals12,000BTU.Atypicalwindowairconditionermightberatedat10,000BTU.Forcomparison,atypical2,000-square-foot(185.8m2)housemighthavea5-ton(60,000-BTU)airconditioningsystem,implyingthatyoumightneedperhaps30BTUpersquarefoot.(Keepinmindthattheseareroughestimates.Tosizeanairconditionerforyourspecificneeds,contactanHVACcontractor.)Theenergyefficiencyrating(EER)ofanairconditionerisitsBTUratingoveritswattage.Forexample,ifa10,000-BTUairconditionerconsumes1,200watts,itsEERis8.3(10,000BTU/1,200watts).Obviously,youwouldliketheEERtobeashighaspossible,butnormallyahigherEERisaccompaniedbyahigherprice.Let'ssaythatyouhaveachoicebetweentwo10,000-BTUunits.OnehasanEERof8.3andconsumes1,200watts,andtheotherhasanEERof10andconsumes1,000watts.Let'salsosaythatthepricedifferenceis$100.Tounderstandwhatthepaybackperiodisonthemoreexpensiveunit,youneedtoknow:ApproximatelyhowmanyhoursperyearyouwillbeoperatingtheunitHowmuchakilowatt-hour(kWh)costsinyourareaLet'ssaythatyouplantousetheairconditionerinthesummer(fourmonthsayear)anditwillbeoperatingaboutsixhoursaday.Let'salsoimaginethatthecostinyourareais$0.10/kWh.Thedifferenceinenerg

yconsumptionbetweenthetwounitsis200watts,whichmeansthateveryfivehoursthelessexpensiveunitwillconsume1additionalkWh(andtherefore$0.10more)thanthemoreexpensiveunit.Assumingthatthereare30daysinamonth,youfindthatduringthesummeryouareoperatingtheairconditioner:4mo.x30days/mo.x6hr/day=720hours[(720hrsx200watts)/(1000watts/kW)]x$0.10/kWh=$14.40


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Split-systemACUnitsAsplit-systemairconditionersplitsthehotsidefromthecoldsideofthesystem,likethis:

Thecoldside,consistingoftheexpansionvalveandthecoldcoil,isgenerallyplacedintoafurnaceorsomeotherairhandler.Theairhandlerblowsairthroughthecoilandroutestheairthroughoutthebuildingusingaseriesofducts.Thehotside,knownasthecondensingunit,livesoutsidethebuilding.Theunitconsistsofalong,spiralcoilshapedlikeacylinder.Insidethecoilisafan,toblowairthroughthecoil,alongwithaweather-resistantcompressorandsomecontrollogic.Thisapproachhasevolvedovertheyearsbecauseitislow-cost,andalsobecauseitnormallyresultsinreducednoiseinsidethehouse(attheexpenseofincreasednoiseoutsidethehouse).Besidesthefactthatthehotandcoldsidesaresplitapartandthecapacityishigher(makingthecoilsandcompressorlarger),thereisnodifferencebetweenasplit-systemandawindowairconditioner.Inwarehouses,businesses,malls,largedepartmentstoresandthelike,thecondensingunitnormallylivesontheroofandcanbequitemassive.Alternatively,theremaybemanysmallerunitsontheroof,eachattachedinsidetoasmallairhandlerthatcoolsaspecificzoneinthebuilding.Inlargerbuildingsandparticularlyinmulti-storybuildings,thesplit-systemapproachbeginstorunintoproblems.Eitherrunningthepipebetweenthecondenserandtheairhandlerexceedsdistancelimitations(runsthataretoolongstarttocauselubricationdifficultiesinthecompressor),ortheamountofductworkandthelengthofductsbecomesunmanageable.


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Chilled-waterandCooling-towerACUnits

Inachilled-watersystem,theentireairconditionerlivesontherooforbehindthebuilding.Itcoolswatertobetween40and45F(4.4and7.2C).Thischilledwateristhenpipedthroughoutthebuildingandconnectedtoairhandlersasneeded.Thereisnopracticallimittothelengthofachilled-waterpipeifitiswell-insulated.

A-ExpansionvalveB-CompressorC-HeatExchangerD-ChilledwatertothebuildingCoolingTowersInallofthesystemsdescribedearlier,airisusedtodissipatetheheatfromtheoutsidecoil.Inlargesystems,theefficiencycanbeimprovedsignificantlybyusingacoolingtower.Thecoolingtowercreatesastreamoflowertemperaturewater.Thiswaterrunsthroughaheatexchangerandcoolsthehotcoilsoftheairconditionerunit.Itcostsmoretobuythesysteminitially,buttheenergysavingscanbesignificantovertime(especiallyinareaswithlowhumidity),sothesystempaysforitselffairlyquickly.Coolingtowerscomeinallshapesandsizes.Theyallworkonthesameprinciple:Generally,thewatertricklesthroughathicksheetofopenplasticmesh.Airblowsthroughthemeshatrightanglestothewaterflow.Theevaporationcoolsthestreamofwater.Becausesomeofthewaterislosttoevaporation,thecoolingtowerconstantlyaddswatertothesystemtomakeupthedifference.


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Air-DistributionSystemsTherearevarioustypesofair-distributionsystems,likefans,filters,ductwork,outlets,dampersetc.

HVACHVACisaninitialism/acronymthatstandsfor"heating,ventilating,andairconditioning".HVACissometimesreferredtoas"climatecontrol"andisparticularlyimportantinthedesignofmediumtolargeindustrialandofficebuildingssuchasskyscrapersandinmarineenvironmentssuchasaquariums,wherehumidityandtemperaturemustallbecloselyregulatedwhilstmaintainingsafeandhealthyconditionswithin.Heating,ventilating,andairconditioningisbasedonthebasicprinciplesofthermodynamics,fluidmechanics,andheattransferTheinventionofthecomponentsofHVACsystemsgoeshand-in-handwiththeindustrialrevolution,andnewmethodsofmodernization,higherefficiency,andsystemcontrolareconstantlyintroducedbycompaniesandinventorsallovertheworld.Thethreefunctionsofheating,ventilating,andair-conditioningarecloselyinterrelated.Allseektoprovidethermalcomfort,acceptableindoorairquality,andreasonableinstallation,operation,andmaintenancecosts.HVACsystemscanprovideventilation,reduceairinfiltration,andmaintainpressurerelationshipsbetweenspaces.Howairisdeliveredto,andremovedfromspacesisknownasroomairdistribution.Inmodernbuildingsthedesign,installation,andcontrolsystemsofthesefunctionsareintegratedintooneormoreHVACsystems.Forverysmallbuildings,contractorsnormally"size"andselectHVACsystemsandequipment.Forlargerbuildingswhererequiredbylaw,"buildingservices"designersandengineers,suchasmechanical,architectural,orbuildingservicesengineersanaly

ze,design,andspecifytheHVACsystems,andspecialtymechanicalcontractorsbuildandcommissionthem.Inallbuildings,buildingpermitsfor,andcode-complianceinspectionsoftheinstallationsarethenorm.


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HVACsystemsuseventilationairductsinstalledthroughoutabuildingthatsupplyconditionedairtoaroomthroughrectangularorroundoutletvents,called"diffusers";andductsthatremoveairthroughreturn-air"grilles.

HeatingHeatingsystemsmaybeclassifiedascentralorlocal.Centralheatingisoftenusedincoldclimatestoheatprivatehousesandpublicbuildings.Suchasystemcontainsaboiler,furnace,orheatpumptoheatwater,steam,orair,allinacentrallocationsuchasafurnaceroominahomeoramechanicalroominalargebuilding.Thesystemalsocontainspipingorductworktodistributetheheatedfluid,andradiatorstotransferthisheattotheair.Thetermradiatorinthiscontextismisleadingsincemostheattransferfromtheheatexchangerisbyconvection,notradiation.Theradiatorsmaybemountedonwallsorburiedinthefloortogiveunder-floorheat.Inboilerfedorradiantheatingsystems,allbutthesimplestsystemshaveapumptocirculatethewaterandensureanequalsupplyofheattoalltheradiators.Theheatedwatercanalsobefedthroughanotherheatexchangerinsideastoragecylindertoprovidehotrunningwater.Forcedairsystemssendheatedairthroughductwork.Duringwarmweatherthesameductworkcanbereusedforairconditioning.Theforcedaircanalsobefilteredorputthroughaircleaners.Mostductscannotfitahumanbeing(astheydoinmanyfilms)sincethiswouldrequireagreaterduct-structuralintegrityandcreateapotentialsecurityliability.Heatingcanalsobeprovidedfromelectric,orresistanceheatingusingafilamentthatglowshotwhenyoucauseelectricitytopassthroughit.Thistypeofheatcan


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befoundinelectricbaseboardheaters,portableelectricheaters,andasbackuporsupplementalheatingforheatpump(orreverseheating)system.Theheatingelements(radiatorsorvents)shouldbelocatedinthecoldestpartoftheroomandtypicallynexttothewindowstominimizecondensation.Popularretaildevicesthatdirectventsawayfromwindowstoprevent"wasted"heatdefeatthisdesignparameter.Draftscontributemoretothesubjectivefeelingofcoldnessthanactualroomtemperature.Therefore,ratherthanimprovingtheheatingofaroom/building,itisoftenmoreimportanttocontroltheairleaks.TheinventionofcentralheatingisoftencreditedtotheancientRomans,whoinstalledasystemofairductscalled"hypocaust"inthewallsandfloorsofpublicbathsandprivatevillas.Theductswerefedwithhotairfromacentralfire.Generally,theseheatedbyradiation;abetterphysiologicapproachtoheatingthanconventionalforcedairconvectiveheating.

VentilatingVentilatingistheprocessof"changing"orreplacingofairinanyspacetoremovemoisture,odors,smoke,heat,dustandairbornebacteria.Ventilationincludesboththeexchangeofairtotheoutsideaswellascirculationofairwithinthebuilding.Itisoneofthemostimportantfactorsformaintainingacceptableindoorairqualityinbuildings.Methodsforventilatingabuildingmaybedividedintomechanical/forcedandnaturaltypes.Ventilationisusedtoremoveunpleasantsmellsandexcessivemoisture,introduceoutsideair,andtokeepinteriorbuildingaircirculating,topreventstagnationoftheinteriorair.

Air-Handlingunit


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HVACSystemsDesignandSafetyHeating,ventilatingandair-conditioning(HVAC)systemscanplayseveralrolestoreducetheenvironmentalimpactofbuildings.TheprimaryfunctionofHVACsystemsistoprovidehealthyandcomfortableinteriorconditionsforoccupants.Well-designed,efficientsystemsdothiswithminimalnon-renewableenergyandairandwaterpollutantemissions.Coolingequipmentthatavoidschlorofluorocarbonsandhydrochlorofluorocarbons(CFCsandHCFCs)mayeliminateamajorcauseofdamagetotheozonelayer.However,eventhebestHVACequipmentandsystemscannotcompensateforabuildingdesignwithinherentlyhighcoolingandheatingneeds.Thegreatestopportunitiestoconservenon-renewableenergyarethrougharchitecturaldesignthatcontrolssolargain,whiletakingadvantageofpassiveheating,daylighting,naturalventilationandcoolingopportunities.Thecriticalfactorsinmechanicalsystems'energyconsumption-andcapitalcost-arereducingthecoolingandheatingloadstheymusthandle.

AirChangeperHour(ACH)Thenumberoftimesperhourthatthevolumeofaspecificroomorbuildingissuppliedorremovedfromthatspacebymechanicalandnaturalventilation.Airhandler,orairhandlingunit(AHU)Centralunitconsistingofablower,heatingandcoolingelements,filterracksorchamber,dampers,humidifier,andothercentralequipmentindirectcontactwiththeairflow.Thisdoesnotincludetheductworkthroughthebuilding.Britishthermalunit(BTU)Anyofseveralunitsofenergy(heat)intheHVACindustry,eachslightlymorethan1kJ.OneBTUistheenergyrequiredtoraiseonepoundofwateronedegreeFahrenheit,butthemanydifferenttypesofBTUarebasedondifferentinterpre

tationsofthisdefinition.IntheUnitedStatesthepowerofHVACsystems(therateofcoolinganddehumidifyingorheating)issometimesexpressedinBTU/hourinsteadofwatts.


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ChillerAdevicethatremovesheatfromaliquidviaavapor-compressionorabsorptionrefrigerationcycle.Thiscooledliquidflowsthroughpipesinabuildingandpassesthroughcoilsinairhandlers,fan-coilunits,orothersystems,coolingandusuallydehumidifyingtheairinthebuilding.Chillersareoftwotypes;air-cooledorwater-cooled.Air-cooledchillersareusuallyoutsideandconsistofcondensercoilscooledbyfan-drivenair.Water-cooledchillersareusuallyinsideabuilding,andheatfromthesechillersiscarriedbyrecirculatingwatertooutdoorcoolingtowers.ControllerAdevicethatcontrolstheoperationofpartorallofasystem.Itmaysimplyturnadeviceonandoff,oritmaymoresubtlymodulateburners,compressors,pumps,valves,fans,dampers,andthelike.Mostcontrollersareautomaticbuthaveuserinputsuchastemperaturesetpoints,e.g.athermostat.Controlsmaybeanalog,ordigital,orpneumatic,oracombinationofthese.Fan-coilunit(FCU)Asmallterminalunitthatisoftencomposedofonlyablowerandaheatingand/orcoolingcoil(heatexchanger),asisoftenusedinhotels,condominiums,orapartments.CondenserAcomponentinthebasicrefrigerationcyclethatejectsorremovesheatfromthesystem.Thecondenseristhehotsideofanairconditionerorheatpump.Condensersareheatexchangers,andcantransferheattoairortoanintermediatefluid(suchaswateroranaqueoussolutionofethyleneglycol)tocarryheattoadistantsink,suchasground(earthsink),abodyofwater,orair(aswithcoolingtowers).Constantairvolume(CAV)Asystemdesignedtoprovideaconstantairvolumeperunittime.ThistermisappliedtoHVACsystemsthathavevariablesupply-airtemperaturebutconstantairflowrates.Mostresidentialforced-airsystemsaresmallCAVsystemswithon/offcontrol.


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DamperAplateorgateplacedinaducttocontrolairflowbyintroducingaconstrictionintheduct.EvaporatorAcomponentinthebasicrefrigerationcyclethatabsorbsoraddsheattothesystem.Evaporatorscanbeusedtoabsorbheatfromair(byreducingtemperatureandbyremovingwater)orfromaliquid.Theevaporatoristhecoldsideofanairconditionerorheatpump.FurnaceAcomponentofanHVACsystemthataddsheattoairoranintermediatefluidbyburningfuel(naturalgas,oil,propane,butane,orotherflammablesubstances)inaheatexchanger.Freshairintake(FAI)Anopeningthroughwhichoutsideairisdrawnintothebuilding.Thismaybetoreplaceairinthebuildingthathasbeenexhaustedbytheventilationsystem,ortoprovidefreshairforcombustionoffuel.GrilleAfacingacrossaductopening,usuallyrectangularisshape,containingmultipleparallelslotsthroughwhichairmaybedeliveredorwithdrawnfromaventilatedspace.Heatload,heatloss,orheatgainTermsfortheamountofheating(heatloss)orcooling(heatgain)neededtomaintaindesiredtemperaturesandhumiditiesincontrolledair.Regardlessofhowwell-insulatedandsealedabuildingis,buildingsgainheatfromwarmairorsunlightorloseheattocoldairandbyradiation.EngineersuseaheatloadcalculationtodeterminetheHVACneedsofthespacebeingcooledorheated.LouversBlades,sometimesadjustable,placedinductsorductentriestocontrolthevolumeofairflow.Thetermmayalsorefertobladesinarectangularframeplacedindoorsorwallstopermitthemovementofair.


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Makeupairunit(MAU)Anairhandlerthatconditions100%outsideair.MAUsaretypicallyusedinindustrialorcommercialsettings,orinonce-through(blowersectionsthatonlyblowairone-wayintothebuilding),lowflow(airhandlingsystemsthatblowairatalowflowrate),orprimary-secondary(airhandlingsystemsthathaveanairhandlerorrooftopunitconnectedtoanadd-onmakeupunitorhood)commercialHVACsystems.Packagedterminalairconditioner(PTAC)Anairconditionerandheatercombinedintoasingle,electrically-poweredunit,typicallyinstalledthroughawallandoftenfoundinhotels.Roof-topunit(RTU)Anair-handlingunit,definedaseither"recirculating"or"once-through"design,madespecificallyforoutdoorinstallation.Theymostofteninclude,internally,theirownheatingandcoolingdevices.RTUsareverycommoninsomeregions,particularlyinsingle-storycommercialbuildings.Variableairvolume(VAV)systemAnHVACsystemthathasastablesupply-airtemperature,andvariestheairflowratetomeetthetemperaturerequirements.ComparedtoCAVsystems,thesesystemswastelessenergythroughunnecessarily-highfanspeeds.MostnewcommercialbuildingshaveVAVsystems.ThermalzoneAsingleorgroupofneighboringindoorspacesthattheHVACdesignerexpectswillhavesimilarthermalloads.Buildingcodesmayrequirezoningtosaveenergyincommercialbuildings.ZonesaredefinedinthebuildingtoreducethenumberofHVACsubsystems,andthusinitialcost.Forexample,forperimeteroffices,ratherthanonezoneforeachoffice,allofficesfacingwestcanbecombinedintoonezone.Smallresidencestypicallyhaveonlyoneconditionedthermalzone,plusunconditionedspacessuchasunconditionedgarages,attics,andcrawlspaces,andunconditionedbasements.


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CoilsTheselectionofhotandchilledwatercoilswillhaveasubstantialimpactonthefanenergyuse.ThincoildesignTraditionalAHUdesignspecifiescoilsizesassumingafacevelocityofbetween400and500feetperminute.Anewdesigntechniquecalledlowfacevelocity,highcoolantvelocityorLFV/HCVhasbeenresearchedattheUniversityofAdelaide,Australia.Thistechniqueusesa"thin"coildesignthatisroughlyhalfthenumberoftubesindepthasinconventionaldesignsbutdoublethecoilfacearea.Thenetresultisafacevelocityintherangeof150to200feetperminute(FPM)withmuchhigherheattransferefficiencyandlowerpressuredropthaninconventionaldesigns.Becausethecoil'spressurelossisproportionaltothevelocityatasquarerate,facevelocityreductioncanresultinpressuredropsofone-fourthorlesscomparedtotheequivalent,traditionallydesignedcoil.

PreheatcoilsApreheatcoiliscommonlyusedtocontrolcondensationinsidetheHVACsystemforlaboratoriesthatuse100percentoutsideairorwhentheoutsideairtemperaturefallsbelowfreezing.Ifaheatingcoilisuseddownstream,thepreheatcoilshouldbecomeinactivetosaveenergywhenoutdoortemperaturesreach45degreesF.Preheatcoilsarealsousedtowarmtheoutsideairstream,assuringbetterairstreammixingandprovidingfreehumidification.


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DamperAdamperisavalveorplatethatstopsorregulatestheflowofairinsideaduct,chimney,VAVbox,airhandler,orotherairhandlingequipment.Adampermaybeusedtocutoffcentralairconditioning(heatingorcooling)toanunusedroom,ortoregulateitforroom-by-roomtemperatureandclimatecontrol.Itsoperationcanbemanualorautomatic.Manualdampersareturnedbyahandleontheoutsideofaduct.Automaticdampersareusedtoregulateairflowconstantlyandareoperatedbyelectricorpneumaticmotors,inturncontrolledbyathermostatorbuildingautomationsystem.Inachimneyflue,adamperclosesoffthefluetokeeptheweather(andbirdsandotheranimals)outandwarmorcoolairin.Thisisusuallydoneinthesummer,butalsosometimesinthewinterbetweenuses.Insomecases,thedampermayalsobepartlyclosedtohelpcontroltherateofcombustion.Thedampermaybeaccessibleonlybyreachingupintothefireplacebyhandorwithawoodpoker,orsometimesbyaleverorknobthatsticksdownorout.Onawoodburningstoveorsimilardevice,itisusuallyahandleontheventductasinanairconditioningsystem.Forgettingtoopenadamperbeforebeginningafirecancauseserioussmokedamagetotheinteriorofahome,ifnotahousefire.

Opposedbladedampersinamixingduct


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Dampersmustbeinstalledinplaceswhereairflowneedstobecontrolledand/orblocked.Damperslocateddirectlybehindanoutlettendtobenoisy.Abetterlocationisinthefinalbranchneartheconnectiontothetrunkduct.Whereverabalancingorvolumedamperislocated,itshouldbeaccessible.Lay-inceilingtilesprovidegoodaccess;inafixedceiling,anaccessdoorisneeded.Dampersshouldnotbeinstalledinhoodexhaustsystemseveniftheexhaustductpassesthroughafirewall.UsetheULapprovedalternative--aproperlysupported,heavy-gaugesteel,unobstructedduct.Dampershavetowithstandthemaximumstaticpressureinasystem.Themaximumstaticpressureisthemaximumthatcanbeexperiencedinasystem,notsimplythepressureintroducedbythefanduringnormaloperation.Maximumstaticpressureusuallyoccurswhenalldampersinasystemareclosedexceptthoseononeflowpath.

AutomatedzonedampersAzonedamper(alsoknownasaVolumeControlDamperorVCD)isaspecifictypeofdamperusedtocontroltheflowofairinanHVACheatingorcoolingsystem.Inordertoimproveefficiencyandoccupantcomfort,HVACsystemsarecommonlydividedupintomultiplezones.Forexample,inahouse,themainfloormaybeservedbyoneheatingzonewhiletheupstairsbedroomsareservedbyanother.Inthisway,theheatcanbedirectedprincipallytothemainfloorduringthedayandprincipallytothebedroomsatnight,allowingtheunoccupiedareastocooldown.ZonedampersasusedinhomeHVACsystemsareusuallyelectricallypowered.Inlargecommercialinstallations,vacuumorcompressedairmaybeusedinstead.Ineithercase,themotorisusuallyconnectedtothedamperviaamechanicalcoupling.Advantages:Cost.Powerconsumption.


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Disadvantages:Zonedampersarenot100%reliable.Themotor-to-open/motor-to-closedstyleofelectricallyoperatedzonedampersaren't"failsafe"(thatis,theydonotfailtotheopencondition).However,zonedampersthatareofthe"NormallyOpen"typearefail-safe,inthattheywillfailtotheopencondition.Noinherentredundancyforthefurnace.Asystemwithzonedampersisdependentuponasinglefurnace.Ifitfails,thesystembecomescompletelyinoperable.Thesystemcanbehardertodesign,requiringbothSPDTthermostats(andrelays)andtheabilityofthesystemtowithstandthefaultconditionwherebyallzonedampersareclosedsimultaneously.

FiredampersFiredampersarefittedwhereductworkpassesthroughfirecompartmentwalls/firecurtainsaspartofafirecontrolstrategy.Innormalcircumstances,thesedampersareheldopenbymeansoffusiblelinks.Whensubjectedtoheat,theselinksfractureandallowthedampertocloseundertheinfluenceoftheintegralclosingspring.Thelinksareattachedtothedampersuchthatthedamperscanbereleasedmanuallyfortestingpurposes.Thedamperisprovidedwithanaccessdoorintheadjacentductworkforthepurposeofinspectionandresettingintheeventofclosure.


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DuctsDuctsareusedinheating,ventilation,andairconditioning(HVAC)todeliverandremoveair.Theseneededairflowsinclude,forexample,supplyair,returnair,andexhaustair.Ductsalsodeliver,mostcommonlyaspartofthesupplyair,ventilationair.Assuch,airductsareonemethodofensuringacceptableindoorairqualityaswellasthermalcomfort.Aductsystemisoftencalledductwork.Planning('layingout'),sizing,optimizing,detailing,andfindingthepressurelossesthroughaductsystemiscalledductdesign.DuctmaterialsLikemodernsteelfoodcans,atonetimeairductswereoftenmadeoftin,like'tincans'weremadeforfood.Tinismorecorrosionresistantthanplainsteel,butisalsomoreexpensive.Withimprovementsinmildsteelproduction,anditsgalvanizationtoresistrust,steel'sheetmetal'hasreplacedtininductsaswellasfoodcans..GalvanizedsteelDuctsarestillmostoftenmadeofgalvanizedsteel.Variousfittingsallowtransitioningbetweenthevariousshapesandsizes.A"tee"connection,forexample,iswheretheairflowcanbedividedintotwoormoredownstreambranches.Manyfactory-madeshapesandsizesareavailablebutgalvanizedsteelcaneasilybecutandbenttoformadditionalshapeswhenrequired.Steelductsarecommonlywrappedorlinedwithfiberglassthermalinsulation,bothtoreduceheatlossorgainthroughtheductwallsandwatervaporfromcondensingontheexterioroftheductwhentheductiscarryingcooledair.Insulation,particularlyductliner,alsoreducesduct-bornenoise.Bothtypesofinsulationreduce'breakout'noisethroughtheducts'sidewalls.Polyurethaneductboard(Preinsulatedaluminumducts)Whileasmentionedabove,galvanizedsteelisstillverycommon,alwaysmorerectangularductsarebeingmanufacturedfromduct

board,thankstothefactthatcustomorspecialshapesandsizesofductscaneasilybeshoporfield


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fabricated.Inadditiontothefactthatductsmadewithductboarddonotneedanyfurtherinsulation.Amongthevarioustypesofrigidpolyurethanefoampanelsavailable,anewwaterformulatedpanelstandsout.Inthisparticularpanel,thefoamingprocessisobtainedthroughtheuseofwaterinsteadoftheCFC,HCFC,HFCandHCgasses.Thefoampanelsarethencoatedwithaluminumsheetsoneitherside,withthicknessesthatcanvaryfrom50micrometresforindooruseto200micrometresforexternaluseinordertoguaranteethehighmechanicalcharacteristicsoftheduct.Theductsconstructionstartswiththeplottingofthesinglepiecesonthepanel.Thepiecesarethencutfromthepanel(witha45cutasexplainedbelow),bentifnecessaryinordertoobtainthedifferentfittings,andfinallyclosedthroughanoperationofgluing,pressingandtaping.Havingobtainedthevariousductsections,theycaneasilybeinstalledbyusinganinvisiblealuminumflangesystem.Fiberglassductboard(Preinsulatednonmetallicductwork)Alsothefiberglasspanelsprovidebuilt-inthermalinsulationandtheinteriorsurfaceabsorbssound,helpingtoprovidequietoperationoftheHVACsystem.Theductboardisformedbyslidingaspecially-designedknifealongtheboardusingastraightedgeasaguide;theknifeautomaticallytrimsouta"valley"with45sides;thevalleydoesnotquitepenetratetheentiredepthoftheductboard,providingathinsectionthatactsasahinge.Theductboardcanthenbefoldedalongthevalleystoproduce90folds,makingtherectangularductshapeinthefabricator'sdesiredsize.Theductisthenclosedwithstaplesandspecialaluminumorsimilar'metal-backed'tape.Commonlyavailableducttapeshouldnotbeusedonairducts,metal,fiberglass,orotherwise,thatareintendedforlong-termuse;theadhesiveonsocalled'ducttape'driesandreleaseswith

time.FlexibletubingFlexibleducts,knownasflex,haveavarietyofconfigurations,butforHVACapplications,theyaretypicallyflexibleplasticoverametalwirecoiltomakeround,flexibleduct.Mostoftenalayeroffiberglassinsulationcoverstheduct,andthenathinplasticlayerprotectstheinsulation.Flexibleductisveryconvenientforattachingsupplyairoutletstotherigidductwork.However,thepressurelossthroughflexishigherthanformostothertypesofducts.Assuch,designersandinstallersattempttokeeptheirinstalledlengths(runs)short,e.g.,


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lessthan15feetorso,andtominimizeturns.Kinksinflexmustbeavoided.FlexibleductisnormallynotusedonthenegativepressureportionsofHVACductsystems.DUCTDESIGNOBJECTIVESTheobjectivesofgoodductdesignareoccupantcomfort,properairdistribution,economicalheatingandcoolingsystemoperation,andeconomicalductinstallation.Theoutcomeoftheductdesignprocesswillbeaductsystem(supplyandreturnplenums,ducts,fittings,boots,grilles,andregisters)thatProvidesconditionedairtomeetallroomheatingandcoolingloads.Isproperlysizedsothatthepressuredropacrosstheairhandleriswithinmanufactureranddesignspecifications.Issealedtoprovideproperairflowandtopreventairfromenteringthehouseorductsystemfrompollutedzones.Hasbalancedsupplyandreturnairflowstomaintainaneutralpressureinthehouse.Minimizesductairtemperaturegainsorlossesbetweentheairhandlerandsupplyoutlets,andbetweenthereturnregisterandairhandler.

SUPPLYDUCTSYSTEMSSupplyductsdeliverairtothespacesthataretobeconditioned.Thetwomostcommonsupplyductsystemsforresidencesarethetrunkandbranchsystemandtheradialsystembecauseoftheirversatility,performance,andeconomy.Thespiderandperimeterloopsystemsareotheroptions.TRUNKANDBRANCHSYSTEMInthetrunkandbranchsystem,alargemainsupplytrunkisconnecteddirectlytotheairhandleroritssupplyplenumandservesasasupplyplenumoranextensiontothesupplyplenum.Smallerbranchductsandrunoutsareconnectedtothetrunk.Thetrunkandbranchsystemisadaptabletomosthouses,butithasmoreplaceswhereleakscanoccur.Itprovidesairflowsthatareeasilybalancedandcanbeeasilydesignedtobelocatedinsidetheconditionedspa

ceofthehouse.Thereareseveralvariationsofthetrunkandbranchsystem.Anextended


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plenumsystemusesamainsupplytrunkthatisonesizeandisthesimplestandmostpopulardesign.Thelengthofthetrunkisusuallylimitedtoabout24feetbecauseotherwisethevelocityoftheairinthetrunkgetstoolowandairflowintobranchesandrunoutsclosetotheairhandlerbecomespoor.Therefore,withacentrallylocatedairhandler,thisductsystemcanbeinstalledinhomesuptoapproximately50feetlong.Areducingplenumsystemusesatrunkreductionperiodicallytomaintainamoreuniformpressureandairvelocityinthetrunk,whichimprovesairflowinbranchesandrunoutsclosertotheairhandler.Similarly,areducingtrunksystemreducesthecross-sectionalareaofthetrunkaftereverybranchductorrunout,butitisthemostcomplexsystemtodesign.SPIDERSYSTEMAspidersystemisamoredistinctvariationofthetrunkandbranchsystem.Largesupplytrunks(usuallylarge-diameterflexibleducts)connectremotemixingboxestoasmall,centralsupplyplenum.Smallerbranchductsorrunoutstakeairfromtheremotemixingboxestotheindividualsupplyoutlets.Thissystemisdifficulttolocatewithintheconditionedspaceofthehouse.RADIALSYSTEMInaradialsystem,thereisnomainsupplytrunk;branchductsorrunoutsthatdeliverconditionedairtoindividualsupplyoutletsareessentiallyconnecteddirectlytotheairhandler,usuallyusingasmallsupplyplenum.Theshort,directductrunsmaximizeairflow.Theradialsystemismostadaptabletosingle-storyhomes.Traditionally,thissystemisassociatedwithanairhandlerthatiscentrallylocatedsothatductsarearrangedinaradialpattern.However,symmetryisnotmandatory,anddesignsusingparallelrunoutscanbedesignedsothatductrunsremainintheconditionedspace(e.g.,installedaboveadroppedceiling).PERIMETERLOOPSYSTEMAperimeterloopsystemusesaperimeter

ductfedfromacentralsupplyplenumusingseveralfeederducts.Thissystemistypicallylimitedtohousesbuiltonslabincoldclimatesandismoredifficulttodesignandinstall.


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RETURNDUCTSYSTEMSReturnductsremoveroomairanddeliveritbacktotheheatingandcoolingequipmentforfilteringandreconditioning.Returnductsystemsaregenerallyclassifiedaseithercentralormultiple-roomreturn.MULTIPLE-ROOMRETURNSYSTEMAmultiple-roomreturnsystemisdesignedtoreturnairfromeachroomsuppliedwithconditionedair,especiallythosethatcanbeisolatedfromtherestofthehouse(exceptbathroomsandperhapskitchensandmechanicalrooms).Whenproperlydesignedandinstalled,thisistheultimatereturnductsystembecauseitensuresthatairflowisreturnedfromallrooms(evenwithdoorsclosed),minimizespressureimbalances,improvesprivacy,andisquiet.However,designandinstallationcostsofamulti-roomreturnsystemaregenerallyhigherthancostsforacentralreturnsystem,andhigherfrictionlossescanincreaseblowerrequirements.CENTRALRETURNSYSTEMAcentralreturnsystemconsistsofoneormorelargegrilleslocatedincentralareasofthehouse(e.g.,hallway,understairway)andoftenclosetotheairhandler.Inmulti-storyhouses,acentralreturnisoftenlocatedoneachfloor.To


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ensureproperairflowfromallrooms,especiallywhendoorsareclosed,transfergrillesorjumperductsmustbeinstalledineachroom(undercuttinginteriordoorstoprovide1inchofclearancetothefloorisusuallynotsufficientbyitself).Transfergrillesarethrough-the-wallventsthatareoftenlocatedabovetheinteriordoorframes,althoughtheycanbeinstalledinafullwallcavitytoreducenoisetransmission.Thewallcavitymustbewellsealedtopreventairleakage.Jumperductsareshortductsroutedthroughtheceilingtominimizenoisetransfer.


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DUCTANDREGISTERLOCATIONSLocatingtheairhandlerunitandairdistributionsysteminsidetheconditionedspaceofthehouseisthebestwaytoimproveductsystemefficiencyandishighlyrecommended.Withthisdesign,anyductleakagewillbetotheinsideofthehouse.Itwillnotsignificantlyaffecttheenergyefficiencyoftheheatingandcoolingsystembecausetheconditionedairremainsinsidethehouse,althoughairdistributionmaysuffer.Also,ductslocatedinsidetheconditionedspaceneedminimalinsulation(inhotandhumidclimates),ifanyatall.Thecostofmovingductsintotheconditionedspacecanbeoffsetbysmallerheatingandcoolingequipment,smallerandlessductwork,reducedductinsulation,andloweroperatingcosts.Thereareseveralmethodsforlocatingductsinsidetheconditionedspace.Placetheductsinafurred-downchasebelowtheceiling(e.g.,droppedceilinginahallway),achasefurred-upintheattic,orothersuchchases.Thesechasesmustbespeciallyconstructed,air-sealed,andinsulatedtoensuretheyarenotconnectedtounconditionedspaces.Locateductsbetweenthefloorsofamulti-storyhome(runthroughthefloortrussesorjoists).Theexteriorwallsofthesefloorcavitiesmustbeinsulatedandsealedtoensuretheyarewithintheconditionedspace.Holesinthecavityforwiring,plumbing,etc.,mustbesealedtopreventairexchangewithunconditionedspaces.Locateductsinaspecially-constructedsealedandinsulatedcrawlspace(wherethewallsofthecrawlspaceareinsulatedratherthantheceiling).Ductsshouldnotberuninexteriorwallsasameansofmovingthemintotheconditionedspacebecausethisreducestheamountofinsulationthatcanbeappliedtotheductandthewallitself.Asupplyoutletispositionedtomixconditionedairwithroomairandisresponsibleformostoftheairmovementwithinaroom.Occ

upantcomfortrequiresthatsupplyregisterlocationsbecarefullyselectedforeachroom.Incoldclimates,perimeterflooroutletsthatblanketportionsoftheexteriorwall(usuallywindows)withsupplyairaregenerallypreferred.However,intodaysbetterinsulatedhomes,theneedtolocateoutletsneartheperimeterwhereheatlossoccursisbecominglessimportant.Inhotclimates,ceilingdiffusersorhighwalloutletsthatdischargeairparalleltotheceilingaretypicallyinstalled.Inmoderateclimates,outletlocationislesscritical.Outletlocationsnearinteriorwallscan


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significantlyreduceductlengths(decreasingcosts),thermallosses(ifductsarelocatedoutsidetheconditionedspace),andblowerrequirements.Topreventsupplyairfrombeingsweptdirectlyupbykitchen,bathroom,orotherexhaustfans,thedistancebetweensupplyregistersandexhaustventsshouldbekeptaslargeaspossible.Thelocationofthereturnregisterhasonlyasecondaryeffectonroomairmotion.However,returnscanhelpdefeatstratificationandimprovemixingofroomairiftheyareplacedhighwhencoolingisthedominantspaceconditioningneedandlowwhenheatingisdominant.Inmulti-storyhomeswithbothheatingandcooling,upper-levelreturnsshouldbeplacedhighandlowerlevelreturnsshouldbeplacedlow.Otherwise,thelocationofthereturnregistercanbedeterminedbywhatwillminimizeductruns,improveaircirculationandmixingofsupplyair,andimpactotherconsiderationssuchasaesthetics.

DESIGNRECOMMENDATIONSANDKEYDESIGNELEMENTSIndesigningtheairdistributionthefollowingrecommendationsbeforefinalizingthedesignshouldbeconsidered:Designtheairdistributionsystemtobelocatedinsidetheconditionedspaceofthehousetothegreatestextentpossible.Donotlocateductsinexteriorwalls.Theentireairdistributionsystemshouldbehardducted,includingreturns(i.e.,buildingcavities,closets,raised-floorairhandlerplenums,platformreturns,wallstudspaces,pannedfloorjoists,etc.,shouldnotbeused).Intwo-storyandverylargehouses,considerusingtwoormoreseparateheatingandcoolingsystems,eachwithitsownductsystem.Intwo-storyhomes,forexample,upperstoriestendtogainmoreheatinsummerandlosemoreheatinwinter,sothebestcomfortandperformanceisoftenachievedbyusingseparate

systemsfortheupperandlowerstories.Considersupplyoutletlocationsnearinteriorwallstoreduceductlengths.Locatesupplyoutletsasfarawayfromexhaustventsaspossibleinbathroomsandkitchenstopreventsupplyairfrombeingsweptdirectlyupbytheexhaustfans.Considerinstallingvolumedamperslocatedatthetakeoffendoftheductratherthanatthesupplyregistertofacilitatemanualbalancingofthesystemafterinstallation.Volumedampersshouldhaveameansoffixingthepositionofthedamperaftertheairdistributionsystemisbalanced.


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Whenusingacentralreturnsystem,include(a)areturnoneachlevelofamultistoryhouse,(b)aspecificationtoinstalltransfergrillesorjumperductsineachroomwithadoor(undercuttinginteriordoorstoallow1inchofclearancetothefloorisusuallynotsufficient),and(c)ifatallpossible,areturninallroomswithdoorsthatrequiretwoormoresupplyducts.Specifyhigherductinsulationlevelsinductslocatedoutsidetheconditionedspacethanthosespecifiedbythe2000InternationalEnergyConservationCode,especiallywhenvariable-speedairhandlingequipmentisbeingused.Lowerairflowsprovidedbyvariable-speedheatingandcoolingsystemstoimproveoperatingefficiencyincreasetheresidenttimeofairwithintheairdistributionsystem,whichinturnincreasesthermallossesinthewinterandthermalgainsinthesummer.Atticinsulationplacedoverductshelpswhereitispossible.Specifythatallductjointsmustbemechanicallyfastenedandsealedpriortoinsulationtopreventairleakage,preferablywithmasticandfiberglassmesh.Considertestingofductsusingaductblowertoensurethattheairdistributionsystemistight,especiallyifductsareunavoidablylocatedinanunconditionedspace.Atypicalrequirementisthatductleakage(measuredusingaductblowerinunitsofcubicfeetperminutewhentheductsarepressurizedto25Pascals)shouldnotexceed5%ofthesystemairflowrate.


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CONTENTSPrinciplesofAir-Conditioning.PsychometricChartRefrigerationCycleVaporCompressioncycleVaporabsorptioncycleAircycleComfortcoolingCoolingsupplydevicesAirconditioningApplicationTypesofACunitsCentralair-conditioningindowACunitsHVACAirdistributionsystems


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20285100 Air Conditioning Principles and Concepts