暖通空调英文

英文文献AirConditioningSystemsAirconditioninghasrapidlygrownoverthepast50years,fromaluxurytoastandardsystemincludedinmostresidentialandcommercialbuildings.In1970,36%ofresidencesintheU.S.wereeitherfullyairconditionedorutilizedaroomairconditionerforcooling(Blue,etal.,1979).By1997,thisnumberhadmorethandoubledto77%,andthatyearalsomarkedthefirsttimethatoverhalf(50.9%)ofresidencesintheU.S.hadcentralairconditioners(CensusBureau,1999).Anestimated83%ofallnewhomesconstructedin1998hadcentralairconditioners(CensusBureau,1999).Airconditioninghasalsogrownrapidlyincommercialbuildings.From1970to1995,thepercentageofcommercialbuildingswithairconditioningincreasedfrom54to73%(JacksonandJohnson,1978,andDOE,1998).Airconditioninginbuildingsisusuallyaccomplishedwiththeuseofmechanicalor heatactivatedequipment.Inmostapplications,theairconditionermustprovidebothcoolinganddehumidificationtomaintaincomfortinthebuilding.Airconditioningsystemsarealsousedinotherapplications,suchasautomobiles,trucks,aircraft,ships,andindustrialfacilities.However,thedescriptionofequipmentinthischapterislimitedtothosecommonlyusedincommercialandresidentialbuildings.Commercialbuildingsrangefromlargehighriseofficebuildingstothecornerconveniencestore.Becauseoftherangeinsizeandtypesofbuildingsinthecommercialsector,thereisawidevarietyofequipmentappliedinthesebuildings.Forlargerbuildings,theairconditioningequipmentispartofatotalsystemdesignthatincludesitemssuchasapipingsystem,airdistributionsystem,andcoolingtower.Properdesignofthesesystemsrequiresaqualifiedengineer.Theresidentialbuildingsectorisdominatedbysinglefamilyhomesandlowriseapartments/condominiums.Thecoolingequipmentappliedinthesebuildingscomesinstandard“packages”thatareoftenbothsizedandinstalledbytheairconditioningcontractor.Thechapterstartswithageneraldiscussionofthevaporcompressionrefrigerationcyclethenmovestorefrigerantsandtheirselection,followedbypackagedChilledWaterSystems。1.1VaporCompressionCycleEventhoughthereisalargerangeinsizesandvarietyofairconditioningsystemsusedinbuildings,mostsystemsutilizethevaporcompressioncycletoproducethedesiredcoolinganddehumidification.Thiscycleisalsousedforrefrigeratingandfreezingfoodsandforautomotiveairconditioning.ThefirstpatentonamechanicallydrivenrefrigerationsystemwasissuedtoJacobPerkinsin1834inLondon,andthefirstviablecommercialsystemwasproducedin1857byJamesHarrisonandD.E.Siebe.Besidesvaporcompression,therearetwolesscommonmethodsusedtoproducecoolinginbuildings:theabsorptioncycleandevaporativecooling.Thesearedescribedlaterinthechapter.Withthevaporcompressioncycle,aworkingfluid,whichiscalledtherefrigerant,evaporatesandcondensesatsuitablepressuresforpracticalequipmentdesigns.Thefourbasiccomponentsineveryvaporcompressionrefrigerationsystemarethecompressor,condenser,expansiondevice,andevaporator.Thecompressorraisesthepressureoftherefrigerantvaporsothattherefrigerantsaturationtemperatureisslightlyabovethetemperatureofthecoolingmediumusedinthecondenser.Thetypeofcompressoruseddependsontheapplicationofthesystem.Largeelectricchillerstypicallyuseacentrifugalcompressorwhilesmallresidentialequipmentusesareciprocatingorscrollcompressor.Thecondenserisaheatexchangerusedtorejectheatfromtherefrigeranttoacoolingmedium.Therefrigerantentersthecondenserandusuallyleavesasasubcooledliquid.Typicalcoolingmediumsusedincondensersareairandwater.Mostresidential-sizedequipmentusesairasthecoolingmediuminthecondenser,whilemanylargerchillersusewater.Afterleavingthecondenser,theliquidrefrigerantexpandstoalowerpressureintheexpansionvalve.Theexpansionvalvecanbeapassivedevice,suchasacapillarytubeorshorttubeorifice,oranactivedevice,suchasathermalexpansionvalveorelectronicexpansionvalve.Thepurposeofthevalveistoregulatetheflowofrefrigeranttotheevaporatorsothattherefrigerantissuperheatedwhenitreachesthesuctionofthecompressor.Attheexitoftheexpansionvalve,therefrigerantisatatemperaturebelowthatofthemedium(airorwater)tobecooled.Therefrigeranttravelsthroughaheatexchangercalledtheevaporator.Itabsorbsenergyfromtheairorwatercirculatedthroughtheevaporator.Ifairiscirculatedthroughtheevaporator,thesystemiscalledadirectexpansionsystem.Ifwateriscirculatedthroughtheevaporator,itiscalledachiller.Ineithercase,therefrigerantdoesnotmakedirectcontactwiththeairorwaterintheevaporator.Therefrigerantisconvertedfromalowquality,two-phasefluidtoasuperheatedvaporundernormaloperatingconditionsintheevaporator.Thevaporformedmustberemovedbythecompressoratasufficientratetomaintainthelowpressureintheevaporatorandkeepthecycleoperating.Allmechanicalcoolingresultsintheproductionofheatenergythatmustberejectedthroughthecondenser.Inmanyinstances,thisheatenergyisrejectedtotheenvironmentdirectlytotheairinthecondenserorindirectlytowaterwhereitisrejectedinacoolingtower.Withsomeapplications,itispossibletoutilizethiswasteheatenergytoprovidesimultaneousheatingtothebuilding.Recoveryofthiswasteheatattemperaturesupto65C(150F)canbeusedtoreducecostsforspaceheating.Capacitiesofairconditioningareoftenexpressedineithertonsorkilowatts(kW)ofcooling.Thetonisaunitofmeasurerelatedtotheabilityofaniceplanttofreezeoneshortton(907kg)oficein24hr.Itsvalueis3.51kW(12,000Btu/hr).ThekWofthermalcoolingcapacityproducedbytheairconditionermustnotbeconfusedwiththeamountofelectricalpower(alsoexpressedinkW)requiredtoproducethecoolingeffect.2.1RefrigerantsUseandSelectionUpuntilthemid-1980s,refrigerantselectionwasnotanissueinmostbuildingairconditioningapplicationsbecausetherewerenoregulationsontheuseofrefrigerants.Manyoftherefrigerantshistoricallyusedforbuildingairconditioningapplicationshavebeenchlorofluorocarbons(CFCs)andhydrochlorofluorocarbons(HCFCs).Mostoftheserefrigerantsarenontoxicandnonflammable.However,recentU.S.federalregulations(EPA1993a;EPA1993b)andinternationalagreements(UNEP,1987)haveplacedrestrictionsontheproductionanduseofCFCsandHCFCs.Hydrofluorocarbons(HFCs)arenowbeingusedinsomeapplicationswhereCFCsandHCFCswereused.Havinganunderstandingofrefrigerantscanhelpabuildingownerorengineermakeamoreinformeddecisionaboutthebestchoiceofrefrigerantsforspecificapplications.Thissectiondiscussesthedifferentrefrigerantsusedinorproposedforbuildingairconditioningapplicationsandtheregulationsaffectingtheiruse.TheAmericanSocietyofHeating,RefrigeratingandAirConditioningEngineers(ASHRAE)hasastandardnumberingsystem,foridentifyingrefrigerants(ASHRAE,1992).ManypopularCFC,HCFC,andHFCrefrigerantsareinthemethaneandethaneseriesofrefrigerants.Theyarecalledhalocarbons,orhalogenatedhydrocarbons,becauseofthepresenceofhalogenelementssuchasfluorineorchlorine(King,1986).Zeotropesandazeotropesaremixturesoftwoormoredifferentrefrigerants.Azeotropicmixturechangessaturationtemperaturesasitevaporates(orcondenses)atconstantpressure.Thephenomenaiscalledtemperatureglide.Atatmosphericpressure,R-407Chasaboiling(bubble)pointof44C(47F)andacondensation(dew)pointof37C(35F),whichgivesitatemperatureglideof7C(12F).Anazeotropicmixturebehaveslikeasinglecomponentrefrigerantinthatthesaturationtemperaturedoesnotchangeappreciablyasitevaporatesorcondensesatconstantpressure.R-410Ahasasmallenoughtemperatureglide(lessthan5.5C,10F)thatitisconsideredanear-azeotropicrefrigerantmixture.ASHRAEgroupsrefrigerantsbytheirtoxicityandflammability(ASHRAE,1994).GroupA1isnonflammableandleasttoxic,whileGroupB3isflammableandmosttoxic.Toxicityisbasedontheuppersafetylimitforairborneexposuretotherefrigerant.Iftherefrigerantisnontoxicinquantitieslessthan400partspermillion,itisaClassArefrigerant.Ifexposuretolessthan400partspermillionistoxic,thenthesubstanceisgiventheBdesignation.Thenumericaldesignationsrefertotheflammabilityoftherefrigerant.ThelastcolumnofTable4.2.1showsthetoxicityandflammabilityratingofcommonrefrigerants.Refrigerant22isanHCFC,isusedinmanyofthesameapplications,andisstilltherefrigerantofchoiceinmanyreciprocatingandscrewchillersaswellassmallcommercialandresidentialpackagedequipment.ItoperatesatamuchhigherpressurethaneitherR-11orR-12.RestrictionsontheproductionofHCFCswillstartin2004.In2010,R-22cannotbeusedinnewairconditioningequipment.R-22cannotbeproducedafter2020(EPA,1993b).R-407CandR-410AarebothmixturesofHFCs.BothareconsideredreplacementsforR-22.R-407Cisexpectedtobeadrop-inreplacementrefrigerantforR-22.ItsevaporatingandcondensingpressuresforairconditioningapplicationsareclosetothoseofR-22(Table4.2.3).However,replacementofR-22withR-407Cshouldbedoneonlyafterconsultingwiththeequipmentmanufacturer.Ataminimum,thelubricantandexpansiondevicewillneedtobereplaced.Thefirstresidential-sizedairconditioningequipmentusingR-410AwasintroducedintheU.S.in1998.SystemsusingR-410Aoperateatapproximately50%higherpressurethanR-22(Table4.2.3);thus,R-410Acannotbeusedasadrop-inrefrigerantforR-22.R-410Asystemsutilizecompressors,expansionvalves,andheatexchangersdesignedspecificallyforusewiththatrefrigerant. Ammoniaiswidelyusedinindustrialrefrigerationapplicationsandinammoniawaterabsorptionchillers.ItismoderatelyflammableandhasaclassBtoxicityratingbuthashadlimitedapplicationsincommercialbuildingsunlessthechillerplantcanbeisolatedfromthebuildingbeingcooled(Toth,1994,Stoecker,1994).Asarefrigerant,ammoniahasmanydesirablequalities.Ithasahighspecificheatandhighthermalconductivity.Itsenthalpyofvaporizationistypically6to8timeshigherthanthatofthecommonlyusedhalocarbons,anditprovideshigherheattransfercomparedtohalocarbons.Itcanbeusedinbothreciprocatingandcentrifugalcompressors.Researchisunderwaytoinvestigatetheuseofnaturalrefrigerants,suchascarbondioxide(R-744)andhydrocarbonsinairconditioningandrefrigerationsystems(Bullock,1997,andKramer,1991).CarbondioxideoperatesatmuchhigherpressuresthanconventionalHCFCsorHFCsandrequiresoperationabovethecriticalpointintypicalairconditioningapplications.Hydrocarbonrefrigerants,oftenthoughtofastoohazardousbecauseofflammability,canbeusedinconventionalcompressorsandhavebeenusedinindustrialapplications.R-290,propane,hasoperatingpressuresclosetoR-22andhasbeenproposedasareplacementforR-22(Kramer,1991).Currently,therearenocommercialsystemssoldintheU.S.forbuildingoperationsthatuseeithercarbondioxideorflammablerefrigerants.3.1ChilledWaterSystemsChilledwatersystemswereusedinlessthan4%ofcommercialbuildingsintheU.S.in1995.However,becausechillersareusuallyinstalledinlargerbuildings,chillerscooledover28%oftheU.S.commercialbuildingfloorspacethatsameyear(DOE,1998).Fivetypesofchillersarecommonlyappliedtocommercialbuildings:reciprocating,screw,scroll,centrifugal,andabsorption.Thefirstfourutilizethevaporcompressioncycletoproducechilledwater.Theydifferprimarilyinthetypeofcompressorused.Absorptionchillersutilizethermalenergy(typicallysteamorcombustionsource)inanabsorptioncyclewitheitheranammonia-waterorwater-lithiumbromidesolutiontoproducechilledwater.3.2OverallSystemAnestimated86%ofchillersareappliedinmultiplechillerarrangementslikethatshowninthefigure(BitondoandTozzi,1999).Inchilledwatersystems,returnwaterfromthebuildingiscirculatedthrougheachchillerevaporatorwhereitiscooledtoanacceptabletemperature(typically4to7C)(39to45F).Thechilledwateristhendistributedtowater-to-airheatexchangersspreadthroughoutthefacility.Intheseheatexchangers,airiscooledanddehumidifiedbythecoldwater.Duringtheprocess,thechilledwaterincreasesintemperatureandmustbereturnedtothechiller(s).Thechillersarewater-cooledchillers.Wateriscirculatedthroughthecondenserofeachchillerwhereitabsorbsheatenergyrejectedfromthehighpressurerefrigerant.Thewateristhenpumpedtoacoolingtowerwherethewateriscooledthroughanevaporationprocess.Coolingtowersaredescribedinalatersection.Chillerscanalsobeaircooled.Inthisconfiguration,thecondenserwouldbearefrigerant-to-airheatexchangerwithairabsorbingtheheatenergyrejectedbythehighpressurerefrigerant.Chillersnominallyrangeincapacitiesfrom30to18,000kW(8to5100ton).MostchillerssoldintheU.S.areelectricandutilizevaporcompressionrefrigerationtoproducechilledwater.Compressorsforthesesystemsareeitherreciprocating,screw,scroll,orcentrifugalindesign.Asmallnumberofcentrifugalchillersaresoldthatuseeitheraninternalcombustionengineorsteamdriveinsteadofanelectricmotortodrivethecompressor.Thetypeofchillerusedinabuildingdependsontheapplication.Forlargeofficebuildingsorinchillerplantsservingmultiplebuildings,centrifugalcompressorsareoftenused.Inapplicationsunder1000kW(280tons)coolingcapacities,reciprocatingorscrewchillersmaybemoreappropriate.Insmallerapplications,below100kW(30tons),reciprocatingorscrollchillersaretypicallyused.3.3VaporCompressionChillersThenominalcapacityrangesforthefourtypesofelectricallydrivenvaporcompressionchillers.Eachchillerderivesitsnamefromthetypeofcompressorusedinthechiller.Thesystemsrangeincapacitiesfromthesmallestscroll(30kW;8tons)tothelargestcentrifugal(18,000kW;5000tons).ChillerscanutilizeeitheranHCFC(R-22andR-123)orHFC(R-134a)refrigerant.Thesteadystateefficiencyofchillersisoftenstatedasaratioofthepowerinput(inkW)tothechillingcapacity(intons).Acapacityratingofonetonisequalto3.52kWor12,000btu/h.Withthismeasureofefficiency,thesmallernumberisbetter.centrifugalchillersarethemostefficient;whereas,reciprocatingchillershavetheworstefficiencyofthefourtypes.Theefficiencynumbersprovidedinthetablearethesteadystatefull-loadefficiencydeterminedinaccordancetoASHRAEStandard30(ASHRAE,1995).Theseefficiencynumbersdonotincludetheauxiliaryequipment,suchaspumpsandcoolingtowerfansthatcanaddfrom0.06to0.31kW/tontothenumbersshownChillersrunatpartloadcapacitymostofthetime.Onlyduringthehighestthermalloadsinthebuildingwillachilleroperatenearitsratedcapacity.Asaconsequence,itisimportanttoknowhowtheefficiencyofthechillervarieswithpartloadcapacity.arepresentativedatafortheefficiency(inkW/ton)asafunctionofpercentagefullloadcapacityforareciprocating,screw,andscrollchillerplusacentrifugalchillerwithinletvanecontrolandonewithvariablefrequencydrive(VFD)forthecompressor.Thereciprocatingchillerincreasesinefficiencyasitoperatesatasmallerpercentageoffullload.Incontrast,theefficiencyofacentrifugalwithinletvanecontrolisrelativelyconstantuntiltheloadfallstoabout60%ofitsratedcapacityanditskW/tonincreasestoalmosttwiceitsfullyloadedvalue.In1998,theAirConditioningandRefrigerationInstitute(ARI)developedanewstandardthatincorporatesintotheirratingspartloadperformanceofchillers(ARI1998c).Partloadefficiencyisexpressedbyasinglenumbercalledtheintegratedpartloadvalue(IPLV).TheIPLVtakesdatasimilartothatinFigure4.2.3andweightsitatthe25%,50%,75%,and100%loadstoproduceasingleintegratedefficiencynumber.Theweightingfactorsattheseloadsare0.12,0.45,0.42,and0.01,respectively.TheequationtodetermineIPLVis:IPLV=1/(0. 01A+0.42B+0.45C+0.12D)Where:A=efficency at 100% loadB=efficency at 75% loadC=efficency at 50% loadD=efficency at 25% loadMostoftheIPLVisdeterminedbytheefficiencyatthe50%and75%partloadvalues.Manufacturerswillprovide,onrequest,IPLVsaswellaspartloadefficiencies.Thefourcompressorsusedinvaporcompressionchillersareeachbrieflydescribedbelow.Whilecentrifugalandscrewcompressorsareprimarilyusedinchillerapplications,reciprocatingandscrollcompressorsarealsousedinsmallerunitarypackagedairconditionersandheatpumps.3.4ReciprocatingCompressorsThereciprocatingcompressorisapositivedisplacementcompressor.Ontheintakestrokeofthepiston,afixedamountofgasispulledintothecylinder.Onthecompressionstroke,thegasis