暖通空调专业外文翻译.docx

暖通空调专业外文翻译.docx

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暖通空调专业外文翻译

暖通空调专业外文翻译

英文文献

AirConditioningSystems

Airconditioninghasrapidlygrownoverthepast50years,fromaluxurytoastandardsystemincludedinmostresidentialandcommercialbuildings.In1970,36%ofresidencesintheU.S.wereeitherfullyairconditionedorutilizedaroomairconditionerforcooling（Blue,etal.,1979）.By1997,thisnumberhadmorethandoubledto77%,andthatyearalsomarkedthefirsttimethatoverhalf（50.9%）ofresidencesintheU.S.hadcentralairconditioners（CensusBureau,1999）.Anestimated83%ofallnew

homesconstructedin1998hadcentralairconditioners（CensusBureau,1999）.Airconditioninghasalsogrownrapidlyincommercialbuildings.From1970to1995,thepercentageofcommercialbuildingswithairconditioningincreasedfrom54to73%（JacksonandJohnson,1978,andDOE,1998）.

Airconditioninginbuildingsisusuallyaccomplishedwiththeuseofmechanicalorheat-activatedequipment.Inmostapplications,theairconditionermustprovidebothcoolinganddehumidificationtomaintaincomfortinthebuilding.Airconditioningsystemsarealsousedinotherapplications,suchasautomobiles,trucks,aircraft,ships,andindustrialfacilities.However,thedescriptionofequipmentinthischapterislimitedtothosecommonlyusedincommercialandresidentialbuildings.

Commercialbuildingsrangefromlargehigh-riseofficebuildingstothecornerconveniencestore.Becauseoftherangeinsizeandtypesofbuildingsinthecommercialsector,thereisawidevarietyofequipmentappliedinthesebuildings.Forlargerbuildings,theairconditioningequipmentispartofatotalsystemdesignthatincludesitemssuchasapipingsystem,airdistributionsystem,andcoolingtower.Properdesignofthesesystemsrequiresaqualifiedengineer.Theresidentialbuildingsectorisdominated

bysinglefamilyhomesandlow-riseapartments/condominiums.Thecoolingequipmentappliedinthesebuildingscomesinstandard“packages”thatare

oftenbothsizedandinstalledbytheairconditioningcontractor.

Thechapterstartswithageneraldiscussionofthevaporcompressionrefrigerationcyclethenmovestorefrigerantsandtheirselection,followedbypackagedChilledWaterSystems。

1.1VaporCompressionCycle

Eventhoughthereisalargerangeinsizesandvarietyofairconditioningsystemsusedinbuildings,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,suchasacapillarytubeor

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shorttubeorifice,oranactivedevice,suchasathermalexpansionvalveorelectronicexpansionvalve.Thepurposeofthevalveistoregulatetheflowofrefrigeranttotheevaporatorsothattherefrigerantissuperheatedwhenitreachesthesuctionofthecompressor.

Attheexitoftheexpansionvalve,therefrigerantisatatemperaturebelowthatofthemedium（airorwater）tobecooled.Therefrigeranttravelsthroughaheatexchangercalledtheevaporator.Itabsorbsenergyfromtheairorwatercirculatedthroughtheevaporator.Ifairiscirculatedthroughtheevaporator,thesystemiscalleda.Ifwateriscirculatedthroughdirectexpansionsystem

theevaporator,itiscalledachiller.Ineithercase,therefrigerantdoes

notmakedirectcontactwiththeairorwaterintheevaporator.Therefrigerantisconvertedfromalowquality,two-phasefluidtoasuperheated

vaporundernormaloperatingconditionsintheevaporator.Thevaporformedmustberemovedbythecompressoratasufficientratetomaintainthelowpressureintheevaporatorandkeepthecycleoperating.

Allmechanicalcoolingresultsintheproductionofheatenergythatmustberejectedthroughthecondenser.Inmanyinstances,thisheatenergyisrejectedtotheenvironmentdirectlytotheairinthecondenserorindirectlytowaterwhereitisrejectedinacoolingtower.Withsomeapplications,itispossibletoutilizethiswasteheatenergytoprovidesimultaneousheatingtothebuilding.Recoveryofthiswasteheatattemperaturesupto65?

C（150?

F）canbeusedtoreducecostsforspaceheating.

Capacitiesofairconditioningareoftenexpressedineithertonsorkilowatts（kW）ofcooling.Thetonisaunitofmeasurerelatedtotheabilityofaniceplanttofreezeoneshortton（907kg）oficein24hr.Itsvalueis3.51kW（12,000Btu/hr）.ThekWofthermalcoolingcapacityproducedbytheairconditionermustnotbeconfusedwiththeamountofelectricalpower（alsoexpressedinkW）requiredtoproducethecoolingeffect.

2.1RefrigerantsUseandSelection

Upuntilthemid-1980s,refrigerantselectionwasnotanissueinmost

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buildingairconditioningapplicationsbecausetherewerenoregulationsontheuseofrefrigerants.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）pointof–44?

C（–47?

F）

andacondensation（dew）pointof–37?

C（–35?

F）,whichgivesitatemperature

glideof7?

C（12?

F）.Anazeotropicmixturebehaveslikeasinglecomponent

refrigerantinthatthesaturationtemperaturedoesnotchangeappreciablyasitevaporatesorcondensesatconstantpressure.R-410Ahasasmallenoughtemperatureglide（lessthan5.5?

C,10?

F）thatitisconsidereda

near-azeotropicrefrigerantmixture.

ASHRAEgroupsrefrigerantsbytheirtoxicityandflammability（ASHRAE,1994）.GroupA1isnonflammableandleasttoxic,whileGroupB3isflammableand

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mosttoxic.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,thelubrican