暖通空调专业 毕业设计外文翻译5文档格式.docx

暖通空调专业 毕业设计外文翻译5文档格式.docx

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Chilledwatersystemswereusedinlessthan4%ofcommercialbuildingsintheU.S.in1995.However,becausechillersareusuallyinstalledinlargerbuildings,chillerscooledover28%oftheU.S.commercialbuildingfloorspacethatsameyear（DOE,1998）.Fivetypesofchillersarecommonlyappliedtocommercialbuildings:

reciprocating,screw,scroll,centrifugal,andabsorption.Thefirstfourutilizethevaporcompressioncycletoproducechilledwater.Theydifferprimarilyinthetypeofcompressorused.Absorptionchillersutilizethermalenergy（typicallysteamorcombustionsource）inanabsorptioncyclewitheitheranammonia-waterorwater-lithiumbromidesolutiontoproducechilledwater.

OverallSystem

Figure4.2.2showsasimplerepresentationofadualchillerapplicationwithallthemajorauxiliaryequipment.Anestimated86%ofchillersareappliedinmultiplechillerarrangementslikethatshowninthefigure（BitondoandTozzi,1999）.Inchilledwatersystems,returnwaterfromthebuildingiscirculatedthrougheachchillerevaporatorwhereitiscooledtoanacceptabletemperature（typically4to7°

C）（39to45°

F）.Thechilledwateristhendistributedtowater-to-airheatexchangersspreadthroughoutthefacility.Intheseheatexchangers,airiscooledanddehumidifiedbythecoldwater.Duringtheprocess,thechilledwaterincreasesintemperatureandmustbereturnedtothechiller（s）.

ThechillersshowninFigure4.2.2arewater-cooledchillers.Wateriscirculatedthroughthecondenserofeachchillerwhereitabsorbsheatenergyrejectedfromthehighpressurerefrigerant.Thewateristhenpumpedtoacoolingtowerwherethewateriscooledthroughanevaporationprocess.Coolingtowersaredescribedinalatersection.Chillerscanalsobeaircooled.Inthisconfiguration,thecondenserwouldbearefrigerant-to-airheatexchangerwithairabsorbingtheheatenergyrejectedbythehighpressurerefrigerant.

Chillersnominallyrangeincapacitiesfrom30to18,000kW（8to5100ton）.MostchillerssoldintheU.S.areelectricandutilizevaporcompressionrefrigerationtoproducechilledwater.Compressorsforthesesystemsareeitherreciprocating,screw,scroll,orcentrifugalindesign.Asmallnumberofcentrifugalchillersaresoldthatuseeitheraninternalcombustionengineorsteamdriveinsteadofanelectricmotortodrivethecompressor.

FIGURE4.2.2Adualchillerapplicationwithmajorauxiliarysystems（courtesyofCarrierCorporation）.

Thetypeofchillerusedinabuildingdependsontheapplication.Forlargeofficebuildingsorinchillerplantsservingmultiplebuildings,centrifugalcompressorsareoftenused.Inapplicationsunder1000kW（280tons）coolingcapacities,reciprocatingorscrewchillersmaybemoreappropriate.Insmallerapplications,below100kW（30tons）,reciprocatingorscrollchillersaretypicallyused.

VaporCompressionChillers

Table4.2.5showsthenominalcapacityrangesforthefourtypesofelectricallydrivenvaporcompressionchillers.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.AsseeninTable4.2.5,centrifugalchillersarethemostefficient;

whereas,reciprocatingchillershavetheworstefficiencyofthefourtypes.Theefficiencynumbersprovidedinthetablearethesteadystatefull-loadefficiencydeterminedinaccordancetoASHRAEStandard30（ASHRAE,1995）.Theseefficiencynumbersdonotincludetheauxiliaryequipment,suchaspumpsandcoolingtowerfansthatcanaddfrom0.06to0.31kW/tontothenumbersshown（Smitetal.,1996）.

Chillersrunatpartloadcapacitymostofthetime.Onlyduringthehighestthermalloadsinthebuildingwillachilleroperatenearitsratedcapacity.Asaconsequence,itisimportanttoknowhowtheefficiencyofthechillervarieswithpartloadcapacity.Figure4.2.3showsarepresentativedatafortheefficiency（inkW/ton）asafunctionofpercentagefullloadcapacityforareciprocating,screw,andscrollchillerplusacentrifugalchillerwithinletvanecontrolandonewithvariablefrequencydrive（VFD）forthecompressor.Thereciprocatingchillerincreasesinefficiencyasitoperatesatasmallerpercentageoffullload.Incontrast,theefficiencyofacentrifugalwithinletvanecontrolisrelativelyconstantuntilthe

loadfallstoabout60%ofitsratedcapacityanditskW/tonincreasestoalmosttwiceitsfullyloadedvalue.

FIGURE4.2.3Chillerefficiencyasafunctionofpercentageoffullloadcapacity.

In1998,theAirConditioningandRefrigerationInstitute（ARI）developedanewstandardthatincorporatesintotheirratingspartloadperformanceofchillers（ARI1998c）.Partloadefficiencyisexpressedbyasinglenumbercalledtheintegratedpartloadvalue（IPLV）.TheIPLVtakesdatasimilartothatinFigure4.2.3andweightsitatthe25%,50%,75%,and100%loadstoproduceasingleintegratedefficiencynumber.Theweightingfactorsattheseloadsare0.12,0.45,0.42,and0.01,respectively.TheequationtodetermineIPLVis:

MostoftheIPLVisdeterminedbytheefficiencyatthe50%and75%partloadvalues.Manufacturerswillprovide,onrequest,IPLVsaswellaspartloadefficienciessuchasthoseshowninFigure4.2.3.

FIGURE4.2.4Volume-pressurerelationshipsforareciprocatingcompressor.

Thefourcompressorsusedinvaporcompressionchillersareeachbrieflydescribedbelow.Whilecentrifugalandscrewcompressorsareprimarilyusedinchillerapplications,reciprocatingandscrollcompressorsarealsousedinsmallerunitarypackagedairconditionersandheatpumps.

ReciprocatingCompressors

Thereciprocatingcompressorisapositivedisplacementcompressor.Ontheintakestrokeofthepiston,afixedamountofgasispulledintothecylinder.Onthecompressionstroke,thegasiscompresseduntilthedischargevalveopens.Thequantityofgascompressedoneachstrokeisequaltothedisplacementofthecylinder.Compressorsusedinchillershavemultiplecylinders,dependingonthecapacityofthecompressor.Reciprocatingcompressorsuserefrigerantswithlowspecificvolumesandrelativelyhighpressures.MostreciprocatingchillersusedinbuildingapplicationscurrentlyemployR-22.

Modernhigh-speedreciprocatingcompressorsaregenerallylimitedtoapressureratioofapproximatelynine.Thereciprocatingcompressorisbasicallyaconstant-volumevariable-headmachine.Ithandlesvarious

dischargepressureswithrelativelysmallchangesininlet-volumeflowrateasshownbytheheavyline（labeled16cylinders）inFigure4.2.4.Condenseroperationinmanychillersisrelatedtoambientconditions,forexample,throughcoolingtowers,sothatoncoolerdaysthecondenserpressurecanbereduced.Whentheairconditioningloadislowered,lessrefrigerantcirculationisrequired.TheresultingloadcharacteristicisrepresentedbythesolidlinethatrunsfromtheupperrighttolowerleftofFigure4.2.4.

Thecompressormustbecapableofmatchingthepressureandflowrequirementsimposedbythesystem.Thereciprocatingcompressormatchestheimposeddischargepressureatanyleveluptoitslimitingpressureratio.Varyingcapacityrequirementscanbemetbyprovidingdevicesthatunload

individualormultiplecylinders.Thisunloadingisaccomplishedbyblockingthesuctionordischargevalvesthatopeneithermanuallyorautomatically.Capacitycanalsobecontrolledthroughtheuseofvariablespeedormulti-speedmotors.Whencapacitycontrolisimplementedonacompressor,otherfactorsatpart-loadconditionsneedtoconsidered,suchas（a）effectoncompressorvibrationandsoundwhenunloadersareused,（b）theneedforgoodoilreturnbecauseoflowerrefrigerantvelocities,and（c）properfunctioningofexpansiondevicesatthelowercapacities.

Withmostreciprocatingcompressors,oilispumpedintotherefrigerationsystemfromthecompressorduringnormaloperation.Systemsmustbedesignedcarefullytoreturnoiltothecompressorcrankcasetoprovideforcontinuouslubricationandalsotoavoidcontaminatingheat-exchangersurfaces.

Reciprocatingcompressorsusuallyarearrangedtostartunloadedsothatnormaltorquemotorsareadequateforstarting.Whengasenginesareusedforreciprocatingcompressordrives,carefulmatchingofthetorquerequirementsofthecompressorandenginemustbeconsidered.

FIGURE4.2.5Illustrationofatwin-screwcompressordesign（courtesyofCarrierCorporation）.

ScrewCompressors

Screwcompressors,firstintroducedin1958（Thevenot,1979）,arepositivedisplacementcompressors.Theyareavailableinthecapacityrangesthatoverlapwithreciprocatingcompressorsandsmallcentrifugalcompressors.Bothtwin-screwandsingle-screwcompressorsareusedinchillers.Thetwin-screwcompressorisalsocalledthehelicalrotarycompressor.Figure4.2.5showsacutawayofatwin-screwcompressordesign.Therearetwomainrotors（screws）.Oneisdesignatedmale（4inthefigure）andtheotherfemale（6inthefigure）.

Thecompressionprocessisaccomplishedbyreducingthevolumeoftherefrigerantwiththerotarymotionofscrews.Atthelowpressuresideofthecompressor,avoidiscreatedwhentherotorsbegintounmesh.Lowpressuregasisdrawnintothevoidbetweentherotors.Astherotorsco