冷水机组毕业设计外文翻译.docx
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冷水机组毕业设计外文翻译.docx
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冷水机组毕业设计外文翻译
英文翻译
ChilledWaterSystems[1]
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.Astherotorsconti
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