暖通空调专业毕业设计外文翻译

ChilledWaterSystems[1][1]节选自JamesB.Bradfordetal.“HVACEquipmentandSystems”.HandbookofHeating,Ventilation,andAir-Conditioning.Ed.JanF.Kreider.BocaRaton,CRCPressLLC.2001Chilledwatersystemswereusedinlessthan4%ofcommercialbuildingsintheU.S.in1995.However,becausechillersareusuallyinstalledinlargerbuildings,chillerscooledover28%oftheU.S.commercialbuildingfloorspacethatsameyear(DOE,1998).Fivetypesofchillersarecommonlyappliedtocommercialbuildings:reciprocating,screw,scroll,centrifugal,andabsorption.Thefirstfourutilizethevaporcompressioncycletoproducechilledwater.Theydifferprimarilyinthetypeofcompressorused.Absorptionchillersutilizethermalenergy(typicallysteamorcombustionsource)inanabsorptioncyclewitheitheranammonia-waterorwater-lithiumbromidesolutiontoproducechilledwater.OverallSystemFigure4.2.2showsasimplerepresentationofadualchillerapplicationwithallthemajorauxiliaryequipment.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.VaporCompressionChillersTable4.2.5showsthenominalcapacityrangesforthefourtypesofelectricallydrivenvaporcompressionchillers.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.Asseenin,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,theefficiencyofacentrifugalwithinletvanecontrolisrelativelyconstantuntiltheloadfallstoabout60%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,IPLVsaswellaspartloadefficienciessuchasthoseshownin.FIGURE4.2.4Volume-pressurerelationshipsforareciprocatingcompressor.Thefourcompressorsusedinvaporcompressionchillersareeachbrieflydescribedbelow.Whilecentrifugalandscrewcompressorsareprimarilyusedinchillerapplications,reciprocatingandscrollcompressorsarealsousedinsmallerunitarypackagedairconditionersandheatpumps.ReciprocatingCompressorsThereciprocatingcompressorisapositivedisplacementcompressor.Ontheintakestrokeofthepiston,afixedamountofgasispulledintothecylinder.Onthecompressionstroke,thegasiscompresseduntilthedischargevalveopens.Thequantityofgascompressedoneachstrokeisequaltothedisplacementofthecylinder.Compressorsusedinchillershavemultiplecylinders,dependingonthecapacityofthecompressor.Reciprocatingcompressorsuserefrigerantswithlowspecificvolumesandrelativelyhighpressures.MostreciprocatingchillersusedinbuildingapplicationscurrentlyemployR-22.Modernhigh-speedreciprocatingcompressorsaregenerallylimitedtoapressureratioofapproximatelynine.Thereciprocatingcompressorisbasicallyaconstant-volumevariable-headmachine.Ithandlesvariousdischargepressureswithrelativelysmallchangesininlet-volumeflowrateasshownbytheheavyline(labeled16cylinders)in.Condenseroperationinmanychillersisrelatedtoambientconditions,forexample,throughcoolingtowers,sothatoncoolerdaysthecondenserpressurecanbereduced.Whentheairconditioningloadislowered,lessrefrigerantcirculationisrequired.Theresultingloadcharacteristicisrepresentedbythesolidlinethatrunsfromtheupperrighttolowerleftof.Thecompressormustbecapableofmatchingthepressureandflowrequirementsimposedbythesystem.Thereciprocatingcompressormatchestheimposeddischargepressureatanyleveluptoitslimitingpressureratio.Varyingcapacityrequirementscanbemetbyprovidingdevicesthatunloadindividualormultiplecylinders.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).ScrewCompressorsScrewcompressors,firstintroducedin1958(Thevenot,1979),arepositivedisplacementcompressors.Theyareavailableinthecapacityrangesthatoverlapwithreciprocatingcompressorsandsmallcentrifugalcompressors.Bothtwin-screwandsingle-screwcompressorsareusedinchillers.Thetwin-screwcompressorisalsocalledthehelicalrotarycompressor.Figure4.2.5showsacutawayofatwin-screwcompressordesign.Therearetwomainrotors(screws).Oneisdesignatedmale(4inthefigure)andtheotherfemale(6inthefigure).Thecompressionprocessisaccomplishedbyreducingthevolumeoftherefrigerantwiththerotarymotionofscrews.Atthelowpressuresideofthecompressor,avoidiscreatedwhentherotorsbegintounmesh.Lowpressuregasisdrawnintothevoidbetweentherotors.Astherotorscontinuetoturn,thegasisprogressivelycompressedasitmovestowardthedischargeport.Oncereachingapredeterminedvolumeratio,thedischargeportisuncoveredandthegasisdischargedintothehighpressuresideofthesystem.Atarotationspeedof3600rpm,ascrewcompressorhasover14,000dischargesperminute(ASHRAE,1996).Fixedsuctionanddischargeportsareusedwithscrewcompressorsinsteadofvalves,asusedinreciprocatingcompressors.Thesesetthebuilt-involumeratio—theratioofthevolumeoffluidspaceinthemeshingrotorsatthebeginningofthecompressionprocesstothevolumeintherotorsasthedischargeportisfirstexposed.Associatedwiththebuilt-involumeratioisapressureratiothatdependsonthepropertiesoftherefrigerantbeingcompressed.Screwcompressorshavethecapabilitytooperateatpressureratiosofabove20:1(ASHRAE,1996).Peakefficiencyisobtainedifthedischargepressureimposedbythesystemmatchesthepressuredevelopedbytherotorswhenthedischargeportisexposed.Iftheinterlobepressureinthescrewsisgreaterorlessthandischargepressure,energylossesoccurbutnoharmisdonetothecompressor.Capacitymodulationisaccomplishedbyslidevalvesthatprovideavariablesuctionbypassordelayedsuctionportclosing,reducingthevolumeofrefrigerantcompressed.Continuouslyvariablecapacitycontrolismostcommon,butsteppedcapacitycontrolisofferedinsomemanufacturers’machines.Variabledischargeportingisavailableonsomemachinestoallowcontrolofthebuilt-involumeratioduringoperation.Oilisusedinscrewcompressorstosealtheextensiveclearancespacesbetweentherotors,tocoolthemachines,toprovidelubrication,andtoserveashydraulicfluidforthecapacitycontrols.Anoilseparatorisrequiredforthecompressordischargeflowtoremovetheoilfromthehigh-pressurerefrigerantsothatperformanceofsystemheatexchangerswillnotbepenalizedandtheoilcanbereturnedforreinjectioninthecompressor.Screwcompressorscanbedirectdrivenattwo-polemotorspeeds(50or60Hz).Theirrotarymotionmakesthesemachinessmoothrunningandquiet.Reliabilityishighwhenthemachinesareappliedproperly.Screwcompressorsarecompactsotheycanbechangedoutreadilyforreplacementormaintenance.Theefficiencyofthebestscrewcompressorsmatchesorexceedsthatofthebestreciprocatingcompressorsatfullload.Highisentropicandvolumetricefficienciescanbeachievedwithscrewcompressorsbecausetherearenosuctionordischargevalvesandsmallclearancevolumes.ScrewcompressorsforbuildingapplicationsgenerallyuseeitherR-134aorR-22.译文冷水机组1995年，在美国，冷水机组应用在至少4％的商用建筑中。而且，由于制冷机组通常安装在较大的建筑中，在同一年里，制冷机组冷却了多于28％的商用建筑的地板空间（DOE，1998）。在商用建筑中普遍采用五种型式的制冷机：往复式、螺杆式、旋涡式、离心式和吸收式。前四种利用蒸汽压缩式循环来制得冷冻水。它们的不同主要在于使用的压缩机种类的不同。吸收式制冷机在吸收循环中利用热能（典型的是来自蒸汽或燃料燃烧）并利用氨－水或水－锂溴化物制得冷冻水。总的系统图4.2.2两台制冷机同时作用的系统图及辅助设备（格力有限公司）图4.2.2给出了包括主要辅助设备在内的复式制冷机的简图。大约86％的制冷机和表所示的一样用在多台制冷机系统中（Bitondo和Tozzi，1999）。在冷冻水系统中，建筑物的回水通过每个蒸发器循环流动，在蒸发器中，回水被冷却到合意的温度（典型的为4～7℃－）（39～45℉）。然后，冷冻水通过各设备传送到水－空气换热器。在换热器中，空气被冷冻水冷却和加湿。在这个过程中，冷水的温度升高，然后必须回送到蒸发器中。图4.2.2所示的制冷机组是冷水机组。水通过每个机组的冷凝器循环，在冷凝器中，水吸收了来自高压制冷剂的热量。接着，水用水泵打到冷却塔中，水通过蒸发而降温。冷却塔将在后一部分讲述。冷凝器也可以是空冷式的。在这种循环中，冷凝器应是制冷剂－空气热交换器，空气吸收来自高压制冷剂的热量。制冷机组名义制冷量为30～18000kw（8～5100tons）。在美国，出售的大部分制冷机组是用电的，利用蒸汽压缩制冷循环来制得冷冻水。在设计中，这种系统所使用的压缩机也有往复式、螺杆式、旋涡式和离心式。一小部分的离心式制冷机利用内燃机或蒸汽机代替电来启动压缩机。在建筑中所使用的制冷机组类型根据应用场所来确定。对于大的办公室建筑或制冷机组需服务于多个建筑时，通常使用离心式压缩机。在所需制冷量小于1000kw（280tons）时，使用往复式或螺杆式制冷机组较合适。在小的应用场合，若低于100kw（30tons）时，使用往复式或旋涡式制冷机组。蒸汽压缩式制冷机图4.2.3制冷机在各种不同满负荷百分数时的效率表4.2.5表示了四种电启动的蒸汽压缩式制冷机组的名义制冷量范围。每种制冷机以所使用的压缩机类型来命名。各种系统的制冷能力范围从最小的旋涡式（30kw，8tons）到最大的离心式（18000kw，5000tons）。制冷机可使用HCFCs（R22，R123）或HFCs（R－134a）制冷剂。制冷机的效率通常用输入功（用kw表示）与制冷量（用tons表示）的比值表示。1tons的制冷量等于3.52kw或1200btu／h。用这种方法衡量效率，其数值越小越好。从表4.2.5可以看出，离心式制冷机的效率最高。而往复式是这四种类型中效率最低的。表中所提供的效率是根据ASHRAEStandard30（ASHRAE，1995）在稳定状态下测得满负荷时的效率，这些效率中不包括辅助设备的能耗，比如泵，冷却塔的风机，而这些设备可以增加0.06～0.31kw/ton（Smitetal..，1996）。制冷机组在大部分时候是在部分负荷下运行的。只有在建筑物的最高热负荷时，制冷机才在额定制冷量附近运行。知道制冷机在部分负荷下效率是怎样变化的，这是很重要的。图4.2.3给出了往复式、螺杆式、旋涡式、带叶片控制的离心式制冷机组、压缩机频繁启动的制冷机组在满负荷时的百分比下相应的效率（用kw/ton表示）。往复式制冷机在占满负荷较小的百分比运行时，效率增加。相反地，带叶片控制的离心式的效率在负荷为额定负荷的60％以后是基本不变的,它的kw/ton值随百分数的减小而增加到满负荷时的两倍.1998年，空调制冷学会提出了一项新的标准，用来划归在部分负荷下制冷机组的运行情况。部分负荷时的效率用综合部分负荷值（IPLV）这个简单的数值来表示。IPLV在数值上和图4.2.3相似。用25%，50%，75%，100%负荷时的效率来计算这个简单的综合效率。在这些负荷下的度量值分别为0.12，0.45，0.42，0.01。IPLV的计算公式为IPLV=1/（0.01/A+0.42/B+0.45/C+0.12/D）其中A——100%负荷时的效率B——75%负荷时的效率C——50%负荷时的效率D——25%负荷时的效率大多数的IPLV由满负荷的50%，75%时的效率决定的，根据要求，制造商除了提供如图4.2.3所示部分负荷时的效率，还会提供IPLV值。以下对使用在蒸汽压缩式制冷机中的四种压缩机做简要的讲述。离心式和螺杆式压缩机主要应用在制冷机组上。往复式和旋涡式压缩机应用在整体式空调和热泵中。往复式压缩机图4.2.4往复式压缩机容积-压力的关系往复式压缩机是一种有确定排量的压缩机。在活塞的进气冲程时，一定量的气体被吸进气缸。在压缩冲程时，气体被压缩直到排气阀打开。在每个冲程被压缩的气体数量等于气缸的体积。在制冷机中使用的压缩机根据压