本科畢業(yè)設(shè)計外文文獻(xiàn)及譯文文獻(xiàn)、資料題目：CoolingTowers文獻(xiàn)、資料來源：HVACEquipmentandSystems文獻(xiàn)、資料發(fā)表（出版）日期：院（部）：專業(yè)：班級：姓名：學(xué)號：指導(dǎo)教師：翻譯日期：山東XX大學(xué)外文文獻(xiàn)及譯文PAGE39-外文資料CoolingTowersIfachillerisusedtoprovidechilledwaterforbuildingairconditioning,thentheheatenergythatisabsorbedthroughthatprocessmustberejected.Thetwomostcommonwaystorejectthermalenergyfromthevaporcompressionprocessareeitherdirectlytotheairorthroughacoolingtower.Inacoolingtower,waterisrecirculatedandevaporativelycooledthroughdirectcontactheattransferwiththeambientair.Thiscooledwatercanthenbeusedtoabsorbandrejectthethermalenergyfromthecondenserofthechiller.ThemostcommoncoolingtowerusedforHVACapplicationsisthemechanicaldraftcoolingtower(Figure).Themechanicaldrafttowerusesoneormorefanstoforceairthroughthetower,aheattransfermediaorfillthatbringstherecirculatedwaterintocontactwiththeair,awaterbasin(sump)tocollecttherecirculatedwater,andawaterdistributionsystemtoensureevendispersalofthewaterintothetowerfill.Figureshowstherelationshipbetweentherecirculatingwaterandairastheyinteractinacounterflowcoolingtower.Theevaporativecoolingprocessinvolvessimultaneousheatandmasstransferasthewatercomesintocontactwiththeatmosphericair.Ideally,thewaterdistributionsystemcausesthewatertosplashoratomizeintosmallerdroplets,increasingthesurfaceareaofwateravailableforheattransfer.Theapproachtothewet-bulbisacommonlyusedindicatoroftowersizeandperformance.Itisdefinedasthetemperaturedifferencebetweenthecoolingwaterleavingthetowerandthewet-bulboftheairenteringthetower.Theoretically,thewaterbeingrecirculatedinatowercouldreachthewetbulbtemperature,butthisdoesnotoccurinactualtoweroperations.FIGURE4.2.14Air/watertemperaturerelationshipinacounterflowcoolingtower.Therangeforachiller/towercombinationisdeterminedbythecondenserthermalloadandthecoolingwaterflowrate,notbythecapacityofthecoolingtower.Therangeisdefinedasthetemperaturedifferencebetweenthewaterenteringthecoolingtowerandthatleaving.Thedriveroftowerperformanceistheambientwet-bulbtemperature.Thelowertheaveragewet-bulbtemperature,the“easier”itisforthetowertoattainthedesiredrange,typically6°C(10°F)forHVACapplications.Thus,inahot,dryclimatetowerscanbesizedsmallerthanthoseinahotandhumidareaforagivenheatload.Coolingtowersarewidelyusedbecausetheyallowdesignerstoavoidsomecommonproblemswithrejectionofheatfromdifferentprocesses.Theprimaryadvantageofthemechanicaldraftcoolingtowerisitsabilitytocoolwatertowithin3–6°C(5–10°F)oftheambientwet-bulbtemperature.Thismeansmoreefficientoperationoftheconnectedchillingequipmentbecauseofimproved(lower)headpressureoperationwhichisaresultofthelowercondensingwatertemperaturessuppliedfromthetower.CoolingTowerDesignsTheASHRAESystemsandEquipmentHandbook(1996)describesover10typesofcoolingtowerdesigns.ThreebasiccoolingtowerdesignsareusedformostcommonHVACapplications.Baseduponairandwaterflowdirectionandlocationofthefans,thesetowerscanbeclassifiedascounterflowinduceddraft,crossflowinduceddraft,andcounterflowforceddraft.Onecomponentcommontoallcoolingtowersistheheattransferpackingmaterial,orfill,installedbelowthewaterdistributionsystemandintheairpath.Thetwomostcommonfillsaresplashandfilm.Splashfilltendstomaximizethesurfaceareaofwateravailableforheattransferbyforcingwatertobreakapartintosmallerdropletsandremainentrainedintheairstreamforalongertime.Successivelayersofstaggeredsplashbarsarearrangedthroughwhichthewaterisdirected.Filmfillachievesthiseffectbyforcingwatertoflowinthinlayersoverdenselypackedfillsheetsthatarearrangedforverticalflow.Towersusingfilmtypefillareusuallymorecompactforagiventhermalload,anadvantageifspaceforthetowersiteislimited.Splashfillisnotassensitivetoairorwaterdistributionproblemsandperformsbetterwherewaterqualityissopoorthatexcessivedepositsinthefillmaterialareaproblem.CounterflowInducedDraftAirinacounterflowinduceddraftcoolingtowerisdrawnthroughthetowerbyafanorfanslocatedatthetopofthetower.Theairentersthetoweratlouversinthebaseandthencomesintocontactwithwaterthatisdistributedfrombasinsatthetopofthetower.Thus,therelativedirectionsarecounter(downforthewater,upfortheair)inthisconfiguration.ThisarrangementisshowninFigure.Inthisconfiguration,thetemperatureofthewaterdecreasesasitfallsdownthroughthecounterflowingair,andtheairisheatedandhumidified.Dropletsofwaterthatmighthavebeenentrainedintheairstreamarecaughtatthedrifteliminatorsandreturnedtothesump.Airandsomecarryoverdropletsareejectedthroughthefansandoutthetopofthetower.Thewaterthathasbeencooledcollectsinthesumpandispumpedbacktothecondenser.FIGURECounterflowinduceddraftcoolingtower.Counterflowtowersgenerallyhavebetterperformancethancrossflowtypesbecauseoftheevenairdistributionthroughthetowerfillmaterial.Thesetowersalsoejectairathighervelocitieswhichreducesproblemswithexhaustairrecirculationintothetower.However,thesetowersarealsosomewhattallerthancrossflowtypesandthusrequiremorecondenserpumphead.CrossflowInducedDraftAsinthecounterflowcoolingtower,thefaninthecrossflowtowerislocatedatthetopoftheunit(Figure).Airentersthetoweratsideorendlouversandmoveshorizontallythroughthetowerfill.Waterisdistributedfromthetopofthetowerwhereitisdirectedintothefillandiscooledbydirectcontactheattransferwiththeairincrossflow(airhorizontalandwaterdown).Watercollectedinthesumpispumpedbacktothechillercondenser.Theincreasedairflowpossiblewiththecrossflowtowerallowsthesetowerstohaveamuchloweroverallheight.Thisresultsinlowerpumpheadrequiredonthecondenserwaterpumpcomparedtothecounterflowtower.Thereducedheightalsoincreasesthepossibilityofrecirculatingtheexhaustairfromthetopofthetowerbackintothesideorendairintakeswhichcanreducethetower’seffectiveness.CounterflowForcedDraftCounterflowforceddraftcoolingtowershavethefanmountedatornearthebottomoftheunitneartheairintakes(Figure).Asintheothertowers,waterisdistributeddownthroughthetoweranditsfill,andthroughdirectcontactwithatmosphericairitiscooled.Thermaloperationofthistowerissimilartothecounterflowinduceddraftcoolingtower.Fanvibrationisnotassevereforthisarrangementcomparedtoinduceddrafttowers.Thereisalsosomeadditionalevaporativecoolingbenefitbecausethefandischargesairdirectlyacrossthesumpwhichfurthercoolsthewater.Therearesomedisadvantagestothistower.First,theairdistributionthroughthefillisuneven,whichreducestowereffectiveness.Second,thereisriskofexhaustairrecirculationbecauseofthehighsuctionvelocityatthefaninlets,whichcanreducetowereffectiveness.Thesetowersfindapplicationsinsmallandmedium-sizedsystems.MaterialsCoolingtowersoperateinacontinuouslywetconditionthatrequiresconstructionmaterialstomeetchallengingcriteria.Besidesthewetconditions,recirculatingwatercouldhaveahighconcentrationofmineralsaltsduetotheevaporationprocess.Coolingtowermanufacturersbuildtheirunitsfromacombinationofmaterialsthatprovidethebestcombinationofcorrosionresistanceandcost.Woodisatraditionalmaterialusedincoolingtowerconstruction.Redwoodorfirareoftenusedandareusuallypressuretreatedwithpreservativechemicals.Chemicalssuchaschromatedcopperarsenateoracidcopperchromatehelppreventdecayduetofungiordestructionbytermites.FIGURECrossflowinduceddraftcoolingtower.FIGURECounterflowforceddraftcoolingtower.Galvanizedsteeliscommonlyusedforsmall-tomid-sizedcoolingtowerstructures.Hardwareisusuallymadeofbrassorbronze.Criticalcomponents,suchasdriveshafts,hardwaremountingpoints,etc.,maybemadefrom302or304stainlesssteel.Castironcanbefoundinbasecastings,motorhousings,andfanhubs.Metalscoatedwithplasticsarefindingapplicationforspecialcomponents.Manymanufacturersmakeextensiveuseoffiberglass-reinforcedplastic(FRP)intheirstructure,pipe,fanblades,casing,inletlouvers,andconnectioncomponents.Polyvinylchloride(PVC)isusedforfillmedia,drifteliminators,andlouvers.Fillbarsandfloworificesarecommonlyinjectionmoldedfrompolypropyleneandacrylonitrilebutadienestyrene(ABS).Concreteisnormallyusedforthewaterbasinorsumpoffielderectedtowers.Tilesormasonryareusedinspecialtytowerswhenaestheticsareimportant.PerformanceRejectionoftheheatloadproducedatthechillingequipmentistheprimarygoalofacoolingtowersystem.Thisheatrejectioncanbeaccomplishedwithanoptimizedsystemthatminimizesthetotalcompressorpowerrequirementsofthechillerandthetowerloadssuchasthefansandcondenserpumps.Severalcriteriamustbedeterminedbeforethedesignercancompleteathoroughcoolingtoweranalysis,includingselectionoftowerrange,water-to-airratio,approach,filltypeandconfiguration,andwaterdistributionsystem.Tablelistssomeofthecommondesigncriteriaandnormallyacceptedrangesforcoolingtowers.MostcommonHVACapplicationsrequiringacoolingtowerwillusean“offtheshelf”unitfromacoolingtowermanufacturer.Manufacturerrepresentativesareusuallywellinformedabouttheirproductsandtheirproperapplication.AftertheprojectdesignprocesshasproducedtheinformationcalledforinTable,itistimetocontactoneormorecoolingtowerrepresentativesandseektheirinputoncorrecttowerselection.ControlSchemewithChillersMostcoolingtowersaresubjecttolargechangesinloadandambientwet-bulbtemperatureduringnormaloperations.Foratypicalcoolingtower,thetowerfanenergyconsumptionisapproximately10%oftheelectricpowerusedbythechillercompressor.Thecondenserpumpsareabout2–5%ofthecompressorpower.Controllingthecapacityofatowertosupplyadequatelycooledwatertothecondenserwhileminimizingenergyuseisadesirableoperationalscheme.ProbablythemostcommoncontrolschemeemployedfortowersservinganHVACloadistomaintainafixedleavingwatertemperature,usually27°C(80°F).Fancyclingisacommonmethodtoachievethiscoolingtowercontrolstrategyandisapplicabletomultiunitandmulticelltowerinstallations.However,thiscontrolmethoddoesnotminimizetotalenergyconsumedbythechiller/coolingtowersystemcomponents.Loweringthecondensingwatertemperatureincreasesachiller’sefficiency.Aslongastheevaporatortemperatureisconstant,areducedcondensertemperaturewillyieldalowerpressuredifferencebetweentheevaporatorandcondenserandreducetheloadonthecompressor.However,itisimportanttorecognizethattheefficiencyimprovementsinitiallygainedthroughlowercondensertemperaturesarelimited.Improvedchillerefficiencymaybeoffsetbyincreasedtowerfanandpumpingcosts.Maintainingaconstantapproachatsomeminimumtemperatureisdesirableaslongasthecondensingtemperaturedoesnotfallbelowthechillermanufacturer’srecommendations.Sincemostmoderntowersusetwo-orthree-speedfans,anearoptimalcontrolschemecanbedevelopedasfollows(BraunandDiderrich,1990):?Towerfansshouldbesequencedtomaintainaconstantapproachduringpartloadoperationtominimizechiller/coolingtowerenergyuse.?Theproductofrangeandcondensingwaterflowrate,ortheheatenergyrejected,shouldbeusedtodeterminethesequencingofthetowerfans.?Developasimplerelationshipbetweentowercapacityandtowerfansequencing.DeSaullesandPearson(1997)foundthatsavingsforasetpointcontrolversusthenearoptimalcontrolforacoolingtowerwereverysimilar.Theircontrolschemecalledforthetowertoproducewateratthelowestsetpointpossible,butnotlessthanthechillermanufacturerwouldallow,andtocomparethatoperationtothesavingsobtainedusingnearoptimalcontrolasdescribedabove.Theyfoundthatthelevelofsavingsthatcouldbeachievedwasdependentontheloadprofileandthemethodofoptimization.Theirsimulationsshowed2.5to6.5%energysavingsforthesinglesetpointmethodwhilethenearoptimalcontrolyieldedsavingsof3to8%.Useofvariablespeedfanswouldincreasethesavingsonlyinmosttowerinstallations.Itismoreeconomicaltooperatemultiplecoolingtowerfansatthesamespeedthantooperateoneatmaximumbeforestartingthenextfan.Variablespeedfansshouldbeusedwhenpossibleincoolingtowers.ThesystemdesignershouldensurethatanynewlyinstalledcoolingtoweristestedaccordingtoASMEStandardPTC23(ASME1986)orCTIStandardATC-105.Thesefieldtestsensurethatthetowerisperformingasdesignedandcanmeettheheatrejectionrequirementsfortheconnectedchillerorrefrigerationload.SelectionCriteriaThecriterialistedinTable4.2.6areusuallyknownaprioribythedesigner.Ifnotknownexplicitly,thencommonlyacceptedvaluescanbeused.Thesecriteriaareusedtodeterminethetowercapacityneededtorejecttheheatloadatdesignconditions.Otherconsiderationsbesidesthetower’scapacityincludeeconomics,servicing,environmentalconsiderations,andaesthetics.Manyofthesefactorsareinterrelated,but,ifpossible,theyshouldallbeevaluatedwhenselectingaparticulartowerdesign.Becauseeconomicsisanimportantpartoftheselectionprocess,twomethodsarecommonlyused—life-cyclecostingandpaybackanalysis.Theseprocedurescompareequipmentonthebasisofowning,operation,andmaintenancecosts.Othercriteriacanalsoaffectfinalselectionofacoolingtowerdesign:buildingcodes,structuralconsiderations,serviceability,availabilityofqualifiedservicepersonnel,andoperationalflexibilityforchangingloads.Inaddition,noisefromtowerscanbecomeasensitiveenvironmentalissue.Iflocalbuildingcodesoundlimitsareanissue,soundattenuatorsattheairintakesandthetowerfanexitshouldbeconsidered.Aestheticscanbeaproblemwithmodernarchitecturalbuildingsoronsiteswithlimitedlandspace.Severaltowermanufacturerscanerectcustomunitsthatcancompletelymaskthecoolingtoweranditsoperation.Applications[1][1]節(jié)選自JamesB.Bradfordetal.“HVACEquipmentandSystems”.HandbookofHeating,Ventilation,andAir-Conditioning.Ed.JanF.Kreider.BocaRaton,CRCPressLLC.2001Unlikechillers,pumps,andairhandlers,thecoolingtowermustbeinstalledinanopenspacewithcarefulconsiderationoffactorsthatmightcauserecirculation(recaptureofaportionofwarmandhumidexhaustairbythesametower)orrestrictairflow.Apoortowersitingsituationmightleadtorecirculation,aproblemnotrestrictedtowetcoolingtowers.Similarrecirculationcanoccurwithair-cooledcondensingequipmentaswell.Withcoolingtowerrecirculation,performanceisadverselyaffectedbytheincreaseinenteringwet-bulbtemperature.Theprimarycausesofrecirculationarepoorsitingofthetoweradjacenttostructures,inadequateexhaustairvelocity,orinsufficientseparationbetweentheexhaustandintakeofthetower.Multipletowerinstallationsaresusceptibletointerference—whentheexhaustairfromonetowerisdrawnintoatowerlocateddownwind.Symptomssimilartotherecirculationphenomenonthenplaguethedownwindtower.Forrecirculation,interference,orphysicallyblockingair-flowtothetowertheresultislargerapproachandrangewhichcontributetohighercondensingpressureatthechiller.Bothrecirculationandinterferencecanbeavoidedthroughcarefulplanningandlayout.Anotherimportantconsiderationwhensitingacoolingtowerinstallationistheeffectoffogging,orplume,andcarryover.Foggingoccursduringcoolerweatherwhenmoistwarmairejectedfromthetowercomesintocontactwiththecoldambientair,condenses,andformsfog.Fogfromcoolingtowerscanlimitvisibilityandcanbeanarchitecturalnuisance.Carryoveriswhensmalldropletsofentrainedwaterintheairstreamarenotcaughtbythedrifteliminatorsandareejectedintheexhaustairstream.Thesedropletsthenprecipitateoutfromtheexhaustairandfalltothegroundlikealightmistorrain(inextremecases).Carryoverordriftcontainsmineralsandchemicalsfromthewatertreatmentinthetowerandcancausestainingordiscolorationofthesurfacesitsettlesupon.Tomitigateproblemswithfogorcarryover,aswithrecirculation,thedesignershouldconsidernearbytrafficpatterns,parkingareas,prevailingwinddirection,largeglassareas,orotherarchitecturalconsiderations.OperationandMaintenanceWinterOperationIfchillersorrefrigerationequipmentarebeingusedincoldweather,freezeprotectionshouldbeconsideredtoavoidformationoficeonorinthecoolingtower.Capacitycontrolisonemethodthatcanbeusedtocontrolwatertemperatureinthetoweranditscomponents.Electricimmersionheatersareusuallyinstalledinthetowersumptoprovideadditionalfreezeprotection.Sinceicingoftheairintakescanbeespeciallydetrimentaltotowerperformance,thefanscanbereversedtode-icetheseareas.Ifthefansareoperatinginextremelycoldweather,icecanaccumulateontheleadingedgesofthefanblades,whichcancauseseriousimbalanceinthefansystem.Instrumentationtodetectout-of-limitsvibrationoreccentricityinrotationalloadsshouldbeinstalled.Aswithanyoperationalequipment,frequentvisualinspectionsduringextremeweatherarerecommended.WaterTreatmentThewatercirculatinginacoolingtowermustbeatanadequatequalityleveltohelpmaintaintowereffectivenessandpreventmaintenanceproblemsfromoccurring.Impuritiesanddissolvedsolidsareconcentratedintowerwaterbecauseofthecontinuousevaporationprocessasthewateriscirculatedthroughthetower.Dirt,dust,andgasescanalsofindtheirwayintothetowerwaterandeitherbecomeentrainedinthecirculatingwaterorsettleintothetowersump.Toreducetheconcentrationofthesecontaminants,apercentageofthecirculatingwaterisdrainedorblown-down.Insmallerevaporativelycooledsystems,thisprocessiscalledableed-offandiscontinuous.Blow-downisusually0.8to1.2%ofthetotalwatercirculationrateandhelpstomaintainreducedimpurityconcentrationsandtocontrolscaleformation.Ifthetowerisservedwithverypoorwaterquality,additionalchemicaltreatmentsmightbeneededtoinhibitcorrosion,controlbiologicalgrowth,andlimitthecollectionofsilt.Ifthetowerinstallationpresentscontinuingwaterqualityproblems,awatertreatmentspecialistshouldbeconsulted.LegionellosisLegionellosishasbeenconnectedwithevaporativecondensers,coolingtowers,andotherbuildinghydroniccomponents.Researchershavefoundthatwell-maintainedtowerswithgoodwaterqualitycontrolwerenotusuallyassociatedwithcontaminationbyLegionellapneumophilabacteria.InapositionpaperconcerningLegionellosis,theCoolingTowerInstitute(CTI,1996)statedthatcoolingtowersarepronetocolonizationbyLegionellaandhavethepotentialtocreateanddistributeaerosoldroplets.Optimumgrowthofthebacteriawasfoundtobeatabout37°C(99°F)whichisaneasilyattainedtemperatureinacoolingtower.TheCTIproposedrecommendationsregardingcoolingtowerdesignandoperationtominimizethepresenceofLegionella.TheydonotrecommendfrequentorroutinetestingforLegionellapneumophilabacteriabecausethereisdifficultyinterpretingtestresults.Acleantowercanquicklybereinfected,andacontaminatedtowerdoesnotmeananoutbreakofthediseasewilloccur.MaintenanceThecoolingtowermanufacturerusuallyprovidesoperatingandmaintenance(O&M)manualswithanewtowerinstallation.Thesemanualsshouldincludeacompletelistofallpartsusedandreplaceableinthetowerandalsodetailsontheroutinemaintenancerequiredforthecoolingtower.Ataminimum,thefollowingshouldalsobeincludedaspartofthemaintenanceprogramforacoolingtowerinstallation.?Periodicinspectionoftheentireunittoensureitisingoodrepair.?Completeperiodicdrainingandcleaningofallwettedsurfacesinthetower.Thisgivestheopportunitytoremoveaccumulationsofdirt,slime,scale,andareaswherealgaeorbacteriamightdevelop.?Periodicwatertreatmentforbiologicalandcorrosioncontrol.?Continuousdocumentationonoperationandmaintenanceofthetower.ThisdevelopsthebaselineforfutureO&Mdecisionsandisveryimportantforapropermaintenancepolicy.4.2.4PackagedEquipmentCentralHVACsystemsarenotalwaysthebestapplicationforaparticularcoolingorheatingload.Initialcostsforcentralsystemsareusuallymuchhigherthanunitaryorpackagedsystems.Theremayalsobephysicalconstraintsonthesizeofthemechanicalcomponentsthatcanbeinstalledinthebuilding.Unitaryorpackagedsystemscomefactoryassembledandprovideonlycoolingorcombinedheatingandcooling.Thesesystemsaremanufacturedinavarietyofconfigurationsthatallowthedesignertomeetalmostanyapplication.Cabinetorskid-mountedforeasyinstallation,typicalunitsgenerallyconsistofanevaporator,blower,compressor,condenser,and,ifacombinedsystem,aheatingsection.Thecapacitiesoftheunitsrangesfromapproximately5kWto460kW(1.5to130tons).Typicalunitarysystemsaresingle-packagedunits(windowunits,rooftopunits),split-systempackagedunits,heatpumpsystems,andwatersourceheatpumpsystems.Unitarysystemsdonotlastaslong(only8to15years)ascentralHVACequipmentandareoftenlessefficient.Unitarysystemsfindapplicationinbuildingsuptoeightstoriesinheight,buttheyaremoregenerallyusedinone-,two-,orthree-storybuildingsthathavesmallercoolingloads.Theyaremostoftenusedforretailspaces,smallofficebuildings,andclassrooms.Unitaryequipmentisavailableonlyinpreestablishedcapacityincrementswithsetperformancecharacteristics,suchastotalL/s(cfm)deliveredbytheunit’sairhandler.SomedesignerscombinecentralHVACsystemswithpackagedequipmentusedonperimeterbuildingzones.Thiscompositecansolvehumidityandspacetemperaturerequirementsbetterthanpackagedunitsalone.Thisalsoworkswellinbuildingswhereitisimpracticalforpackagedunitstoserveinteriorspaces.TablelistssomeoftheadvantagesanddisadvantagesofpackagedandunitaryHVACequipment.Tablelistsenergyefficiencyratings(EERs)fortypicalelectricair-andwater-cooledsplitandsinglepackageunitswithcapacitygreaterthan19kW(65,000Btuh).Typically,commercialbuildingsuseunitarysystemswithcoolingcapacitiesgreaterthan18kW(5tons).Insomecases,however,duetospacerequirements,physicallimitations,orsmalladditions,residential-sizedunitarysystemsareused.Ifaunitarysystemis10yearsorolder,energysavingscanbeachievedbyreplacingunitarysystemswithproperlysized,energy-efficientmodels.aElectricair-andwater-cooledsplitsystemandsinglepackageunitswithcapacityover19kW(65,000Btuh)arecoveredhere.bEER,orenergyefficiencyratio,isthecoolingcapacityinkW(Btu/h)oftheunitdividedbyitselectricalinput(inwatts)atstandard(ARI)conditionsof35°C(95°F)forair-cooledequipment,and29°C(85°F)enteringwaterforwater-cooledmodels.cBasedonARI210/240testprocedure.dSEER(seasonalenergyefficiencyratio)isthetotalcoolingoutputkW(Btu)providedbytheunitduringitsnormalannualusageperiodforcoolingdividedbythetotalenergyinput(inWh)duringthesameperiod.eSplitsystemandsinglepackageunitswithtotalcapacityunder19kW(65,000Btuh)arecoveredhere.Thisanalysisexcludeswindowunitsandpackagedterminalunits.FIGUREComparisonbetweenTXVandshort-tubeorificesystemscapacityforarangeofchargingconditionsand95°F(35°C)outdoortemperature.(FromRodriquezetal.,1996).AswithanyHVACequipment,propermaintenanceandoperationwillensureoptimumperformanceandlifeforasystem.Split-systemairconditionersandheatpumpsarethemostcommonunitsappliedinresidentialandsmallcommercialapplications.Theseunitsaretypicallyshippedtotheconstructionsiteasseparatecomponents;afterthecondenser(outdoorunit)andtheevaporator(indoorunit)aremounted,therefrigerantpipingisconnectedbetweenthem.Theairconditioningtechnicianmustensurethattheunitisproperlychargedwithrefrigerantandcheckforproperoperation.Ifthesystemisunder-orovercharged,performancecanbeadverselyaffected.Rodriquezetal.(1996)foundthatperformanceofanairconditioningsystemequippedwithashorttubeorificewasaffectedbyimpropercharge(Figure).TheplotinFigureclearlyshowsthatfora20%under-chargeinrefrigerant,aunitwithashorttubeorificesuffersa30%decreaseincoolingcapacity.Thissamestudyalsoinvestigatedtheeffectsofreturn-airleakage.Acommonproblemwithnewinstallationsisimpropersealingofductconnectionsatthediffusersandgrillsaswellasaroundthereturn-airplenum.Leakageamountsaslowas5%inthereturnairductsresultedincapacityandefficiencyreductionsofalmost20%forhighhumidityclimates.Thesereductionsdroppedtoabout7%forlowhumidityclimates.Theresultsofthechargingandleakagestudiessuggesttheneedfortheinstallationcontractor,maintenancecontractor,andsystemownertoensuretheproperinstallationoftheairconditioningsystem.FIGURERooftoppackagedheatingandairconditioningunit.(AdaptedfromCarrierCorporation).PackagedUnitsPackagedunitsarecompleteHVACunitsthatareusuallymountedontheexteriorofastructure(rooforwall)freeingupvaluableindoorfloorspace(Figure).Theycanalsobeinstalledonaconcretehousekeepingpadatgroundlevel.Becausetheyareself-contained,completemanufacturedunits,installationcostsareusuallylowerthanf