轮机英语新教材电子版Lesson18air conditioning system

轮机英语新教材电子版Lesson18airconditioningsystemLesson18 airconditioningsystemShipstraveltheworldandarethereforesubjecttovariousclimaticconditons.thecrewoftheshipmustbeprovidedwithreasonableconditionsinwihichtowork regardless oftheweather.Temperturealoneisnotsufficentmeasureofconditionacceptabletothehumanbody.Relative humidityiinconjunctionwithtemperaturemoretrulydeterminestheenvironment forhumancomfort.Relativehumidity,expressedasapercentage,istheratioofthewatervapourpressureintheairtested,tothesaturatedvapourpressureofairatthesametemperature.Thefactthatlesswatercanbeabsorbedasairiscooledandmorecanbeabsoredwhenitisheatedisthemajorconsiderationinairconditioningsystemdesign.Otherfactorsarethenearnessofheatsources,exposuretosunlight,sourcesofcoldandtheinsulationprovidedaroundthespace(Fig.18-1).Anairconditioningsystemaimstoprovideacomfortableworkingenvironmentregardlessofoutsideconditions.Satisfactoryairtreatmentmustinvolvearelatively"closed"systemwheretheairiscirculatedandreturned.However,someairis"consume"byhumansandsomemachinerysothereisarequirementforrenewal.Publicroomsandaccommodationwilloperatewithareducedpercentageofairrenewalsincetheconditionigcostof100%renewalwouldbeconsiderable.Airconditioningsystemsfallintotwomainclasses:individualunitsystem,inwhicheachroomcontainsitsownsmallrefrigerationplantandfanandaircooler;andcentralsystems,wherelargerrefrigerationmachineryunitareinstalledandtheiroutputdistributedabouttheshipbyavarietyofmeans.Self-containedunitsarenoisierthancentralsystems,requiremoremaintenanceandhavebeenfoundtohavearelativelyshortlife(about7years).Thesingleductsystemonlyallowsforadjustmentoftemperatureineachroombytheoccupantmanuallycontrollingtheairvolumeadmitted.Itisthuslessflexiblethananyoftheothersystems,whichallowindividualtemperaturecontrol,atleastofsectionsoftheshipifnotindividualrooms.Withductedsystems,themoderntendencyistouse"highvelocity"intheairductswithfansgeneratingupto2550mbar(250mmH²O)pressurecomparedto"lowvelocity"systemswithfansgeneratingupto520mbar(50mmH²O).Thistendencyhelpsinstallationasthesizeofductsisreducedandprefabricatedstandardductscanbeused,butitincurstheheavierrunningcostsofmorepowerfulfans.Airterminalslinedwithsoundinsulationmaterialarenecessarytoreducethenoisepassingintotheroomwithhighvelocitysystems.Inatypicalmarinepatternself-containedunit,aircirculationisusuallyeffectedbymeansofacentrifugalfan,forquietrunning,andadirectexpansioncoolerservedbyahermeticcompressor.Watercooledcondensersareused.Asthesecontainsmallwaterpassages,chokingdevelopsrapidlywithdirectseawatercirculationandabettermethodistocirculatewithfreshwater,itselfcooledinaseawater/freshwaterheatexchanger.Controlison/offbyathermostatsensingthetemperatureofairreturningtotheunit.Thecoolingcoilofthecentralunitmaybeofthedirectexpansion,brineorchilledwatercooledtype.Whencoolingisbydirectexpansion,aseparatesteamheatercoilisfittedintheunitforwinterheating.Withbrineorwatercoolers,acentralheaterisusedsothatthesamecoilservesforsummerorwinter.Thermostaticcontrolisprovidedsensingairdeliverytemperatureitself,thetemperatureoftheroom,orthereturnairtemperture.Alltypesofthermostatsarefoundinairconditioningsystems,directacting,pneumaticandelectrical.Inthemselves,theyareallsatisfactoryinstruments,buttheresultstheyachievearedependentonthecorrectsittingoftheirsensingelements.Eventhesiteforadirectactingthermoststtocontrolonesingleberthcabinmustbechosenwithcare-ifitismaskedbehindcurtains,ortoofarawayfromtheairinletcontrolwillbetoosluggish.Thecorrectlocationforathermostattocontrolablockofcabinsismoredifficulttofind.Onecanpickona"typical"cabin,butiftheoccupationopenshisportholehecanupsetthewholeblock.Anotherpossibililityistositethethermostatinthealleywayoftheblockofcabin.Thispositionmaybeaffectedmorebyanopendoorordraguhtinthealleywaytthanbythetemperatureofthecabins.Yetanotherpossiblilityistositethethermostatintherecirculationgrillistoclosetoanoutsidedoor,thispositiontoocanbeaffectedbyoutsideairtemperaturewhenthedoorisopen,ratherthanbycabintemperature.GeneraloperationoftheairconditoninginstallationThefirstenssentialinoperatingtheaircoolingappliancesthroughouttheshipistohhaveallthermostatscorrectlysetandcorrectlyfunctioning.Inextremeweatherconditions,eitherhotorcoold,controloftheplantusuallypresentfewdiffculties.Thecapacityofmanyinstallationsissuchthatundertropicalconditionsnearlyallcontrolvalvesmovetothefullopenposition.Althoughautomatedcontrolhasbeenlost,internalconditionsarebyandlargeacceptable.Controldiffcultiesariseinintermediateweatherconditionswhenthereisacallforonlyasmallamoutofcooling.Theworstcaseiswhenpartoftheship,sayinboardcabinsagainsttheengineroom,requirecoolingandotherparts,sayexposeduppercabins,requirewarming.Forthisintermediatecondition,thermostatsmustbecorrectlysetbytrialanderror.Itisfoundthatauniformsettingofsay21°Cthroughouttheshipisnotsatisfactory,butslightvariationsofafewdegreesupordownareneededtosuitparticularregionsoftheship.Unfortunately,thesevariationsinthermostatsettingarenotalwaysthesameforthecoolingandheatingconditionandfrequentresettingmaybeneededforashiprepeatedlypassingfromcoldtowarmweather.Thecontrolproblemiseasedifthechilledbrine(orwater)ofsystemsusingchilledliquidcirculationisheldatabout13°Cintheintermediateweatherconditionsandloweredprogressivelytoabout5°Castropicalweatherconditionsareapproached.Whenaircoolingisinuseitisgoodpracticetokeepallportholes,windowsanddoorsshut.Onpassengerships,somepublicannouncementrequestingthatthisbedoneisworthwhile.Awiseprecautionforanengineertotakeistogothroughaccommodationandpublicroomsperiodicallyrecordingwetanddrybulbtemperature.Keepingalogofthesereadingthenservestoidentifyanymalfunctioningoftheinstallationassoonasitarises.Thequantityofcooledairdeliveredbyanairconditioningunitshouldbablancethesumofthequantityoftheairrecirculatedtothequantitymechanicallyexhausted.Thecorrectbablancebetweensupplyandexhaustfansshouldbecheckedperiodically.Evenwithfiltersfittedductscanbecomepartiallyblockedcanfanperformancecanfallofftoupsetthebablance.Onolderships,temperaturemaintenancecanbemadeeasierbyincreasingtheratioofrecirculatedtofreshair.Mostairconditioningunitshavedampersforadjustingthisratioandtheeffectofthesecanbeextendedaftertheyhavereached,fulltravelbypartiallyblockingfreshairinlets.Caremustbetakennottoreducethefreshairsothatstuffinessorsmellsarise.Cleaningorrenewaloffiltersisnecessaryatbaout3-monthlyintervals,thetimevaryingaccordingtolocationontheship.Disposablefilterscanbevacuum-cleanedsothatinfacttwoorthree"lives"areobtainedbeforetheyneedtobethrownaway.Inadditiontonormalmechanicalattention,suchaslubricationofbearings,andadjustmentoffanbeltsandcleaningofmotors,carefulgreasingoflinkagesofautomaticcontrolsisnecessary.Cooledairductsshouldbeexaminedtoseethattheinsulationvapoursealremainsingoodorder.Ifaplasticfilm:vapoursealbecomesdamsged,condensationformswithinthefilm.Aswellasmakingtheinsulationwetandineffective,thecondensationmaybecomeseriousenoughtocausedripsanddampparchesonceilings.VentilationVentilationistheprovisionofasupplyoffreshuntreatedairthroughaspace.Naturalventilationoccurswhenchangesintemperatureorairdensitycausecirculationinthespace.Mechanicalorforcedventilationusesfansforapositivemovementoflargequantitiesofair(Fig.18-2).Naturalventilationisusedforsomesmallworkshopsandstoresbutisimpracticalforworkingareaswheremachineryispresentoranumberofpeopleareemployed.Forcedventilationmaybeusedincargospaceswherethemovementofairremovesmoistureoravoidscondensation,removesodorsorgases,etc.Themachineryspacepresentsanotherareawhichrequiresventilation.Asaresultofitslargesizeandthefactthatlargevolumesofairareconsumedatreatmentplantwouldbeextremelycostlytorun.Ventilationisthereforeprovidedthesufficientquantitiesformachineryairconsumptionandalsotoeffectcooling.Severalaxialflowfansprovideairthroughductingtothevariousworkingplatforms.Thehotairrisesinthecentreandleavesthroughlouversoropenings,usuallyinthefunnel.Themachinerycontrolroom,asaseparatespace,maywellbearrangedforairconditioningwithanindividualunitwhichdrawsairthroughtruckingfromtheoutsideandexhaustsbacktotheatmosphere.Notes1. Airconditioningsystemsfallintotwomainclasses:individualunitsystem,inwhicheachroomcontainsitsownsmallrefrigerationplantandfanandaircooler;andcentralsystems,wherelargerrefrigerationmachineryunitareinstalledandtheiroutputdistributedabouttheshipbyavarietyofmeans.空调系统分为两大类，独立空调系统和中央空调系统。在独立空调系统中，各房间有各自的小型制冷装置、风机和空气冷却器；在中央空调系统中，装备了较大型的制冷装置，它的输出以各种方式传遍及全船。2.Thistendencyhelpsinstallationasthesizeofductsisreducedandprefabricatedstandardductscanbeused,butitincurstheheavierrunningcostsofmorepowerfulfans.这种趋势有利于安装工作，因为所采用的风管尺寸减小，预制的标准尺寸也可使用，但也因大功率鼓风机的运行，成本增加。3.Inatypicalmarinepatternself-containedunit,aircirculationisusuallyeffectedbymeansofacentrifugalfan,forquietrunning,andadirectexpansioncoolerservedbyahermeticcompressor.在典型的单体船用空调装置中，空气的循环是通过低噪声的离心式风机和由气密式压缩机辅助的直接膨胀式冷却器来实现的。4.Eventhesiteforadirectactingthermoststtocontrolonesingleberthcabinmustbechosenwithcare-ifitismaskedbehindcurtains,ortoofarawayfromtheairinletcontrolwillbetoosluggish.即使是控制一单人卧室的直接作用式的恒温器的位置也必须精心选择，如果它被遮蔽在窗帘后面，或者离空气进口太远，控制就会不灵敏。5.Thispositionmaybeaffectedmorebyanopendoorordraguhtinthealleywaytthanbythetemperatureofthecabins.这个位置可能会因门开着或受走廊通风的影响比受舱室温度的影响更大。6.Thequantityofcooledairdeliveredbyanairconditioningunitshouldbablancethesumofthequantityoftheairrecirculatedtothequantitymechanicallyexhausted.由空调装置供给的冷风量应当同回风数量和机械排风量之和平衡。NewwordsandexpressionsRenewal    n.       更新，恢复Thermostat   n.       温度调节装置Pneumatic    n       空气的，气动的Alleyway    n.      小巷，（船上的）通道Porthole     n.      舷窗Filter      n      过滤；过滤器，滤波器Damper     n.      起抑制作用的因素，节气闸Stuffiness    n.      不通风Odor      n.       气味，名声Louver     n.       天窗，屋顶窗