新版轮机英语unit18-air-conditioning-system.ppt

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1、Lesson18AirconditioningsystemShipstraveltheworldandarethereforesubjecttovariousclimatic(气候）conditions.Thecrewoftheshipmustbeprovidedwithreasonableconditionsinwhichtoworkregardlessoftheweather.Temperaturealoneisnotasufficientmeasureofconditionsacceptabletothehumanbody.Relativeh

2、umidityinconjunctionwith（与…协力）temperaturemoretrulydeterminestheenvironmentforhumancomfort(人体).Relativehumidity,expressedasapercentage,istheratioofthewatervapourpressureintheairtested,tosaturated（饱和的）vapourpressureofairatthesametemperature.Thefactthatlesswatercanbeabsorbedasair

3、iscooledandmorecanbeabsorbedwhenitisheatedisthemajorconsiderationinairconditioningsystemdesign.Otherfactorsarethenearness（靠近）ofheatsources,exposuretosunlight,sourcesofcoldandtheinsulation（绝热）providedaroundthespace.(Fig.18-1)Airconditioningsystemaimstoprovideacomfortableworking

4、environmentregardlessofoutsideconditions.Satisfactoryairtreatmentmustinvolvearelatively“closed”systemwheretheairiscirculatedandreturned.However,someairis“consumed”byhumansandsomemachinerysothereisarequirementforrenewal（补充）.Publicroomsandaccommodation(住处)willoperatewithareduced

5、percentageofairrenewalsincetheconditioningcostof100%renewalwouldbeconsiderable.Airconditioningsystemsfallintotwomainclasses:individualunitsystems(独立空调系统)inwhicheachroomcontainsitsownsmallrefrigerationplantandfanandaircooler;andcentralsystems,wherelargerrefrigerationmachineryun

6、itsareinstalledandtheiroutputdistributedabouttheshipbyavarietyofmeans.Self-containedunitsarenoisierthancentralsystems,requiremoremaintenanceandhavebeenfoundtohavearelativelyshortlife(about7years.)Thesingleductsystem(单风管系统）onlyallowsforadjustmentoftemperatureineachroombytheoccu

7、pant（居住者）manuallycontrollingtheairvolumeadmitted.Itisthuslessflexiblethananyoftheothersystems,whichallowindividualtemperaturecontrol,atleastofsectionsoftheshipifnotindividualrooms.Withductedsystems,themoderntendencyistouse“highvelocity”intheairductswithfansgeneratingupto2550mb

8、ar(250mmH2Og)pressurecomparedto“lowvelocity”intheairductswith