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1、Unit21Pumps1•IntroductionPump,deviceusedtoraise,transfer,orcompressliquidsandgases.Four5generalclassesofpumpsforliquidsaredescribedbelowtInallofthem,stepsaretakentopreventcavitation(theformationofavacuulll),whichwouldreducetheflowanddamagethestructureofthepump,・pumpsusedforgasesandvap
2、orsareusuallyknownascompressors•Thestudyoffluidsinmotioniscalledfluiddynamics.1•介绍泵是提出，转移或压缩液体和气体的设备。下而介绍四种类型的泵。在所有的这些屮，我们一步步采取措施防止气蚀，气蚀将减少流量并且破坏泵的结构。用來处理气体和蒸汽的泵称为压缩机，研究流体的运动的科学成为流体动力学。WaterPump,devicelormovingwaterfromonelocationtoanother,usingtubesorothermachinery.Waterpumpsoperateu
3、nderpressuresrangingfromafractionofapoundtomorethan10,000poundspersquareinch.Everydayexamplesofwaterpumpsrangefromsmallelectricpumpsthatcirculateandaeratewaterinaquariumsandfountainstosumppumpsthatremove'Waterfrombeneaththefoundationsofhomes.水泵是用管子或其他机械把水从一个地方传到另一个地方。水泵的操作压力从一磅到一万磅每平方
4、英尺。日常生活中，泵是很多的，有用于在鱼池和喷泉使水循环和向水屮充气的电泵，还有用于从住宅处把水引走的污水泵。Twotypesofmodernpumpsusedtomovewaterarethepositive-displacementpumpandthecentrifugalpump.Positive-displacementpumpsusesuctioncreatedbyavacuumtodrawwaterintoaclosedspace・Anexampleofthistypeofpumpisthelift,orforce,pumpusedcommonlyin
5、theruralUnitedStatesuntilthemid-1900s.Theliftpumpisoperatedbyraisingahandlethatisattachedtoapistonencasedinapipe.Liftingthepistoncreatesapartialvacuumbeneathitinthepipe,causingwatertobedrawnfromawellbelow、throughthepipe,andintoachamberinthepump.Aone-wayvalveclosesafterwaterispumpedint
6、othechamber,keepingthewaterfromflowingbackdownintothewell.Subsequentpumpsofthepistonpullmorewaterintothechamber,whicheventuallyoverflows,spillingwateroutofaspout.Centrifugalpumpsusemotor-drivenpropellersthatcreateaflowofwaterwhentheyrotate.Thebladesofthepropellerareimmersedinthewatert
7、obepumped・Asthepropellerturns,waterentersthepumpneartheaxisofthebladesandissweptouttowardtheirendsathighpressure.Analternative,earlyversionofthecentrifugalpump,thescrewpump,consistsofacorkscrew-shapedmechanisminapipethat,whenrotated,pullswaterupward.Screwpumpsareoftenusedinwaste-water
8、treat
