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Unit21Pumps1.IntroductionPump,deviceusedtoraise,transfer,orcompressliquidsandgases.Four'generalclassesofpumpsforliquidsaredescribedbelowtInallofthem,stepsaretakentopreventcavitation(theformationofavacuull1),whichwouldreducetheflowanddamagethestructureofthepump,-pumpsusedforgasesandvaporsareusuallyknownascompressors.Thestudyoffluidsinmotioniscalledfluiddynamics.1．介绍泵是提出，转移或压缩液体和气体的设备。下面介绍四种类型的泵。在所有的这些中，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。用来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.Waterpumpsoperateunderpressuresrangingfromafractionofapoundtomorethan10,000poundspersquareinch.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中充气的电泵，还有用于从住宅处把水引走的污水泵。Twotypesofmodernpumpsusedtomovewaterarethepositive-displacementpumpandthecentrifugalpump.Positive-displacementpumpsusesuctioncreatedbyavacuumtodrawwaterintoaclosedspace.Anexampleofthistypeofpumpisthelift,orforce,pumpusedcommonlyintheruralUnitedStatesuntilthemid-1900s.Theliftpumpisoperatedbyraisingahandlethatisattachedtoapistonencasedinapipe.Liftingthepistoncreatesapartialvacuumbeneathitinthepipe,causingwatertobedrawnfromawellbelow,throughthepipe,andintoachamberinthepump.Aone-wayvalveclosesafterwaterispumpedintothechamber,keepingthewaterfromflowingbackdownintothewell.Subsequentpumpsofthepistonpullmorewaterintothechamber,whicheventuallyoverflows,spillingwateroutofaspout.Centrifugalpumpsusemotor-drivenpropellersthatcreateaflowofwaterwhentheyrotate.Thebladesofthepropellerareimmersedinthewatertobepumped.Asthepropellerturns,waterentersthepumpneartheaxisofthebladesandissweptouttowardtheirendsathighpressure.Analternative,earlyversionofthecentrifugalpump,thescrewpump,consistsofacorkscrew-shapedmechanisminapipethat.whenrotated,pullswaterupward.Screwpumpsareoftenusedinwaste-watertreatmentplantsbecausetheycanmovelargeamountsofwaterwithoutbecomingcloggedwithdebris.IntheancientMiddleEasttheneedforirrigationoffarmlandwasastronginduceDlenttodevelopawaterpump.Earlypumpsinthisregionweresimpledevicesforliftingbucketsofwaterfromasourcetoacontaineroratrench.GreekmathematicianandinventorArchimedesisthoughttohavedevisedthefirstscrewpumpinthe3rdcenturyBe.Later,GreekinventorCtesibiusdevelopedthefirstliftpump.Duringthelate17thandearly18thcenturiesAD,BritishengineerThomasSavery,FrenchphysicistDenisPapin..andBritishblacksmithandinventorThomasNewcornen.contributedtothedevelopmentofawaterpumpthatusedsteamtopowerthepumpspiston.Thesteam-poweredwaterpump'sfirstwideusewasinpumpingwateroutofmines.Modern-dayexamplesofcentrifugalpumpsarethoseusedattheGrandCouleeDarnontheColumbiaRiver.Thispumpsystemhasthepotentialtoirrigateoveronemillionacresofland.现在，两种典型的排水泵是容积泵和离心泵。容积泵通过由真空产生的吸力把水引到一个紧凑的地方。这种类型泵的一个实例就是提升或压力泵，在20世纪中叶美国农村普遍使用。提升泵的操作是通过一个与被管子包住的活塞手柄来进行的。当我们提升活塞时在管子下部产生一个局部的真空，这样我们就用管子从下面的取水，并且送到泵的一个空间。当水被泵吸入时，单向阀关闭，阻止水流回到井下。接着泵的活塞吸入更多的水进入泵的膛体中。这样最后形成溢流，水从管口处流出。而离心泵时使用了一种螺旋推进器，旋转时使水流动，而且推进器的切片是在泵送水时侵入水中的。而且，当推进器旋转时，水进入位于刃片的轴部的间隙并且以很高的压力甩向底部。与它类似，离心泵的早期形式，螺杆泵，通过一个管子螺丝钉的组成，当旋转时，把水提升上去。螺旋泵经常用在污水处理厂中，因为他们可以运输大量的水，而不会因为碎片而堵塞。在远古的中东，因为对农场进行灌溉的需求，所以有一种强大的动力去推进水泵的进程。在这些区域里，早期的泵是为了将水一桶一桶的从水源或河渠中提升到容器中。古希腊的发明家和数学家的阿基米德泵认为是公元前3世纪首先提出螺旋泵的发明家。之后，古希腊发明家发明了第一个提水泵。在十七世纪末和十八世纪初，英国的工程师ThomasSavory，法国的物理学家DenisPa]pin，和英国的铁匠和发明家TomasNewcomen,它们发明了用蒸汽驱动活塞的水泵。蒸汽驱动的水泵首先广泛的被应用在从煤矿往外输水过程中。现在离心泵使用的例子，可以是在哥伦比亚河上使用的大古利水坝。这个泵有超过灌溉一百万英亩的土地能力。2.ReciprocatingPumpsReciprocatingpumpsconsistofapistonmovingbackandforthinacylinderthathasvalvestoregulatetheflowofliquidintoandoutofthecylinder.Thesepumpsmaybesingleordoubleacting.Inthesingleactingpump,thepumpingactiontakesplaceononlyonesideofthepiston,asinthecaseoftheCOD1ll1onliftPUDlP,inwhichthepistonismovedupanddownbyhand.Inthedoubleactingpump,thepumpingactiontakesplaceonbothsidesofthepiston,asintheelectricalorsteam-drivenboilerfeedpump,inwhichwaterissuppliedtoasteamboilerunderhighpressure.Thesepumpscanbesingle-stageormultistaged,Multistagedreciprocatingpumpshavemultiplecylindersinseries.2.往复泵往复泵有一个在圆筒中上下移动的活塞，可以使水规则的流入或流出圆筒。这些泵可以是单作用的，也可以是双作用的。在单作用的泵中，泵的作用仅仅发生在活塞的一侧，典型的例子就是升液泵。在升液泵中，活塞通过手上下移动。在双作用的水泵中，泵的作用发生在活塞的两侧，比如说电动的或气动的锅炉给水泵，水以高压通过蒸汽锅炉供给。这些泵可以是单级的也可以是多级的。多级的往复泵的泵系列有多个刚体。3.CentrifugalPumpsAlsoknownasrotarypumps,centrifugalpumpshavearotatingimpeller,alsoknownasablade,thatisimmersedintheliquid.Liquidentersthepumpneartheaxisoftheimpeller,andtherotatingimpellersweepstheliquidouttowardtheendsoftheimpellerbladesathighpressure.Theimpelleralsogivestheliquidarelativelyhighvelocitythatcanbeconvertedintopressureinastationarypartofthepump,knownasthediffuser.Inhigh-pressurepumps,anumberofimpellersmaybeusedinseries,andthediffusersfollowingeachimpellermaycontainguidevanestograduallyreducetheliquidvelocity.Forlower-pressurepumps,thediffuserisgenerallyaspiralpassage,knownasavolute,withitscross-sectionalareaincreasinggraduallytoreducethevelocityefficient