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CompressorsThemaindifferencebetweenpumpsandcompressorsisthatthefluiddeliveredbycompressorsairiscompressedandunderpressureatthetimeitisdelivered,evenifthereisnoloadonthesystem.Mostdevicesusedtocompressairareverysimilarinconceptandperhapseveninhardwaretohydraulicpumps,andselectionconsiderationsaresimilar.Theonlyothersubstantivedifferenceisthatmosthydraulicsystemsarepoweredbyasinglepumpthatisactuallyapartofthesystem,whereasahostofpneumaticsystemsareoftenpoweredbyasinglecompressor,whichisalmostautilityintheplantlikewaterorelectricservice.Nevertheless,manysmallcompressorsareavailableforspecific,discretejobstypicallytheyarepositivedisplacementcompressors.Dynamic,ornonpositivedisplacementcompressorsaretypicallylarger,facilitytypeunits.Compressorsarefairlysimpledevices,capableoflongperiodsofmaintenancefreeoperationifproperlyintegratedintopneumaticsystems.Yettimeandagaintheysufferfromearlyfailuresbecauseobviousprecautionswereignoredduringsystemdesign.FourbasicrulescanprovidesubstantialimprovementincompressorlifewithonlymoderatedesigneffortPumpsandcompressorsshouldbesizedtoprovideatleasttherequiredpressureandflow,andpreferably10to25more.Filtersshouldbeselectedtoprotectthepumpingunit,andsometimestoprotectdownstreamcomponentsorproductsaswell.Reliefvalvesshouldbeselectedtokeeppressureorvacuumatappropriatelevels.Pumpingunitsshouldbeplacedinaclean,cool,dryenvironment.Bellowscompressorsconsistofaweldedmetalbellowsconnectedtoinletandoutletportswithcheckvalves.Thesecompressorstypicallycoverthepressurerangeupto10psig,andareusedinpollutiondetectingandmeasuringdevices,gassamplinginstruments,andmedicalapplications.Lubricationisnotneeded,allowinghighpuritiestobemaintained.Vanecompressorsaresimplemachineswithfewmovingparts.Liketheirhydrauliccounterparts,vanepumps,thecompressorsareinexpensive,withlowoperatingcost,andlowstartingtorquerequirement.Theyarecompactandrelativelyvibrationfree,withlittlepulsationinthecompressoroutput.Theslidingvanesarecloselyfittedintherotorslotsandwearverylittleduringoperation.Thesecompressorsareavailableinpowerrangesfrom10to500hp,atpressuresto150psi.Reciprocatingcompressorsconsistofapistonmovingwithinthecylindertotrapandcompressthegas.Inprinciple,suchaunitislikeanautomobileengine,withthepistonscompressingthegasandvalvescontrollingitsinletandoutflows.Sizesrangefromlessthan1toover5,000hp.Reciprocatingcompressorshavegoodpartloadefficienciesandareusefulforwidevariationsinoperatingconditions.Diaphragmcompressorsareamodificationofthereciprocatingcompressor.Compressionisperformedbytheflexingofametalorfabricateddiaphragmwhichiscausedbythemotionofareciprocatingpistoninacylinderunderthediaphragm.Thespacebetweenthediaphragmandthepistonisusuallyfilledwithliquid.Lobedrotorcompressorshavetworotatingelementsthatrevolveinoppositedirectionsinachamber.Inmostcompressors,therotorsdonotactuallytouchanddonotdriveeachother,beingdriveninsteadbytiminggears.Becausetherotorsdonotactuallytouch,airleaksbetweenthematasmallbutconstantrate.Thisleakage,calledslip,isconstantforagivencompressoratagivenpressure.Forhighestefficiency,thesecompressorsshouldbeoperatedatmaximumspeed.Theyareavailableinpowerrangesfrom7to3,000hp,deliveringpressuresto250psi.Becausetheinternallobesdonotcontact,theyneednolubrication.Liquidpistoncompressorshavenomovingpartsinwearingcontact.Arotorwithmultipleforwardcurvedbladesrotatesinanellipticalcasing.Fluid,trappedwithinthecasing,iscarriedaroundtheinnerperipherybytheblades.Spacebetweenthebladeschangesvolumeduetotheellipticalfluidpath,andtheinnersurfaceoftheliquidringtrappedbetweenthebladesservesasthefaceofaliquidpiston.Thesecompressorsacceptliquidslugsandfineparticleswithoutseriousdamage.Lubricationisrequiredonlyinbearingslocatedoutsidethepumphousing.Thesecompressorsdeliverupto150psithroughouttherangeof10to500hp.Centrifugalcompressorsarebestsuitedtomovinglargevolumesofairatrelativelylowpressures.Basically,theyconsistofahighspeedrotatingimpeller,adiffusersectionwherevelocityisreducedandpressureincreased,andacollectorsectionthatfurtherreducesvelocityandincreasespressure.Centrifugalcompressorscanhandlehighflowdemandswell,butwhendemanddecreasesmuchbelowratedflowandoutputpressurerises,thecompressorscansurge.Insurge,thepressurefieldatthecompressoroutletvariesrandomly.Ifallowedtocontinue,thisconditioncandamagebearings,blades,andeventhehousingitself.Centrifugalcompressorstypicallyusefromtwotosixstages,supplyingfrom400to3,000cfmatspeedsto20,000rpm.Regenerativeblowersalsoknownasperipheralblowersuseadisclikeimpellerwithbladesmountedarounditsoutsideedge.Astheimpellerrevolves,airisdrawnintothespacebetweentheblades.Centrifugalforcemovestheairinaspiralpathoutwardtothehousing,whereitslipsbytheinitialbladeandreturnstothebaseofthesucceedingblade,wheretheprocessisrepeated.Insomemodels,aflowsplittercreatestwoflowpaths,sothattheairmustmaketwocircuitsaroundtheimpeller.Inothermodels,thesplitterisomitted,andtheairmakesonlyonecircuitbeforeexiting.Regenerativeblowersprovideairflowsupto1,000cfmandpressuresto8psi.Helicalcompressorslookliketwogiantscrewsmeshingtogethertheyworkmuchlikehydraulicscrewpumps.Maximumpressurefromthesemachinesisapproximately125psiinsinglestageconfigurations.Helicalcompressorsmaybeeitheroilfloodedordry.Dryhelicalcompressors,likelobedunits,requiretiminggearstomaintainproperclearancebetweentherotatingelements.Theseunitsaremostefficientlyoperatedathighcontinuousspeeds.Floodedcompressorsdonotrequireanytiminggears,becausetheoilladenscrewsurfacescandriveeachother.However,oilseparatorsareneededtoremovetheoilfromtheairasitleavesthecompressor.Theyareavailableoverapowerrangeofabout7to300hp.Singlescrewcompressorsarebasedonthesameprincipleashelicalcompressors.Asthecentralscrewrotates,airtrappedbetweenthescrewteethiscompressedagainstthestarshapedrotors.Thesecompressorstendtohavelowvibrationandnoiselevels,andlowdischargepressures.Lubricationisrequired.PumpsvacuumpumpsInprinciple,industrialvacuumpumpsaremerelycompressorsrunwiththeinletattachedtothevacuumsystemandtheoutletopentoexhaust.Insmallersizes,compressorsandvacuumpumpsareoftenidenticalmachines.However,inthelargesizesthatmightpoweraplantwidevacuumsystem,themachinesdifferinminorwaysthatareintendedtoenhanceefficiencyforoneapplicationortheother.Manufacturersstronglyadvisethatthesamemachinenotbeusedforbothvacuumandcompressionatthesametime.Theheavyloadswilldamageit.Threecriteriacontrolpumpselectiondegreeofvacuumproduced,rateofairremoval,andpowerrequirement.However,applicationssuchasfiltrationmaysubjecttheunittotheingestionofforeignmaterial.Thefirstpumpperformancecriterionisthevacuumitproduces.ManufacturersprovideamaximumvacuumratingexpressedasabsolutepressureinmmHg,orvacuuminin.Hg.Largerunitsareusuallyratedonlyforcontinuousduty,butsmallerunitsmayhaveahighervacuumratingforintermittentduty.Insmallerunits,temperatureriseconsiderationslimitthevacuumthatcanbeproduced.Continuousandintermittentvacuumratingsaredeterminedforstandardatmosphericpressure29.92in.Hg.Lowerambientpressuresreducethevacuumthatcanbeproduced.TheratingisdeterminedfromwhereVaadjustedvacuumrating,in.HgVooriginalvacuumratingatstandardconditions,in.HgandPaanticipatedatmosphericpressureattheapplicationsite,in.Hg.Rateofairremovalisthesecondcriterion.Vacuumpumpsareflowratedaccordingtothevolumeofairexhaustedwithnopressuredifferentialacrossthepump.Manufacturersprovidecurvesshowingfreeairdeliveryatratedspeedforvacuumlevelsrangingfrom0in.Hgsocalledopencapacitytomaximumvacuumrating.Somemanufacturersalsoprovidecurvesofcapacityatdifferentspeedsforagivenvacuum.Thelastpumpcriterionispowerrequirement.Comparedwithaircompressors,vacuumpumpsrequirerelativelylittlepower.Atlowflows,vacuumorpressuredifferentialishighathighflows,vacuumislow.Therefore,power,whichisproportionaltoflowandpressuredifferential,isgenerallylow.Poweroutputofthepumpcanbefoundfrompressureflowcurvesprovidedbymanufacturers.Inputpowerandspeedrequirementsarealsoshowninthedata.Overallpumpefficiencyincludingbothvolumetricandmechanicalefficiencycanbeevaluatedbycombiningthisdata.Thisisdonebydividingthefreeaircapacityofthepumpattherequiredvacuumlevelbydrivepowerrequiredatthatcondition.Theresultisproportionaltotheproductofgagevacuumandairflowrateandisrepresentativeofefficiency.Allthreeperformancecriteriavacuum,flowandpowercanbeaffectedbypumptemperature.Athighervacuumlevels,littleairflowsthroughthepump,solittleheatistransferredtotheair.Muchoftheheatgeneratedbyfrictionmustbedissipatedbythepump.Thisheatgraduallyraisespumptemperatureandcandrasticallyreduceservicelife.Temperatureexcursionsareespeciallyimportanttointermittentdutypump,whichcanoverheatifontimegreatlyexceedsofftime.Vacuumpumpsareclassifiedaseitherpositiveornonpositivedisplacement.Apositivedisplacementpumpcreatesvacuumbyisolatingandcompressingadistinct,constantvolumeofair.Thecompressedairisventedoutoneport,andavacuumiscreatedattheotherportwheretheairisdrawnin.Thisgeneratesrelativelyhighvacuum,butlittleflow.Anonpositivedisplacementpump,ontheotherhand,usesrotatingimpellerbladestoaccelerateairandcreateavacuumattheinletport.Whilenonpositivedisplacementpumpscannotproducehighlevelsofvacuum,theyprovidehighflowrates.Principaltypesofpositivedisplacementvacuumpumpsincludepiston,diaphragm,rockingpiston,rotaryvane,lobedrotor,rotaryscrew,andliquidringdesigns.Reciprocatingpistonpumpsgeneraterelativelyhighvacuumsfrom27tomorethan29in.Hgunderavarietyofoperatingconditions.Typicalpumpsofthistypehaveoneormorepistonslinkedtoarotatingcrankshaft.Thealternatingpistonactionmovesairpastcheckvalvesinthecylinderheadtocreateavacuumattheinletport.Lubricatedpistonpumpsarequieter,producelessvibration,haveahighercapacity,andfeatureamuchlongerlifethanoillessdesigns,buttheyarealsoheavierandmoreexpensive.Diaphragmpumpsoffertheadvantageofthefluidchamberbeingtotallysealedfromthepumpingmechanisms.Aneccentricconnectingrodmechanicallyflexesadiaphragminsidetheclosedchambertocreateavacuum.Thisresultsinsomewhatlowervacuumcomparedtothatproducedbyareciprocatingpiston.However,thediaphragmslowercompressionratiolowflow,largediameter,andshortstrokemakesforquiet,economical,andreliableoperation.Thedesignisavailableinbothoneandtwostageversions.Singlestagepumpsprovidevacuumupto25.5in.Hg,whiletwostageunitsareratedto29in.Hg.Rockingpistonpumpscombinethecompactsizeandquiet,oillessoperationofthediaphragmpumpwiththehighvacuumcapabilitiesofthereciprocatingpistonpump.Here,apistonisrigidlymountednowristpinontopofthediaphragmunitseccentricconnectingrod.Anelastomericcupskirtsthepistonandfunctionsbothasasealequivalenttotheringsonapistoncompressorandasaguidememberfortherod.Thecupexpandsasthepistontravelsupward,thusmaintainingcontactwiththecylinderwallsandcompensatingfortherockingmotion.Theabsenceofawristpinisthekeytothepumpslightweightandcompactsize.Singlestagerockingpistonpumpsproducevacuumto27.5in.Hgtwostagedesignscangenerate29in.Hgormoreofvacuum.Rockingpistonpumpsarealsorelativelyquiet,operatingatsoundlevelsaslowas50dBA.Adrawbacktorockingpistonpumpsisthattheycannotgeneratealotofairflow.Eventhelargesttwincylindermodelshaveflowratesoflessthan10cfm.Rotaryvanepumpsuseaseriesofsliding,flatvanesrotatinginacylindricalcasetogeneratevacuum.Asaneccentricallymountedrotorturns,thevanesslideinandout,trappingaquantityofairandmovingitfromtheinletsideofthepumptotheoutlet.
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