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外文翻译--螺杆式压缩机.doc

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外文翻译--螺杆式压缩机.doc

英文原文1IntroductionThescrewcompressorisoneofthemostcommontypesofmachineusedtocompressgases.Itsconstructionissimpleinthatitessentiallycomprisesonlyapairofmeshingrotors,withhelicalgroovesmachinedinthem,containedinacasing,whichfitscloselyroundthem.Therotorsandcasingareseparatedbyverysmallclearances.Therotorsaredrivenbyanexternalmotorandmeshlikegearsinsuchamannerthat,astheyrotate,thespaceformedbetweenthemandthecasingisreducedprogressively.Thus,anygastrappedinthiscaseiscompressed.Thegeometryofsuchmachinesiscomplexandtheflowofthegasbeingcompressedwithinthemoccursinthreestages.Firstly,gasentersbetweenthelobes,throughaninletportatoneendofthecasingduringthestartofrotation.Asrotationcontinues,thespacebetweentherotorsnolongerlinesupwiththeinletportandthegasistrappedandthuscompressed.Finally,afterfurtherrotation,theoppositeendsoftherotorspassasecondportattheotherendofthecasing,throughwhichthegasisdischarged.Thewholeprocessisrepeatedbetweensuccessivepairsoflobestocreateacontinuousbutpulsatingflowofgasfromlowtohighpressure.Thesemachinesaremainlyusedforthesupplyofcompressedairinthebuildingindustry,thefood,processandpharmaceuticalindustriesand,whererequired,inthemetallurgicalindustryandforpneumatictransport.Theyarealsousedextensivelyforcompressionofrefrigerantsinrefrigerationandairconditioningsystemsandofhydrocarbongasesinthechemicalindustry.Theirrelativelyrapidacceptanceoverthepastthirtyyearsisduetotheirrelativelyhighrotationalspeedscomparedtoothertypesofpositivedisplacementmachine,whichmakesthemcompact,theirabilitytomaintainhighefficienciesoverawiderangeofoperatingpressuresandflowratesandtheirlongservicelifeandhighreliability.Consequently,theyconstituteasubstantialpercentageofallpositivedisplacementcompressorsnowsoldandcurrentlyinoperation.Themainreasonsforthissuccessarethedevelopmentofnovelrotorprofiles,whichhavedrasticallyreducedinternalleakage,andadvancedmachinetools,whichcanmanufacturethemostcomplexshapestotolerancesoftheorderof3micrometersatanacceptablecost.Rotorprofileenhancementisstillthemostpromisingmeansoffurtherimprovingscrewcompressorsandrationalproceduresarenowbeingdevelopedbothtoreplaceearlierempiricallyderivedshapesandalsotovarytheproportionsoftheselectedprofiletoobtainthebestresultfortheapplicationforwhichthecompressorisrequired.Despitetheirwideusage,duetothecomplexityoftheirinternalgeometryandthenonsteadynatureoftheprocesseswithinthem,uptillrecently,onlyapproximateanalyticalmethodshavebeenavailabletopredicttheirperformance.Thus,althoughitisknownthattheirelementsaredistortedbothbytheheavyloadsimposedbypressureinducedforcesandthroughtemperaturechangeswithinthem,nomethodswereavailabletopredictthemagnitudeofthesedistortionsaccurately,norhowtheyaffecttheoverallperformanceofthemachine.Inaddition,improvedmodellingofflowpatternswithinthemachinecanleadtobetterportingdesign.Also,moreaccuratedeterminationofbearingloadsandhowtheyfluctuateenablebetterchoicesofbearingstobemade.Finally,ifrotorandcasingdistortion,asaresultoftemperatureandpressurechangeswithinthecompressor,canbeestimatedreliably,machiningprocedurescanbedevisedtominimisetheiradverseeffects.Screwmachinesoperateonavarietyofworkingfluids,whichmaybegases,dryvapourormultiphasemixtureswithphasechangestakingplacewithinthemachine.Theymayinvolveoilflooding,orotherfluidsinjectedduringthecompressionorexpansionprocess,orbewithoutanyformofinternallubrication.Theirgeometrymayvarydependingonthenumberoflobesineachrotor,thebasicrotorprofileandtherelativeproportionsofeachrotorlobesegment.Itfollowsthatthereisnouniversalconfigurationwhichwouldbethebestforallapplications.Hence,detailedthermodynamicanalysisofthecompressionprocessandevaluationoftheinfluenceofthevariousdesignparametersonperformanceismoreimportanttoobtainthebestresultsfromthesemachinesthanfromothertypeswhichcouldbeusedforthesameapplication.Asetofwelldefinedcriteriagovernedbyanoptimisationprocedureisthereforeaprerequisiteforachievingthebestdesignforeachapplication.Suchguidelinesarealsoessentialforthefurtherimprovementofexistingscrewmachinedesignsandbroadeningtheirrangeofuses.Flemingetal.,1998givesagoodcontemporaryreviewofscrewcompressormodelling,designandapplication.Amathematicalmodelofthethermodynamicandfluidflowprocesseswithinpositivedisplacementmachines,whichisvalidforboththescrewcompressorandexpandermodesofoperation,ispresentedinthisMonograph.Itincludestheuseoftheequationsofconservationofmass,momentumandenergyappliedtoaninstantaneouscontrolvolumeoftrappedfluidwithinthemachinewithallowanceforfluidleakage,oilorotherfluidinjection,heattransferandtheassumptionofrealfluidproperties.Bysimultaneoussolutionoftheseequations,pressurevolumediagramsmaybederivedoftheentireadmission,dischargeandcompressionorexpansionprocesswithinthemachine.Ascrewmachineisdefinedbytherotorprofilewhichisheregeneratedbyuseofageneralgearingalgorithmandtheportshapeandsize.Thisalgorithmdemonstratesthemeshingconditionwhich,whensolvedexplicitly,enablesavarietyofrotorprimaryarcstobedefinedeitheranalyticallyorbydiscretepointcurves.Itsusegreatlysimplifiesthedesignsinceonlyprimaryarcsneedtobespecifiedandthesecanbelocatedoneitherthemainorgaterotororevenonanyotherrotorincludingarack,whichisarotorofinfiniteradius.Themostefficientprofileshavebeenobtainedfromacombinedrotorrackgenerationprocedure.Therotorprofilegenerationprocessor,thermofluidsolverandoptimizer,togetherwithpreprocessingfacilitiesfortheinputdataandgraphicalpostprocessingandCADinterface,havebeenincorporatedintoadesigntoolintheformofageneralcomputercodewhichprovidesasuitabletoolforanalysisandoptimizationofthelobeprofilesandothergeometricalandphysicalparameters.TheMonographoutlinestheadoptedrationaleandmethodofmodelling,comparestheshapesofthenewandconventionalprofilesandillustratespotentialimprovementsachievedwiththenewdesignwhenappliedtodryandoilfloodedaircompressorsaswellastorefrigerationscrewcompressors.ThefirstpartoftheMonographgivesareviewofrecentdevelopmentsinscrewcompressors.Thesecondpartpresentsthemethodofmathematicaldefinitionofthegeneralcaseofscrewmachinerotorsanddescribesthedetailsoflobeshapespecification.Itfocusesonanewlobeprofileofaslendershapewiththinnerlobesinthemainrotor,whichyieldsalargercrosssectionalareaandshortersealinglinesresultinginhigherdeliveryratesforthesametipspeed.Thethirdpartdescribesamodelofthethermodynamicsofthecompressionexpansionprocesses,discussessomemodellingissuesandcomparestheshapesofnewandconventionalprofiles.Itillustratesthepotentialimprovementsachievablewiththenewdesignappliedtodryandoilfloodedaircompressorsaswellastorefrigerationscrewcompressors.Theselectionofthebestgaterotortipradiusisgivenasanexampleofhowmathematicalmodellingmaybeusedtooptimisethedesignandthemachinesoperatingconditions.Thefourthpartdescribesthedesignofahighefficiencyscrewcompressorwithnewrotorprofiles.Awellprovenmathematicalmodelofthecompressionprocesswithinpositivedisplacementmachineswasusedtodeterminetheoptimumrotorsizeandspeed,thevolumeratioandtheoilinjectionpositionandjetdiameter.Inaddition,moderndesignconceptssuchasanopensuctionportandearlyexposureofthedischargeportwereincluded,togetherwithimprovedbearingandsealspecification,tomaximisethecompressorefficiency.Theprototypesweretestedandcomparedwiththebestcompressorscurrentlyonthemarket.Themeasuredspecificpowerinputappearedtobelowerthananypublishedvaluesforotherequivalentcompressorscurrentlymanufactured.Boththepredictedadvantagesofthenewrotorprofileandthesuperiorityofthedesignprocedureweretherebyconfirmed.1.1BasicConceptsThermodynamicmachinesforthecompressionandexpansionofgasesandvapoursarethekeycomponentsofthevastmajorityofpowergenerationandrefrigerationsystemsandessentialfortheproductionofcompressedairandgasesneededbyindustry.Suchmachinescanbebroadlyclassifiedbytheirmodeofoperationaseitherturbomachinesorthoseofthepositivedisplacementtype.Turbomachineseffectpressurechangesmainlybydynamiceffects,relatedtothechangeofmomentumimpartedtothefluidspassingthroughthem.Theseareassociatedwiththesteadyflowoffluidsathighvelocitiesandhencethesemachinesarecompactandbestsuitedforrelativelylargemassflowrates.Thuscompressorsandturbinesofthistypearemainlyusedinthepowergenerationindustry,where,asaresultofhugeinvestmentinresearchanddevelopmentprogrammes,theyaredesignedandbuilttoattainthermodynamicefficienciesofmorethan90inlargescalepowerproductionplant.However,theproductionrateofmachinesofthistypeisrelativelysmallandworldwide,isonlyoftheorderofsometensofthousandsofunitsperannum.Positivedisplacementmachineseffectpressurechangesbyadmittingafixedmassoffluidintoaworkingchamberwhereitisconfinedandthencompressedorexpandedand,fromwhichitisfinallydischarged.Suchmachinesmustoperatemoreorlessintermittently.Suchintermittentoperationisrelativelyslowandhencethesemachinesarecomparativelylarge.Theyarethereforebettersuitedforsmallermassflowratesandpowerinputsandoutputs.Anumberoftypesofmachineoperateonthisprinciplesuchasreciprocating,vane,scrollandrotarypistonmachines.Ingeneral,positivedisplacementmachineshaveawiderangeofapplication,particularlyinthefieldsofrefrigerationandcompressedairproductionandtheirtotalworldproductionrateisinexcessof200millionunitsperannum.Paradoxically,butpossiblybecausethesemachinesareproducedbycomparativelysmallcompanieswithlimitedresources,relativelylittleisspentonresearchanddevelopmentprogrammesonthemandthereareveryfewacademicinstitutionsintheworldwhichareactivelypromotingtheirimprovement.Oneofthemostsuccessfulpositivedisplacementmachinescurrentlyinuseisthescrewortwinscrewcompressor.Itsprincipleofoperation,asindicatedinFig.1.1,isbasedonvolumetricchangesinthreedimensionsratherthantwo.Asshown,itconsists,essentially,ofapairofmeshinghelicallobedrotors,containedinacasing.Thespacesformedbetweenthelobesoneachrotorformaseriesofworkingchambersinwhichgasorvapouriscontained.Beginningatthetopandinfrontoftherotors,showninthelightshadedportionofFig.1.1a,thereisastartingpointforeachchamberwherethetrappedvolumeisinitiallyzero.Asrotationproceedsinthedirectionofthearrows,thevolumeofthatchamberthenincreasesasthelineofcontactbetweentherotorwithconvexlobes,knownasthemainrotor,andtheadjacentlobeofthegaterotorFig.1.1.ScrewCompressorRotorsadvancesalongtheaxisoftherotorstowardstherear.Oncompletionofonerevolutioni.e.360◦bythemainrotor,thevolumeofthechamberisthenamaximumandextendsinhelicalformalongvirtuallytheentirelengthoftherotor.Furtherrotationthenleadstoreengagementofthemainlobewiththesucceedinggatelobebyalineofcontactstartingatthebottomandfrontoftherotorsandadvancingtotherear,asshowninthedarkshadedportionsinFig.1.1b.Thus,thetrappedvolumestartstodecrease.Oncompletionofafurther360◦ofrotationbythemainrotor,thetrappedvolumereturnstozero.ThedarkshadedportionsinFig.1.1showtheenclosedregionwheretherotorsaresurroundedbythecasing,whichfitscloselyroundthem,whilethelightshadedareasshowtheregionsoftherotors,whichareexposedtoexternalpressure.ThusthelargelightshadedareainFig.1.1acorrespondstothelowpressureportwhilethesmalllightshadedregionbetweenshaftendsBandDinFig.1.1bcorrespondstothehighpressureport.Exposureofthespacebetweentherotorlobestothesuctionport,astheirfrontendspassacrossit,allowsthegastofillthepassagesformedbetweenthemandthecasinguntilthetrappedvolumeisamaximum.Furtherrotationthenleadstocutoffofthechamberfromtheportandprogressivereductioninthetrappedvolume.Thisleadstoaxialandbendingforcesontherotorsandalsotocontactforcesbetweentherotorlobes.Thecompressionprocesscontinuesuntiltherequiredpressureisreachedwhentherearendsofthepassagesareexposedtothedischargeportthroughwhichthegasflowsoutatapproximatelyconstantpressure.ItcanbeappreciatedfromexaminationofFig.1.1,isthatifthedirectionofrotationoftherotorsisreversed,thengaswillflowintothemachinethroughthehighpressureportandoutthroughthelowpressureportanditwillactasanexpander.Themachinewillalsoworkasanexpanderwhenrotatinginthesamedirectionasacompressorprovidedthatthesuctionanddischargeportsarepositionedontheoppositesidesofthecasingtothoseshownsincethisis

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