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外文翻译--离心式和往复式压缩机的工作效率特性.doc

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外文翻译--离心式和往复式压缩机的工作效率特性.doc

英文原文EfficiencyAndOperatingCharacteristicsOfCentrifugalAndReciprocatingCompressorsByRainerKurz,BernhardWinkelmann,andSaeidiVIokhatabReciprocatingcompressorsandcentrifugalcompressorshavedifferentoperatingcharacteristicsandusedifferenteificiencydefinitions.Thisarticleprovidesguidelinesforanequitablecomparison,resultinginauniversalefficiencydefinitionforbothtypesofmachines.Thecomparisonisbasedontherequirementsinwhichauserisultimatelyinterested.Further,theimpactofactualpipelineoperatingconditionsandtheimpactonefficiencyatdifferentloadlevelsisevaluated.Atfirstglance,calculatingtheefficiencyforanytypeofcompressionseemstobestraightforwardcomparingtheworkrequiredofanidealcompressionprocesswiththeworkrequiredofanactualcompressionprocess.Thedifficultyiscorrectlydefiningappropriatesystemboundariesthatincludelossesassociatedwiththecompressionprocess.Unlesstheseboundariesareappropriatelydefined,comparisonsbetweencentrifugalandreciprocatingcompressorsbecomeflawed.Wealsoneedtoacknowledgethattheefficiencydefinitions,evenwhenevaluatedequitably,stilldontcompletelyansweroneoftheoperatorsmainconcernsWhatisthedriverpowerrequiredforthecompressionprocessToaccomplishthis,mechanicallossesinthecompressionsystemsneedtobediscussed.Trendsinefficiencyshouldalsobeconsideredovertime,suchasoffdesignconditionsastheyareimposedbytypicalpipelineoperations,ortheimpactofoperatinghoursandassociateddegradationonthecompressors.Thecompressionequipmentusedforpipelinesinvolveseitherreciprocatingcompressorsorcentrifugalcompressors.Centrifugalcompressorsaredrivenbygasturbines,orbyelectricmotors.Thegasturbinesusedare,ingeneral,twoshaftenginesandtheelectricmotordrivesuseeithervariablespeedmotors,orvariablespeedgearboxes.Reciprocatingcompressorsareeitherlowspeedintegralunits,whichcombinethegasengineandthecompressorinonecrankcasing,orseparablehighspeedunits.Thelatterunitsoperateinthe7501,200rpmrange1,800rpmforsmallerunitsandaregenerallydrivenbyelectricmotors,orfourstrokegasengines.EfficiencyTodeterminetheisentropicefficiencyofanycompressionprocessbasedontotalenthalpiesh,totalpressuresp,temperaturesTandentropiessatsuctionanddischargeofthecompressoraremeasured,andtheisentropicefficiencyr\thenbecomes,,,,suctsuctdischdischsuctsuctsuctdischsTphTphTphsphEq.1and,withmeasuringthesteadystatemassflowm,theabsorbedshaftpoweris,,.suctsuctdischdischmTphTphmpEq.2consideringthemechanicalefficiencyr\.Thetheoreticalisentropicpowerconsumptionwhichisthelowestpossiblepowerconsumptionforanadiabaticsystemfollowsfrom,,.suctsuctsuctdischtheorTphsphmPEq.3Theflowintoandoutofacentrifugalcompressorcanbeconsideredassteadystate.Heatexchangewiththeenvironmentisusuallynegligible.Systemboundariesfortheefficiencycalculationsareusuallythesuctionanddischargenozzles.Itneedstobeassuredthatthesystemboundariesenvelopeallinternalleakagepaths,inparticularrecirculationpathsfrombalancepistonordivisionwallleakages.Themechanicalefficiencyr.,,describingthefrictionlossesinbearingsandseals,aswellaswindagelosses,istypicallybetween98and99.Forreciprocatingcompressors,theoreticalgashorsepowerisalsogivenbyEq.3,giventhesuctionanddischargepressureareupstreamofthesuctionpulsationdampenersanddownstreamofthedischargepulsationdampeners.Reciprocatingcompressors,bytheirverynature,requiremanifoldsystemstocontrolpulsationsandprovideisolationfromneighboringunitsbothreciprocatingandcentrifugal,aswellasfrompipelineflowmetersandyardpipingandcanbeextensiveinnature.Thedesignofmanifoldsystemsforeitherslowspeedorhighspeedunitsusesacombinationofvolumes,pipinglengthsandpressuredropelementstocreatepulsationacousticfilters.Thesemanifoldsystemsfilterscauseapressuredrop,andthusmustbeconsideredinefficiencycalculations.Potentially,additionalpressuredeductionsfromthesuctionpressurewouldhavetomadetoincludetheeffectsofresidualpulsations.Likecentrifugalcompressors,heattransferisusuallyneglected.Forintegralmachines,mechanicalefficiencyisgenerallytakenas95.Forseparablemachinesa97mechanicalefficiencyisoftenused.Thesenumbersseemtobesomewhatoptimistic,giventhefactthatanumberofsourcesstatethatreciprocatingenginesincurbetween815mechanicallossesandreciprocatingcompressorsbetween612Ref1Kurz,R.,K.Bun,2007.OperatingConditionsForasituationwhereacompressoroperatesinasystemwithpipeofthelengthLuupstreamandapipeofthelengthLddownstream,andfurtherwherethepressureatthebeginningoftheupstreampipepuandtheendofthedownstreampipepeareknownandconstant,wehaveasimplemodelofacompressorstationoperatinginapipelinesystemFigure1.Figure1ConceptualmodelofapipelinesegmentRef.2Kurz,R.,M.Lubomirsky.2006.Foragiven,constantflowcapacityQstdthepipelinewillthenimposeapressurepsatthesuctionandpdatthedischargesideofthecompressor.Foragivenpipeline,theheadHsflowQrelationshipatthecompressorstationcanbeapproximatedby11112243skkdsppQCCTCH(Eq.4)whereC3andC4areconstantsforagivenpipelinegeometrydescribingthepressureateitherendsofthepipeline,andthefrictionlosses,respectivelyRef2Kurz,R.,M.Lubomirsky,2006.Amongotherissues,thismeansthatforacompressorstationwithinapipelinesystem,theheadforarequiredflowisprescribedbythepipelinesystemFigure2.Inparticular,thischaracteristicrequiresthecapabilityforthecompressorstoallowareductioninheadwithreducedflow,andviceversa,inaprescribedfashion.Thepipelinewillthereforenotrequireachangeinflowatconstantheadorpressureratio.Figure2StafionHeadFlowrelationshipbasedonEq.4.Intransientsituationsforexampleduringlinepacking,theoperatingconditionsfollowinitiallyaconstantpowerdistribution,i.e.theheadflowrelationshipfollowsconstHPssm(Eq.5)QconstHss1andwillasymptoticallyapproachthesteadystaterelationshipRef3Ohanian,S.,R.Kurz,2002.Basedontherequirementsabove,thecompressoroutputmustbecontrolledtomatchthesystemdemand.Thissystemdemandischaracterizedbyastrongrelationshipbetweensystemflowandsystemheadorpressureratio.Giventhelargevariationsinoperatingconditionsexperiencedbypipelinecompressors,animportantquestionishowtoadjustthecompressortothevaryingconditions,and,inparticular,howdoesthisinfluencetheefficiency.Centrinagalcompressorstendtohaveratherflatheadvs.flowcharacteristic.ThismeansthatchangesinpressureratiohaveasignificanteffectontheactualflowthroughthemachineRef4Kurz,R.,2004.Foracentrifugalcompressoroperatingataconstantspeed,theheadorpressureratioisreducedwithincreasingflow.ControllingtheflowthroughthecompressorcanbeaccomplishedbyvaryingtheoperatingspeedofthecompressorThisisthepreferredmethodofcontrollingcentrifugalcompressors.Twoshaftgasturbinesandvariablespeedelectricmotorsallowforspeedvariationsoverawiderangeusuallyfrom4050to100ofmaximumspeedormore.Itshouldbenoted,thatthecontrolledvalueisusuallynotspeed,butthespeedisindirectlytheresultofbalancingthepowergeneratedbythepowerturbinewhichiscontrolledbythefuelflowintothegasturbineandtheabsorbedpowerofthecompressor.Virtuallyanycentrifugalcompressorinstalledinthepast15yearsinpipelineserviceisdrivenbyavariablespeeddriver,usuallyatwoshaftgasturbine.Olderinstallationsandinstallationsinotherthanpipelineservicesometimesusesingleshaftgasturbineswhichallowaspeedvariationfromabout90100speedandconstantspeedelectricmotors.Intheseinstallations,suctionthrottlingorvariableinletguidevanesareusedtoDrovidemeansofcontrol.Figure3Typicalpipelineoperatingpointsplottedintoatypicalcentrifugalcompressorperformancemap.Theoperatingenvelopeofacentrifugalcompressorislimitedbythemaximumallowablespeed,theminimumflowsurgeflow,andthemaximumflowchokeorstonewallFigure3.Anotherlimitingfactormaybetheavailabledriverpower.Onlytheminimumflowrequiresspecialattention,becauseitisdefinedbyanaerodynamicstabilitylimitofthecompressorCrossingthislimittolowerflowswillcauseaflowreversalinthecompressor,whichcandamagethecompressor.Modemcontrolsystemspreventthissituationbyautomaticallyopeningarecyclevalve.Forthisreason,virtuallyallmoderncompressorinstallationsusearecyclelinewithcontrolvalvethatallowstheincreaseoftheflowthroughthecompressorifitcomesnearthestabilitylimit.Thecontrolsystemsconstantlymonitortheoperatingpointofthecompressorinrelationtoitssurgeline,andautomaticallyopenorclosetherecyclevalveifnecessary.Formostapplications,theoperatingmodewithanopen,orpartiallyopenrecyclevalveisonlyusedforstartupandshutdown,orforbriefperiodsduringupsetoperatingconditions.AssumingthepipelinecharacteristicderivedinEq.4,thecompressorimpellerswillbeselectedtooperateatornearitsbestefficiencyfortheentirerangeofheadandflowconditionsimposedbythepipeline.ThisispossiblewithaspeedNcontrolledcompressor,becausethebestefficiencypointsofacompressorareconnectedbyarelationshipthatrequiresapproximatelyfanlawequation525CNH6CNQ26525CCQHEq.6Foroperatingpointsthatmeettheaboverelationship,theabsorbedgaspowerPgisduetothefactthattheefficiencystaysapproximatelyconstant37653726557gNCCCQCCCQHCPEq.7Asitis,thispowerspeedrelationshipallowsthepowerturbinetooperateat,orveryclosetoitsoptimumspeedfortheentirerange.Thetypicaloperatingscenariosinpipelinesthereforeallowthecompressorandthepowerturbinetooperateatitsbestefliciencyformostofthetime.Thegasproducerofthegasturbinewill,however,losesomethermalefficiencywhenoperatedinpartload.Figure3showsatypicalrealworldexamplePipelineoperatingpointsfordifferentflowrequirementsareplottedintotheperformancemapofthespeedcontrolledcentrifugalcompressorusedinthecompressorstation.Reciprocatingcompressorswillautomaticallycomplywiththesystempressureratiodemands,aslongasnomechanicallimitsrodloadpowerareexceeded.Changesinsystemsuctionordischargepressurewillsimplycausethevalvestoopenearlierorlater.Theheadislowered

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