Compressed Air and Gas Handbook Chapter Two

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CompressedAir&GasInstitute•1300SumnerAvenue•Cleveland,OH44115Phone:216/241-7333•Fax:216/241-0105•E-mail:cagi@cagi.org™82CompressedAirProduction(Compressors)ChAPter22CompressedAirProduction(Compressors)OILINJECTEDROTARYSCREWCOMPRESSORSTheoilinjectedrotaryscrewcompressor,withelectricmotordriver,hasbecomeadominanttypeforawidevarietyofindustrialandminingapplications.Theenginedrivenportableversionalsoenjoysgreatpopularityformining,constructionandenergyexplorationapplications.Naturalgasenginesalsoareused.Thestationaryversionischaracterizedbylowvibration,requiringonlyasimpleloadbearingfoundationandprovidinglonglifewithminimalmaintenanceinbroadrangesofcapacityandpressure.Theyalsoareusedwidelyinvacuumservice.Theoilinjectedrotaryscrewcompressorisapositivedisplacementtype,whichmeansthatagivenquantityofairorgasistrappedinacompressionchamberandthespacethatitoccupiesismechanicallyreduced,causingacorre-spondingriseinpressurepriortodischarge.CompressionPrincipleTheoilinjectedrotaryscrewcompressorconsistsoftwointermeshingrotorsinastatorhousinghavinganinletportatoneendandadischargeportattheother.Themalerotorhaslobesformedhelicallyalongitslengthwhilethefemalerotorhascorrespondinghelicalgroovesorflutes.Thenumberofhelicallobesandgroovesmayvaryinotherwisesimilardesigns.CompressedAir&GasInstitute•1300SumnerAvenue•Cleveland,OH44115Phone:216/241-7333•Fax:216/241-0105•E-mail:cagi@cagi.org™83CompressedAirProductionAirflowinginthroughtheinletportfillsthespacesbetweenthelobesoneachrotor.Rotationthencausestheairtobetrappedbetweenthelobesandthestatorastheinter-lobespacespassbeyondtheinletport.Asrotationcontinues,alobeononerotorrollsintoagrooveontheotherrotorandthepointofintermeshingmovesprogressivelyalongtheaxiallengthoftherotors,reducingthespaceoccupiedbytheair,resultinginincreasedpressure.Compressioncontinuesuntiltheinter-lobespacesareexposedtothedischargeportwhenthecompressedairisdischarged.ThiscycleisillustratedinFigure2.1.Figure2.1HelicalScrew-TypeCompressor;CompressionCycle,SingleStageOilisinjectedintothecompressionchamberduringcompressionandservesthreebasicfunctions:1)Itlubricatestheintermeshingrotorsandassociatedbearings.2)Ittakesawaymostoftheheatcausedbycompression.3)Itactsasasealintheclearancesbetweenthemeshingrotorsandbetweenrotorsandstator.LubricationThegenerictermoilmaybeahydrocarbonproductbutmostcompressorsnowusecleanerandlonger-lifesyntheticlubricants,includingdiesters,polyglycols,polyalphaolefins,polyolestersandsiliconbasedfluids.Thesenewerproductsaresuitableforawiderrangeoftemperaturesandhavehigherflashpoints.Thelubri-cantchosenshouldbecompatiblewiththecompressorgasketsandseals.Amixtureofcompressedairandinjectedoilleavestheairendandispassedtoasump/separatorwheremostoftheoilisremovedfromthecompressedair.Directionalandvelocitychangesareusedtoseparatemostoftheliquid.Theremainingaerosolsinthecompressedairthenareseparatedbymeansofacoalesc-ingfilter,resultinginonlyafewpartspermillionofoilcarry-over(usuallyintherange2-5).Aminimumpressuredevice,oftencombinedwithadischargecheckvalve,preventsexcessivevelocitiesthroughtheseparatorelementuntilanormalsystempressureisachievedatstart-up.Mostoilinjectedrotaryscrewcompressorpackagesusetheairpressureintheoilsump/separator,afterthedischargeoftheairend,tocirculatetheoilthroughafilterandcoolerpriortore-injectiontothecompressionchamber.Somedesignsmayuseanoilpump.CompressedAir&GasInstitute•1300SumnerAvenue•Cleveland,OH44115Phone:216/241-7333•Fax:216/241-0105•E-mail:cagi@cagi.org™84CompressedAirProduction(Compressors)ChAPter2Bearingsateachendofeachrotoraredesignedtocarrytheradialandaxialthrustloadsgenerated.SeeFigure2.2A.Thesebearingsarelubricateddirectlywiththesamefilteredoilasisinjectedintothecompressionchamber.Asimilararrange-mentwithbuilt-inSpiralorTurnValveforcapacitycontrol,isshowninFigure2.2B.Figure2.2ASingleStageOilInjectedScrewCompressorFigure2.2BSingleStageOilInjectedScrewCompressorwithIntegralVariableDisplacementControlValveMulti-stagecompressorsmayhavetheindividualstagesmountedsidebyside,eitherinseparatestatorsorwithinacommonmulti-borestatorhousing.SeeFigure2.2C.Alternatively,thestagesmaybemountedintandemwiththesecondstagedrivendirectlyfromtherearofthefirststage.SeeFigure2.2D.Multiplestagesareusedeitherforimprovedefficiencyatagivenpressureortoachievehigherpressures.CompressedAir&GasInstitute•1300SumnerAvenue•Cleveland,OH44115Phone:216/241-7333•Fax:216/241-0105•E-mail:cagi@cagi.org™85CompressedAirProductionFigure2.2CTwoStage“Over/Under”DesignofOilInjectedRotaryScrewCompressorFigure2.2DTwoStage“Tandem”OilInjectedRotaryScrewCompressorCoolingThetemperatureoftheoilinjectedintothecompressionchambergenerallyiscontrolledtosomewhereintheareaof140˚F,eitherdirectlyorindirectlybycon-trollingthedischargetemperature.Thedischargetemperaturemustremainabovethepressuredewpointtoavoidcondensationofmoisturethatwouldmixwiththeoil.Athermostaticbypassvalveallowssomeoralloftheoilbeingcirculatedtobypasstheoilcoolertomaintainthedesiredtemperatureoverawiderangeofambienttemperatures.CompressedAir&GasInstitute•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