CompressorsThemaindifferencebetweenpumpsandcompressorsisthatthefluiddeliveredbycompressors-air-iscompressedandunderpressureatthetimeitisdelivered,evenifthereisnoloadonthesystem.Mostdevicesusedtocompressairareverysimilarinconceptand-perhapseveninhardware-tohydraulicpumps,andselectionconsiderationsaresimilar.Theonlyothersubstantivedifferenceisthatmosthydraulicsystemsarepoweredbyasinglepumpthatisactuallyapartofthesystem,whereasahostofpneumaticsystemsareoftenpoweredbyasinglecompressor,whichisalmostautilityintheplantlikewaterorelectricservice.Nevertheless,manysmallcompressorsareavailableforspecific,discretejobs；typicallytheyarepositive-displacementcompressors.Dynamic,ornonpositive-displacementcompressorsaretypicallylarger,facility-typeunits.Compressorsarefairlysimpledevices,capableoflongperiodsofmaintenance-freeoperationifproperlyintegratedintopneumaticsystems.Yettimeandagaintheysufferfromearlyfailuresbecauseobviousprecautionswereignoredduringsystemdesign.Fourbasicrulescanprovidesubstantialimprovementincompressorlifewithonlymoderatedesigneffort:Pumpsandcompressorsshouldbesizedtoprovideatleasttherequiredpressureandflow,andpreferably10to25%more.Filtersshouldbeselectedtoprotectthepumpingunit,andsometimestoprotectdownstreamcomponentsorproductsaswell.Reliefvalvesshouldbeselectedtokeeppressureorvacuumatappropriatelevels.Pumpingunitsshouldbeplacedinaclean,cool,dryenvironment.Bellowscompressorsconsistofaweldedmetalbellowsconnectedtoinletandoutletportswithcheckvalves.Thesecompressorstypicallycoverthepressurerangeupto10psig,andareusedinpollutiondetectingandmeasuringdevices,gas-samplinginstruments,andmedicalapplications.Lubricationisnotneeded,allowinghighpuritiestobemaintained.Vanecompressorsaresimplemachineswithfewmovingparts.Liketheirhydrauliccounterparts,vanepumps,thecompressorsareinexpensive,withlowoperatingcost,andlowstarting-torquerequirement.Theyarecompactandrelativelyvibrationfree,withlittlepulsationinthecompressoroutput.Theslidingvanesarecloselyfittedintherotorslotsandwearverylittleduringoperation.Thesecompressorsareavailableinpowerrangesfrom10to500hp,atpressuresto150psi.Reciprocatingcompressorsconsistofapistonmovingwithinthecylindertotrapandcompressthegas.Inprinciple,suchaunitislikeanautomobileengine,withthepistonscompressingthegasandvalvescontrollingitsinletandoutflows.Sizesrangefromlessthan1toover5,000hp.Reciprocatingcompressorshavegoodpartloadefficienciesandareusefulforwidevariationsinoperatingconditions.Diaphragmcompressorsareamodificationofthereciprocatingcompressor.Compressionisperformedbytheflexingofametalorfabricateddiaphragmwhichiscausedbythemotionofareciprocatingpistoninacylinderunderthediaphragm.Thespacebetweenthediaphragmandthepistonisusuallyfilledwithliquid.Lobed-rotorcompressorshavetworotatingelementsthatrevolveinoppositedirectionsinachamber.Inmostcompressors,therotorsdonotactuallytouchanddonotdriveeachother,beingdriveninsteadbytiminggears.Becausetherotorsdonotactuallytouch,airleaksbetweenthematasmallbutconstantrate.Thisleakage,calledslip,isconstantforagivencompressoratagivenpressure.Forhighestefficiency,thesecompressorsshouldbeoperatedatmaximumspeed.Theyareavailableinpowerrangesfrom7to3,000hp,deliveringpressuresto250psi.Becausetheinternallobesdonotcontact,theyneednolubrication.Liquidpistoncompressorshavenomovingpartsinwearingcontact.Arotorwithmultipleforward-curvedbladesrotatesinanellipticalcasing.Fluid,trappedwithinthecasing,iscarriedaroundtheinnerperipherybytheblades.Spacebetweenthebladeschangesvolumeduetotheellipticalfluidpath,andtheinnersurfaceoftheliquidringtrappedbetweenthebladesservesasthefaceofaliquidpiston.Thesecompressorsacceptliquidslugsandfineparticleswithoutseriousdamage.Lubricationisrequiredonlyinbearingslocatedoutsidethepumphousing.Thesecompressorsdeliverupto150psithroughouttherangeof10to500hp.Centrifugalcompressorsarebestsuitedtomovinglargevolumesofairatrelativelylowpressures.Basically,theyconsistofahigh-speedrotatingimpeller,adiffusersectionwherevelocityisreducedandpressureincreased,andacollectorsectionthatfurtherreducesvelocityandincreasespressure.Centrifugalcompressorscanhandlehighflowdemandswell,butwhendemanddecreasesmuchbelowratedflowandoutputpressurerises,thecompressorscansurge.Insurge,thepressurefieldatthecompressoroutletvariesrandomly.Ifallowedtocontinue,thisconditioncandamagebearings,blades,andeventhehousingitself.Centrifugalcompressorstypicallyusefromtwotosixstages,supplyingfrom400to3,000cfmatspeedsto20,000rpm.Regenerativeblowers(alsoknownasperipheralblowers)useadisclikeimpellerwithbladesmountedarounditsoutsideedge.Astheimpellerrevolves,airisdrawnintothespacebetweentheblades.Centrifugalforcemovestheairinaspiralpathoutwardtothehousing,whereitslipsbytheinitialbladeandreturnstothebaseofthesucceedingblade,wheretheprocessisrepeated.Insomemodels,aflowsplittercreatestwoflowpaths,sothattheairmustmaketwocircuitsaroundtheimpeller.Inothermodels,thesplitterisomitted,andtheairmakesonlyonecircuitbeforeexiting.Regenerativeblowersprovideairflowsupto1,000cfmandpressuresto8psi.Helicalcompressorslookliketwogiantscrewsmeshingtogether；theyworkmuchlikehydraulicscrewpumps.Maximumpressurefromthesemachinesisapproximately125psiinsingle-stageconfigurations.Helicalcompressorsmaybeeitheroilfloodedordry.Dryhelicalcompressors,likelobedunits,requiretiminggearstomaintainproperclearancebetweentherotatingelements.Theseunitsaremostefficientlyoperatedathighcontinuousspeeds.Floodedcompressorsdonotrequireanytiminggears,becausetheoil-ladenscrewsurfacescandriveeachother.However,oilseparatorsareneededtoremovetheoilfromtheairasitleavesthecompressor.Theyareavailableoverapowerrangeofabout7to300hp.Single-screwcompressorsarebasedonthesameprincipleashelicalcompressors.Asthecentralscrewrotates,airtrappedbetweenthescrewteethiscompressedagainstthestar-shapedrotors.Thesecompressorstendtohavelowvibrationandnoiselevels,andlowdischargepressures.Lubricationisrequired.PumpsvacuumpumpsInprinciple,industrialvacuumpumpsaremerelycompressorsrunwiththeinletattachedtothevacuumsystemandtheoutletopentoexhaust.Insmallersizes,compressorsandvacuumpumpsareoftenidenticalmachines.However,inthelargesizesthatmightpoweraplant-widevacuumsystem,themachinesdifferinminorwaysthatareintendedtoenhanceefficiencyforoneapplicationortheother.Manufacturersstronglyadvisethatthesamemachinenotbeusedforbothvacuumandcompressionatthesametime.Theheavyloadswilldamageit.Threecriteriacontrolpumpselection:degreeofvacuumproduced,rateofairremoval,andpowerrequirement.However,applicationssuchasfiltrationmaysubjecttheunittotheingestionofforeignmaterial.Thefirstpumpperformancecriterionisthevacuumitproduces.ManufacturersprovideamaximumvacuumratingexpressedasabsolutepressureinmmHg,orvacuuminin.Hg.Largerunitsareusuallyratedonlyforcontinuousduty,butsmallerunitsmayhaveahighervacuumratingforintermittentduty.Insmallerunits,temperature-riseconsiderationslimitthevacuumthatcanbeproduced.Continuousandintermittentvacuumratingsaredeterminedforstandardatmosphericpressure:29.92-in.Hg.Lowerambientpressuresreducethevacuumthatcanbeproduced.Theratingisdeterminedfrom:whereVa=adjustedvacuumrating,in.Hg；Vo=originalvacuumratingatstandardconditions,in.Hg；andPa=anticipatedatmosphericpressureattheapplicationsite,in.Hg.Rateofairremovalisthesecondcriterion.Vacuumpumpsareflowratedaccordingtothevolumeofairexhaustedwithnopressuredifferentialacrossthepump.Manufacturersprovidecurvesshowingfreeairdeliveryatratedspeedforvacuumlevelsrangingfrom0-in.Hg(so-calledopencapacity)tomaximumvacuumrating.Somemanufacturersalsoprovidecurvesofcapacityatdifferentspeedsforagivenvacuum.Thelastpumpcriterionispowerrequirement.Comparedwithaircompressors,vacuumpumpsrequirerelativelylittlepower.Atlowflows,vacuum(orpressuredifferential)ishigh；athighflows,vacuumislow.Therefore,power,whichisproportionaltoflowandpressuredifferential,isgenerallylow.Poweroutputofthepumpcanbefoundfrompressure-flowcurvesprovidedbymanufacturers.Inputpowerandspeedrequirementsarealsoshowninthedata.Overallpumpefficiency(includingbothvolumetricandmechanicalefficiency)canbeevaluatedbycombiningthisdata.Thisisdonebydividingthefree-aircapacityofthepumpattherequiredvacuumlevelbydrivepowerrequiredatthatcondition.Theresultisproportionaltotheproductofgagevacuumandair-flowrateandisrepresentativeofefficiency.Allthreeperformancecriteria-vacuum,flowandpower-canbeaffectedbypumptemperature.Athighervacuumlevels,littleairflowsthroughthepump,solittleheatistransferredtotheair.Muchoftheheatgeneratedbyfrictionmustbedissipatedbythepump.Thisheatgraduallyraisespumptemperatureandcandrasticallyreduceservicelife.Temperatureexcursionsareespeciallyimportanttointermittent-dutypump,whichcanoverheatifontimegreatlyexceedsofftime.Vacuumpumpsareclassifiedaseitherpositiveornonpositivedisplacement.Apositive-displacementpumpcreatesvacuumbyisolatingandcompressingadistinct,constantvolumeofair.Thecompressedairisventedoutoneport,andavacuumiscreatedattheotherportwheretheairisdrawnin.Thisgeneratesrelativelyhighvacuum,butlittleflow.Anonpositive-displacementpump,ontheotherhand,usesrotatingimpellerbladestoaccelerateairandcreateavacuumattheinletport.Whilenonpositive-displacementpumpscannotproducehighlevelsofvacuum,theyprovidehighflowrates.Principaltypesofpositive-displacementvacuumpumpsincludepiston,diaphragm,rocking-piston,rotary-vane,lobed-rotor,rotary-screw,andliquid-ringdesigns.Reciprocating-pistonpumpsgeneraterelativelyhighvacuums-from27tomorethan29in.Hg-underavarietyofoperatingconditions.Typicalpumpsofthistypehaveoneormorepistonslinkedtoarotatingcrankshaft.Thealternatingpistonactionmovesairpastcheckvalvesinthecylinderheadtocreateavacuumattheinletport.Lubricatedpistonpumpsarequieter,producelessvibration,haveahighercapacity,andfeatureamuchlongerlifethanoillessdesigns,buttheyarealsoheavierandmoreexpensive.Diaphragmpumpsoffertheadvantageofthefluidchamberbeingtotallysealedfromthepumpingmechanisms.Aneccentricconnectingrodmechanicallyflexesadiaphragminsidetheclosedchambertocreateavacuum.Thisresultsinsomewhatlowervacuumcomparedtothatproducedbyareciprocatingpiston.However,thediaphragmslowercompressionratio-lowflow,largediameter,andshortstroke-makesforquiet,economical,andreliableoperation.Thedesignisavailableinbothoneandtwo-stageversions.Single-stagepu