Air Compressors Section II空气压缩机教程英文原版教程教材教案

Air Compressors Section II空气压缩机教程英文原版教程教材教案 21AIR PRESSORSTokeep youoriented aswe discusspressure,the basioncepts aretreated inthissequence:generation,transmission,storage,and utilizationof pressedairin a pneumatic system.This sectioncovers air pressors,the devicesthatgenerate airpressure.Pressure Generation:CompressionThe pressureexerted by a confinedgas resultsfrom rapidand repeatedbom-bardment of the containerwalls by the enormousnumber of gas moleculespresent.The pressurecan beincreased byincreasing the number orforce of thecollisions.Increasing thetemperature doesthis byspeeding upthe molecules(CharlesLaw).Another wayis toincrease theaverage numberof moleculesin agivenvolume.This is pression.It can be done by eitherdecreasing thevol-ume(Boyles Law)or increasingthe amount ofgas.Liquids andsolids can be pressed only withdifficulty.But gasesare easilypressedbecause theirmolecules arerelatively farapart andmove freelyandrandomly within a confinedspace.Compression decreasesthe volumeavailable toeach molecule.This meansthateach particlehas ashorter distanceto travelbefore collidingwith anotherparticleor thewall.Thus,proportionately morecollisions ourin a given spanoftime,resulting in a higherpressure.Compression WorkRequirementsAn air pressor doesmost of its workduring the pression stroke.Thisadds energy to the air byincreasing itspressure.Compression alsogeneratesheat,however,and the amount of work required to pressa quantity of airto a givenpressure dependson howfast thisheat isremoved.The pressionworkdone willlie between the theoreticalwork requirementsof two processes:?Adiabatic-a processhaving nocooling;the heatremains in the air,caus-ing a pressure risethat increasespression workrequirements to a maximumvalue.?Isothermal-a processthat providesperfect cooling;thus,there isno changein airtemperature and the work required forpression is held toa minImum.The differencein theamountofworkrequiredto press air to100psi bythese twoprocessesis about36percent.Most industrialair pressors arenear adiabatic,since theprocess is too fastto allowmuch heat to escapethrough the pressorcasing.Section II22Air Compressors:Basic OperationAnair pressoroperates byconverting mechanicalenergy intopneumatic energyviapression.The inputenergy coulde froma drivemotor,gasoline engine,or powertakeoff.The ordinaryhand bellowsused byearly smeltersand blacksmithswas asimple type ofair pressor.It admittedair throughlarge holesas itexpanded.As thebellows werepressed,it expelledair througha smallnozzle,thus increasingthe pressure inside thebellowsand the velocity of the expelledair.Modern pressorsuse pistons,vanes,and otherpumping mechanismsto drawairfrom the atmosphere,press it,and dischargeit intoa receiveror pressuresystem.Table2summarizes thecapabilities ofvarious pressortypes.The mostbasic types of air pressors aredesignated as.positive displacementandnonpositive displacement(sometimes calleddynamic).The characteristicaction of apositive displacement pressor isthus adistinct volumetrihange-a literaldisplace-ment actionby whichsuessive volumes of air are confinedwithin aclosed chamberoffixed volumeand the pressure isgradually increased by reducingthe volumeof thespace.The forcesare static-that is,the pumpingrate isessentially constant,given afixedoperating speed.The principleis the same as the actionof a piston/cylinder assemblyin asimplehand pump.Positive DisplacementCompressorsPositive displacement pressors generallyprovide themost economicsolution forsys-tems requiringrelatively high pressures.Their chiefdisadvantage is that thedisplacingmechanism provideslower mass flow ratesthan nonpositive displacement pressors(see pages29-33).Pressure Characteristics-A pressorwith apositive displacementpumpingmechanism has these importantpressure characteristics:?The pressureagainst which the pressorworks risesto higherand highervalues aspumping continues.It mustbe limitedby someexternal pressure control device.?The rateof free air deliveryis highestat0psig and very graduallydrops tolower valuesas pressure increases.?The amountof heatgenerated progressivelyrises aspressure increases,causing substantialincreases intemperature ofboth the air handledand the pressor structure.23Types ofPositive DisplacementCompressorsPositive displacement pressors aredivided into those which pressair with a recip-rocating motion and thosewhichpressair witha rotary motion.The principaltypes ofpositive displacementpressors are the piston,diaphragm,rocking piston,rotary vane,lobed rotor,and rotaryscrew.Reciprocating Piston-This design(Fig.11)is widelyused in mercial airpres-sors because ofitshighpressurecapabilities,flexibility,and abilityto rapidlydissipate heatof pression.And it is oil-less.Compression is aomplished by the reciprocatingmovement of a pistonwithin acylin-der(Fig.12).This motionalternately fillsthe cylinderand thenpresses the air.A con-necting rodtransforms the rotarymotionof the crankshaft intoreciprocating pistonmotionin the cylinder.Depending on the application,the rotatingcrank(or eentric)is driven atconstant speedby asuitable primemover.Separate inlet and discharge valves reacttovariations inpressure produced by the piston movement.As Fig.12shows,the suction stroke beginswith the piston at the valveside of thecylinder,in aposition providingminimum(or clearance)volume.As the piston movesto amaximumvolume position,outside airflows into the cylinderthrough the inlet valve.Thedischarge valveremains closedduring thisstroke.During thepression stroke,thepistonmoves in the oppositedirection,decreasingthe volumeof airas thepiston returnsto theminimum position.During thisaction,the spring-loaded inlet and discharge valves areautomatically acti-vated by pressure differentials.That is,during thesuctionstroke,thepistonmotion re-duces the pressure in the cylinderbelow atmosphericpressure.The inlet valve thenopensagainst the pressures ofits springand allowsair toflow into the cylinder.Figure11Typical reciprocating piston airpressor24Reciprocating motionof thepiston pressesairwitheach revo-lution of thecrankshaft.When thepiston beginsits return(pression)stroke,the inlet valve springcloses theinletvalve because there isno pressuredifferential tohold the valve open.As pressureincreasesin thecylinder,the valve isheldfirmly inits seat.The discharge valve functionssimilarly.When pressurein thecylinder beesgreaterthan thebined pressuresof the valve springand the delivery pipe,thevalveopensand thepressed airflows into the system.In short,the inletvalve isopened byreduced pressure,and thedischargevalveisopened byincreased pressure.Some piston pressors aredouble-acting.As thepiston travelsin a given direction,air ispressedonone sidewhile suctionis producedon theother side.On thereturnstroke thesame thinghappens withthe sidesreversed.In a single-acting pressor,bycontrast,only oneside of thepistonis active.Single-acting pressorsare generallyconsidered light-duty machines,regardless ofwhetherthey operatecontinuously orintermittently.Larger double-acting pressors(usually watercooled)are consideredheavy-duty machinescapable ofcontinuous opera-tion.Sizes ofreciprocating piston pressors rangefrom less than1hp to6000hp.Goodpart-load efficiencymakes themvery usefulwhere widevariations incapacity areneeded.Figure12a)dischargevalve closesb)inletvalveopensc)inletvalveclosesd)dischargevalveopens25Their disadvantages?Reciprocating pistonpressors inherentlygenerate inertialforcesthat shakethe machine.Thus,a rigidframe,fixed toa solidfoundation,is oftenrequired.Also,these machinesdeliver apulsating flow of airthat may be objectionableunder someconditions.Properly sizedpulsation dampingchambers orreceiver tanks,however,willeliminate suchproblems.In general,the reciprocating pistonpressor is best suited topression ofrela-tively smallvolumesof air tohigh pressures.Diaphragm-The diaphragmdesign(Fig.13)isamodification of the reciprocatingpistonprinciple.An outstandingcharacteristic of the diaphragmdesign isthat thebasic press-ing mechanismdoes notrequire asliding sealbetween movingparts.A diaphragm-pressorisalso oil-less and it is therefore oftenselected whenno oilcontamination of theline or atmosphere can be tolerated.Compression isperformed bythe flexingof adiaphragm backand forthinaclosedchamber.Fig.14indicates howthis flexingaction isgenerated bythe motionof aconnect-ing rodunder the diaphragm.Only ashort strokeis requiredto producepressure effectssimilar tothoseproduced by a reciprocatingpiston ina cylinder.Intake anddischarge valvesconvert thevolume changesproducedbythe reciprocatingmovementinto pumping action.The reed-type valveswork likethose in thepistondesign.Figure13Typical diaphragmpressor.The heavy-duty diaphragmis madeofheat-resistant elastomerwith fabricreinforcement.26Dual-chamber diaphragmpressor.Fig.15shows adual-chamber machine.The contourof the diaphragm in the separatechambersindicates differentstroke positionsat thesame instant.The pressurecapabilities of thediaphragmpressor arelessthanthose of the pistontype,but usuallyexceed those of the rotary vaype.Figure14Figure15Cross-section showsdiaphragm flexingin responseto up/down motionofconnecting rod.ECCENTRICCHAMBERDIAPHRAGMCONNECTINGRODCONNECTOR RODMOTIONECCENTRICCONNECTINGROD27The rockingpiston principlecan be viewed as a bination of thereciprocatingpiston and diaphragm Ideas.Rocking Piston-The rockingpiston principle(Fig.16)is anothervariation ofreciprocalpression.In fact,it can beviewedas abinationof thediaphragmand pistonprin-ciples.The rockingpiston pumpessentially mountsapistonrigidly(no wrist pin)on topof thediaphragmunits eentrionnecting rod.This pistonis surmountedbyacup madeofTeflon,for instance.The cupfunctions bothas aseal-equivalent to the rings of apistonpressor-and asa guidemember for the rod.It expandsasthepiston travelsupward,thus maintainingcontact withthecylinderwalls andpensating forthe rockingmotion.Teflon isa registeredtrademark ofDuPont.Figure16?28The rockingpistonpressornot onlybines themechanical featuresof therecipro-catingpistonanddiaphragm types,but italso binesmany oftheir bestperformancefeatures.Like thediaphragmtype,it isquiet,pact,and oil-less.Like thereciprocatingpiston unit,it canprovide pressuresto100psi.The absenceof awristpinis thekey to the lightweight andpact sizeoftherock-ingpistonpressor.This makesthe entirepiston-connecting rodassembly muchshorterand sharplyreduces the overall dimensionsand theweight ofthe unit.As fordurability,the cupis(perhaps surprisingly)more durablethan theringsof aconventional oil-less pistonunit.And,on Gastmodels,when thecup needsreplacing itcanbe removedand replacedin minutes.Rotary Vane-Some applicationsrequire that there belittle orno pulsationin the airoutput,and perhapsa minimumof vibrationalso.The rotary vane pressor(Fig.17)provides this.It ismonly used for moderatelyhigh airflows at pressures under30psig,although somerotary vane designs canprovide pressuresof200psig.Rotary vaneunitsgenerally havelower pressure ratings thanpiston unitsbecause ofmore difficultsealingproblems andgreater sensitivityto thermaleffects.Fig.18shows howpumpingactionis producedbya series ofsliding,flat vanesasthey rotate inacylindrical case.As therotor turns,the individualvanes slidein and out,trapping aquantityof air andmoving itfrom theinlet sideofthe pressor to the outletside.Figure17Typical rotary vane airpressor.29There areno valvesintherotaryvanedesign.The entireflowof air intoandoutof theindividualpartments is controlled bythe movementofthe vanes acrossseparate inletanddischarge ports.The rotoris mountedeentrically-that is,not inthe centerofthe casing.As therotorrotates,thevanes are flungoutwards andheld against the bodybore by centrifugal andpres-sure-loading forces.This createsaseriesof airpartments ofunequal volume(because oftherotors eentricity).The partmentsformed betweenadjacent vanesgradually beelargerduring thesuction partofthecycle,and air is drawninto the partment from the inletport.During thedischarge portion ofthecycle,thepartmentvolumes graduallybeesmaller,pressing theair.When a rotating partmentreaches thedischarge port,thepressed airescapes to thedeliverysystem.The suctionand exhaustflows arerelatively free of pulsationbecause theinletanddis-charge portsdo nothave valves,and theair ismoved continuouslyrather thanintermittently.Rotary vane pressors havecertain significantadvantages.In additionto providingsmooth,pulse-free airflow withoutreceiver tanks,they arepact(or,equivalently,offer highflowFigure18In arotary-vanepressor,the eentricallymounted rotorcre-ates smallerpression partmentsasthevanes arepushed inbychamber walls.ROTATIONDISCHARGE PORTINTAKEPORTVANEDISCHARGESTROKESUCTIONSTROKEHOUSINGROTOR30capacities foragivensize),are simpleand economicalto installand operate,have lowstartingand runningtorque requirements,and producelittle noiseor vibration.Rotary Screwand LobedRotor-Two other typesof positivedisplacement-pressorsaretherotaryscrew andlobed rotor.Neither isas widelyused,especially insmallersizes,as arerotaryvaneand pistonpressors.Rotary screwpressorsareused whennearly pulselesshigh-volume air is required.The pressionmechanism isposed of two meshingrotors thathave helicalcon-tours.When therotors aredrivenatthesamespeed,airistrapped between the lobesasthe screwsturn.The volumebetweentheadvancing rotorhelix and the endplatedimin-ishes,forming continuouscavities until the endofthehelix passesover thedischarge port.In alobed rotorpressor,a pairof matinglobes onseparate shaftsrotateinoppositedirections totrap iningair andpress itagainst thecasing.Lobed rotorunits pro-vide very high airflows atpressures betweenthoseof nonpositive displacementpres-sors andother typesofpositivedisplacement units.Multistage CompressionCompressionmay beaomplished inone ormore stages.That is,air canbe pressedonceor severaltimes beforeit reachesthe pressoroutlet and is deliveredtothesys-tem devices.Each stageprovides aproportional increaseintheoutput pressure.Positive displacementpressors havethe advantageof providingrelatively largepressurechanges ina single stage,andvery large pressurechanges ina fewstages.However,the pressureoutput of nonpositive displacementpressors can also beraisedby staging.Single Stage-Fig.19is anotherway ofillustrating howthepressionprocess iscarriedout ina singlepass througha pumpingchamber.This piston-type pressorhastwo cylinders,but thepression actionours ina singlestage.The cylinders are con-nected inparallel betweenthe atmosphereand thedischarge manifold.The normalmaximum pressure rating forsingle-stage pressorsis about100psig.Operation abovethis levelincreases the heat ofpression(caused byleakage andrepression)to levelsthat couldharm the pressor andtheoverallsystem.Multiple Stage-In multiple-stage pression,the gasmoves fromone chambertoanother.This sequentialaction providesthe finalpressure.For generalutility andprocess purposes,two-stage pression is usuallyjustifiedwhen thepression ratio(R,)exceeds six.When Rcexceeds20,pression is31Basic operationofa singlestage/two cylinderairpressor.usually aomplishedin threestages.To putthis inpressure units,the upperlimit forutilitytwo-stage pressorsis between280and300psig.A gaugepressure of500psi hasanRC valueof35.Some multistage pressors eliminatethe problemof increasedheat ofpressionabove100psig.This isdoneby:?Compressing theair toan intermediatepressureinthe large-diameter low-pressure cylinder.?Removing aportionoftheheatofpressionbefore theairisfed tothe nextstage(this isknown asintercoolingandisnormally doneby anair-cooled orwater-cooled heatexchanger).?Further pressingtheair to finalpressureina smallerhigh-pressure cylinder.As Fig.20shows,thesetwo cylindersareconnected inseries throughthe intercooler(pare withFig.19).Intercooling greatlydecreases boththe totaltemperature riseof thepressed air andtheamountofworkrequired forits pression.But theadded costofan intercoolercannot alwaysbe justifiedonasmall pressor.Some two-stagepressorshave threecylinders:two low-pressure cylinderscon-nected toone high-pressure cylinderthrough anintercooler.Figure19INTAKE FILTERINTAKEFILTERDISCHARGEPORTSINGLE STAGETWO CYLINDERPRESSORDISCHARGEMANIFOLDDISCHARGEPORT32Basic operationoftwostage/twocylinderair pressorLubricationand ExhaustAir QualityContamination intheaircan affectmany applications.A laboratoryprocess,for ex-ample,powered bypressedairmay beextremely sensitiveto moisture,oil,or dustparticles.Or insuch placesas foodprocessing plants,even theair exhaustedfrom thepneumaticsystem mayhave tobe entirelyfreeofoil vaporand contaminants.A varietyof filters,generally expensive,have beendeveloped tosolve suchproblems.An alternativeis touse an oil-less airpressor.Oil-Less Compressors-Compressors designedwithdryself-lubricating materials,such asgraphite orTeflon,produce oil-freeairboth inthe lineand atthe exhaust.Theyeffectively eliminatethe presenceof air/oil vaporsin applicationswhere evena veryfine oilmistcan causecontamination,stains,deterioration,ora safety hazard.Oil-less pneumaticsystems areparticularly usefulinthefood,textile,paper,pharmaceu-tical,and chemicalindustries.And sinceno maintenancelubrication isrequired,these unitscanbe mountedinthebest,rather than the easiestto reach,location.Figure20INLETPORTINTAKEFILTERDISCHARGEPORTDISCHARGEPORTLP CYLINDERHPCYLINDERINTERCOOLER(HEAT EXCHANGER)TWO STAGETWO CYLINDERPRESSOR33Oil-Lubricated Compressors-if,for somereason,an oillessairpressor is notpracticalin an application wherecontarnination isprohibited,then anoil-lubricated unitmustbe usedand equippedwith appropriatefilters toremove theoil aftertheairis-pressed.In anoil-lubricated pressor,a thinfilm ofoil ismaintained betweenthe walls of thepumpingchamber andthe pistons,vanes orother movingparts.Siphon orwick-type lubri-cators areused inlight-duty operations.Pressure typelubricators areused inheavy-duty orcontinuous-duty applications.In general,oil-lubricated pressorshave higherpressure ratingsthan oil-free -pressors.They alsorun coolerand maythe