樓宇第07次課-CHAPTER-2-Pneumatic-Control-Fundamentals-電課件

CHAPTER2PneumaticControlFundamentalsCHAPTER21INTRODUCTIONThissectionprovidesbasicinformationonpneumaticcontrolsystemsandcomponentscommonlyusedtocontrolequipmentincommercialheatingandairconditioningapplications.Theinformationinthissectionisofageneralnatureinordertoexplainthefundamentalsofpneumaticcontrol.Sometermsandreferencesmayvarybetweenmanufacturers(e.g.,switchportnumbers).INTRODUCTION2Pneumaticcontrolsystemsusecompressedairtooperateactuators,sensors,relays,andothercontrolequipment.Pneumaticcontrolsdifferfromothercontrolsystemsinseveralwayswithsomedistinctadvantages:—Pneumaticequipmentisinherentlyproportionalbutcanprovidetwo-positioncontrolwhenrequired.—Manycontrolsequencesandcombinationsarepossiblewithrelativelysimpleequipment.—Pneumaticequipmentissuitablewhereexplosionhazardsexist.Pneumaticcontrolsyst3—Theinstalledcostofpneumaticcontrolsandmaterialsmaybelower,especiallywherecodesrequirethatlowvoltageelectricalwiringforsimilarelectriccontrolsberuninconduit.—Quality,properlyinstalledpneumaticequipmentisreliable.However,ifapneumaticcontrolsystemrequirestroubleshootingorservice,mostbuilding-maintenancepeoplehavethenecessarymechanicalknowledge.—Theinstalledcostofp4DEFINITIONSActuator:Amechanicaldevicethatoperatesafinalcontrolelement(e.g.,valve,damper).Branchline:Theairlinefromacontrollertothecontrolleddevice.Branchlinepressure(BLP):Avaryingairpressuresignalfromacontrollertoanactuatorcarriedbythebranchline.Cangofromzerotofullmainlinepressure.Dischargeair:Conditionedairthathaspassedthroughacoil.Also,airdischargedfromasupplyductoutletintoaspace.DEFINITIONS5Directacting(DA):Adirect-actingthermostatorcontrollerincreasesthebranchlinepressureonanincreaseinthemeasuredvariableanddecreasesthebranchlinepressureonadecreaseinthevariable.Adirect-actingactuatorextendstheshaftonanincreaseinbranchlinepressureandretractstheshaftonadecreaseinpressure.Mainline:Theairlinefromtheairsupplysystemtocontrollersandotherdevices.Usuallyplasticorcoppertubing.Directacting(DA):Adirect6Mixedair:Typicallyamixtureofoutdoorairandreturnairfromthespace.Resetchangeover:Thepointatwhichthecompensationeffectisreversedinactionandchangesfromsummertowinterorviceversa.Thepercentofcompensationeffect(authority)mayalsobechangedatthesametime.Restrictor:Adeviceinanairlinethatlimitstheflowofair.Returnair:Airenteringanairhandlingsystemfromtheoccupiedspace.Mixedair:Typicallyamixt7Reverseacting(RA):Areverse-actingthermostatorcontrollerdecreasesthebranchlinepressureonanincreaseinthemeasuredvariableandincreasesthebranchlinepressureonadecreaseinthevariable.Areverse-actingvalveactuatorretractstheshaftonanincreaseinbranchlinepressureandextendstheshaftonadecreaseinpressure.SensorSpan:Thevariationinthesensedmediathatcausesthesensoroutputtovarybetween3and15psi.Reverseacting(RA):Areve8ABBREVIATIONSThefollowingportabbreviationsareusedindrawingsofrelaysandcontrollers:B—BranchC—Common

E—ExhaustM—Main

O—Normallyconnected*X—Normallydisconnected*P—Pilot(P1andP2fordual-pilotrelays)S—Sensor(S1andS2fordual-inputcontrollers)N.C.—NormallyclosedN.O.—NormallyopenABBREVIATIONS9SYMBOLSSYMBOLS10BASICPNEUMATICCONTROLSYSTEMGENERALApneumaticcontrolsystemismadeupofthefollowingelements:—Compressedairsupplysystem—Mainlinedistributionsystem—Branchlines—Sensors—Controllers—ActuatorsBASICPNEUMATICCONTROLSYSTE11—Finalcontrolelements(e.g.,valves,dampers)Abasicpneumaticcontrolsystemconsistsofanairsupply,acontrollersuchasathermostat,andanactuatorpositioningavalveordamper(Fig.1).—Finalcontrolelements(e.12Thecontrollerreceivesairfromthemainlineandregulatesitsoutputpressure(branchlinepressure)asafunctionofthetemperature,pressure,humidity,orothervariable.Thebranchlinepressurefromthecontrollercanvaryfromzerotofullmainlinepressure.Theregulatedbranchlinepressureenergizestheactuator,whichthenassumesapositionproportionaltothebranchlinepressureapplied.Theactuatorusuallygoesthroughitsfullstrokeasthebranchlinepressurechangesfrom3psito13psi.Otherpressurerangesareavailable.Thecontrollerreceives13Inatypicalcontrolsystem,thefinalcontrolelement(avalveoradamper)isselectedfirstbecauseitmustproducethedesiredcontrolresults.Forexample,asystemdesignedtocontroltheflowofwaterthroughacoilrequiresacontrolvalve.Thetypeofvalve,however,dependsonwhetherthewaterisintendedforheatingorcooling,thewaterpressure,andthecontrolandflowcharacteristicsrequired.Anactuatoristhenselectedtooperatethefinalcontrolelement.Acontrollerandrelayscompletethesystem.WhenallcontrolsystemsInatypicalcontrolsyst14forabuildingaredesigned,theairsupplysystemcanbesizedanddesigned.AIRSUPPLYANDOPERATIONThemainlineairsupplyisprovidedbyanelectricallydrivencompressorpumpingairintoastoragetankathighpressure(Fig.2).Apressureswitchturnsthecompressoronandofftomaintainthestoragetankpressurebetweenfixedlimits.Thetankstorestheairuntilitisneededbycontrolequipment.Theairdryerremovesmoisturefromtheair,andthefilterremovesoilandotherimpurities.Thepressurereducingvalveforabuildingaredesigned,t15(PRV)typicallyreducesthepressureto18to22psi.Fortwo-pressure(day/night)systemsandforsystemsdesignedtochangefromdirecttoreverseacting(heating/cooling),thePRVswitchesbetweentwopressures,suchas13and18psi.Themaximumsafeairpressureformostpneumaticcontrolsis25psi.(PRV)typicallyreducesthepr16FromthePRV,theairflowsthroughthemainlinetothecontroller(inFig.1,athermostat)andtoothercontrollersorrelaysinotherpartsofthesystem.Thecontrollerpositionstheactuator.Thecontrollerreceivesairfromthemainlineataconstantpressureandmodulatesthatpressuretoprovidebranchlineairatapressurethatvariesaccordingtochangesinthecontrolledvariable,asmeasuredbythesensingelement.Thecontrollersignal(branchlinepressure)istransmittedviathebranchlinetothecontrolleddevice(inFig.1,avalveactuator).FromthePRV,theairflo17Theactuatordrivesthefinalcontrolelement(valve)toapositionproportionaltothepressuresuppliedbythecontroller.Whentheproportionalcontrollerchangestheairpressuretotheactuator,theactuatormovesinadirectionanddistanceproportionaltothedirectionandmagnitudeofthechangeatthesensingelement.Theactuatordrivesthef18RESTRICTORTherestrictorisabasiccomponentofapneumaticcontrolsystemandisusedinallcontrollers.Arestrictorisusuallyadiscwithasmallholeinsertedintoanairlinetorestricttheamountofairflow.Thesizeoftherestrictorvarieswiththeapplication,butcanhaveaholeassmallas0.003inches.RESTRICTOR19NOZZLE-FLAPPERASSEMBLYThenozzle-flapperassembly(Fig.3)isthebasicmechanismforcontrollingairpressuretothebranchline.Airsuppliedtothenozzleescapesbetweenthenozzleopeningandtheflapper.Atagivenairsupplypressure,theamountofairescapingisdeterminedbyhowtightlytheflapperisheldagainstthenozzlebyasensingelement,suchasabimetal.Thus,controllingthetensiononthespringalsocontrolstheamountofairescaping.Verylittleaircanescapewhentheflapperisheldtightlyagainstthenozzle.NOZZLE-FLAPPERASSEMBLY20樓宇第07次課-CHAPTER-2-Pneumatic-Control-Fundamentals-電課件21PILOTBLEEDSYSTEMThepilotbleedsystemisameansofincreasingaircapacityaswellasreducingsystemairconsumption.Therestrictorandnozzlearesmallerinapilotbleedsystemthaninanozzleflappersystembecauseinapilotbleedsystemtheysupplyaironlytoacapacityamplifierthatproducesthebranchlinepressure(Fig.4).Thecapacityamplifierisapilotbleedcomponentthatmaintainsthebranchlinepressureinproportiontothepilotpressurebutprovidesgreaterairflowcapacity.PILOTBLEEDSYSTEM22樓宇第07次課-CHAPTER-2-Pneumatic-Control-Fundamentals-電課件23Thepilotpressurefromthenozzleentersthepilotchamberofthecapacityamplifier.InthestateshowninFigure4,noairentersorleavesthebranchchamber.Ifthepilotpressurefromthenozzleisgreaterthanthespringforce,thepilotchamberdiaphragmisforceddown,whichopensthefeedvalveandallowsmainairintothebranchchamber.Thepilotpressurefromt24Whenthepilotpressuredecreases,thepilotchamberdiaphragmrises,closingthefeedvalve.Ifthepilotchamberdiaphragmrisesenough,itliftsthebleedvalveoffthefeedvalvedisc,allowingairtoescapefromthebranchchamberthroughthevent,thusdecreasingthebranchlinepressure.Mainairisusedonlywhenbranchlinepressuremustbeincreasedandtosupplytheverysmallamountexhaustedthroughthenozzle.Whenthepilotpressured25SENSINGELEMENTSBIMETALAbimetalsensingelementisoftenusedinatemperaturecontrollertomovetheflapper.AbimetalconsistsoftwostripsofdifferentmetalsweldedtogetherasshowninFigure6A.Asthebimetalisheated,themetalwiththehighercoefficientofexpansionexpandsmorethantheothermetal,andthebimetalwarpstowardthelower-coefficientmetal(Fig.6B).Asthetemperaturefalls,thebimetalwarpsintheotherdirection(Fig.6C).SENSINGELEMENTS26樓宇第07次課-CHAPTER-2-Pneumatic-Control-Fundamentals-電課件27Atemperaturecontrollerconsistsofabimetalelementlinkedtoaflappersothatachangeintemperaturechangesthepositionoftheflapper.Figure7showsadirect-actingthermostat(branchlinepressureincreasesastemperatureincreases)inwhichthebranchlinepressurechangeisproportionaltothetemperaturechange.Anadjustmentscrewonthespringadjuststhetemperatureatwhichthecontrolleroperates.Ifthetensionisincreased,thetemperaturemustbehigherforthebimetaltodeveloptheforcenecessarytoopposethespring,lifttheflapper,Atemperaturecontroller28andreducethebranchpressure.andreducethebranchpressure29RELAYSANDSWITCHESRelaysareusedincontrolcircuitsbetweencontrollersandcontrolleddevicestoperformafunctionbeyondthecapacityofthecontrollers.Relaystypicallyhavediaphragmlogicconstruction(Fig.9)andareusedtoamplify,reverse,average,select,andswitchcontrolleroutputsbeforebeingsenttovalveanddamperactuators.RELAYSANDSWITCHES30樓宇第07次課-CHAPTER-2-Pneumatic-Control-Fundamentals-電課件31Thecontrollingpressureisconnectedatthepilotport(P),andpressurestobeswitchedareconnectedatthenormallyconnectedport(O)orthenormallydisconnectedport(X).Theoperatingpointoftherelayissetbyadjustingthespringpressureatthetopoftherelay.Whenthepressureatthepilotportreachestherelayoperatingpoint,itpushesuponthediaphragminthecontrolchamberandconnectspressureonthenormallydisconnectedport(X)tothecommonportasshown.Thecontrollingpressure32Ifthepilotpressurefallsbelowtherelaysetpoint,thediaphragmmovesdown,blocksthenormallydisconnected(X)port,andconnectsthenormallyconnectedport(O)tothecommonport.Ifthepilotpressurefal33AIRSUPPLYEQUIPMENTGENERALApneumaticcontrolsystemrequiresasupplyofclean,dry,compressedair.Theairsourcemustbecontinuousbecausemanypneumaticsensors,controllers,relays,andotherdevicesbleedair.Atypicalairsupplysystemincludesacompressor,anairdryer,anairfilter,apressurereducingvalve,andairtubingtothecontrolsystem(Fig.10).AIRSUPPLYEQUIPMENT34Thefollowingparagraphsdescribethecompressor,filter,pressurereducingvalves,andairdryingtechniques.Forinformationondeterminingthemoisturecontentofcompressedair,refertotheGeneralEngineeringDatasection.AIRCOMPRESSORTheaircompressorprovidesthepowerneededtooperateallcontroldevicesinthesystem.Thecompressormaintainspressureinthestoragetankwellabovethemaximumrequiredinthecontrolsystem.Thefollowingparagraphs35樓宇第07次課-CHAPTER-2-Pneumatic-Control-Fundamentals-電課件36Whenthetankpressuregoesbelowaminimumsetting(usually70to90psi),apressureswitchstartsthecompressormotor.Whenthetankpressurereachesahighlimitsetting,thepressureswitchstopsthemotor.Astandardtankistypicallylargeenoughsothatthemotorandcompressoroperatenomorethan50percentofthetime,withuptotwelvemotorstartsperhour.Whenthetankpressuregoe37Someapplicationsrequiretwocompressorsoradualcompressor.Inadualcompressor,twocompressorsoperatealternately,sowearisspreadoverbothmachines,eachcapableofsupplyingtheaveragerequirementsofthesystemwithoutoperatingmorethanhalfthetime.Intheeventoffailureofonecompressor,theotherassumesthefullload.

Someapplicationsrequire38Contaminationintheatmosphererequiresacompressorintakefiltertoremoveparticlesthatwoulddamagethecompressorpump.Thefilterisessentialonoil-lesscompressorsbecauseacontaminatedinletaircancauseexcessivewearonpistonrings.Theintakefilterisusuallylocatedintheequipmentroomwiththecompressor,butitmaybelocatedoutdoorsifcleanoutdoorairisavailable.Aftertheairiscompressed,coolingandsettlingactionsinthetankcondensesomeoftheexcessmoistureandallowfalloutofthelargeroildropletsgeneratedbythecompressorpump.Contaminationintheatmo39Ahighpressuresafetyreliefvalvewhichopensonexcessivelyhightankpressuresisalsorequired.Ahandvalveorautomatictrapperiodicallyblowsoffanyaccumulatedmoisture,oilresidue,orotherimpuritiesthatcollectinthebottomofthetank.AIRDRYINGTECHNIQUESGENERALAirshouldbedryenoughtopreventconden-sation.Condensationcausescorrosionthatcanblockorificesandvalvemechanisms.Ahighpressuresafetyre40Inaddition,dryairimprovestheabilityoffilterstoremoveoilanddirt.Moistureincompressedairisremovedbyincreasingpressure,decreasingtemperature,orboth.Whenairiscompressedandcooledbelowitssaturationpoint,moisturecondenses.drainingthecondensatefromthestoragetankcausessomedryingoftheairsupply,butanairdryerisoftenrequired.Anairdryerisselectedaccordingtotheamountofmoistureintheairandthelowesttemperaturetowhichanairlinewillbeexposed.Inaddition,dryairimpr41CONDENSINGDRYINGThetwomethodsofcondensingdryingarehigh-pressuredryingandrefrigerantdrying.High-PressureDryingHigh-pressuredryingmaybeusedwhenmainairpipingiskeptawayfromoutsidewallsandchillingequipment.Duringcompressionandcoolingtoambienttemperatures,airgivesupmoisturewhichthencollectsinthebottomofthestoragetank.CONDENSINGDRYING42Thehigherthetankpressure,thegreatertheamountofmoisturethatcondenses.Maintainingahighpressureremovesthemaximumamountofmoisture.Thecompressorshouldhaveahigheroperatingpressurethanisrequiredforairsupplypurposesonly.However,higherairpressurerequiresmoreenergytorunthecompressor.Thetankmustincludeamanualdrainvalveoranautomatictraptocontinuallydrainoffaccumulatedmoisture.Withtankpressuresof70to90psi,adewpointofapproximately70Fat20psicanbeobtained.Thehigherthetankpress43RefrigerantDrying(作業(yè)）DESICCANTDRYINGAdesiccantisachemicalthatremovesmoisturefromair.AdesiccantdryerisinstalledbetweenthecompressorandthePRV.Dewpointsbelow–100Farepossiblewithadesiccantdryer.Thedesiccantrequiresaboutone-thirdoftheprocessairtoregenerateitself,oritmaybeheated.Toregenerate,desiccantdryersmayrequirealargercompressortoproducetheneededairflowtosupplythecontrolsystemandthedryer.RefrigerantDrying(作業(yè)）44Itmaybenecessarytoinstalladesiccantdryeraftertherefrigerantdryerinapplicationswherethe12Fdewpointat20psimainlinepressuredoesnotpreventcondensationinairlines(e.g.,aroof-topunitexposedtoseverewinters).Thedesiccantdryermostapplicabletocontrolsystemsusestheadsorbentprincipleofoperationinwhichporousmaterialsattractwatervapor.Thewatervaporiscondensedandheldasaliquidintheporesofthematerial.Thedryingactioncontinuesuntilthedesiccantissaturated.ThedesiccantisregeneratedbyremovingthemoisturefromtheItmaybenecessarytoin45poresofthedesiccantmaterial.Themostcommonadsorbentdesiccantmaterialissilicagel,whichadsorbsover40percentofitsownweightinwaterandistotallyinert.Anothertypeofadsorbentdesiccantisthemolecularsieve.Adesiccantisregeneratedeitherbyheatingthedesiccantmaterialandremovingtheresultingwatervaporfromthedesiccantchamberorbyflushingthedesiccantchamberwithairatalowervaporpressureforheatlessregeneration.Toporesofthedesiccantmateria46provideacontinuoussupplyofdryair,adesiccantdryerhastwodesiccantchambers(Fig.14).Whileonechamberisbeingregenerated,theothersuppliesdryairtothesystem.Thecyclingisaccomplishedbytwosolenoidvalvesandanelectrictimer.Duringonecycle,airpassesfromthecompressorintotheleftdesiccantchamber(A).Theairisdried,passesthroughthecheckvalve(B),andflowsouttothePRVvideacontinuoussupplyo47樓宇第07次課-CHAPTER-2-Pneumatic-Control-Fundamentals-電課件48Simultaneously,someofthedriedairpassesthroughtheorifice(G)totherightdesiccantchamber(E).Theairisdryandthedesiccantchamberisopentotheatmosphere,whichreducesthechamberpressuretonearatmosphericpressure.Reducingtheairpressurelowersthevaporpressureoftheairbelowthatofthedesiccant,whichallowsthemoisturetotransferfromthedesiccanttotheair.Thetimercontrolsthecycle,whichlastsapproximately30minutes.Simultaneously,someoft49Duringthecycle,thedesiccantintheleftchamber(A)becomessaturated,andthedesiccantintherightchamber(E)becomesdry.Thetimerthenreversestheflowbyswitchingbothofthesolenoidvalves(DandH).Thedesiccantintherightchamber(E)thenbecomesthedryingagentconnectedtothecompressorwhilethedesiccantintheleftchamber(A)isdried.Duringthecycle,thede50CHAPTER2PneumaticControlFundamentalsCHAPTER251INTRODUCTIONThissectionprovidesbasicinformationonpneumaticcontrolsystemsandcomponentscommonlyusedtocontrolequipmentincommercialheatingandairconditioningapplications.Theinformationinthissectionisofageneralnatureinordertoexplainthefundamentalsofpneumaticcontrol.Sometermsandreferencesmayvarybetweenmanufacturers(e.g.,switchportnumbers).INTRODUCTION52Pneumaticcontrolsystemsusecompressedairtooperateactuators,sensors,relays,andothercontrolequipment.Pneumaticcontrolsdifferfromothercontrolsystemsinseveralwayswithsomedistinctadvantages:—Pneumaticequipmentisinherentlyproportionalbutcanprovidetwo-positioncontrolwhenrequired.—Manycontrolsequencesandcombinationsarepossiblewithrelativelysimpleequipment.—Pneumaticequipmentissuitablewhereexplosionhazardsexist.Pneumaticcontrolsyst53—Theinstalledcostofpneumaticcontrolsandmaterialsmaybelower,especiallywherecodesrequirethatlowvoltageelectricalwiringforsimilarelectriccontrolsberuninconduit.—Quality,properlyinstalledpneumaticequipmentisreliable.However,ifapneumaticcontrolsystemrequirestroubleshootingorservice,mostbuilding-maintenancepeoplehavethenecessarymechanicalknowledge.—Theinstalledcostofp54DEFINITIONSActuator:Amechanicaldevicethatoperatesafinalcontrolelement(e.g.,valve,damper).Branchline:Theairlinefromacontrollertothecontrolleddevice.Branchlinepressure(BLP):Avaryingairpressuresignalfromacontrollertoanactuatorcarriedbythebranchline.Cangofromzerotofullmainlinepressure.Dischargeair:Conditionedairthathaspassedthroughacoil.Also,airdischargedfromasupplyductoutletintoaspace.DEFINITIONS55Directacting(DA):Adirect-actingthermostatorcontrollerincreasesthebranchlinepressureonanincreaseinthemeasuredvariableanddecreasesthebranchlinepressureonadecreaseinthevariable.Adirect-actingactuatorextendstheshaftonanincreaseinbranchlinepressureandretractstheshaftonadecreaseinpressure.Mainline:Theairlinefromtheairsupplysystemtocontrollersandotherdevices.Usuallyplasticorcoppertubing.Directacting(DA):Adirect56Mixedair:Typicallyamixtureofoutdoorairandreturnairfromthespace.Resetchangeover:Thepointatwhichthecompensationeffectisreversedinactionandchangesfromsummertowinterorviceversa.Thepercentofcompensationeffect(authority)mayalsobechangedatthesametime.Restrictor:Adeviceinanairlinethatlimitstheflowofair.Returnair:Airenteringanairhandlingsystemfromtheoccupiedspace.Mixedair:Typicallyamixt57Reverseacting(RA):Areverse-actingthermostatorcontrollerdecreasesthebranchlinepressureonanincreaseinthemeasuredvariableandincreasesthebranchlinepressureonadecreaseinthevariable.Areverse-actingvalveactuatorretractstheshaftonanincreaseinbranchlinepressureandextendstheshaftonadecreaseinpressure.SensorSpan:Thevariationinthesensedmediathatcausesthesensoroutputtovarybetween3and15psi.Reverseacting(RA):Areve58ABBREVIATIONSThefollowingportabbreviationsareusedindrawingsofrelaysandcontrollers:B—BranchC—Common

E—ExhaustM—Main

O—Normallyconnected*X—Normallydisconnected*P—Pilot(P1andP2fordual-pilotrelays)S—Sensor(S1andS2fordual-inputcontrollers)N.C.—NormallyclosedN.O.—NormallyopenABBREVIATIONS59SYMBOLSSYMBOLS60BASICPNEUMATICCONTROLSYSTEMGENERALApneumaticcontrolsystemismadeupofthefollowingelements:—Compressedairsupplysystem—Mainlinedistributionsystem—Branchlines—Sensors—Controllers—ActuatorsBASICPNEUMATICCONTROLSYSTE61—Finalcontrolelements(e.g.,valves,dampers)Abasicpneumaticcontrolsystemconsistsofanairsupply,acontrollersuchasathermostat,andanactuatorpositioningavalveordamper(Fig.1).—Finalcontrolelements(e.62Thecontrollerreceivesairfromthemainlineandregulatesitsoutputpressure(branchlinepressure)asafunctionofthetemperature,pressure,humidity,orothervariable.Thebranchlinepressurefromthecontrollercanvaryfromzerotofullmainlinepressure.Theregulatedbranchlinepressureenergizestheactuator,whichthenassumesapositionproportionaltothebranchlinepressureapplied.Theactuatorusuallygoesthroughitsfullstrokeasthebranchlinepressurechangesfrom3psito13psi.Otherpressurerangesareavailable.Thecontrollerreceives63Inatypicalcontrolsystem,thefinalcontrolelement(avalveoradamper)isselectedfirstbecauseitmustproducethedesiredcontrolresults.Forexample,asystemdesignedtocontroltheflowofwaterthroughacoilrequiresacontrolvalve.Thetypeofvalve,however,dependsonwhetherthewaterisintendedforheatingorcooling,thewaterpressure,andthecontrolandflowcharacteristicsrequired.Anactuatoristhenselectedtooperatethefinalcontrolelement.Acontrollerandrelayscompletethesystem.WhenallcontrolsystemsInatypicalcontrolsyst64forabuildingaredesigned,theairsupplysystemcanbesizedanddesigned.AIRSUPPLYANDOPERATIONThemainlineairsupplyisprovidedbyanelectricallydrivencompressorpumpingairintoastoragetankathighpressure(Fig.2).Apressureswitchturnsthecompressoronandofftomaintainthestoragetankpressurebetweenfixedlimits.Thetankstorestheairuntilitisneededbycontrolequipment.Theairdryerremovesmoisturefromtheair,andthefilterremovesoilandotherimpurities.Thepressurereducingvalveforabuildingaredesigned,t65(PRV)typicallyreducesthepressureto18to22psi.Fortwo-pressure(day/night)systemsandforsystemsdesignedtochangefromdirecttoreverseacting(heating/cooling),thePRVswitchesbetweentwopressures,suchas13and18psi.Themaximumsafeairpressureformostpneumaticcontrolsis25psi.(PRV)typicallyreducesthepr66FromthePRV,theairflowsthroughthemainlinetothecontroller(inFig.1,athermostat)andtoothercontrollersorrelaysinotherpartsofthesystem.Thecontrollerpositionstheactuator.Thecontrollerreceivesairfromthemainlineataconstantpressureandmodulatesthatpressuretoprovidebranchlineairatapressurethatvariesaccordingtochangesinthecontrolledvariable,asmeasuredbythesensingelement.Thecontrollersignal(branchlinepressure)istransmittedviathebranchlinetothecontrolleddevice(inFig.1,avalveactuator).FromthePRV,theairflo67Theactuatordrivesthefinalcontrolelement(valve)toapositionproportionaltothepressuresuppliedbythecontroller.Whentheproportionalcontrol