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9.翻译Standard62.12004SystemOperationDynamicResetOptionSeventeenyearsago,theventilationratesestablishedbyANSI/ASHRAEStandard621989,ventilationforAcceptableindoorairquality,increasedsubstantiallyoverthosepreviouslyrequiredbythe1981versionofthestandard.Fifteenyearslater,standard622004prescribednewminimumbreathingzoneventilationrateandanewcalculationproceduretofindtheminimumintakeairflowneededfordifferentventilationsystems.Describedinpreviousarticles,thesenewratesandproceduresmustbeusedtofindthedesignorworstcaseoutdoorairintakeflow,whichestablishestherequiredcapacityofmechanicalsystemequipment.Inthisarticle,weturnfromventilationsystemdesigntooperation.VariationinoccupancyorventilationairflowThenumberofoccupantsinmanyventilationzoneschangesduringnormal.Actualzonepopulationoftenfallsshortofparkdesignpopulationusedtoestablishthehighestzoneoutdoorairflowandoutdoorairintakeflowneeded.Consequently,standard62.1allowsbreathingzoneand/orintakeairflowtoberesetinresponsetochangesinzonepopulationortheresultingchangesinthezoneventilationairflowperperson.Toresetventilationbasedonthecurrentpopulationwithinazonerequiresareasonablyaccurateestimateofpopulationorbreathingzoneoutdoorairrate.Standard62.1listsfourexamplemeasuresusedtoestimatevariationsinoccupancyandonemeasuretoestimatevariationinventilationairflow.DirectCountofPeople.Insomezones,occupantentryandexitmaybeorderly.Usingappropriatesensorsandcountingcontrols,thedifferencebetweenentryandexiteventscanbeusedtoestimatepopulationbasedonadirectcount.Ticketsalesmightalsobeusedasanestimateofpopulation.Someretailstoresorauditoriumsmaybeconfiguredtousetheseapproaches.Sensorsthatactuallysenseindividualoccupantsalsomightbeused,althoughsucharenotcommon.PresenceofPeople.Avariationtocountingoccupantsdirectlyinvolvessimplysensingthepresenceofoccupants.Motiondetectorscansensehumanactivityinoffices,conferenceroomsandsoon.Upondetectingmotion,thecontrolsystemassumesthatthezoneisoccupiedatparkpopulationandcallsforventilationaccordingly.Timeofdayschedule.Insomezones,populationcanbepredictedbasedontimeofday.Forinstance,thepopulationinagivenclassroominanelementaryschoolmaybeestimatedquiteaccuratelyduringanyhouroftheday.ThedailyscheduleforMr.BrownsfifthgradersplacestheminMr.Brownsclassroomfrom8a.m.until10a.m.,intheartroomfrom10a.m.until11a.m.,thenbackintheclassroomfrom11a.m.untilnoon,andsoon.Ifthebuildingcontrolsystemincludesanoccupancyschedulefortheclassroom,currentpopulationcanbeestimateandoutdoorairflowcanberesettomatchprescribedventilationairflowtocurrentpopulation.EstimateofOccupancyBasedonCO2andAirflow.AsMummahasshown,inasinglezonesystem,zoneandoutdoorCO2levelalongwithintakeairflowcouldbesensedandassuminganoccupantactivitylevel,usedtoestimatecurrentpopulation.AtsteadystateCO2concentrationzonepopulationcouldbefound,butinpractice,zonesseldomreachsteadystatecondition.So,inadditiontoaccurateCO2andairflowsensors,thisapproachrequiresacontrollercapableofsolvinganonsteadystateequation.And,sinceoccupantactivitylevelandCO2generationratecanvarywidely,calculatedpopulationmaynotbeaccurate.Duetoitscost,itscomplexityanditspotentialforpooraccuracy,notmanydesignersusethisapproach.However,donotconfusethisCO2basedpeoplecountingmeasurewithournexttopictraditionCO2–baseddemandcontrolledventilation.CO2–BasedEstimateofCurrentOutdoorAirflowperPerson.Sincechangesinpopulationchangeventilationdemand,anydynamicresetapproachthatrespondstozonepopulationcouldbereferredtoasdemandcontrolledventilationDCV.However,thisacronymhasbeenusedforyearstoreferspecificallytocontrolmeasuresthatsenseCO2concentrationasasurrogatefortheconcentrationofhumanbioeffluents.Themostpopulardynamicresetapproachtopartloadventilation,DCVcontrolsadjustoutdoorairflowratetomaintainzoneCO2–andtherebybioeffluentsatlevelsthatwouldresultfromventilatingat,orabove,prescribedminimumoutdoorairflowrates.AsexplainedbyTaylor,CO2–basedCO2DVCassumesthatpeopleproducebothodorsandCO2inproportiontotheiractivitylevel,thatoccupantactivitylevelinazonecanbeestimatedwithreasonableaccuracy.Andthatsteadystateequationscanbeusedtoestimateventilationload.Muchthesameassteadystateequationscanbeusedtoestimateheating/coolingload.Forsinglezonesystems.TheminimumrequiredoutdoorairintakeflowcanberelatedtothedifferencebetweenindoorandoutdoorCO2level.TaylorgoesotoexplainhowdifferentialCO2levelcanbeincorporatedinonepossibleDVCapproach–alsodiscussedinthe62.1Usersmanualanddetailedbelowasoneofseveralapproaches–tocontrolintakeairflowtoequalorexceedtheminimumrequiredventilationrate.ThepopulationestimateresultingfromanyofthefirstfourmeasurescanbeusedtorecalculatetheminimumbreathingzoneoutdoorairflowrequiredbyStandard62.1forthecurrentpopulation.Thisnewoutdoorairflowvaluethencanbeusedtofindcurrentminimumzoneoutdoorairflowneeded.Forsinglezonesystems,thezoneoutdoorairflowrequirementequalstheintakeairflow.Anycontrollerusedtoimplementthesefourresetapproachesmustbecapableofdoingsomesimplecalculation,sincethepopulationestimatemustbetranslatedtoacurrentoutdoorairflowsetpoint,andmustincludemeanstoadjustoutdoorairflowtomaintainthecurrentsetpoint.Thefifthmeasuredescribedearliercanbeusedtocontrolintakeairflowdirectly,withoutactuallycalculatingeithercurrentpopulationortheminimumoutdoorairintakeflowcurrentrequiredbyStandard62.1.inasense,anyDCVapproachthatmaintainscontrolsbioeffluentsdirectlyinresponsetobothcurrentpopulationandcurrentintakeairflow.DVCapproachesneedcontrolsthatsensedifferentialCO2andmaintainitbyadjustingoutdoorairflowincrementallywithoutsensingit.WediscussseveralpossibleDVCimplementationbelow.VariationinefficiencyThesecondoperatingconditionlistedinSection6.2.7relatestoventilationefficiency.Standard62.1requiresaccountingforbothzoneairdistributioneffectivenessandsystemventilationefficiency.Zoneairdistributioneffectivenessappliestoallventilationzonesinanysystemandindicatesthefractionofoutdoorairdeliveredtoazonethatactuallyentersthebreathingzone.Systemventilationefficiency,ontheotherhand,onlyappliestomultiplezonerecirculationsystemsandindicatesthefractionofoutdoorairenteringthesystemthatactuallydilutescontaminantsinthebreathingzone.Changesintheseefficiencyvaluesduringoperationeffectivelychangeventilationdemandnotasaresultofvaryingpopulationbutasaresultofvaryingzoneandsystemairflowatdifferentloadconditions.Thedefaultvalueforzoneairdistributioneffectivenesscanchangeduringnormaloperation.Forinstance,ifthesameoverheaddiffusersdeliverwarmairduringheatingmodeandcoolairduringcoolingmode.Thelikelihoodofairbypassingthebreathingzonechanges.Whenheating,warmdischargeairtendstofloatabovecoolerzoneair.Ifitstoowarm,aportionofitbypassesthebreathingzone,inwhichcaseE0.8,accordingtoStandard62.1.whencooling,however,allofthedischargeairdropsintothebreathingzone,soE1.0.Fordesignpurposes,zoneoutdoorairflowmayneedtoexceedthebreathingzoneoutdoorairflowbyafactorof1.25intheheatingmode.However,duringnormaloperation,zoneairdistributioneffectivenessmayincreasefrom0.8whenheatingto1.0whencooling,sozoneoutdoorairflowcouldberesetdownwardtoequaltherequiredbreathingzoneoutdoorairflow.Variationsinzoneairdistributioneffectivenesscanbeusedtorecalculateanddynamicallyresetzoneoutdoorairflowinanyventilationsystem,butsinglezoneheatingcoolingsystemsarelikelytobenefitmostfromthisresetapproach.SinglezonesystemswithincreasedEneedlesszoneoutdoorairflowandlessoutdoorairintakeflowduringcoolingoperationthanduringheating,whichreducescoolingenergy.Dedicatedoutdoorairsystemssupplying100outdoorair,ontheotherhand,usuallydeliverventilationairatorbelowzonetemperature,soEdoesntvarywithloadandintakeairflowisusuallyconstantnodynamicresetopportunities.And,formultiplezonerecirculatingsystems,increasedEinthecriticaloneduringcanmeanlessintakeairflow..SystemventilationefficiencyETtheratioofoutdoorairusedinallbreathingzonestooutdoorairintakeforthesystemEvVou/VotvarieswidelyinVAVmultiply–zonesystems,sincebothzoneandsystemairflowchangeinresponsetoload.Fordesignpurposes,minimumoutdoorairintakeflowforthesesystemsmustdeterminedusingthelowestefficiencyandthehighestoutdoorairintakeflow.Forproperoperation,however,minimumintakeairflowmaybedeterminedusingthesystemventilationefficiencyatcurrentoperatingconditions.ThisarticlefocusesonDCVforsinglezonesystems.Adetaileddiscussionofdynamicresetapproachesinmultiplezonesystemsisleftforafuturearticle.VariationsinIntakeAirflowStandard62.1citesathirdconditionthatmaybeusedasthebasisfordynamicresetcontrol.ItrelatestovariationsinthefractionofoutdoorairintheprimaryairstreamduetotheintroductionofexcessventilationairintheprocessofprovidingfreecoolingwithoutdoorairorprovidingintaketoVAVmultiplezonesystemssinceitallowsresettingzoneminimumprimaryairflowsettingscansavelocalreheatenergyinsomesystemsatsomeloadconditions.Sincethisapproachworldcommonlybecombinedwithvariationsinefficiencyformultiplezonesystems,amoredetaileddiscussionremainsforafuturearticle.WhatChangedThisarticlefocusesondynamicresetofoutdoorairintakeflowinresponsetovariationsinpopulationinsinglezonesystems.Since1989,standard62hasspecifiedminimumventilationratesintermsofoutdoorairflowperpersonformostoccupancycategories.ThiswashandyforCO2basedDCVbecauseitallowedtheventilationrateperpersontobeheldconstantbysimplyadjustingzoneoutdoorairflowasnecessarytomaintainthesensedzoneCO2levelconstant.AlthoughnotexplicitlyallowedbyearlierversionsofStandard62.1,DCVhasbeenusedeffectivelybymanysystemmanufacturersanddesigners,anditwassupportedbyseveralofficialinterpretationsofthestandard.HowdiditworkForexample,assumethatasinglezone,constantvolumerooftopunitservesa1.000ft2lectureclassroomdesignedfor65people.Standard622001requiredaminimumbreathingzoneoutdoorairflowof970cfmatdesign.Withzoneairdistributioneffectivenessof1.0,zoneoutdoorairflowatdesignequaledbreathingzoneoutdoorairflow.Assumingaverageoccupantactivitylevelof1.25metandatypicalCO2generationrate,therequiredrateof15cfm/personresultedinadifferentialCO2levelof0.000700.withoutDCV,intakeairflowwouldbemaintainedat970cfmregardlessofchangesinpopulation.Ifpopulationdroppedto50people,differentialCO2woulddropto540ppmFigure/.But,50peoplerequiredintakeairflowofonly750cfm.Introducing750cfmofoutdoorairfor50occupantsagainwouldhaveresultedinadifferentialCO2levelof700ppm.DifferentialCO2levelcouldbecontrolledtoaconstantvaluebecauseitwasrelatedtotheconstantventilationrateof15cfm/person.bysensingdifferentialCO2levelandadjustingintakeairflowtomaintainitat700ppm,aproperlydesignedcontrollercouldmaintainthezoneoutdoorairflowattherequiredpersonrate,withoutanyknowledgeofactualpopulationoractualoutdoorairflow.ButStandard62.12004introducedanewwaytofindtheminimumbreathingzoneoutdoorairflow.Insteadofprescribingasingleperpersonoutdoorairflowrate,thestandardprescribesbothaperpersonrateandaperunitarearateforeachoccupancycategory.Theperpersonrateresultsinaminimumoutdoorairflowrateintendedtodilutecontaminantsgeneratedbyoccupantsandtheiractivities,whiletheperunitarearateresultsinanoutdoorairflowrateintendedtodilutecontaminantsgeneratedbythebuilding,itsfinishesandfurnishings.Thesumoftheseairflowrateshelpstoestablishtherequiredintakeairflow.Therequiredeffectiverateperpersonvarieswithpopulation.
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