文献翻译系统设置动态重置操作.docx
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文献翻译系统设置动态重置操作.docx
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文献翻译系统设置动态重置操作
9.翻译
Standard62.1-2004:
SystemOperation:
DynamicResetOption
Seventeenyearsago,theventilationratesestablishedbyANSI/ASHRAEStandard62-1989,ventilationforAcceptableindoorairquality,increasedsubstantiallyoverthosepreviouslyrequiredbythe1981versionofthestandard.Fifteenyearslater,standard62-2004prescribednewminimumbreathingzoneventilationrateandanewcalculationproceduretofindtheminimumintakeairflowneededfordifferentventilationsystems.Describedinpreviousarticles,thesenewratesandproceduresmustbeusedtofindthedesignor“worst-case”outdoorairintakeflow,whichestablishestherequiredcapacityofmechanicalsystemequipment.Inthisarticle,weturnfromventilationsystemdesigntooperation.
Variationinoccupancyorventilationairflow
Thenumberofoccupantsinmanyventilationzoneschangesduringnormal.Actualzonepopulationoftenfallsshortofparkdesignpopulationusedtoestablishthehighestzoneoutdoorairflowandoutdoorairintakeflowneeded.Consequently,standard62.1allowsbreathingzoneand/orintakeairflowtoberesetinresponsetochangesinzonepopulationortheresultingchangesinthezoneventilationairflowperperson.Toresetventilationbasedonthecurrentpopulationwithinazonerequiresareasonablyaccurateestimateofpopulationorbreathingzoneoutdoorairrate.Standard62.1listsfourexamplemeasuresusedtoestimatevariationsinoccupancyandonemeasuretoestimatevariationinventilationairflow.
DirectCountofPeople.
Insomezones,occupantentryandexitmaybeorderly.Usingappropriatesensorsandcountingcontrols,thedifferencebetweenentryandexiteventscanbeusedtoestimatepopulationbasedonadirect“count”.Ticketsalesmightalsobeusedasanestimateofpopulation.Someretailstoresorauditoriumsmaybeconfiguredtousetheseapproaches.Sensorsthatactuallysenseindividualoccupantsalsomightbeused,althoughsucharenotcommon.
PresenceofPeople.
Avariationtocountingoccupantsdirectlyinvolvessimplysensingthepresenceofoccupants.Motiondetectorscansensehumanactivityinoffices,conferenceroomsandsoon.Upondetectingmotion,thecontrolsystemassumesthatthezoneisoccupiedatparkpopulationandcallsforventilationaccordingly.
Time-of-dayschedule.
Insomezones,populationcanbepredictedbasedontime-of-day.Forinstance,thepopulationinagivenclassroominanelementaryschoolmaybeestimatedquiteaccuratelyduringanyhouroftheday.ThedailyscheduleforMr.Brown’sfifthgradersplacestheminMr.Brown’sclassroomfrom8a.m.until10a.m.,intheartroomfrom10a.m.until11a.m.,thenbackintheclassroomfrom11a.m.untilnoon,andsoon.Ifthebuildingcontrolsystemincludesanoccupancyschedulefortheclassroom,currentpopulationcanbeestimateandoutdoorairflowcanberesettomatchprescribedventilationairflowtocurrentpopulation.
EstimateofOccupancyBasedonCO2andAirflow.
AsMummahasshown,inasingle-zonesystem,zoneandoutdoorCO2levelalongwithintakeairflowcouldbesensedandassuminganoccupantactivitylevel,usedtoestimatecurrentpopulation.Atsteady-stateCO2concentrationzonepopulationcouldbefound,butinpractice,zonesseldomreachsteady-statecondition.So,inadditiontoaccurateCO2andairflowsensors,thisapproachrequiresacontrollercapableofsolvinganon-steadystateequation.And,sinceoccupantactivitylevelandCO2generationratecanvarywidely,calculatedpopulationmaynotbeaccurate.Duetoitscost,itscomplexityanditspotentialforpooraccuracy,notmanydesignersusethisapproach.However,donotconfusethisCO2-based“peoplecounting”measurewithournexttopic:
traditionCO2–based“demand-controlledventilation.”
CO2–BasedEstimateofCurrentOutdoorAirflowperPerson.
Sincechangesinpopulationchangeventilation“demand”,anydynamicresetapproachthatrespondstozonepopulationcouldbereferredtoasdemand-controlledventilation(DCV).However,thisacronymhasbeenusedforyearstoreferspecificallytocontrolmeasuresthatsenseCO2concentrationasasurrogatefortheconcentrationofhumanbioeffluents.Themostpopulardynamic-resetapproachtopart-loadventilation,DCVcontrolsadjustoutdoorairflowratetomaintainzoneCO2–andtherebybioeffluents-atlevelsthatwouldresultfromventilatingat,orabove,prescribedminimumoutdoorairflowrates.AsexplainedbyTaylor,CO2–basedCO2DVCassumesthatpeopleproducebothodorsandCO2inproportiontotheiractivitylevel,thatoccupantactivitylevelinazonecanbeestimatedwithreasonableaccuracy.Andthatsteady-stateequationscanbeusedtoestimateventilationload.Muchthesameassteady-stateequationscanbeusedtoestimateheating/coolingload.Forsingle-zonesystems.TheminimumrequiredoutdoorairintakeflowcanberelatedtothedifferencebetweenindoorandoutdoorCO2level.TaylorgoesotoexplainhowdifferentialCO2levelcanbeincorporatedinonepossibleDVCapproach–alsodiscussedinthe62.1User’smanualanddetailedbelowasoneofseveralapproaches–tocontrolintakeairflowtoequalorexceedtheminimumrequiredventilationrate.
ThepopulationestimateresultingfromanyofthefirstfourmeasurescanbeusedtorecalculatetheminimumbreathingzoneoutdoorairflowrequiredbyStandard62.1forthecurrentpopulation.Thisnewoutdoorairflowvaluethencanbeusedtofindcurrentminimumzoneoutdoorairflowneeded.Forsingle-zonesystems,thezoneoutdoorairflowrequirementequalstheintakeairflow.Anycontrollerusedtoimplementthesefourresetapproachesmustbecapableofdoingsomesimplecalculation,sincethepopulationestimatemustbetranslatedtoacurrentoutdoorairflowsetpoint,andmustincludemeanstoadjustoutdoorairflowtomaintainthecurrentsetpoint.
Thefifthmeasuredescribedearliercanbeusedtocontrolintakeairflowdirectly,withoutactuallycalculatingeithercurrentpopulationortheminimumoutdoorairintakeflowcurrentrequiredbyStandard62.1.inasense,anyDCVapproachthatmaintainscontrolsbioeffluentsdirectlyinresponsetobothcurrentpopulationandcurrentintakeairflow.DVCapproachesneedcontrolsthatsensedifferentialCO2andmaintainitbyadjustingoutdoorairflowincrementallywithoutsensingit.WediscussseveralpossibleDVCimplementationbelow.
Variationinefficiency
ThesecondoperatingconditionlistedinSection6.2.7relatestoventilationefficiency.Standard62.1requiresaccountingforbothzoneair-distributioneffectivenessandsystemventilationefficiency.Zoneair-distributioneffectivenessappliestoallventilationzonesinanysystemandindicatesthefractionofoutdoorairdeliveredtoazonethatactuallyentersthebreathingzone.Systemventilationefficiency,ontheotherhand,onlyappliestomultiple-zonerecirculationsystemsandindicatesthefractionofoutdoorairenteringthesystemthatactuallydilutescontaminantsinthebreathingzone.Changesintheseefficiencyvaluesduringoperationeffectivelychangeventilation“demand”notasaresultofvaryingpopulationbutasaresultofvaryingzoneandsystemairflowatdifferentloadconditions.
Thedefaultvalueforzoneair-distributioneffectivenesscanchangeduringnormaloperation.Forinstance,ifthesameoverheaddiffusersdeliverwarmairduringheatingmodeandcoolairduringcoolingmode.Thelikelihoodofairbypassingthebreathingzonechanges.Whenheating,warmdischargeairtendstofloatabovecoolerzoneair.Ifit’stoowarm,aportionofitbypassesthebreathingzone,inwhichcaseE=0.8,accordingtoStandard62.1.whencooling,however,allofthedischargeairdropsintothebreathingzone,soE=1.0.Fordesignpurposes,zoneoutdoorairflowmayneedtoexceedthebreathingzoneoutdoorairflowbyafactorof1.25intheheatingmode.However,duringnormaloperation,zoneair-distributioneffectivenessmayincreasefrom0.8whenheatingto1.0whencooling,sozoneoutdoorairflowcouldberesetdownwardtoequaltherequiredbreathingzoneoutdoorairflow.
Variationsinzoneair-distributioneffectivenesscanbeusedtorecalculateanddynamicallyresetzoneoutdoorairflowinanyventilationsystem,butsingle-zoneheatingcoolingsystemsarelikelytobenefitmostfromthisresetapproach.Single-zonesystemswithincreasedE=needlesszoneoutdoorairflowandlessoutdoorairintakeflowduringcoolingoperationthanduringheating,whichreducescoolingenergy.Dedicatedoutdoorairsystemssupplying100%outdoorair,ontheotherhand,usuallydeliverventilationairatorbelowzonetemperature,soE=doesn’tvarywithloadandintakeairflowisusuallyconstant-nodynamicresetopportunities.And,formultiple-zonerecirculatingsystems,increasedE=inthe“critical=one”duringcanmeanlessintakeairflow..
Systemventilationefficiency(E)-T=theratioofoutdoorairusedinallbreathingzonestooutdoorairintakeforthesystem(Ev=Vou/Vot)-varieswidelyinVAVmultiply–zonesystems,sincebothzoneandsystemairflowchangeinresponsetoload.Fordesignpurposes,minimumoutdoorairintakeflowforthesesystemsmustdeterminedusingthelowestefficiencyandthehighestoutdoorairintakeflow.Forproperoperation,however,minimumintakeairflowmaybedeterminedusingthesystemventilationefficiencyatcurrentoperatingconditions.ThisarticlefocusesonDCVforsingle-zonesystems.Adetaileddiscussionofdynamicresetapproachesinmultiple-zonesystemsisleftforafuturearticle.
VariationsinIntakeAirflow
Standard62.1citesathirdconditionthatmaybeusedasthebasisfordynamicresetcontrol.Itrelatestovariationsinthefractionofoutdoorair
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