HVAC暖通空调外文文献

(HVAC)SystemsforUseinVerifyingAcousticalPredictionMethods

StevenR.RyherdandLilyM.Wang

ArchitecturalEngineeringProgram,UniversityofNebraska—Lincoln,PKI,1110S.Copyright ASCE 200667thSt.,Omaha,NE68182;PH(402)554-2065;FAX(402)554-2080;email:sryherd@mail.unomaha.eduAbstract

Thispaperdiscussesissuesregardingin-situmethodsofobtainingsoundpoweratapointinaheating,ventilatingandair-conditioning(HVAC)ductsystem.Suchamethodisbeingusedaspartofalargerinvestigationonacousticalpredictionmethodstoallowforcomparisonofmeasureddatatoresultsfromattenuationpredictionsofindividualductelements,suchaselbowsordampers.SoundpropagationinHVACductworkiscomplex.Anymeasurementsofsoundenergyintheductmustaddressthecharacteristicsofsoundpropagationinducts,endreflections,andairturbulence.Investigationsarebeingconductedtounderstandtheextentstowhichtheseacousticalissuesaffectmeasurementresults.ThestudyprovidesabetterunderstandingofsoundpropagationinHVACductsforfutureinvestigationofacousticalpredictionmethods.Introduction

Thestudyoutlinedinthispaperispartofalargerinvestigationofacoustical

predictionsoftwareforheating,ventilating,andair-conditioning(HVAC)systems.TheauthorshavepreviouslyinvestigatedpredictionsoftwareforHVACsystemsalongentireacousticalpathsfromthefansourcetothereceiverinaroom(Ryherd&Wang,2005).Suchverificationofthepredictionsoftwarerequiredcontrolledenvironmentswithwell-documentedinformationabouttheactualsoundsourcepower,ductelements,andreceivingroomcharacteristics.Inanattempttoverifythealgorithmsusedagainstactualdatameasuredinfieldinstallations,though,itisdesiredtolimitotherpotentialsourcesoferror.

Thispaperpresentstheissuessurroundinganin-situmethodofinvestigatingtheacousticalinfluenceofeachelementintheductpathseparately.Toverifyeachductelement’scontribution,theremustbedependablemeansofobtainingasoundpowerlevelattheinletandoutletofthespecifiedelement.Thesesoundpowerlevelscouldthenbecomparedtotheexpectedattenuationofthatelement,ascurrently

1

Building Integration Solutions AEI 2006Downloaded from ascelibrary.org by Huazhong University of Science & Technology on 12/26/13. Copyright ASCE. For personal use only; projectedinHVACnoisepredictionsoftware.However,in-situmeasurementsofthiskindarenotcommon,especiallyininstalledHVACsystems.Background

all rights reserved.SoundpredictionsoftwarefornoiseinHVACsystemsutilizeacollectionofalgorithmsthatcalculatetheattenuationcontributionsofeachelementofasystem.Ingeneral,thealgorithmisanempiricalblackboxthattakesanincomingsoundpowerlevelandcharacteristicinformationoftheelementtoproduceanoutputsoundpowerCopyright ASCE 2006level.Forexample,ifafanisusedasthesoundsourceatthebeginningofalengthofduct,thesoundpowerlevelsateachoctavebandareputintothealgorithmforthespecifictypeofduct(e.g.rectangular,circular,etc.)alongwiththeductdimensions,ductlengthandamountofabsorption.Thealgorithmcalculatestheamountofsoundattenuationandprojectstheoutputsoundpowerlevel.ThisprocesscontinuesdownthepathoftheHVACsystemforeachelementofthepath(silencers,elbows,branches,etc.)untilthesoundreachesthereceiverroom.Atthereceivingroom,correctionfactorsareappliedtotheestimatedsoundpowerleveltocalculatetheequivalentsoundpressurelevelperceivedbyareceiverinthespace.

Althoughtheexampleissimplystated,theprocessofpredictingthenoisein

HVACsystemsiscomplexwithmanypotentialsourcesoferror.Thereisinherenterrorwhenalgorithmsbasedonempiricaldatamadeincontrolledenvironmentsareusedforin-situapplications;andunfortunately,usersofsoftwareprogramsoftenarenotabletoaccessevenwhatalgorithmisbeingusedtoknowifitsapplicationisappropriate.Additionally,theinitialsourcedatafromafanisnotalwaysreliableandmayvarybasedonoperatingconditionsandinstallation.Ultimately,anyinaccuraciesatonepointintheanalysisofthesystemcancompounderrorsfurtherdownthesystempath.ToimproveacousticalpredictionsalongHVACductwork,oneshouldfirstbesuretounderstandsoundpropagationinducts.RelationshipofSoundPressureandSoundPower

Inacousticsofbuildingmechanicalsystems,soundenergyisoftenrepresented

asoneoftwoquantities:soundpressurelevelsorsoundpowerlevels.Soundpressurelevelisthemostcommonformofdescribingthehumanresponsetoairbornesound,andmeasuresthechangesinpressurewithrespecttostaticpressure.Thisquantityisdependentuponthedistanceofthereceivertothesourceandtheenvironmentinwhichitismeasured.However,anotherquantity,soundpowerlevel,isindependentofdistancetothesourceandtheenvironmentalcharacteristicsofthespace.Soundpowerdescribestherateatwhichsoundenergyisproducedbyasourceandisusedtoquantifythesoundenergyrelatingtomechanicalequipment.Usinganacousticalpredictorthatisindependentoftheenvironmentallowsforsimplecomparisonofacousticalcharacteristicsofmechanicalequipment.

2

Building Integration Solutions AEI 2006Downloaded from ascelibrary.org by Huazhong University of Science & Technology on 12/26/13. Copyright ASCE. For personal use only; all rights reserved.Althoughsoundpowerisacommondescriptorofacousticalenergy,thereisnoeasywaytomeasuresoundpowerdirectly.Acollectionofsoundpressuremeasurementscanbeusedtocalculatesoundpower.Otherinformationisnecessaryincludingthedirectivityofthesource,thedistanceofthemeasurements,environmentalcharacteristics,andtheareathatthemeasurementscover.Theactualequationusedforconvertingbetweensoundpressurelevelandsoundpowerlevelvarieswiththesituation(Bies&Hansen,2003).SoundPropagationinDucts

Copyright ASCE 2006Therearethreeissuesassociatedwiththesoundpropagationinductsthat

affectin-ductmeasurements—thecharacteristicsofacousticalenergyinducts,endreflections,andturbulence.Thefirstissueofacousticalcharacteristicsdependsonthedimensionsoftheductandthefrequenciesbeingmeasured.Atlowerfrequencieswithlargewavelengths,onlyplanewavespropagateinaductandasimplerelationshipcanbeshownbetweensoundpressureandsoundpower.Athighfrequencieswithshorterwavelengths,planemodesandhigherordermodescanexist.Thismeansthatsoundispropagatingnotonlyparalleltotheaxisoftheductbutalsoinvariousanglesduetoreflectionsoffthewalloftheduct.Thesemodescausevariationsinthesoundpressurelevelatparticularlocationsinacross-sectionalareaoftheduct.Modesintheductwillvarybasedonthedimensionsoftheductandthefrequencyofthemeasurements.Thesemodescancauseinterferencethatresultsinachangeinmeasuredenergy.

Thesecondissuewhentakingin-ductmeasurementsisendreflectionfactorsduetoducttermination.Anopeningatthedischargeofaductcancreateendreflectionsthatsendasoundwavebackintotheductagainsttheairflowbecauseofanimpedancemismatch.Thereflectionscancauseinterferenceandgeneratestandingwavesthatfurthercomplicatethepatternsofsoundenergybeingtransmittedthrougheachelementoftheductsystem.Suchstandingwavesintheductcancauseinaccuracieswithin-ductmeasurementsofelementcontributions.

Thethirdissuewithin-ductmeasurementsisturbulencecausedbythemovementofairintheduct.Turbulencecanbecausedbyobstructionstotheflowandotherchangesinpressure.Theresultingturbulenteddieshaveflowthatmaynotbeparalleltotheaxisoftheduct.Theturbulentfluctuationsinpressurecannotbedifferentiatedbyamicrophonemeasuringthepressurechangesassociatedwithacousticalenergy.Thesepressurefluctuationsaffectrandomfrequenciesofmeasurementstakeninsuchacondition(Liao,1990).MeasuringSoundEnergyinDucts

Allthreesoundpropagationissuesmustbeaddressedwhenmakingin-duct

measurementstoobtainreliableacousticaldata.Inexistingstandards,thethreeissuesareaddressedbyobtainingthedatainacontrolledlaboratoryenvironment.These

3

Building Integration Solutions AEI 2006Downloaded from ascelibrary.org by Huazhong University of Science & Technology on 12/26/13. Copyright ASCE. For personal use only; existingstandardscanserveasaguidelinetocreateamethodofin-ductmeasurementforobtainingthedesiredsoundpowermeasurementsataparticularpointinaduct.Althoughmanystandardsexistforthemeasuringofsoundpowerinducts,theonemostpertinenttothisresearchisISO5136—Determinationofsoundpowerradiatedintoaductbyfansandotherair-movingdevices—In-duct.Thisstandardrequiresthattheductbeorientedinastraightlinewithminimaltransitionstoreduceturbulence,andmeasurementsaretakenataspecifieddistancefromthesourcetoassurethattheairhaslaminarflow.Turbulenceeffectsarefurtherreducedbyusingaall rights reserved.Copyright ASCE 2006foamball,nosecone,orturbulencescreenontheendofthemicrophone.Endreflectionfactorsareminimizedbyhavingonlyoneinletandoutletoftheductwithan anechoictermination.Thespeciallydesignedterminationlimitstheabilityforasoundwavetoreflectbackintotheductbyeliminatingtheplaneofreflectionandflaringtheedgesoftheremainingduct.Finally,theacousticalcharacteristicsareaddressedbyvaryingthelocationofmeasurementsintheduct,limitingthefrequencyrangemeasured,andusingamodalcorrectionfactor.

Unfortunately,in-situmeasurementsofHVACductsystemsdonotprovidethe

necessarycontrolledenvironment;however,considerationofallthreeissuescanbeaddressed.Theeffectsofthemodalcharacteristicsintheductcanbeminimizedbyvaryingthelocationofmeasurementsandaveragingdataforanequivalentvalue.Themeasurementlocationswillvarywithinthecross-sectionalareaoftheductandalongthelengthofaduct.Also,measurementsmustbetakenataconsiderabledistancefromanymajordisturbancebothupstreamanddownstream.Thestandardsuggestsapproximately6feetorfourductwidthstoensureundisturbedflowconditions,andthisdistancerequirementshouldalsobeobservedforin-situmeasurements.Theissueofendreflectionscanalsobelimitedbyobservingtherecommendeddistances.Theinfluenceofthereflectedsoundwouldbediminishedbythedistancetothemeasurementlocation.

Finally,theturbulenceeffectscanbefurtherdiminishedbyutilizingoneofthemicrophoneprotectiondevicesspecifiedbythestandard.Thefoamballisdesignedformeasurementsinairvelocitiesupto3000feet/minute(fpm),andthenoseconeisdesignedforupto4000fpm.Bothofthesedevicesareconsideredtomaintaintheomni-directionalcharacteristicsofthemicrophone.Thethirdprotectivedeviceisreferredtoasasamplingtubewhichisalongcylinderthatencasesthemicrophonewithaslitdownthesideandanoseconeontheend.Thesamplingtubeisdesignedforflowvelocitiesofapproximately7800fpmandisstronglysuggestedformeasurementsatorbelowthe125Hzoctaveband.Conclusion

Pursuingareliablein-situmethodofobtainingsoundpoweratapointinan

HVACductwillprovideawayofverifyingalgorithmsusedtoaccountfortheacousticbehaviorofindividualductelements.Theproposedmethodispartofa

4

Building Integration Solutions AEI 2006Downloaded from ascelibrary.org by Huazhong University of Science & Technology on 12/26/13. Copyright ASCE. For personal use only; largerstudytotestsuchalgorithmsusedtopredictHVACacousticsinmanysoftwareprograms.Thenatureofsuchapredictionisverycomplexwithmanysourcesoferror.Bydevelopingamethodofmeasuringsoundpressuretoobtainsoundpower,verificationofthealgorithmscanbeobtainedforanyelementoftheductedsystem.

all rights reserved.Threemainissuesmustbeaddressedtoobtainreliablesoundenergydatafrom

aduct.Thosethreeissues—characteristicsofacousticalenergy,endreflections,andturbulence,affectsoundpropagationinductsandanymeasurementsmadeintheduct.Toaddresstheseconcerns,measurementsmustbemadeatagreatenoughdistanceCopyright ASCE 2006fromadisturbanceupstreamordownstreamfromtheduct;theymustbemadeinvariouslocations;andtheymustbemadewithappropriateprotectionfromturbulence.Furtherinvestigationisrequiredtodeterminethebestapproachtoaccomplishthegoalofobtainingsoundpoweratapointintheductwhileaddressingeachissue.References

Bies,D.A.andHansen,C.H.(2003).EngineeringNoiseControl—Theoryand

Practice,SponPress,NewYork.

InternationalOrganizationforStandardization.(2003).ISOStandard5136:2003(E).

Acoustics—Determinationofsoundpowerradiatedintoaductbyfansandotherair-movingdevices—In-ductmethod,InternationalOrganizationforStandardization,Switzerland.

Liao,J.(1990).AnalysisofAcousticEnergyPropagationinaCircularDuctto

ImprovetheAccuracyofIn-ductNoiseMeasurement.Ph.D.dissertation,TennesseeTechnologicalUniversity.

Ryherd,S.andWang,L.M.(2005).“Acousticalpredictionmethodsforheating,

ventilating,andair-conditioning(HVAC)systems.”ProceedingsofNoise-Con2005,Minneapolis,MN,InstituteofNoiseControlEngineering,Ames,IA.

5

Building Integration Solutions AEI 2006Downloaded from ascelibrary.org by Huazhong University of Science & Technology on 12/26/13. Copyright ASCE. For personal use only;