4 Ways to Measure Airflow _ Service Content From Contracting Business
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7/1/2015 4WaystoMeasureAirflow|ServicecontentfromContractingBusiness
http://contractingbusiness.com/service/4waysmeasureairflow 1/4
Oct3,2014
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Whereexpertsandprofessionalsshareinsightsandinspirationstogrowbusinessandbuildcareers
4WaystoMeasureAirflowbyRobFalkePresident,NationalComfortInstitute
Its hard to find a practical standard describing how to measure ventilation airflows that can be applied inthe field. Heres how you can determine residential and commercial exhaust fan airflow on a job site.
Whileindustrystandardsspecifyingventilationratesarecomingoutofthewoodwork,apracticalstandarddescribinghowtomeasureventilationairflowsthatcanbeappliedinthefieldishardtofind.Letstakealookhowyoucandetermineresidentialandcommercialexhaustfanairflowinthefield.
FansatEitherEndoftheDuct
Thismaybeobvious,butitisagoodplacetobeginthisdiscussion.Althoughinlineexhaustfansexist,mostexhaustfansarefoundateitherthestartofaductorattheendofaduct.Thisisadefiningcharacteristicofsomeexhaustfansthataffectsthewaywemaydeterminefanairflowascomparedtoaforcedairsystemwherefansarelocatedbetweenthesupplyandreturnducts.
TheDangerousAssumption
Moststandardsarewrittentopersuadedesignerstospecifytheneededairfloworairchangerates.Thesestandardsrequireacalculation,andaspecificationrequiringthatthefanshoulddothejob.Withthesetaskscompleted,mostoftheengineeringworldbelievesthestandardismetandthatventilationneedsareassureddreamonfellas.
Theassumptionthatthefanandductthatwasspecifiedwilldothejobisfarfromrealized.Thedesignandspecificationmustrequirethattheventilationsystemalsoreceivetheverificationstepofmeasuring,adjustingandbalancingairflowbyaqualifiedairbalancer.Withoutthisstepthedesignisonlyawish.
TypicalMeasurementMethods
Therearemanydifferenttypesofventilationsystemsandnotalltherequiredmeasurementmethodscanbediscussedinashortarticle,sowewillreviewthemostcommontestmethods.
1.AirBalancingHoodMeasurement
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7/1/2015 4WaystoMeasureAirflow|ServicecontentfromContractingBusiness
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Exhaustfanperformancechart.
Ifyourelucky,andairexhaustfanairflowisbeingpulledfromtheroomthroughagrillethatisflatagainstaceilingorwall,andthevolumeofairflowisbetween30and2000CFM,useacalibratedairbalancinghood.Setyouthebalancinghoodtoreadinexhaustmode,securelyplacethehoodoverthegrilletocaptureallofthefanairflowcomingthroughthegrille.Thenreadandrecordairflow.
Airflowspecificationsareplusorminus10%ofdesignairflow.Withmostsmallerfansthisspecisadequate.Ifyoureallcaughtupinexactness,forgetaboutit.Ifyoureconcernedaboutthedeterminingthepreciseamountofductleakage,youreffortisbetterspentininstallingtheductsothatisdoesntleak.
Whenairflowcannotbemeasureddirectlywithabalancinghood,airflowcanbemeasuredintheexhaustductbyperforminganairflowtraverse.
2.TraversetheAirflowintheExhaustDuct
Anairflowtraverserequiresatleast5lengthsofstraightexhaustduct.Twoormore3/8in.testholesarethendrilledintotheduct.Ananemometer,atestinstrumentthatmeasuresairvelocityisusedtodeterminetheaverageairspeedintheduct.Thentheaveragefeetperminuteismultipliedbytheareaoftheductinsquarefeettodeterminetheairflowmovingthroughtheduct.
Traversetheairflowintheexhaustduct.
Example:Youhaveabathroomexhaustfandesignedfor200CFM.Thesystemhasan8in.exhaustduct.Theareaofan8in.ductis.35sq.ft.Youmeasurethevelocityatapointsintheductandfindtheaveragevelocityintheexhaustductis400FPM.Multiply400FeetperMinutetimestheareaoftheduct,whichis.35squarefeet,tofind140CFMexhaustfanairflow.
3.PlotFanAirflow
Eachfanmanufacturerpublishesfanperformancetablesforeachfan.Normally,thesefantablesareshippedwiththefan,ortheinformationcanbeeasilyfoundonlineatthemanufacturerswebsite.Inordertointerpretexhaustfanairflow,youmustmeasurethefanoperatingstaticpressureandthefanspeedorRevolutionsPerMinute.Thisfielddataisthenusedtoplotfanairflow.
Smallerfansareoftenconstantorsinglespeed.Withthesefans,youarenotrequiredtomeasurefanRPM.LargerfansrequirethatyoumeasurethefanRPM.ThisisnormallydoneusinganoncontacttachometerthatreadsRPMfromreflectivetapeaffixedtothefan.
Fanstaticpressureismeasuredusingamanometer(apressuregauge)andahoseortubeandastaticpressuretip.(Seestaticpressurekitspecificationsavailableattheendofthearticle.)
Sincemanyfansarelocatedateitherendoftheduct,theoperatingfanstaticpressureismeasuredasairentersorexitstheexhaustfan.Drillatestholeintheductandreadandrecordthefanstaticpressure.
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Exhaustfantest.
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Armedwiththefanoperatingstaticpressureandthefanspeed,gotothemanufacturersfantablematchingthefanbeingmeasuredandplotfanairflow.
DrawalineintersectingthefanspeedandthefanoperatingstaticpressuretorevealthefanoperatingCFM.
4.ACombinationofOneorMoreTestMethods
Becausesomefansarenotinstalledsuitabletomeasurefanairflowandbecauseyouarethejudgeandjuryresponsibleto
determinefanairflow,youwillfindtimeswhereyoumayemploytwoormoreofthetestmethodsabovetogathersufficientdatatrenderyourjudgmentofoperatingfanairflow.Themoredatayougather,themoreaccurateyourairflowdecision.
AirflowAwareness
Justbybecomingawarethatairflowshouldandcanbemeasuredwillgetyouintouchwiththeairsideofthesystemsyoudesign,installandservice.
Reviewingthespecificationsofthefansyouuseregularlywillmakeyouawareofthelimitationseachfanhasandtheinstallationconditionsthatcancausefanstoperformat50%oftheexpectedcapacity.
Theprimaryreasontomeasureairflowisforyoutodiscoverventilationissuesthatyourcustomershavebeensufferingwithforyears.Measuringexhaustairflowbringsopportunityforyourcustomerstoreceiveimprovedventilationandcomfort.Opportunitycomesyourwaythroughprovidingbetterserviceandincreasedearnings.
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