hepa filter failure modes v5 - isnatt bergman.pdf · 2018. 7. 25. · separatorless hepa filters...

35thInternationalNuclearAirCleaningConference(2018)

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HEPAFilterFailureModesatElevatedPressure,TemperatureandMoisture

by

WernerBergmanandGerardGarcia1

AerosolScience,LLC,Stanwood,WA,[email protected],[email protected]

AbstractThisreportpresentsananalysisofHEPAfilterfailuremodesatelevatedpressure,temperature,andmoistureusingpublishedexperimentalstudies.ThetestresultspresentedbyGiffinetal(2012)andWaggoner(2012a)onfilterpleatcollapseareanalyzedintermsofprobabilityofHEPAfilterfailureasafunctionofdifferentialpressureatvarioustemperaturesandrelativehumidityforseveraldifferentfilterdesigns.PleatcollapseoccursforseparatorlessHEPAfiltersabove8in.WCunderambienttemperatureanddryconditionsandabove6inchesatmoderatelyhightemperatures(130°F).Atheoryforpleatcollapsebasedonstudiesofmechanicalplatedeflectionispresentedandisusedintheanalysis.ThisreportalsoreviewstheHEPAfilterfailuresathigherdifferentialpressuresduetothebendingoftheHEPAfilterpackandduetotheruptureofpleatends.Astatisticalanalysisofthesefilterfailuresisperformedforbothwetanddryconditions.PleatCollapsePleatcollapse,illustratedinFigure1,istheundesirablefeatureofapleatedfilterinwhichtheindividualpleatscollapseduetotheappliedpressureofmovingair.Theuseofrigidseparatorslikecorrugatedaluminumseparatorspreventpleatcollapse.SeparatorlessHEPAfiltersthatuseembossmentsofthefiltermediumpreventpleatcollapseundermildconditionsoftemperatureandpressuredrop.


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Figure1.Crosssectiondrawingofaportionofapleatedfilterpackfor(A)afilterwithnopleatcollapse,andthepackmaintainsadistancefromthefilterscreenand(B)afilterwithpleatcollapse,andthefilterpackiscompressedagainstthefilterscreen.Pleatcollapseoccurswhenthefiltermediumhasinsufficientstiffnesstomaintainthedesiredshapeandisdeflectedontotheadjoiningmediumofthepleatiftherearenoseparatorsoriftheseparatorsarenoteffective.Themediumstiffnesscanbesignificantlyreducedathighertemperaturesandwhenthemediumbecomeswet.Forfiltersthatuseembossmentsofthemediumtoformdimplesorridgesasseparators,thestrengthoftheseseparatorsdecreaseswithincreasingtemperatureandwhenthemediumbecomeswet.Asthepleatsbegintocollapse,thereducedfiltermediumarearesultsinahigherpressuredrop,whichinturncausesevengreaterpleatcollapse.Oncethepleatsbegintocollapse,theunstablefeed-forwardprocessresultsinarapidincreaseinpressuredropandcanleadtomediumtearsifthefanhassufficientpower.ParticleloadingcontributestopleatcollapsebyincreasingthefilterpressuredropasillustratedinFigure2.However,thesincetheparticledepositsdonotinteractwiththefiberbondsthatareresponsibleforthefiltermediumstiffness,thedepositsdonotalterthestiffnessofthemedium.Theparticledepositsonlyincreasethepressuredropacrossthefilterinasimilarfashionasincreasedairflow.

BA.


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Figure2.Crosssectionoffilterpleatsloadedwithparticlesthatincreasethepressuredropacrossthepleatsataconstantvelocity.Theparticledepositsdonotalterthestiffnessofthefiltermedium.PleatCollapsewithRadialFlowHEPAFiltersTheexperimentalmeasurementsonFlandersradialflowHEPAfiltersconductedattheInstituteforCleanEnergyTechnology(ICET)atMississippiStateUniversity(MSU)wereusedtoanalyzepleatcollapseinthisreport(Giffinetal,2012;Waggoner,2012a,2012b).FlandersradialflowHEPAfilters,showninFigure3wereusedintheMSUtests.Twomodelsofthesefiltersweretested:asafe-change(SC)andaremote-change(RC)modelthatdifferedslightlyforusedintwodifferentfilterhousings.


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(A) (B) (C)Figure3.Flanders(A)2,000cfmremotechangefilter,(B)close-upofoutsidepleatsand(C)cutoutportionoffilterpackshowingthe3”deeppleatwithembosseddimples(Giffinetal,2012).Table1.EstimatedfilterparametersforFlandersRadialFlowHEPAfilterusedinthisstudy WTP Radial Filter Type

Dimension Remote Safe Change Units Filter Inlet Opening ID 11.0 13.0 in. Filter Pack OD 18.5 19.8 in. Filter Pack ID 12.5 13.8 in. Pack Medium Width 23 23 in. Estimated Medium Thickness 0.018 0.018 in. Minimum Effective Filter Media Area 308 308 sq. ft. Pleat depth 3.00 3.00 in. Estimated Number of Pleats 321 323

Pleats Per Inch @ Filter Inlet Face 8.18 7.47 ppi Pleats Per inch @ Filter outlet face 5.52 5.20 ppi Average ppi for filter 6.85 6.34 ppi TheinitialpressuredropsoftheFlandersSCandRCfiltersat2000cfmmeasuredattheDOEFilterTestFacility(FTF),Flanders,andMSUisgiveninTable2(Paxtonetal,2012;Waggoner,2012b).TheFTFandFlandersdataagreeclosely,andtheMSUdataisabout0.1in.WClower.Thesedifferencesbetweenthedifferentfacilitiesarelikelyduetothedifferentorificediametersusedinthefiltertesthousings,butthatinformationwasnotavailable.


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ThehigherpressuredropoftheRCfilters(MSUaverage1.39in.WC)comparedtotheSCfilters(MSUaverage0.93in.WC)isduetoamorecompressedfilterpackandamorerestrictiveinletopeningfortheRCfilterasseeninTable1.Sincethesameorificeopeningof13inchesintheMSUfilterhousingwasusedforbothSCandRCfilters,theΔPduetothefilterinletopeningcouldbesubtractedfortheSCfilter,butnotfortheRCfilter.Thehousingorificeopeningwouldhavetobe11in.diametertoremovetheΔPduetotheRCinlet.Computingtheequivalenttareforthe11in.diameterorificebyassumingaconstantdischargecoefficientasthe13in.diameterorificeyieldsanunrealisticpressuredropandwasthereforenotusedhere.ThustheΔPvaluesfortheRCfiltersinthisreportincludestheadditionalresistanceofthe11in.grabringusedforremotehandling.Thisadditionalresistanceintroducesanon-linearityintheΔPversusflowcurvefortheRCfilterandasmallerrorinparametersderivedfromtheRCΔPvalues.SincethecorrectedΔPfortheRCfiltermustbegreaterthantheΔPfortheSCfilter,thecorrectionmustbelessthan0.46in.WCat2000cfm.GeneratingatareΔPcurveforan11in.orificeintheMSUhousingcouldbeappliedinfuturestudiestoeliminatethegrabringcontributiontotheRCfilterΔP.Table2.PressuredropofnewFlandersSCandRCHEPAfilterstestedattheDOEFilterTestFacility(FTF),FlandersandatMSUat2,000cfm.

ΔP,in.WC

ΔP,in.WC

SafeChangeID

DOEFTF

Flanders MSU RemoteChangeID

DOEFTF

Flanders MSU

SC-001 1.08 1.01 0.9 RC-001 1.67 1.45 1.4SC-002 0.96 1.01 0.9 RC-002 1.38 1.45 1.34SC-003 1.08 1.08 1.0 RC-003 1.54 1.49 1.4SC-004 1.12 -- 0.9* RC-004 1.2 1.41 1.4Average= 1.06 1.03 0.93 RC-007 1.73 1.52 1.4

RC-008 1.48 1.46 1.34 RC-009 1.50 1.51 1.4 RC-010 1.3 -- 1.4 Average= 1.48 1.47 1.39

*FromgraphsAmbientTemperatureandHumidityTestsTheMSUtestsconsistedofloadingtheradialflowHEPAfilterswithtestaerosolsofAl(OH)3powder,carbonblack,orArizonaRoadDustataconstant2,000cfmatambienttemperatureandrelativehumidity(Giffinetal,2012).Thepressuredropacrossthefilterwouldbemonitoredduringtheparticleloadinguntilthefilterfailedduetohighaerosolpenetrationasmeasuredwithadownstreamphotometerorwhenthepressuredropreachedthefanlimitof50in.WC.


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AnexampleofaparticleloadingtestunderambientconditionsisshowninFigure4whereAl(OH)3powderisloadedonaRCfilter.

Figure4.Massloadingcurveforaremotechangefilterloadedwithaluminumhydroxide,Al(OH)3.Thearrowsindicatetheapproximatestart(12.9in.WC)andend(20.5in.WC)ofthepleatcollapse.ThemassofAl(OH)3at4in.WCis370g.ThefilterrunIDisRC-DS1-001(Giffinetal,2012)Thedifferentialpressure(DP)acrossthefilterataconstant2,000acfmincreasessteadilyasseeninFigure4untiltheΔPreaches12.9in.WC.Atthatpoint,theHEPAfiltershowsarapidincreaseinΔPduetopleatcollapse.Thepleatcollapseiscompleteat20.5in.WCandsignificantlyincreasesthepressuredropbecausetheeffectivefilterareaisdramaticallyreducedwhilemaintainingthesameairflowrate.Figure5showsaseriesofvideoframestakenatdifferentstagesoffilterloadingfromFigure4.Thevideoispositionedinsidethefilterhousingandenablesmonitoringoftheexitsideofthefilterpleatsduringtestconditions.Thefilterpleatsareopenat4in.WCasseeninFigure5(A)wheretheseparationsbetweenadjacentpleatsareseenasdarkregionsbetweenthedimplepleatsthatactasseparators.AstheDPincreasesto10in.WCinFigure5(B)thepleatseparationsarenearlyclosed.WhenthefilterreachesaDPof20in.WCinFigure5(C)allofthepleatsareincontactasseenbythedisappearanceofthedarklines.Finallyat32in.WCinFigure5(D),thefiltertearsnearthesealantandcausessomeofthepleatstoballoonout.


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Figure5.In-testingvideoimagesofthefilterexhaustsideintestRC-DS1-001at(A)4in.WC,(B)10in.WC,(C)20in.WC,(D)32in.WC.(Giffinetal,2012)AnexampleoftheAl(OH)3powderloadingonaSCfilterisshowninFigure6.


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Figure6.Massloadingcurveforasafechangefilterloadedwithaluminumhydroxide,Al(OH)3.Thearrowsindicatetheapproximatestart(10.2in.WC)andend(15.6in.WC)ofthepleatcollapse.ThemassofAl(OH)3depositsat4in.WCis440g.ThefilterrunIDisSC-DS1-001(Giffinetal,2012)AnadditionalfiveRCandtwoSCfilterswereloadedwithtestaerosolsandtheresultsanalyzedinasimilarfashion.Table3summarizestheresultsfromtheseloadingtests.Table3.Pressuredropestimatesforthestartandendofthepleatcollapsedeterminedfromtheparticleloadingcurvesunderambienttemperature(~70°F)andrelativehumidity(40-60%RH).ThetemperatureandRHarethevaluesatpleatcollapse,andtherangeduringthetestisgiveninparentheses.(Giffinetal,2012,2013;Waggoner,2012b)FilterID

LoadingAerosol

Massofaerosol,kg

Temp,°F RH,% PleatcollapseΔP,in.WC*

Start EndRC-001 Al(OH)3 0.981 70(69-72) 45(43-50) 12.9 20.5RC-002 CarbonB 1.196 70(67-72) 60(48-61) 11.2 16.9RC-003 AZDust 7.760 68(68-72) 45(40-65) 12.6 16.1RC-007 Al(OH)3 0.974 72(69-72) 47(45-52) 13.6 20.0RC-008 CarbonB 1.070 76(68-77) 43(40-59) 10.9 16.0RC-009 AZDust 7.268 68(67-71) 48(40-61) 11.9 15.7SC-001 Al(OH)3 0.808 70(70-72) 50(48-52) 10.2 15.6SC-002 CarbonB 0.874 72(68-73) 57(55-64) 8.8 12.1SC-003 AZDust 5.361 73(70-73) 50(50-56) 8.5 12.3*Visualestimatesfromtheparticleloadinggraphs.Thevaluesofthefilterpressuredropatthestartandendofpleatcollapsewereobtainedbyvisualestimatesoftheloadinggraphsandhaveanuncertaintyduetojudgmenterrorsanderrorsinextractingdatafromgraphs,especiallytheDPvalues


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forthestartofpleatcollapse.Sincesimpleanalyticapproachessuchastakingthefirstandsecondderivativesofthegraphswouldnothelpidentifythestartandendofpleatcollapseduetothenatureofthegraph,suchapproacheswerenotpursued.Table3showstheΔPvaluesforthestartandendofthepleatcollapsearegreaterfortheRCfiltersthanthestartandendofthepleatcollapsefortheSCfilters.ThetighterfilterpackfortheRCfiltersappearstomaketheRCfilterlesspronetopleatcollapsethantheSCfilters.AhigherpressuredropisrequiredfortheRCfilteratboththestartandendofthepleatcollapsecomparedtothepressuredroprequiredfortheSCfilter.Table3alsoshowsthattheΔPforboththestartandendofthepleatcollapseisgreaterwhentheRCandSCfiltersareloadedwithAl(OH)3powdercomparedtothecomparableΔPvalueswhenloadingwithcarbonblackorArizonaRoadDust.TheAl(OH)3depositappearstoformamorerigiddepositonthefilterthanthedepositsofcarbonblackandArizonaRoadDustbecauseboththeSCandRCfiltersrequireahigherΔPtohavepleatcollapse.Themorerigiddepositabsorbssomeofthepressuredropthatwouldbetransmittedthroughthedepositlayertothefiltermedium.ThedatainTable3wasanalyzedbyfirstgroupingthedataintoseparatecategories:SCorRCandstartorendusingalltheloadingdata;SCorRCandstartorendusingalltheloadingdataexceptforAl(OH)3.Akeyassumptionintheanalysiswasthatthevariationofdatawithineachgroupisrandom.TheΔPvaluesineachgrouparethenplacedinascendingorderandassignedacumulativeprobability,PC,valuegivenby(NO-0.5)/NT,whereNOistheordernumberoftheΔPvalueandNTisthetotalnumberofΔPvalues.Forexample,thefourthof6ΔPvaluesinaparticulargroupwouldhaveacumulativeprobabilityof(4-0.5)/6=0.583.Theorderednumberpairsof(ΔP,PC)arethenfittedtoanormaldistributionusingKalaidagraphsoftware.AsimilarfitcanbeobtainedwithExcelbyfirstconvertingtheprobabilityfractiontothenormalizedformusingtheExcelfunctionNORMSINV()withthecumulativeprobabilityintheargument.ThetrendcurveisthenobtainedbythebestlinearfittooftheΔPvaluesversustheNORMSINVvalues.ThegraphicalresultsfromanalyzingthedatainTable3whengroupedaccordingtoSCorRCandpleatStartorEndareshowninFigure7.Figure7showstheRCfiltersrequireahigherΔPforpleatcollapsecomparedtotheSCfiltersforboththestartofthepleatcollapseandtheendofthepleatcollapse.However,theinfluenceofthehigherΔPfortheAl(OH)3loadingtestsatboththestartandendofpleatcollapseisincludedintheresults.ToseparateouttheeffectofthehighΔPduetotheAl(OH)3deposits,Table3isreorderedinTable4.


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Figure7.ProbabilityofthestartandendofpleatcollapseasafunctionofthefilterΔPfortheFlandersSCandRCradialflowHEPAfiltersusingallthedatainTable3.Table4.ReorderingofTable3toseparateeffectsoffilterdesignandtestdustonstartandendofpleatcollapse.Filter Al(OH)3 CarbonBlack ArizonaRoadDust

Start End Start End Start EndRC 12.9 20.5 11.2 16.9 12.6 16.1RC 13.6 20.0 10.9 16.0 11.9 15.7SC 10.2 15.6 8.8 12.1 8.5 12.3

Table4showsthattheΔPvaluesforthecarbonblackandArizonaRoadDustloadingatthestartandendofthepleatcollapsearesimilarandsignificantlylowerthanthecomparableΔPvaluesfortheAl(OH)3loading.ThusalogicalgroupingwouldcombinethecarbonblackandArizonaRoadDustdatafortheRCandSCfiltersandcomparethesegraphstothegraphsfortheAl(OH)3loading.Usingtheanalysismethoddescribed,theprobabilitydataandcurvesareobtainedasshowninFigure8.


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Figure8.ProbabilityoffullpleatcollapsefortheFlandersRCandSCfiltersatroomtemperatureandrelativehumiditywiththeAl(OH)3dataisolated.TheΔPatfinalpleatcollapseat50%probabilityfortheSCandRCfilterswithouttheAl(OH)3dataare12.2and16.2in.WCrespectively.ThecurveforthesingleAl(OH)3testfortheSCfilterisbasedontheSCcurvefortheAZandCBtests.ThemuchhigherΔPvaluesfortheAl(OH)3inFigure8areattributedtotherigidAl(OH)3depositsthathaveaportionofthedepositΔPthatdoesnottransmittothefiltermediumforpleatdisplacement.ArigidparticledepositstructurepreventsthefullDPtobeappliedtothefiltermediumandthusleadstoerroneouslyhigherDPvaluesforpleatcollapse.TheDPvaluesforfullpleatcollapseat50%probabilityvaluesfromFigure8aretabulatedinTable5.ThedifferenceinΔPbetweentheAl(OH)3loadingandtheCBandAZloadingis3.4in.WCfortheSCfilterand4.0in.WCfortheRCfilter.HowevertheSCfilteronlyhas0.804kgdepositatpleatcollapse(Figure6)comparedto0.98kgfortheRCfilteratpleatcollapse(Figure4).WhenthisdifferenceinmassdepositofAl(OH)3inconsidered,thenthepressuredropincrementduetotheAl(OH)3depositsthatisnottransmittedtothemediumisthesameforbothRCandSCfiltersasseeninTable5.ThepressuredropincrementpermassofAl(OH)3depositis4.1and4.2fortheRCandSCfiltersrespectively.


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Table5.ComparisonofpressuredropincrementofAl(OH)3depositsnotcontributingtopleatcollapseforRCandSCfilters.Filtertype ΔP1,in.WC

Al(OH)3ΔP2,in.WCCB&AZ

ΔP1-ΔP2,in.WC

M,kgAl(OH)3

(ΔP1-ΔP2)/M

RC 20.2 16.2 4.0 0.98 4.1SC 15.6 12.2 3.4 0.80 4.2

PhotographsoftheparticledepositssuggestthattheAl(OH)3depositsarestructurallyrigidandcanhaveanindependentpressuredropthatdoesnottransmittothefiltermedium.Figure9showsphotographsofacetylenesootandAl(OH)3particledepositsontheinletsideoffullyloadedHEPAfilters(Waggoner,2012a,2016).AlthoughtheHEPAfiltersarenotthesameastheFlandersradialflowHEPAfilters,thenatureofthedepositswillbesimilar. (A) (B)Figure9.Photographsof(A)acetylenesootand(B)Al(OH)3powderdepositsonfullyloadedHEPAfilters.Waggoner(2012a,2016)TheacetylenesootdepositsinFigure10(A)showthesootdepositscoverthesurfaceofthemediumanddonothaveanindependentstructure.Thusthepressuredropduetothesootdepositstransmitdirectlytothefiltermedium.Incontrast,theAl(OH)3depositsinFigure10(B)showsarigid,cement-likestructurefillingthefilterpleats.SucharigidparticledepositwouldhavesomeindependentpressuredropthatdoesnottransmittotheunderlyingHEPAmedium.ArizonaRoadDust(~5microns)ismuchlargerthanAl(OH)3powder(1micron),andthereforelesspronetoformingrigidstructures.TheArizonaRoadDustismoresand-likeandwouldtransmitthepressuredropfromthedepositontotheHEPAfiltermedium.


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ElevatedTemperatureandHumidityTestsMSUalsoconductedtestsontwoRCandtwoSCradialflowHEPAfiltersinwhichthefilterswerepreloadedunderambientconditionswithAl(OH)3powderat2,000cfmuntilthefilterDPreached4in.WC(Giffinetal,2012).Thefilterswerethensubjectedtoelevatedtemperatureconditionsandthepressuredropmonitoredasafunctionoftimetodeterminepleatcollapseandfilterfailure(Giffinetal,2012).Theelevatedtemperaturewasachievedusinganaturalgasburnerinsidetheductinlet.TopreventcombustionsootfromaddingtothefilterDP,theburnerwasoperatedwithnofilterinplaceuntilthesootwasnegligibleandthetesthousingbecamehot.Thetestfilterwastheninstalledinthehotductandthetestcommenced.WaterwassprayedinsidetheheatedducttoincreasetheRHasneeded.Althoughnotestsoranalyseswereperformedtoensurethewatersprayhadevaporated,nowateraccumulationwasfoundinthetestductoronthefilters.ThetestresultsforthethreefiltersareshowninFigures10-12.

Figure10.ExposureoffilterRC-004toelevatedtemperatureandincreasingRHafterpre-loadingthefilterwithAl(OH)3to4in.WCat2,000cfm.ThearrowindicatesthefilterΔPatwhichpleatcollapsebeginsandiscomplete.(Giffinetal,2012)


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Figure11.ExposureoffilterRC-010toelevatedtemperatureandincreasingRHafterpre-loadingthefilterwithAl(OH)3to4in.WCat2,000cfm.ThetwoarrowsindicatethefilterΔPatwhichpleatcollapsebeginsandiscomplete.(Giffinetal,2012)

Figure12.ExposureoffilterSC-004toelevatedtemperatureandlowRHafterpre-loadingthefilterwithAl(OH)3to4in.WCat2,000cfm.ThetwoarrowsindicatethefilterΔPatwhichpleatcollapsebeginsandiscomplete.(Giffinetal,2012)


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ThetestconditionsandtheΔPvaluesatthestartandendofthepleatcollapsefromFigures10-13aretabulatedinTable6.Table6.Pressuredropestimatesforthestartandendofthepleatcollapsedeterminedfromexposuretoelevatedtemperature(130-144°F)afterpre-loadingthefilterswithAl(OH)3to4in.WC.ThetemperatureandRHareattheendofthepleatcollapse,andtherangeduringthetestisgiveninparentheses.(Giffinetal,2012,2013;Waggoner,2012b)FilterID Mass,kg

Al(OH)3Temp,°Fatcollapse

RH%atcollapse

PleatcollapseΔP,in.WCStart End

RC-004 0.411 130(127-140)

75(15-78) 6.8 6.8

RC-010 0.375 129(128-137)

79(14-85) 6.2 7.2

SC-004 0.459 137(136-139)

14(13-15) 8.8 11.3

Table6showsthattheSCfilterhasaslightlylargerΔPforpleatcollapsethantheRCfiltersattheelevatedtemperaturetestwithAl(OH)3loading.TheoppositetrendwasobservedinTables3and4andinFigure8forloadingwithAl(OH)3powderandtheothertwoaerosolsatambienttemperatures.ThedatainTable6wasanalyzedusingthesameproceduredescribedpreviouslyforfilterloadingunderambienttemperatureconditions.Figure13showstheresultinggraphsofprobabilityofpleatcollapseatthestartandcompletionofthepleatcollapseatelevatedtemperature.SincethefiltersinFigure13werepreloadedwithAl(OH)3powderpriortotheelevatedtemperatureandrelativehumidityexposure,theissueofarigidparticledepositraisestheconcernthattheΔPvalueatpleatcollapsecouldbelowerthanthatindicated.TheprimaryargumentagainstthatpossibilityisthattheparticleloadingintheelevatedtemperaturetestsislessthanhalfthatintheambienttemperaturetestsasseeninTables3and6.WithoutsufficientmassofAl(OH)3deposits,itwouldnotbepossibletoformthedepositseeninFigure9Bwherethedepositcoversthepleatinlets.ThedepositsthatbridgethepleatinletsaresupportedbythepleatendsandwouldhaveaportionofthedepositΔPtransmittedtothepleatendsandnotcontributetothedeflectionoftheflatmediumwithinthepleatstocollapsethepleats.Photographsofintermediatedepositsonthefilterwouldreadilyconfirmthisassessment.SuchanassessmentissupportedbytheΔPversusmasscurvesinFigures4and6fortheRCandSCfiltersrespectively.ThemoderatedepositofAl(OH)3fallsataboutthemid-pointofthenearlinearΔPcurve.Thissuggeststhatthedepositis


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uniformlydistributedoverthefiltermedium;and,thus,theΔPacrosstheparticledepositisapplieddirectlytothemediumandcontributestopleatcollapse.Figure13.ProbabilityoffullpleatcollapseasafunctionoffilterpressuredropforFlandersSCandRCradialflowHEPAfiltersatelevatedtemperaturetakenfromTable6.ThecurvedrawnthroughthesinglepointwastakenfromtheSCfilteratambientconditionsinFigure8.SummaryofRadialFlowHEPAFilterTestResultsThetestsfromFigures8and13arecombinedandshowninFigure14.Figure14showsthatasthetemperatureisincreasedfromabout70°Fto130-137°F,theΔPvalueforpleatcollapsedecreasesdramaticallyfortheRCfiltersandonlyslightlyfortheSCfilter.Thisdecreaseisduetothesofteningofthemediumatelevatedtemperaturewherebythemediumislessstiffandmoreeasilydeflectedtocausepleatcollapse.ThedramaticdecreaseintheΔPforpleatcollapseseenfortheRCfilterisduetothetightpacking(Table1)ofthepleatswherebyasmalldeflectionofthemediumresultsinpleatcollapse.TheSCfilteralreadyhasalowerΔPvalueforpleatcollapseduetotheloosepleatpacking(Table1)andisonlyslightlyenhancedbysofteningthemediumwithhighertemperature


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Figure14.ProbabilityoffinalpleatcollapseasafunctionoffilterpressuredropforFlandersradialflowHEPAfiltersatroomtemperatureandelevatedtemperaturefromFigures8and13respectively.ThepleatcollapseforbothambientandelevatedtemperatureischaracterizedashavingacriticalΔPCwherethefilterΔPquicklyincreasesinanunstablefeed-forwardprocess.OncetheΔPCisreached,thepressuredropcausesthepleatspacingtodecrease,therebyincreasingthefilterΔP,which,inturn,causesthepleatspacingtodecreasefurtherandtherebyincreasesthefilterΔPfurtherintheunstablecollapseprocess.IfthefilterisclosetotheΔPC,thenanyprocessthatincreasesfilterΔPpasttheΔPCcaninitiatethepleatcollapse.PotentialprocessincludeatransientincreaseinairflowthatresultsinincreasingthefilterΔPandanincreaseintheΔPoftheAl(OH)3depositduetodepositedwaterdropletsoradsorbedmoisture.Attheelevatedtemperaturesinthisstudy(130-137°F),oncetheΔPCisreached,ittakeslessthan2minutesforcompletepleatcollapse.


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Figure15showstheprobabilityofreachingtheΔPCfortheSCandRCfiltersissimilartothefullpleatcollapseinFigure14butatlowerΔPvalues.Figure15.Probabilityofinitialpleatcollapse(ΔPC)asafunctionoffilterpressuredropforFlandersradialflowHEPAfiltersatroomtemperatureandelevatedtemperature.BasedontheanalysisoftheradialflowHEPAfiltersdescribedhere,therearetwofilterdesigncharacteristicsthatdeterminethevalueoftheDPC:(1)thetightnessofthefilterpackand(2)theseparationofthemediumwithinthepleats.Thesetwocharacteristicsproduceoppositeeffectsonthepleatcollapse.Table1showsthattheSCfilterhasalooserfilterpack(lowerppi)thantheRCandthereforehaspleatcollapseatlowerΔPvalues.Howeverthetighterfilterpackhasasmallerpleatspacingandthereforerequireslessdeflectionofthemediumforpleatcollapse.FromFigures14and15,theSCfilterpackissufficientlyloosethattheΔPCdoesnotchangeandthefinalDPatpleatcollapsedecreasesslightlywithincreasingtemperature.Incontrast,theRCfilterhasamajorreductionintheΔPCandfinalΔPwithincreasingtemperature.


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PleatCollapsewithRadialFlowHEPAFiltersAsmallnumberoftestshaveshownthataxialflowHEPAfiltersalsoexhibitpleatcollapse(Waggoner,2012a;Stormoetal,2016).AlthoughtenFlandersseparatorlessHEPAfiltersweretested,sixwiththeDYN-E2design(U-pack)andfourwiththePureformdesign(W-pack),dataonthefilterDPatpleatcollapsewasonlyavailablefortwooftheU-packfilters(Stormoetal,2016).Figures16and17showthefilterΔP,temperature,andRHasafunctionoftimefortheU-packfilterspreloadedwithAl(OH)3to4in.WCat1000cfmand1500cfmrespectivelyandthenexposedtoelevatedtemperatureat1000and1500cfm.

Figure 16. Test data for a 24 x 24 x 11.5 inch Section FC Axial Flow Flanders U-pack HEPA Filter loaded to 4 in. w.c. dP with Al(OH)3 followed by exposure to elevated temperature at the rated flow of 1000 cfm. The arrow marks the start of pleat collapse. (Waggoner, 2012a;Stormo et al, 2016) TheinitialpleatcollapseinFigure16occursat7.3in.WC,122°F,20%RH,and17minutes.


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Figure 17. Test data for a 24 x 24 x 11.5 inch Section FC Axial Flow Flanders U-pack HEPA Filter loaded to 4 in. w.c. dP with Al(OH)3 followed by exposure to elevated temperature and relative humidity at the rated flow of 1500 cfm. The arrow marks the start of pleat collapse. (Waggoner, 2012a; Stormo et al, 2016) TheinitialpleatcollapseinFigure17occursat8.4in.WC,104°F,22%RH,and11.6minutes.ComparingthepleatcollapseinaxialflowHEPAfiltersinFigures16and17withtheradialflowHEPAfiltersinFigures10-12showsthestartofthepleatcollapseoccursatmuchshortertimes(11.6and17minutescomparedto40-80minutes)andatlowertemperatures(104and122°Fcomparedto129-137°F).TheU-packfiltersalsohaveslowerrateofpleatcollapse(asmeasuredbytherateofΔPincreaseversustime)thantheradialflowHEPAfilters.Thepleatcollapsedoesnotseemtocorrelatewithrelativehumidity.TheU-packfiltersshowpleatcollapseatlowRHvalues(20%and22%)whiletheradialflowfiltersshowpleatcollapseatbothlow(14%RHinFigure12)andhigh(75%and79%RHinFigures10and11respectively)relativehumidity.AmoredetailedanalysisofthepleatcollapsewithaxialflowHEPAfilterswasnotpossiblewiththelimiteddataavailable.


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EffectofRelativeHumidityonPleatCollapseFromthetestresultsshowninFigures10and11fortheradialflowfilters,oneistempedtoconcludethatincreasedmoistureisalsoresponsibleforpleatcollapseinadditiontoelevatedtemperature.TheRCfilterspre-loadedwithAl(OH)3powderatelevatedtemperatureshowednoincreaseinΔPuntilawaterspraywasturnedontoincreasetherelativehumidity.OncetheRHreached80%,thefiltersrapidlyincreasedinΔPduetopleatcollapse.Thereisastrongprejudicetoconsiderelevatedhumidityasresponsibleforthepleatcollapsesinceitiswellknownthatboththetensilestrengthandstiffnessoftheglassfibermediumdecreasessignificantlywhenthemediumbecomeswet.However,therapidincreaseinfilterΔPwiththeinjectedwaterspraymaybeduetoanabruptincreaseinairflowduetothefanadjustingtochangingtemperaturewhiletryingtomaintainaconstantmassflowasobservedinpreliminaryMSUtests(Waggoner,2012B).OncethecriticalDPisreached,thefilterquicklyhaspleatcollapseinlessthan2minutes.Figure10showsignificantchangesinairtemperaturewhenthewaterspraywasturnedon,andFigure11showssmalltemperaturechangeswhenthewatersprayisturnedon.Unfortunately,MSUdidnotreporttheairflowduringthesetransientstoclarifythesourceoftheincreaseinfilterΔP.Tohelpunderstandtheeffectofhighhumidityonfilterpleatcollapse,alloftheinitialpressuredropdatafortheradialflowHEPAfilterusedinFigure15plustheadditionaltwopressuredropsfortheU-packfiltersinFigures16and17aregraphedinFigure18asafunctionoftherelativehumidityatpleatcollapse.ThetemperatureatpleatcollapsearealsoshowninFigure18.Unfortunately,thedataisnotsufficienttomakeanystatementsregardingtheeffectofmoistureonpleatcollapse(Giffinetal,2012).TheanalysisoftheSCandRCfiltersshowedthatthefiltershavesignificantlydifferentvaluesofΔPCandthattheRCfiltersaresensitivetotemperaturewhiletheSCfiltersarenot.TheU-packfilteralsoappearstohaveatemperaturedependency,butwithonlytwodatapoints,thereislittleconfidenceinanyspeculation.AlthoughtheMSUdataisinsufficienttoclarifythedependencyofpleatcollapseontherelativehumidity,otherstudieshaveshownthatthereisnegligibleincreaseinfilterΔPwithexposuretoelevatedrelativehumidityupto80%forbothcleanfiltersandfiltersloadedwithroomdust(Rudingeretal,1985).ThelackofΔPincreaseindicatesthatthefilterandparticledepositshavenotadsorbedwater.


22

Figure18.PlotoftheinitialΔPatpleatcollapse(ΔPC)asafunctionoftherelativeforthe10datapointsforSCandRCfiltersfromFigure15plusthetwoadditionalpointsfortheU-packfiltersfromFigure16and17.


23

(A) (B)Figure19.Adsorptionofmoistureon(A)cleanHEPAfiltersandon(B)HEPAfiltersloadedwithroomairdusthavelittleornoimpactonthefilterΔPbelow80%relativehumidity.Thefiltersarenucleargrade,1000cfm,glassfiberHEPAfilterswithaluminumseparators.(Rudingeretal,1985)


24

PlateDeflectionModelofPleatCollapseThecollapseoffilterpleatsinFigure1Bcanbesimulatedwiththemechanicaldeflectionoftwosheetsofmediumthatrepresentsthetwosideofapleat.Thedeflectionofsheetsofmaterialfromuniformforces(i.e.appliedpressure)canbecomputedfromstress-strainanalysisandpropertiesofthematerial.TheformulasforthedeflectionofrectangularsheetsbyauniformforcearederivedbyYoungandBudynas(2002)Forauniformpressurethemediumsheethasamaximumdeflectionof

yP,max =αP PW

4

E t3

where yP,max =maximumdeflectionincenter,inches αP =aconstantthatdependsontheD/Wratio,

W =widthbetweenseparatorpeaksorembossments,inches D =pleatdepth,inches P =appliedpressureonthesheet,pounds/inch2 E =Modulusofelasticity(Young’smodulus),pounds/inch2 t =thicknessofmedium,inchesThevaluesofαPareafunctionoftheD/WratioofthesheetandaregiveninTable7(YoungandBudynas,Table11.4(2002))Table7.ValuesofaPcoefficientsasafunctionofD/WD/W: 1.0 1.2 1.4 1.6 1.8 2.0 3.0 4.0 5.0

€

∞ αP 0.0444 0.0616 0.0770 0.0906 0.1017 0.1110 0.1335 0.1400 0.1417 0.1421

Thedimensionsofthemediumsheettobetesteddependsonthedesignofthefilter.ThekeyfilterparameterstobeusedinthedeflectiontestsaregiveninFigure20.Heret=mediumthickness(inches),D=pleatdepth(inches),2X=spacebetweenthetwosidesofthepleats(inches),andN=numberofpleatsperinch.

(1)


25

Figure20.Keyfilterparametersusedforpleatcollapsetesting.t=mediumthickness(inches),D=pleatdepth(inches),2X=spacebetweenthetwosidesofthepleats(inches),andN=numberofpleatsperinch.Notethatthedistanceofonepleatis1/N.ThusfromFigure20,wehave1N= 4X + 2t

RearrangingEquation2yields,

€

X =14N −

t2

Figure20andEquation3areonlyapplicabletoaxialflowHEPAfilters.ThecomparabledrawingforradialflowHEPAfiltersisshowninFigure21.

(2)

(3)


26

Figure21.DrawingforthepleatconfigurationinradialflowHEPAfilters.Duetotheasymmetryoftheradialflowpleats,theinletandoutletpleatparametersaredifferentasshowninFigure21.Theinletpleatsaretightlyspacedwhiletheoutletpleatsaremorewidelyseparated.TheinletchannelwidthisshowninFigure21asconstantalongthepleatdepth,D.Thisfollowsfromthemanufacturingprocessofpleatingthemediumtoproduceaflatfilterpack.Whenthefilterpackiswrappedaroundintoacylinder,thepleatsnaturallyformtheaccordionshapeshowninFigure21,withaconstantinletpleatchannelwidthandanexpandingoutletpleatchannelwidth.FortheFlandersdimplepleatdesign,thepleatasymmetryislesspronouncedbecausetheembossedportionsofthemediumcanbeslightlycompressedwhenformingthecylindricalfilterpack.ToavoidthecomplexitiesoftheasymmetricalinletandoutletoftheradialflowHEPA,theaverageoftheinletandoutletpleatsperinchareused.AssumingtheaverageN=6.85and6.34pleats/inchfortheFlandersremote-changeandsafe-changedesigns,andt=0.018in.,thenEquation3yieldsX=0.0275in.and0.0304in.fortheRemote-ChangeandSafe-Changedesignsrespectively.Thusdeflectionsofthemediumby0.0275in.and0.0304in.representsacompletepleatcollapseforRCandSCfilters.

D

1/Ni

1/N0

2X0

2Xi


27

ThedimensionsofthepleatsectiontobeusedinEquation1canbedeterminedfromthecutopenpleatsectionshowninFigure3C.Sincetheconsecutiveembossmentsareintoandoutoftheplane,thewidth,W,ofthesheetisdeterminedbythedistancebetweenthefirstandthirdembossments,orW=1.36in..Eachpleatsectionthushasacenterembossedridgethataddsrigiditytothesheet.Thelengthofthepleatsectionisthedepthofthepleat,orD=3in..ThevalueofαpforD/W=2.21fromTable7isαp=0.1157withinterpolation.Equation1canbeusedtodeterminetheeffectivemodulusofelasticity,E,usingthefilterpressuredropatfinalpleatcollapseandtheparametervaluesdeterminedhere.Figure22showstheresultsofthiscalculationasafunctionoftemperature.Figure22.ComputedeffectivemodulusofelasticityfortheSCandRCfilterasafunctionoftemperature.Notethatthecomputedmodulusofelasticitydependsnotonlyontheintrinsicmaterialpropertiesoftheglassfibermedium,butalsotheembossmentsdefiningthemediumsheet,theadditionalembossmentinthecenterofthesheet,andthetightnessofthefilterpack.FurtheranalysisofpleatcollapseondifferentdesignsofseparatorlessHEPAfilters(e.g.U-pack,W-pack,mini-pleat)willberequiredtoallowaseparationofthefilterpackdesignfromtheintrinsicmodulusofelasticityoftheHEPAfiltermedium.


28

FilterMediumTearsWhentheHEPAfilterexperienceshigherpressuredropsorthefilterbecomeswet,thenthefiltermediamaytear.TwodifferentfailuremodeshavebeenidentifiedwithmediumtearsinHEPAfilters:(1)bendingofthefilterpackwithtearsofthemediumperpendiculartothepleatsand(2)ballooningofpleatendandtearingalongthepleat.TearsDuetoBendingoftheFilterPackFigure23illustratesthemediumtearsthatresultfrombendingoftheHEPAfilterpackforfilterswithoutaguardscreen.Figure23.Failureofanew24in.x24x11.5in.deep-pleatedHEPAwithaluminumseparatorsratedat1,000cfmduetobendingofpleatsunderhighflowwithdryair.(Rudingeretal,1987).Notethebreakageoccursatthecenterofthefilter,wherethepressuredropforcesthefilterpacktobulgeoutandthestrainisthegreatest.Suchbreakagewouldnotoccurwhenascreenisusedandpreventsthefilterfrombulging.ThebreakagewouldoccurclosertothefilterpacksealantasshowninFigure24.


29

Figure24.Close-upphotoofremotechange,radialflowHEPAfilter,RC-009,loadedwithArizonaRoadDusttohighpressuredrop.Thefiltersufferedcompletepleatcollapseat15.7in.WCand(Table3)andrupturedat40in.WCat2000cfmdryairflow.Thearrowsshowtheregionofthemediawherethepleatsaretornloosefromthesealant(shownasyellow).(Waggoneretal,2012b)ThebendingforceonthefilterpackinFigure24isthegreatestnearthesealantwherethefilterpackissecuredbutdoesnothavethesupportoftheprotectivescreen.ThebendingofHEPAfilterpacksandthesubsequentmediumtearshasalsobeenseenwithfiltersusingribbonseparatorsasshowninFigure25.Thescreenpreventsthefilterpackfrombulgingoutinthecenter,thusplacingthegreateststrainneartheendcapwherethefilterpackissecuredwiththeurethanesealant.Thepressuredropforcesthefilterpacktobulgetowardthesupportscreen.Theribbonseparatorofthefilterpackmakescontactwiththescreenasmalldistancefromtheendcapandremainsfixed.Betweenthisfixedpointandtheurethanesealant,thefilterbulgesandhasnosupport.NotethatthemediumtearoccursattheedgeoftheribbonseparatorinFigure25B,wherethethicknessoftheribbonseparatorexacerbatesthealreadyhighstressinthefilterpackandcausesthemediumtears.


30

(A) (B)

Figure25.FailureofVokesradialflowHEPAfilterwithribbonseparatorsduetobendingofthefilterpackfollowingparticleloadingat2,000cfmdryair.(Giffinetal,2012).Forseparatorfilters,theseparatorpreventsthecompletecollapseofthefiltermedia.However,separatorfiltersalsohavemediatearsathigherpressuresasseeninFigures26and27.Figures26and27showthattheseparatorlessHEPAfiltersfailatsignificantlylowerpressuredropsthanfilterswithseparators.


31

Figure26.FailureofHEPAfiltersduetomediatearsasafunctionofDPforthreedifferentfilterdesignstestedatambienttemperatureandRHandhighaerosolloadingatconstantfloworhighairflows:dimplepleatradialflow(Giffinetal2012,2013),axialflowseparatorless(W-pleat)andmini-pleatwithstringseparator(Gregoryetal,1983),andaxialflowdeep-pleatedwithaluminumseparators(Rudinger,1990,Gregory,1979,1983).NotethatGregoryetal(1983)lumpedtogetherthemini-pleatfilterandtheseparatorless(W-pleat)filter.NotethatalthoughthethreedifferentfilterdesignshavedifferentΔPfailurecurves,theyallconvergetoathresholdfailurevalueabout25-30in.WC.


32

Figure27.FailureofHEPAfiltersduetomediatearsasafunctionofΔPforseparatorandseparatorlessfilterdesignstestedatambienttemperatureandRH:axialflowseparatorless(W-pleat)andmini-pleatwithstringseparator(Gregoryetal,1983),andaxialflowdeep-pleatedwithaluminumseparators(Rudinger,1990,Gregory,1979,1983).NotethatGregoryetal(1983)lumpedtogetherthemini-pleatfilterandthedimplepleat(W-pleat)filter.NotethatalloftheexamplesofmediumtearsgiveninFigures23-27haveoccurredindryairstreams.IfthefilterbecomeswetthentheinherenttensileoftheHEPAmediumdeceasessignificantly,andfiltermediumtearsatfarlowerfilterΔPvaluesasshowninFigure28.


33

Figure28.FailureofHEPAfiltersduetomediatearsasafunctionofDPfordeeppleatedfilterswithaluminumseparatorsatambienttemperatureandeitherambientRH(dry)orwithhighhumidity(wet).NotethatthewettestdiffersfromthatusedintheMSUtestsorintheAG-1test,andthewetcurvedoesnotrepresentaconstantmoisturechallenge.TheprocedurebyRicketts(1987)andRudinger(1990)todeterminetheΔPatfailureforthewetconditionsconsistedofraisingtheRHoftheratedairflowat122°Ffrom50%to100%untilfailurewasobserved.IfHEPAfailuredidnotoccur,thenadditionalwaterwassprayedontothefilteruntilaruptureoccurs.Theaddedwaterincreasedthefilterpressuredrop.Unfortunately,themoisturechallengeatHEPAfailurewasnotrecorded.

0

20

40

60

80

100

0 20 40 60 80 100 120

Rudinger 1990Gregory 1978Gregory 1982Ricketts 1986Rudinger 1990Best fit

HEP

A fa

ilure

, %

ΔP, in

Wet

Dry


34

Rudingeretal(1987)developedatheorytoexplainthemediumtearsduetobendingofthefilterpack.Figure28showsthebasicconceptofthebendingofthefilterpackandtheresultingtensilestressonthedownstreamsideofthefilter.Figure29.Drawingofthedeflectionofthefilterpackwithhighdifferentialpressurethatinducesatensilestressonthedownstreamfilterpleatsandresultsinmediumtearsperpendiculartothepleatdirection.(Rudingeretal,1987)Thetheoryofmediumtearsduetofilterpackdeflectionisbasedonexaminingthestressonasinglepleatusinggeneralconceptsfrombendingofbeams(Rudingeretal,1987,1990).Figure30showsasketchofasinglepleatthatisdeflectedduetohighpressure.ThetheoryofthemediumstressduetopleatdeflectiondevelopedbyRudingeretal(1987,1990)isgivenbyEquation4.

σ L =W H 2

4At D2 +2pt ε

⎛

⎝⎜

⎞

⎠⎟ΔP

where σL =thetensilestressonthefiltermedium W =widthoftheHEPAfilterpack H =heightoftheHEPAfilterpack D =depthofthemediumpleats t =thicknessofHEPAfiltermedium p =pitchoftheseparatorcorrugation ε =elongationofthemediumduetoloadingin% ΔP =pressuredropacrosstheHEPAfilterpack

(4)


35

Figure30.SchematicofasinglepleatofaHEPAfilterpackthatisdeflectedduetohighpressuredrop.TheQrepresentstheairflowagainstapleatofheight,H,anddepth,D.Rudingeretal(1990)demonstratedthatEquation4gavegoodagreementwithexperimentalHEPAfilterfailuresunderbothdryandhumidairconditionsinFigure31.Figure31.Thecomparisonsofmeasuredstructuralstrength(filterΔP)andcalculatedstructuralstrength(filterΔP)usingEquation4forbothdryairandhumidairtestsshowgoodagreement.


36

TearsDuetoSwellingandRuptureofPleatEndsAnothermajorfailuremodethatresultsinmediumtearsistheswellingandruptureofpleatendsinbothdryandwetenvironments.Filterdesignsthathavethepleatslooselypackedaremorepronetohavepleatswellingthantightlypackedpleats.ThepleatrupturesoccurinbothdryairconditionsasshowninFigure32andinwetconditionsasshowninFigure33.Figure32.Exampleofaclean1000cfmHEPAfiltersubjectedtohighDPunderdryairconditionsthatsufferedpleatswellingandrupture(Rudingeretal,1990).

Figure33.Exampleofaclean1000cfmHEPAfilterthatsufferedpleatrupturefollowingexposuretotheASMEAG-1resistancetopressuretestunderwetconditions.


37

PleattearsfrequentlyoccurinfiltersthathavefailedtheASMEAG-1wetoverpressuretest.TheAG-1resistancetopressuretestrequiresthefilterbeexposedtoairflowat95°F,95%RHandadditionalwaterspraysuchthattheresultingfilterpressuredropis10inchesWC.Sincethetestdoesnotmeasuretheairflow,itisnotpossibletodetermineifpleatcollapseoccurs.Failureisdefinedashavinga0.3micronDOPefficiencyoflessthan99.97%at20%ratedflowfollowingonehourexposureattheelevatedtemperatureandmoisture.Thusthetestonlymeasuresfiltertearsandcannotmeasurepleatcollapse.Theresultsforseveraldifferentfilters,includingtheWandUpackfilters,areshowninFigure34(Senetal,2010).AkeyfindingintheSen(2010)paperisthattheHEPAfailuresoftheAG-1testarestatisticalasshownbythecumulativefailuredistributioninFigure34.ThustheAG-1practiceoftesting4filterssuggeststhatthefilterhaslessthan25%(1/4)probabilityoffailingthewetpressuretest.AreviewoftheFlandersAG-1resistancetopressuretestindicatesthatthefailurerateforWpackfiltersis17%(4/24)andforUpackis0%(0/8).Althoughthe0%failurefortheUpackislikelyduetothesmallnumberoftestsandmaybegreaterwithmoretests,itisclearthattheWpackismorepronetotearfailurethantheUpack.ThisfollowsnotonlybecauseofthetestresultsinFigure34,butalsobecausethemediatearsareduetoweakertensilestrength.TheWpackfiltershavethepleatsformedbybendingovertheembossedmediawhereastheUpackfiltershavepleatsformedbybendingoverflatmedia.Bendingoveranembossmentputsalargestressonthemediaandmakesitpronetotearing.


38

Figure34.PercentofHEPAsfailingtheAG-1resistancetopressuretestsuperimposedonacurveforpercentfailuresofGermanHEPAfiltersduetotornfiltermedia(Rickettsetal,1987)exposedtomoistairatincreasingpressuredrops.Thenumbersinparenthesesarethenumberoffailedfiltersoverthetotalnumberoftestedfilters.(Sen,2010).

0

20

40

60

80

100

0 5 10 15 20 25 30 35 40

Perc

ent H

EPA

Failu

res

ΔP, in. H2O

Flanders W Pack 1,500 cfm(4/24)

Flanders U Pack 1,500 cfm, (0/8)

German Deep Pleated/Al Separators1,000 cfm

Cam. Farr, 500 cfm (1/18)Cam. Farr, 1,500 cfm (4/23)


39

Rudingeretal(1987,1990)alsodevelopedthetheoryforHEPAfilterfailureduetopleatswellingandruptureasshowninFigure35.

Figure35.Sketchoffilterpleatshowingthecriticaltensilestrength,σC,forpleatswellingandruptureisdependentontheproductofthepleatradiusofcurvature,r,andthefilterpressuredropdividedbythemediumthickness.ComparisonoftheexperimentalDPatfailurewithcomputedfailuresshowreasonableagreementinFigure36.Figure36.ComparisonofmeasuredandcalculatedfilterDPatpleatfailureshowgoodagreement.(Rudingeretal,1987)Rickettsetal(2004)continuedtofurtherrefinethemodelsforHEPAfilterfailureduetofilterpackbendingandpleatrupture.


40

ReferencesGiffin,P.K.,Parsons,M.S.,Wilson,J.A.,andWaggoner,C.A.(2012)“PerformancecomparisonofdimplepleatandribbonseparatedradialflowHEPAfilters”32ndNuclearAirCleaningConference,June17-19,2012,Denver,CO,InternationalSocietyforNuclearAirTreatmentTechnologies,www.isnatt.orgGiffin,P.,Waggoner,C.,andParsons,M.(2013)LifecycletestingresultsofdimplepleatradialflowHEPAfiltersunderambientandelevatedconditions,AerosolScienceandTechnology,Vol47,No.7,pp.785-791. Gregory,W.S.,Andrae,R.W.,Duerre,K.H.,Horak,H.L.,Smith,P.R.,Ricketts,C.I.,andGill,W(1979)AircleaningsystemsanalysisandHEPAfilterresponsetosimulatedtornadoloadings,Proceedingsofthe15thNuclearAirCleaningConference,pp.694-706.www.isnatt.orgGregory,W.S.,Martin.,R.A.,Smith,P.R.andFenton,D.E.(1983)ResponseofHEPAfilterstosimulatedaccidentconditions,Proceedingsofthe17thDOENuclearAirCleaningConference,pp.1051-1068.CONF-820833.www.isnatt.org Munson,B.R.,D.F.Young,andT.H.Okiishi(2006)FundamentalsofFluidMechanics,FifthEdition,JohnWiley&Sons. Ricketts, C.I. and Smith, P.R. (2004) “A model for the stresses in the filter medium of Deep-Pleat HEPA filter packs and its practical applications.” In Proceedings of 28th Nuclear Air Cleaning Conference. www.isnatt.org Rudinger, V, Ricketts, C.I., and Wilhelm, J.G.(1985) Limits of HEPA-filter application under high-humidity conditions” Proceedings of the 18th DOE/NRC Nuclear Air Cleaning Conference, M.W. First, Editor, CONF-840806, Vol. 2. pp 1058-1084. www.isnatt.org Rudinger, V, Ricketts, C.I., Wilhelm, J.G., and Alken, W. (1987) Development of glass-fiber HEPA filters of high structural strength on the basis of the establishment of the failure mechanisms” Proceedings of the 19th DOE/NRC Nuclear Air Cleaning Conference, M.W. First, Editor, NUREG/CP-0086, CONF-860820, Vol. 2. pp 947-966. www.isnatt.org Rudinger,V. Ricketts, C.I., Wilhelm, J.G. (1990) High-Strength, High-efficiency particulate air filters for nuclear applications, Nuclear Technology, Vol. 92, October issue, pp. 11-29. Sen,S.,Bergman,W.,Hahn,M.,andStormo,J.(2010)OnthequalificationtestingofHEPAfilters,Proceedingsofthe31stNuclearAirCleaningConference,Charlotte,NC,July19-21,2010www.isnatt.org


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Waggoner,C.(2012a)“Isafilterloadingqualificationtestneeded?”Proceedingsofthe32ndNuclearAirCleaningConference,Denver,CO,June17-19,2012Waggoner,C.(2012b)Personalcommunication.Waggoner,C.(2016)Personalcommunication.Young,W.C.andR.G.Budynas(2002)Roark’sFormulasforStressandStrain,SeventhEdition,page502,McGraw-Hill,NewYork,materiales.azc.uam.mx/gjl/Clases/MA10_I/Roark%27s%20formulas%20for%20stress%20and%20strain.pdf (copy to web browser, do not click on url)

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