Bag filters

1. 3355
BAG FILTERS
J RUSHWORTH

2. 1.
2.
3.
4.
5.
6.
7.
8.
9.
BAGFILTERS
CONTENTS
INTRODUCTION
THEMECHANXSM OFPARTICLECAPTURE
CLEANING MEITIODS
TEMPERATURE LIMITATIONS
BAGFILTERSIZING
5.1FiltrationVelocity
5.2EstimatingDedustingAirFlowRates
CHOICEOFCORRECT FABRICFORAPPLICATION
TROUBLE SHOOTING
COMMENTS ONAPPLICATION
RECENTDEVELOPMENTS
Appendix1: HmdDesign

3. 1. INTRODUCTION
Fabricfiltrationhasbeenappliedformanyyearsonbothindustrialanddomesticfronts.
Inessence,adustbearinggasisinterceptedbyapermeablefabricinsuchamanner
thatallthegaspassesthroughthefabricwhilstthedustimpingesonthefibreofthe
fabricandistherebyretained.
Asthedustaccumulatesonthefabrica‘cake’isformed,whichaidsfiltrationby
improvingparticlecaptureandimprovesthecollectionefficiency.Atthesamethe,
however,theresistancetogasflowincreasesandinordertomaintainthesamegas
flowrateasatstartUpthesystemfanhastoworkharder.
Whentheresistancetogasflowreachesanunacceptablelevel,thefabrichastobe
cleanedtodislodgethecake.Thepressuredropacrossthefabricwillalwaysbegreater
thantheinitialvalue,thatiswithnewfabric,becausesomeofthedustparticleswill
becomepermanentlylodgedinthefabric.Providedsteadystateconditionsbetweenthe
fabricandthequantityoftrappeddustisreachedinareasonablyshorttimetheeffect
isbeneficial,butifthequantityoftrappeddustincreasesaftereverycleaningcycle,
thenultimatelybIindingwilloccur.
2. THEMECHANISM OFPARTICLECAPTURE
Thefiltrationprocessisextremelycompkxandinvohmsacombinationofimpaction,
diffusion,thermal,molecularandelectrostaticforces.Ofthese,themostimportant
are:-
l
l
l
ImRaction-whichoccurswhenaparticle,becauseofitsmomentum,crossesthe
fluidstreamlinesandstrikesafibre.Thelargertheparticleandthesmallerthe
fibre,thegreaterarethechancesofimpactionbyparticleinertia.
Diffusion-whichistheprimarycollectionmechanismforparticlesbelow0.5
micron.
ElectrostaticForces-whichaffectparticlesbelow0.5micron.
Theearlystagesoffiltrationoccurwiththecaptureofindividualparticlesby
singlefibresasaresultofanycombinationoftheabovementionedmechanisms.
Theparticleswhichdepositonfibresprojectingintothegasstreamthenactas
additionalsitesforthecaptureoffurtherparticlesandeventuallychainlike
aggregatesr-ult.Astheprocesscontinues,acompletematrix,orcake,ofdust
particlesisbuiltupuntilfinallyparticlecaptureiseffectedbytruesurface
filtration,orsieving,andthefunctionofthefabric,apartfromactingasa
support,becomesnominal.Followingacleaningaction,furtherparticlesinthe
gasstreamattachthemselvestoparticleswhichhaveremainedonthefibresand
thecakebuildingprocessrecommences.
1

4. Fibresusedinthemanufactureoffabricsforfiltrationarealmostexclusivelysynthetic
andtheyareeitherwovenorneedlefelted-seeFigure1.Wovenfabricsaresmoother
andmoreeasilycleanedthanfeltsandsometimes,atlowloads,nocleaningdevicesare
neededbecausethefabricisselfcleaning.Ontheotherhandtheyoftencamotbe
cleanedtoovigorouslybecausethiswouldbreakdowntheentiredustcakeandforcethe
dustbetweenthefibressothatthedustemissionwouldbehigh.Needlefeltsareless
permeablethanwovenfabrics,buttheycanbeoperatedatconsiderablyhigherfiltration
velocities.Theporesinneedlefeltsareverysmallcomparedwithwovenfabrics,so
dustpenetrationislow.
Generally,thefilterelements,whetherofwovenorfeltedfabric,arecylindrical,but
somemanufacturershaveadoptedflatpanel,orenvelopeelements.
3. CLEANING MEITIODS
Theremovaloftheaccumulatedlayerofdustfromthefilterfabriccanbeachievedin
manywaysincludingcollapseofthefilterelement,mechanicalshaking,reverseair
flow,reverseairpulseandreverseairjet.Anyone,orcombinationofthesemethods
maybeemployedbut,asageneralisation,thereverseairpulseandreverseairjetare
usuallyassociatedonlywithfiltershavingneedlefeltedelements.
Cleaningbycollapseofthefikerelement-seeFigure2isamethodusedwhenthe
fabricisrelativelyweak,asisthecasewithglassfibre,andwhencakereleaseis
relativelyeasy.Strongerfabricsandthenecessityforamorevigorousactioninorder
todislodgethecakeleadstoshakecleaning,oftenwiththeassistanceofareverseair
flow,seeFigure3.
Duringthecollapseofthefilterekment,ortheshakeorreverseairperiod,thegas
flowmustbestoppedinordertoallowthedustcaketofallawayfromthefabric.
Thus,afilterplantmustbemadeupofasufficientnumberofseparatecompartments,
eachcontainingagroupoffilterelements,toallowonecompartmenttobetakenout
ofserviceatatimeforcleaning.Ifthereareonlyafewcompartmentsinthefilter,
thentakingoneoffstreamwillmarkedlyincreasetheflow,andconsequentlythe
pressuredropacrosstheothers,andthisfactormustbetakenintoaccountatthedesign
stage.
Withreverseairpulsecleaning,moderatelypressurisedairfromasecondarybloweris
introducedintotheelement,oftenbymeansofatravelingnozzle(refertoFigure4).
Thereverseairjetmethodutilisesahighpressurejetofairwhichisinjectedintothe
elementforatimeintwwaiofabout0.1second-seeFigures5and6.Cakereleaseis
accomplishedbyacombinationoffabricdeformation,duetotheshockofairblast,and
flowreversal.Bothcleaningmethodsremovethedustwithonlyabriefinterruptionin
thegasflowandbothinvariablyuseneedlefeltfabrics.
Figure7showstherelationshipbetweenpressuredropandtimebothforasectionalised
continuouslyratedfilterandforafilterofthereverseairpuise/jettype.
2

5. WOVEN CLOTH
10TIMES
FPARTICLES
NEEDLE FELT
CROSS SECTION OF
WOVEN ANDFELTED FILTERFABRIC
Fig.
3

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COLLAPSING
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AIR/CLEANED
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FANCLOSED
4
COLECTED DUST
FILTERING
DIRTY
GAS
DIRTYGAS
OUTLET IO
COLLAPSING
FANOPEN
+
COLLECTED DUST
CLEANING
~LLAPSING
FAN
\ DIRTY
\ AIRTO
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+
OTHER
!
smoNs
y....p.:,”,:
‘1
,.,..<.:.>
FABRICFILTERWITHCOLLAPSE BAGCLEANING
Fig.2
4

7. REVERSE
All?FAN,
BAGSHAKING
/DEvlcE-OFF
99 -0
REVERSE AIR
INLETcLOSED
..
CLEANED GAS
OUTLET OPEN $: ;$,.
.,+ ...
t?
t.
fIy
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DIRTY
GAS
C&EANED
GAS
REVERSE
AIRFAN\
BAGSHAKING
DEVICE-ON
\
Am
1-
/
REVERSE AIR-
INLETOPEN
CLEANED GAS
OUTLET CmSED
----F.-7 .&:h.-
..
DIRTYAIRTO
~)+ER +
SECTIONS
COLLECTED DUST
FILTERING
COLLECTED OUST
CLEANING
FABRIC FILTER WITH
SHAKE ANDREVERSE AIRCLEANING
Fig. 3

8. ,REVERSE AIR
CLEANEDGAS*
DIRTYGAS +
w
b
FILTERBAGWITH_
DUSTLAYER
(RLTERCAK=
BUILDINGUP
BAGSUPPORT~
1!
!-....
FAN
/TRAVELLING
AIRTUBE
-FILTERBEBEING
CLEANED AIRW
BRIEFLYREVERSED
INFLATES BAG&
O\SLODGES DUST
COLLECTED DUST
FABRIC FILTERWITH
PULSEAIRCLEANING ANDCYLINDRICAL BAGS
F@ 4

9. CLEANEDGAS~
DIRTYGAS e
FILTERBAGWITH
DUSTLAYER
CFILTERCAKE)
8UILDING UP
BAGSUPP9RT—
DIAPHRAGM VALVES
~.H=~ ‘iR
-—
- (Co100RS.I.)
l--
JETTUBE
INJECTI?4G
BURSTOF
COMPRESSED -
5
AIRINTO
‘FILTER BAG
1-
PILOTVALVES
&/ORTIMERS
FILTERBAGBEING
CLEANED AIRmW
BRIEFLYREVERSED
INFLATES B=&
DIS~DGES DUST
8
COLLECTED DUST
FABRICFILERWITHPULSE
J=CLEANINGANDCYLINDRICALBAGS
Fig.5

10. 01RTY
FLTER BAG—
BAGSUPPOllT—
FILTERBAGWITH
DUSTLAYER
(FILTER CAKE)
BUILDING UP.
FILTERBAGBEING ,
CLEANED AIRFLOW
BRIEFLY REVERSED
lNFLA~S BAG&
DKLODGES DUST.
pox
--RT
&
COUECTED DUST
f
“i
,
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1...:....!...-...-.-+. :::..-.l.;..,....-
J1’
‘..~>.-...
-7..:.
* ““””--..-.-”.‘.:%-
/’
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:’>;’ ‘
~ CLEANED GAS
—JETTUBE
AIRVALVES &
TIMERS
JETTUBEINJECTING
BURSTOFCOMPRESSED
AIRINTOFILTERBAG
FABRICFILTERWITHPULSEJETCLEAN!NGANDFLATBAGS.
Fig.

11. I
i
COMPLETE F[LTER[
I
kLEANING CYCLE 1
I
I I
PRESSURE
DROP
~3RD. SECTION CLEANEO
I
-2ND. SECTION CLEANED
fs7.SECTION CLEANED
TIME
SECTIONALISED CONTINUOUSLY RATEDF[LTER
PRESSURE
DROP
TIME
CONTINUOUSLY RATED FILTER
PRESSURE DROP VARIATION
WITHFILTER CAKE BUILD UP
.
Fig. 7
9

12. TemoeratureMmitatiomandcknkalresistanceoffilterfabrics
KeytoChemicaIResistance:-
Notverygood Good Excellent
Fig.8
10

13. 4. TEMPEMTURE LIMITATIONS
Twoofthemostimportantfactorsindeterminingthelifeandefficiencyofafilterare
thechoiceofthecorrecttypeoffibreandhowitiswovenintoafabric.Theseare
normallychosenaccordingtothetypeofdusttobefilteredandtheoperating
temperatureandnatureofthegasbeingtreated.Themaximumtemperaturesatwhich
variousfiltrationmaterialscanbeoperatedcontinuouslyareshowninFigure8.
Minortemperatureexcursionsabovethesevaluesmaybetolerated,butfabriclifewould
bereduced.Significantincreasesintemperatureabovetheselevelswouldresultin
damagetothefiltrationmaterial.Inthecaseofglassfibre,whichisgenerallysilicone
treated,thiscoatingdecomposes.Oncethishashappenedthefibresrubagainstone
anotherduringthecleaningcycleandmechanicalfailurequicklyfollows.Tolimit
operatingtemperaturestosafevalues,itissometimesnecessarytoprovide
automaticallycontrolledfreshairinletsorwaterspraysystems.
Conversely,excessivelylowtemperaturescanalsoinfluencethelifeofthefabric,since
suchconditionsareconducivetocondensationofacidsoralkalisonthefabric.
Condensationcanalsocausethedusttoadheresostronglytothefabricthatthe
cleaningdeviceisunabletoremoveit.Thisrapidlyleadstocompleteblindingofthe
fabricandthenecessityforitsreplacement.
ThechemicalresistanceofvariousfiltrationmaterialsisalsoshowninFigure8.The
chemicalresistancesshownarebasedondrygasconditions.Whenwatervapouris
present,degradationofsusceptiblefabricswillbeaccelerated.
11

14. 5. BAGFILTERSIZING
5.1FiltrationVelocity >* .
?’
Thisisthevelocityofthedustanditscarriergasclosetothesurfaceofthefilter,
fabric.Itisthevalueofthegasflowratedividedbytheareaoffilterclothsurface,
throughwhichitpasses.
Filtrationvelocity,orairtoclothratio,dictatesthesizeoffiltrationareanecess~
foraparticularvolumeflowrateofgas.Thetypeoffabricanditscleaningmechanism
limitstherangeoffiltrationvelocitiesthatcanbeachievedbythatparticularunit.
Table1givesbasevaluesofairtoclothratiosforvarioustypesoffilterfor%ormal
“dusts.Thesevaluesrelatetoordinarytypesofdustinmoderateconcentrationsfor
“normaltfapplication.ThesevaluesmaybeincreasedbyUpto10%whenthedustis
knowntobeeasytofilter.Anexampleofthiswouldbeclinkerdustwhichisgenerally
coarselysized.Thesevaluesshouldbereducedbyupto20%for“difficult!?dusts.Fine
dustssuchascoaldust,alkali-enrichedfluedustandadditivessuchassilicafumeare
examplesofdifficultdusts.
TABLE1:BaseValuesofAirtoClothRatioforVariousTvtwsofFilterPlant
for“Normal”Dus@
RangeofBase
TvDeofFabricFilter ;ValuesofA/C
i.e.MethodofSelf-Cleaning ProtrietarvExamnle metres/minut~
Mechanicalshaker visco-Beth; 0.65to1.0
Spencer-Halstead
Mechanicalshakerwithlow Visco-Beth; 0.75to1.0
pressurereverseair Norblo
Mediumpressurereverseair SIMLuhq 1.2
Mediumpressurepulsating Luhr 1.8
reverseair
Highpressurereversejet
(a)Envelopebags DCE 1.5
(b)Cylindricalbags<3mlong AirmasteqMikroPul
(c)Cylindricalbags>3mlong
1.8
Cibel,AAF,Flakt,Joy,
First:.3m GBE,etc 1.8*
Next3m 100*
>
I
*Valueforillustrationonly;dependheavilyupondetailsofairpurgesystem.
12’”

15. 5.2EstimatingDedustingAirFlowRates
Therecommendedreferenceonthissubjectis“IndustrialVentilation”publishedbythe
AmericanConferenceofGovernmentHygienists.Someguidelinevaluesaresummarised
below;
Beltconveyortransfers:323cfinperfootofbeltwidthforbeltspeeds
<or=3.3ft/sec.
Beltwipers: 215cfinperfootofbeltwidth.
Vibratingscreens: 66cfrnpersquarefootofscreenarea.
6. CHOICEOFCORRECTFABRICFORAPPLICATION
Table2indicateswhatfiltrationmaterialshavebeenfoundtoperformbestindifferent
applicationswithinthecementmanufacturingprocess.Thebasefiltrationvelocitieshavealso
beenindicatedforeachapplication.Gore-Texfabricsandtheir“lookalikes”appeartobeable
tooperateathighfiltrationvelocities.Howeverthesurfacesofthesefabricsareverydelicate,
andathighgasvelocitiesmaybeeroded.Thefabricpropertywouldthenreverttothatofthe
basefabric,anormalneedlefeltedmedium.Thesefabricsdohaveexcellentdustrelease
propertiesandshouldbeusedwheredustreleaseisaproblem.
13

16. TABLE2:TheRightFabricfortheRightDust
(SubjecttoTemperatureLimitations)
“BASE” VALUE OF
DUST/PROCESS FABRIC AIRTOCLOTH RATIO*
(std/min) (R/rein)
Cementtransportsystems PP;PE 1.5 4.9
Cementrawmaterials PE;NX 1.5 4.9
Whiting(CaCO~) Dry:PE;moist:DT 1.25 4.1
KilnBEDusttransport Dry:PPorPE 1.25 4.1
Enrichedalkaliprecip-dust PP;possiblyDT;NX 1.25 4.1
Clinkertransport PEorNX 1.5 4.9
Clinkercoolerwasteair NXorotherhightemp 1.5 4.9
fabric
Clinkercoolerwasteairwith PEorNXasdesign 1.65 5.4
heatexchanger
.———
Furnacegases GlassNF;PTFE;Ryton 1.4;1.5;1.5? 4.6,4.9,4.9
Rawmealtransport PP;PE 1.4tol.5 4.6to4.9
CoalPFordryrawcoal PE;DT;PEAV600 1.25 4.1
Coalmill (Epitropicor+5%SS)
KilnBEgases Wovenglass;?Tefaire 0.65to0.9;? 2.1to2.95
NXquestionable,has
beenused 1.5 4.9
Additives,extenders,
Limestone,Gypsum PP;PE 1.5 4.9
CAF2,SiO~,fime PP;PE 1.25 4.1
Cement/Rawmill 1.0 3.2
Higheffeciciencyseparator
filter
Cement/Rawmillventfilters 1.2 3.9
r
*
AirtoclothratioforDCEtypefilter.Adduptoapproximately20%forpulsejetfilterswith
cylindricalbags<3mlong(seeTable1).
Key:PP=PolypropylenePE=PolyesterDT=DralonTNX=Nomex
PEAV600=specialfabric,donotspeci@withoutfintheradvice
SS=stainlesssteelfibreNF=Needlefelt

17. ‘7.TROUBLESHOOTING
Ifafilterisconsistentlyfailingforwhateverreasonitisworthwhileobtainingthe
originaldesigndataandcomparingthiswiththecurrentoperatingconditions.Several
modificationsmayhavebeencarriedoutovertheyearsontheplantbeingde-dusted
andthesecouldhavedrasticallychangedthefilterduty.
Increasedemissionlevelsareusuallycausedbybrokenfilterbags.Iftheincreased
emissionlevelhasbeenindicatedbyadustmonitorandnotvisually,itwouldbe
worthwhilefirstcheckingtheemissionvisuallyifthisispossible.Ifthisisnotpossible,
theoperationofthedustmonitorshouldthenbechecked.Thismayrequirethatadust
emissiontestbecarriedouttochecktheaccuracyofthemonitor.
Insmallerfiltersbrokenbagsareusuailylocatedbycheckingeachindividualbag.This
wouldbeaveryarduoustaskhoweveronalargerfilter.Forfilterswithlong
cylindricalbagssuspendedfromatubesheet,abrokenbagcansometimesbedetected
byapileofdustontopofthetubesheetnexttothebrokenbag.Itistherefore
importanttocleanthetubesheetaftereachmaintenance.Forolderfilterswithbags
thatarenotsupportedbytubesheetsthetaskcanbemorearduous.ItispossibletO
locatethecompartmentorcompartmentsthathavebrokenbagsbyselectivelyisolating
eachcompartmentinturnandnoticingthechangeindustemission,especiallyifadust
monitorhasbeeninstalled.
Increaseddustemissionscanalsobecausedbyleaksinthetubesheetorinternal
chambers.Unlessthecrackorholeisrelativelylargethelocationsoftheseleaksare
notalwayseasytofind.MakinguseoffluorescentpowderandaUVlampcangreatly
assistinlocatingtheleaks.Afewkilogramsoffluorescentpowderareintroducedin
totheintakeofthefilterwhilstitisinoperation.Thefilteristhenrunforafew
minutestoallowthepowdertoworkitswaythrough.Thefilteristhenstoppedand
inspectedinternallywithaWlamp.
Increasedpressuredropacrossafilterisusuallycausedbyblindedbags.Ifthepressure
dropsuddenlyincreasesorreduces,asimilarchangeontheexhaustfancurrentdrawn
mayalsobeobserved.Ifthisisnotobservedandthedustemissionhasnotincreased,
thenthepressuretappingsshouldbecheckedtoseeiftheyareblocked.Blindedbags
usuallyresultfromproblemswiththecleaningmechanism.Thiscouldresultfromaloss
incompressedairpressureforpulse-jetfilters.Forproductcollectingfilterson
cementmilIsitisnormaltointeriockthecompressedairsupplylinepressuretomill
operation.Iftheairpressuredropsthemillistrippedout.Lowairpressureapartfrom
compressorfauits,canbecausedbyfaultywatertrapswhichhaveresultedintheline
filtersblocking.Excessiveoilorwaterentrainedintheairisoftenthecauseoffailure
oftheairmanagementsystemandisanindicationoffaultycompressoroperation.
Blindedbagscanalsoresultfromoperatingbelowthedewpointofthegasresulting
incondensationonthebags,whichcanrenderthebagcleaningdeviceineffective.Poor
gasdistributionthroughafiltercanalsobedetrimentaltoitsoperationwithhighflow
areascausingre-entrain.mentofthedustandexcessivepressurelossacrossthefilter.
15

18. Shortbaglifecanbecausedbypoorgasdistribution.Areaswithhighgasvelocitiescan
resultinrapidbagwearduetoexcessiveimpingementofdustonthebags.Highgas
velocitiescancauseattritionbetweenindividualbagsalsoresultinginwear.
Shortbaglifecanresultfromincorrectfabricchoicefortheapplication;highgas
temperaturesandchemicalattackarealsocausesofprematurefailures.
8. COMMENTS ONAPPLICATION
Asmentionedabovemajorproblemscanresultifcondensationoccwsleadingtoblinding
ofafabric.Maintaininggastemperaturesbelowtheratingofthefilterfabricisalso
importanttoavoiditbeingoverheated.Thesefactorsmustbeborneinmindwhen
decidingwhetherornotafabricfiltershouldbeusedtode-dustthegasesfromany
particularprocess.Itispossible,thoughnotnecessarilypracticable,toalterthe
conditionofunsuitablegasesiftheuseofafabricfilterisessential.
WhenthemoisturecontentofthegasesishighatrelativelyIOWtemperatures,asisthe
casewiththeexhauststreamsfromwetandsemi-dryprocesskilns,anelectrostatic
precipitatorwouldbetheobviousfirstchoice.Afabricfiltercouldbeusedif
supplementaryheaterswereinstalledinordertopre-warmthefilter.
Inthecaseofdryprocess,suspensionpreheaterorprecalcinerkilns,thewastegasesare
naturallydryandatfirstsightmightseemtobesuitedforafabricfilter.However,
thetemperatureofthesegasesistoohighfortheuseofbagfiItersandcoolingwould
benecessary.Thisisbestcarriedoutbytheevaporationofwaterintothehotgases
inaconditioningtower.Theincreasedmoisturecontentoftiegasmakesitmore
favorabletouseelectrostaticprecipitation.Afurtherfactortosupportthisarises
whenuseismadeofthewastegasesinthemilling/dryingcircuit.Contactwiththeraw
materialsincreasesitsmoisturecontentandreducesthegastemperature.
Electrostaticprecipitatorsarethepreferredequipmentfordustremovalfromkiln
wastegasesinU.K.andmostofEurope.ThisisnotthecaseintheU.S.A.,where
fabricfiltersoncementkilnexhaustsaremuchmorecommonplace. Thereasonsfor
thisweremainlypoliticalwhentherewereseriousairqualityproblemsintheLehigh
Valleyregionandothers.OtherpossiblereasonsmaybeahistoryintheU.S.A.ofbadly
designedelectrostaticprecipitators,whichgaverisetotheimpressionthathigh
efficiencygascleaningcouldnotbeachievedbyelectrostaticprecipitators.
Theinstallationoflargefabricfiltersentaillowercapitalcoststhanelectrostatic
precipitators(althoughrunningcostsarehigher).
Insomecasesthechimney,orexhauststack,canbedispensedwith.
Thelatterpointisofparticularinterestsince,forexample,intheU.K.theAlkali
Inspectoratedemandthatthewastegasesbeexhaustedtoatmosphereviaanexhaust
stackofadefinedheight.IntheU.S.A.louvreopeningsintheroofofthefilterhousing
16

19. arecurrentlyacceptable.Ithasbeensuggestedthatthelouvredischargesystem
facilitatesthelocationofafaultybag,whereaswhenachimneyiSusedthetaskismore
difficult.ThisislikelytochangeasnewenvironmentallegislationintheU.S.A.
requiresexhauststackstobeinstalledonexistingandnewbagfilterinstallations.This
istoenablethewholeexhauststreamtobemeasured.
AlargequantityofwaterisrequiredtocoolthegasesfromadI’Yprocesskiln(about
200gofwaterperkgofclinker)andinsomepartsoftheworldsuchquantitiesarenot
available.Electrostaticprecipitationcanbeextremelydifficultkthesecircumstances
(duetohighdustresistivity)andafabricfilterthencouldbeconsidered.Itssize
howeverwouldbeexcessiveasthegaswouldbecooledbyambientairandthusresult
inanincreaseinthequantityofgastobetreated.Thefiltermedium,whichis
invariablyglassfibreforsuchapplications,demandsalowfikrationvelocityfor
satisfactoryoperation-typically0.5-0.6metresperminuteandthisalsodictatesa
largesizedfilterplant.
Thewasteairstreamfromagratetypeofclinkercoolerisvery(h’yandtheresistivity
ofthedustsisgenerallyhigh.Thegastemperatureofthisstreamistypicallyabout
300*Cbutthiscanincreaseto500°Cduringakilnflush.Toenabletheuseofafilter
fittedwithNomexorpolyesterneedlefeltbagsamethodofcoolingthegasisrequired.
Gaswascooledinthepastusingwaterspraysbutmostmoderninstallationsincorporate
anairtoairheatexchanger.Acoldairbleedmayalsobeincorporatedinthecircuit
foremergencyuseduringakilnflush.Thecoarsenatureofthedustpermitsafiltration
velocityofabout1.5m/min,thusmakingthefilterrelativelycompact.
Experiencewithwaterspraysystemsonexistingclinkercoolerapplicationshasnotbeen
encouragingandwherefabricfiltershavebeenused,temper~turecontrolbythe
automaticintroductionoffreshairhasbeenoptedfor.
Fabricfiltersandelectrostaticprecipitatorshavebothbeenusedtode-dustcementmill
exhauststreamsformanyyears.Thetrendistowardslargerclosedcircuitmilling
operationswithseparatemillandseparatorventilationcircuits.Freshfeedtothemill
ispartiallycooledbythecoarsereturnsfromtheseparator.Thistogetherwith
improvedmillventilationresultsinlesscoolingwaterbeingrequiredduringthemilling
process.Hencemostrecentcementmillinstallationshaveoptedforbagfilterstode-
dustthemillandseparatorcircuitsinsteadofprecipitators.
Thefabricfilterfindsitsgreatestapplication,inthecementmanufacturingprocess,
intheremovalofdustfromambientair.Examplesoftheseareatconveyortransfer
points,onrailwagontipplers,de-dustingloadingchutesandventingsilos.Allthese
applicationscanbesuccessfullyde-dustedwithcorrectlysizedfilters.Problemshave
beenencounteredde-dustingclinkerconveyorsduetothepassageintothefilterof
glowingparticies,whichburnthefilterelements.Asatisfactorysolutionwouldseem
17

20. <
FIGURE9
18

21. tobetheinstallationofaninertialcollectorbeforethe
glowingparticlesbeforetheyenterthefilter.Ceramic
forthisapplication.
filterinordertoremovethe
fibrefiltersaretobetested
9. RECENT DEVELOPMENTS
DCELtdandNeuEngineeringLimitedmanufacturearigidself-supportingelement
whichcanalsoberetrofittedtoanexistingDalamatictypefilterorinstalledinnew
filters.AnelementandthewayitisinstalledinafilterisshowninFigure9.These
elementsaremouldedfromsinteredplasticgranulesandhaveaprofiledoutersurface
whichistreatedwithapermeablecoatingofPTFE.
Thedutyofeachmoduleisgreaterthanasimilarsizedfabricfilterduetothe
increasedsurfaceareadevelopedbytheprofiling.Thebasefiltrationvelocityforthese
elementsstillremainsat1.5m/min.
Atpresentthefiltermediumislimitedtoamaximumoperatingtemperatureof60°C.
Themanufacturersarecurrentiylookingatmethodsofraisingthisoperating
temperature.
Thereisagrowingnumberofareaswheresinteredceramicfibrefilterelementsmay
haveapplicationswithinthecementindus~.Thesefilterelementsaresuitedtovery
hightemperatureapplicationsandthereforedonotrequireprotectioninthesameway
asabagfilter.Theirdisadvantagesareprimarilythehighcostoftheindividual
elements,therelativelysmalldimensionsoftheindividualfilterelementsandthehigh
costoftheresultingfilterunit.Furtherdevelopmentsinthisareamaychangethe
economicviabilityofthistypeoffilterforhightemperatureapplications.

22. Appendmi
3.HoodDesign
Oncetheprocessesofidentificationand
quantificationhavebeencarriedo@adust
controlengineermayplanhiscampaignboth
fromtheengineeringandeconomicviewpoint
Rarelycanaparticdardustsourcebe
completelyeliminated,althoughthedust
controlengineerandtheprocessengineer
shouldconsiderwhetheranychangeof
productiontechniquecanminim-ifnot
eliminate,theproblem.Thereductionofa
particularemissionsourcebyeither
suppressionorcontainmentis,inpractice,
oftenpossibleandusuallyrepaysinvestigation.
Thenextstepistodesigntheexhaust
enclosure.Formulaeforhooddesigndoexis4
althoughexperiencecountsforagreatdealin
theirapplication.Thestartingpointforahood
designcalculationisdeterminingtheemission
rateorvelocityoftheliberatedd-Fromthis
acapturevelocitymaybedecideduponwhich
willalsobeinfluencedbythetypeofdust
FinaIfythesitingofthecapture~ from
whencethecaptureVdOcityisproduced,must
alsobetakenintoconsideration.
Unfortunatdyd]toooftentheeconomicand
engineeringirnpo~nce oftheavailable*
regardi~thesitingofCX&UXXhoodsiseither
ignoredorcompletelymisunderstoodby-
ofthoseconcernedinthespecificationand
purchaseofdustcontroIplantThefbllowing
briefexcursionintothefieldofhtid~
mayhelpinkktif@g the-m~
Muchoftheavaiktkdata-tothesiting
ofexhausthoodsisbasedonw*atiti
inthe1930’sbyDaIlaWkandnearly50yeZWS
laterbyFletckBymeasuringcontour
Velocitiesinfiontofaninletm formulacan
bederivedforthecentreiineairflow
relatbmhip.Fromtheseformulaethe~
onthecentrelineinfrontofahood~
expressedasapcmtage ofthehoodface
velocityReferencetdg.loshowsthe
terminoiogyusedinthevariousformulae
FIGURE10
FaceareaX=Wx‘U Facevekityistheaverage
Equivalent
exeftedoverfaceofhood-%
diameter‘D*
Emission
veloc@
J
XWXL
47
w
~conveyingvelocity
=Q
\‘:..:../ Areaofduct
-.... k
l,,.---:-------
_,;~u;;,,.
,.:...,-
source$~..-
..........
y.:”..‘:.“.........:.
/....:-:“.:”;..’.
lblumeQ=lAxface
“..:<,....:..
.?.,:.“\
j...
i
-
Di&ncefromdustsource
capture tohoodface=’X
Veiocity‘v
Velocii

23. AppendixzCont.
TheformuiaofDallaVaile,(fig.11),isarelatively
sirnpieone.Althoughsatisfactorywherethe
hoodmouthiseitkcircuiarorsquareit
shouldnotbeusedwhenthehoodmouthis
~guiar andwheretheaspectradioisany
otherthanonetoone.
Cakuiationfortherequiredvolumeofair
forroundorsquarehinds,accordingto
DallaVaile:-
Q=V(10X2+A)
Where:
Q=Quantityofair
V=Thecapturevelocityatthedust
X=Thedistancefromhoodmouthto
dustsource
A=Theopenfaceareaofthehood
Fig.11FormulaofDallaVaIle
l
Fletcher’sformula,whichismorecompiex
doeshowevertakeintoregardvarying_
ratiosanduseofhisnomogram,(fig.12),will
giveamuchmoreaccurateresdtforanygiven
problem.
HoweverthesimplerDaUaVaUeequationcan
providethepracticalengineerwithan
immediateindicationastothelikelyc-
anexperienceddustcontmiengineercan
makeanaccuratepredictionastotheoverall
coUectionefficiencyoftheckvice.Thecloser
thehoodistothedwgenerationPointthe
moreeconomicalthesystemis,,andgenerally
anycapturehoodthatissitedmorethan0.7
diametersawayfromthedustsourcecouldbe
regardedaspoorlypositionedand
uneconomic.Todemonstrateinrealtermsthe
inmiicatioriofthisfactozthefollowingexample
(fig.13)showsthedifferenceinairtilumes
requiredforthesamecollectionproblem
fortwoalternativedisticesbetweenhood
anddustsource.
Thenecessaryairvolumewhenahoodof
400mmdia.isplaced320mmfroma
dustsourcewhereacapturevelocityof
150mperminuteisrequiredis-
Q=V(10X2+A)-DaUaWle
=150(10x0322+rrx022)
=1723m3permm.
Kthesamehoodisnowm_ed
200mmfromthedustsourceandthe
__~@~_~e~
IRAlmebecomes
Q=15010x022+rrx O~)
!l
=79mper*
vekityofanexhaust~tiba~n [email protected] CakuMonsallowingimportanced
positionrelativetothedustSOUME.F-*
---
LOO~
,
1
0s~
0.4-
03-
02-
o.ls-
0.10-
0.07-
.
0.06-
O.M“
0.03-
O.oa-
Om-
0.015+
0.01“
.
.
Oms~ X=Distancefromfaceofhoodtodustsource
A=Openareaofhoodface
-0.!0
-0.4
-030
-1.00
A
s
P
E
c
T