Bag Filters

23
3 3 5 5 BAG FILTERS J RUSHWORTH

Transcript of Bag Filters

Page 1: Bag Filters

3355

BAG FILTERS

J RUSHWORTH

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BAG FILTERS

CONTENTS

INTRODUCTION

THE MECHANXSMOF PARTICLE CAPTURE

CLEANING MEITIODS

TEMPERATURE LIMITATIONS

BAG FILTER SIZING

5.1 FiltrationVelocity5.2 EstimatingDedustingAir Flow Rates

CHOICE OF CORRECT FABRIC FOR APPLICATION

TROUBLE SHOOTING

COMMENTS ON APPLICATION

RECENT DEVELOPMENTS

Appendix 1: Hmd Design

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1. INTRODUCTION

Fabric filtration has been applied for many years on both industrial and domestic fronts.In essence, a dust bearing gas is intercepted by a permeable fabric in such a mannerthat all the gas passes through the fabric whilst the dust impinges on the fibre of thefabric and is thereby retained.

As the dust accumulates on the fabric a ‘cake’ is formed, which aids filtration byimproving particle capture and improves the collection efficiency. At the same the,however, the resistance to gas flow increases and in order to maintain the same gasflow rate as at start Up the system fan has to work harder.

When the resistance to gas flow reaches an unacceptable level, the fabric has to becleaned to dislodge the cake. The pressure drop across the fabric will always be greaterthan the initial value, that is with new fabric, because some of the dust particles willbecome permanently lodged in the fabric. Provided steady state conditions between thefabric and the quantity of trapped dust is reached in a reasonably short time the effectis beneficial, but if the quantity of trapped dust increases after every cleaning cycle,then ultimately bIinding will occur.

2. THE MECHANISM OF PARTICLE CAPTURE

The filtration process is extremely compkx and invohms a combination of impaction,diffusion, thermal, molecular and electrostatic forces. Of these, the most importantare:-

ImRaction - which occurs when a particle, because of its momentum, crosses thefluid streamlines and strikes a fibre. The larger the particle and the smaller thefibre, the greater are the chances of impaction by particle inertia.

Diffusion - which is the primary collection mechanism for particles below 0.5micron.

Electrostatic Forces - which affect particles below 0.5 micron.

The early stages of filtration occur with the capture of individual particles bysingle fibres as a result of any combination of the above mentioned mechanisms.The particles which deposit on fibres projecting into the gas stream then act asadditional sites for the capture of further particles and eventually chain likeaggregates r-ult. As the process continues, a complete matrix, or cake, of dustparticles is built up until finally particle capture is effected by true surfacefiltration, or sieving, and the function of the fabric, apart from acting as asupport, becomes nominal. Following a cleaning action, further particles in thegas stream attach themselves to particles which have remained on the fibres andthe cake building process recommences.

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Fibres used in the manufacture of fabrics for filtration are almost exclusively syntheticand they are either woven or needle felted - see Figure 1. Woven fabrics are smootherand more easily cleaned than felts and sometimes, at low loads, no cleaning devices areneeded because the fabric is self cleaning. On the other hand they often camot becleaned too vigorously because this would break down the entire dust cake and force thedust between the fibres so that the dust emission would be high. Needle felts are lesspermeable than woven fabrics, but they can be operated at considerably higher filtrationvelocities. The pores in needle felts are very small compared with woven fabrics, sodust penetration is low.

Generally, the filter elements, whether of woven or felted fabric, are cylindrical, butsome manufacturers have adopted flat panel, or envelope elements.

3. CLEANING MEITIODS

The removal of the accumulated layer of dust from the filter fabric can be achieved inmany ways including collapse of the filter element, mechanical shaking, reverse airflow, reverse air pulse and reverse air jet. Any one, or combination of these methodsmay be employed but, as a generalisation, the reverse air pulse and reverse air jet areusually associated only with filters having needle felted elements.

Cleaning by collapse of the fiker element - see Figure 2 is a method used when thefabric is relatively weak, as is the case with glass fibre, and when cake release isrelatively easy. Stronger fabrics and the necessity for a more vigorous action in orderto dislodge the cake leads to shake cleaning, often with the assistance of a reverse airflow, see Figure 3.

During the collapse of the filter ekment, or the shake or reverse air period, the gasflow must be stopped in order to allow the dust cake to fall away from the fabric.Thus, a filter plant must be made up of a sufficient number of separate compartments,each containing a group of filter elements, to allow one compartment to be taken outof service at a time for cleaning. If there are only a few compartments in the filter,then taking one off stream will markedly increase the flow, and consequently thepressure drop across the others, and this factor must be taken into account at the designstage.

With reverse air pulse cleaning, moderately pressurised air from a secondary blower isintroduced into the element, often by means of a traveling nozzle (refer to Figure 4).The reverse air jet method utilises a high pressure jet of air which is injected into theelement for a time intwwai of about 0.1 second - see Figures 5 and 6. Cake release isaccomplished by a combination of fabric deformation, due to the shock of air blast, andflow reversal. Both cleaning methods remove the dust with only a brief interruption inthe gas flow and both invariably use needle felt fabrics.

Figure 7 shows the relationship between pressure drop and time both for a sectionalisedcontinuously rated filter and for a filter of the reverse air puise/jet type.

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WOVEN CLOTH

10 TIMESF PARTICLES

NEEDLE FELT

CROSS SECTION OFWOVEN AND FELTED FILTER FABRIC

Fig.

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f

‘4 :,

. ;V). .;. .

.’

. .“,. h..* .... .,

t y ‘.-.... ....,.. ..-..:.

1~! ...

.,. . ..“. . -.

. ... #

t

COLLAPSING:AN

WI

l!=~~ AIR/CLEANED—

Ku I

**.. ...*. .....!

#

I

%...-., .

‘..=*:

J .,

-. . ,~

. . . . . . . . . .’ ..41 s.

FAN CLOSED4

COLECTED DUST

FILTERING

DIRTY

GAS

DIRTY GAS

OUTLET IO

COLLAPSINGFAN OPEN

+

COLLECTED DUST

CLEANING

~LLAPSINGFAN

\ DIRTY\ AIR TO

~

+OTHER

! smoNs

y....p.:,” , :‘1,., ..<.:. >

FABRIC FILTER WITH COLLAPSE BAG CLEANING

Fig. 2

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REVERSEAll? FAN,

BAG SHAKING/DEvlcE-OFF

99 -0

REVERSE AIRINLET cLOSED

..

CLEANED GASOUTLET OPEN $ : ;$,.

., + .. .

t ?t.

f I y:...... .. . ...

4 ●< -.t.. . 1.*.. .. :.... :

.. - .. ,%“. ..... .. -1:4~: : A,.●. .! #-“ .

,“● <

DIRTY

GAS

C&EANED

GAS

REVERSEAIR FAN \

BAG SHAKINGDEVICE -ON\

Am1-

/REVERSE AIR-INLET OPENCLEANED GASOUTLET CmSED

---- F .-7 .& : h .-. .

DIRTY AIR TO~)+ER +

SECTIONS

COLLECTED DUST

FILTERINGCOLLECTED OUST

CLEANING

FABRIC FILTER WITH

SHAKE AND REVERSE AIR CLEANING

Fig. 3

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,REVERSE AIR

CLEANED GAS*

DIRTY GAS +

wb

FILTER BAG WITH_DUST LAYER

(RLTER CAK=BUILDING UP

BAG SUPPORT~

1!

!-. . . .

FAN

/TRAVELLING

AIR TUBE

- FILTER BE BEINGCLEANED AIRW

BRIEF LYREVERSEDINFLATES BAG &O\ SLODGES DUST

COLLECTED DUST

FABRIC FILTER WITH

PULSE AIR CLEANING AND CYLINDRICAL BAGS

F@ 4

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CLEANED GAS ~

DIRTY GAS e

FILTER BAG WITHDUST LAYERCFILTER CAKE)8UILDING UP

BAG SUPP9RT—

DIAPHRAGM VALVES

~.H=~ ‘iR

-—

- (Co 100 R S.I.)

l--JET TUBEINJECTI ?4GBURST OFCOMPRESSED -

5 AIR INTO‘FILTER BAG

1- PILOT VALVES&/OR TIMERS

FILTER BAGBEINGCLEANED AIRmWBRIEFLY REVERSEDINFLATES B=&DIS~DGES DUST

8COLLECTED DUST

FABRIC FILER WITH PULSEJ= CLEANING AND CYLINDRICAL BAGS

Fig. 5

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01 RT Y

F LTER BAG —

BAG SUP POllT—

FILTER BAG WITHDUST LAYER

(FILTER CAKE)

BUILDING UP.

FILTER BAG BEING ,

CLEANED AIRFLOW

BRIEFLY REVERSEDlNFLA~S BAG &DKLODGES DUST.

pox

--RT

&COUECTED DUST

f

“i,

4

1 . . . :. . . . !. ..-... -.-+. : : :..-. l.; ..,....-

J1’

‘..~>.-...-7..:.* ““””--..-.-”. ‘.:%-/’ I:’> ;’ ‘

~ CLEANED GAS

—JETTUBE

AIR VALVES &

TIMERS

JET TUBE INJECTING

BURST OF COMPRESSED

AIR INTO FILTER BAG

FABRIC FILTER WITH PULSE JET CLEAN!NG AND FLAT BAGS.

Fig.

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I iCOMPLETE F[LTER [

IkLEANING CYCLE 1

II I

PRESSURE

DROP~3RD. SECTION CLEANEO

I -2ND. SECTION CLEANED

fs7. SECTION CLEANED

TIME

SECTIONALISED CONTINUOUSLY RATED F[LTE R

PRESSURE

DROP

TIME

CONTINUOUSLY RATED FILTER

PRESSURE DROP VARIATION

WITH FILTER CAKE BUILD UP.

Fig. 7

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Temoerature Mmitatiom and cknkal res istance of filter fabrics

Key to ChemicaI Resistance:-

Not very good Good Excellent

Fig. 8

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4. TEMPEMTURE LIMITATIONS

Two of the most important factors in determining the life and efficiency of a filter arethe choice of the correct type of f ibre and how it is woven into a fabric. These arenormally chosen according to the type of dust to be filtered and the operatingtemperature and nature of the gas being treated. The maximum temperatures at whichvarious filtration materials can be operated continuously are shown in Figure 8.

Minor temperature excursions above these values may be tolerated, but fabric life wouldbe reduced. Significant increases in temperature above these levels would result indamage to the filtration material. In the case of glass f ibre, which is generally siliconetreated, this coating decomposes. Once this has happened the fibres rub against oneanother during the cleaning cycle and mechanical failure quickly follows. To limitoperating temperatures to safe values, it is sometimes necessary to provideautomatically controlled fresh air inlets or water spray systems.

Conversely, excessively low temperatures can also influence the life of the fabric, sincesuch conditions are conducive to condensation of acids or alkalis on the fabric.Condensation can also cause the dust to adhere so strongly to the fabric that thecleaning device is unable to remove it. This rapidly leads to complete blinding of thefabric and the necessity for its replacement.

The chemical resistance of various filtration materials is also shown in Figure 8. Thechemical resistances shown are based on dry gas conditions. When water vapour ispresent, degradation of susceptible fabrics will be accelerated.

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5. BAG FILTER SIZING

5.1 Filtration Velocity >* .?’

This is the velocity of the dust and its carrier gas close to the surface of the filter ,fabric. It is the value of the gas flow rate divided by the area of filter cloth surface ,through which it passes.

Filtration velocity, or air to cloth ratio, dictates the size of filtration area necess~for a particular volume flow rate of gas. The type of fabric and its cleaning mechanismlimits the range of filtration velocities that can be achieved by that particular unit.Table 1 gives base values of air to cloth ratios for various types of filter for %ormal“ dusts. These values relate to ordinary types of dust in moderate concentrations for“normaltf application. These values may be increased by Up to 10% when the dust isknown to be easy to filter. An example of this would be clinker dust which is generallycoarsely sized. These values should be reduced by up to 20% for “difficult!? dusts. Finedusts such as coal dust, alkali-enriched flue dust and additives such as silica fume areexamples of difficult dusts.

TABLE 1: Base Values of Air to Cloth Ratio for Various Tvtws of Filter Plantfor “Normal” Dus@

Rang e of BaseTvDe of Fabric Filter ; Values of A/C

i.e. Method of Self-Cleaning Protrietarv Examnle metres/minut~

Mechanical shaker visco-Beth; 0.65 to 1.0Spencer-Halstead

Mechanical shaker with low Visco-Beth; 0.75 to 1.0pressure reverse air Norblo

Medium pressure reverse air SIM Luhq 1.2

Medium pressure pulsating Luhr 1.8reverse air

High pressure reverse jet(a) Envelope bags DCE 1.5(b) Cylindrical bags <3m long Airmasteq MikroPul(c) Cylindrical bags >3m long

1.8Cibel, AAF, Flakt, Joy,

First :.3m GBE, etc 1.8*Next 3m 100*

>

I

* Value for illustration only; depend heavily upon details of air purge system.

12’”

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5.2 Estimating Dedusting Air Flow Rates

The recommended reference on this subject is “Industrial Ventilation” published by theAmerican Conference of Government Hygienists. Some guideline values are summarisedbelow;

Belt conveyor transfers: 323 cfin per foot of belt width for belt speeds< or = 3.3 ft/sec.

Belt wipers: 215 cfin per foot of belt width.

Vibrating screens: 66 cfrn per square foot of screen area.

6. CHOICE OF CORRECT FABRIC FOR APPLICATION

Table 2 indicates what filtration materials have been found to perform best in differentapplications within the cement manufacturing process. The base filtration velocities have alsobeen indicated for each application. Gore-Tex fabrics and their “lookalikes” appear to be ableto operate at high filtration velocities. However the surfaces of these fabrics are very delicate,and at high gas velocities may be eroded. The fabric property would then revert to that of thebase fabric, a normal needle felted medium. These fabrics do have excellent dust releaseproperties and should be used where dust release is a problem.

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TABLE 2: The Right Fabric for the Right Dust(Subject to Temperature Limitations)

“BASE” VALUE OFDUST/PROCESS FABRIC AIR TO CLOTH RATIO*

(std/min) (R/rein)

Cement transport systems PP; PE 1.5 4.9

Cement raw materials PE; NX 1.5 4.9

Whiting (CaCO~) Dry: PE; moist: DT 1.25 4.1

Kiln BE Dust transport Dry: PP or PE 1.25 4.1

Enriched alkali precip-dust PP; possibly DT; NX 1.25 4.1

Clinker transport PE or NX 1.5 4.9

Clinker cooler waste air NX or other high temp 1.5 4.9fabric

Clinker cooler waste air with PE or NX as design 1.65 5.4heat exchanger .—— —

Furnace gases Glass NF; PTFE; Ryton 1.4; 1.5; 1.5? 4.6,4 .9,4.9

Raw meal transport PP; PE 1.4tol.5 4.6 to 4.9

Coal PF or dry raw coal PE; DT; PEAV 600 1.25 4.1Coal mill (Epitropic or + 5% SS)

Kiln BE gases Woven glass; ?Tefaire 0.65 to 0.9; ? 2.1 to 2.95NX questionable, has

been used 1.5 4.9

Additives, extenders,Limestone, Gypsum PP; PE 1.5 4.9CAF2, SiO~, fime PP; PE 1.25 4.1

Cement/Raw mill 1.0 3.2High effeciciency separatorfilter

Cement/Raw mill vent filters 1.2 3.9r

* Air to cloth ratio for DCE type filter. Add up to approximately 20% for pulse jet filters withcylindrical bags < 3m long (see Table 1).

Key: PP = Polypropylene PE = Polyester DT = Dralon T NX = NomexPEAV 600 = special fabric, do not speci@ without finther adviceSS = stainless steel fibre NF = Needle felt

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‘7. TROUBLESHOOTING

If a filter is consistently failing for whatever reason it is worthwhile obtaining theoriginal design data and comparing this with the current operating conditions. Severalmodifications may have been carried out over the years on the plant being de-dustedand these could have drastically changed the filter duty.

Increased emission levels are usually caused by broken filter bags. If the increasedemission level has been indicated by a dust monitor and not visually, it would beworthwhile first checking the emission visually if this is possible. If this is not possible,the operation of the dust monitor should then be checked. This may require that a dustemission test be carried out to check the accuracy of the monitor.

In smaller filters broken bags are usuaily located by checking each individual bag. Thiswould be a very arduous task however on a larger filter. For filters with longcylindrical bags suspended from a tubesheet, a broken bag can sometimes be detectedby a pile of dust on top of the tube sheet next to the broken bag. It is thereforeimportant to clean the tubesheet after each maintenance. For older filters with bagsthat are not supported by tubesheets the task can be more arduous. It is possible tOlocate the compartment or compartments that have broken bags by selectively isolatingeach compartment in turn and noticing the change in dust emission, especially if a dustmonitor has been installed.

Increased dust emissions can also be caused by leaks in the tubesheet or internalchambers. Unless the crack or hole is relatively large the locations of these leaks arenot always easy to find. Making use of fluorescent powder and a UV lamp can greatlyassist in locating the leaks. A few kilograms of fluorescent powder are introduced into the intake of the filter whilst it is in operation. The filter is then run for a fewminutes to allow the powder to work its way through. The filter is then stopped andinspected internally with a W lamp.

Increased pressure drop across a filter is usually caused by blinded bags. If the pressuredrop suddenly increases or reduces, a similar change on the exhaust fan current drawnmay also be observed. If this is not observed and the dust emission has not increased,then the pressure tappings should be checked to see if they are blocked. Blinded bagsusually result from problems with the cleaning mechanism. This could result from a lossin compressed air pressure for pulse-jet filters. For product collecting filters oncement milIs it is normal to interiock the compressed air supply line pressure to milloperation. If the air pressure drops the mill is tripped out. Low air pressure apart fromcompressor fauits, can be caused by faulty water traps which have resulted in the linefilters blocking. Excessive oil or water entrained in the air is often the cause of failureof the air management system and is an indication of faulty compressor operation.

Blinded bags can also result from operating below the dew point of the gas resultingin condensation on the bags, which can render the bag cleaning device ineffective. Poorgas distribution through a filter can also be detrimental to its operation with high flowareas causing re-entrain.ment of the dust and excessive pressure loss across the filter.

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Short bag life can be caused by poor gas distribution. Areas with high gas velocities canresult in rapid bag wear due to excessive impingement of dust on the bags. High gasvelocities can cause attrition between individual bags also resulting in wear.

Short bag life can result from incorrect fabric choice for the application; high gastemperatures and chemical attack are also causes of premature failures.

8. COMMENTS ON APPLICATION

As mentioned above major problems can result if condensation occws leading to blindingof a fabric. Maintaining gas temperatures below the rating of the filter fabric is alsoimportant to avoid it being overheated. These factors must be borne in mind whendeciding whether or not a fabric filter should be used to de-dust the gases from anyparticular process. It is possible, though not necessarily practicable, to alter thecondition of unsuitable gases if the use of a fabric filter is essential.

When the moisture content of the gases is high at relatively IOW temperatures, as is thecase with the exhaust streams from wet and semi-dry process kilns, an electrostaticprecipitator would be the obvious first choice. A fabric filter could be used ifsupplementary heaters were installed in order to pre-warm the filter.

In the case of dry process, suspension preheater or precalciner kilns, the waste gases arenaturally dry and at first sight might seem to be suited for a fabric filter. However,the temperature of these gases is too high for the use of bag fiIters and cooling wouldbe necessary. This is best carried out by the evaporation of water into the hot gasesin a conditioning tower. The increased moisture content of tie gas makes it morefavorable to use electrostatic precipitation. A further factor to support this ariseswhen use is made of the waste gases in the milling/drying circuit. Contact with the rawmaterials increases its moisture content and reduces the gas temperature.

Electrostatic precipitators are the preferred equipment for dust removal from kilnwaste gases in U.K. and most of Europe. This is not the case in the U.S. A., wherefabric filters on cement kiln exhausts are much more commonplace. The reasons forthis were mainly political when there were serious air quality problems in the LehighValley region and others. Other possible reasons may be a history in the U.S.A. of badlydesigned electrostatic precipitators, which gave rise to the impression that highefficiency gas cleaning could not be achieved by electrostatic precipitators.

The installation of large fabric filters entail lower capital costs than electrostaticprecipitators (although running costs are higher).

In some cases the chimney, or exhaust stack, can be dispensed with.

The latter point is of particular interest since, for example, in the U.K. the AlkaliInspectorate demand that the waste gases be exhausted to atmosphere via an exhauststack of a defined height. In the U.S.A. louvre openings in the roof of the filter housing

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are currently acceptable. It has been suggested that the louvre discharge systemfacilitates the location of a faulty bag, whereas when a chimney iS used the task is moredifficult. This is likely to change as new environmental legislation in the U.S.A.requires exhaust stacks to be installed on existing and new bag filter installations. Thisis to enable the whole exhaust stream to be measured.

A large quantity of water is required to cool the gases from a dI’Yprocess kiln (about200g of water per kg of clinker) and in some parts of the world such quantities are notavailable. Electrostatic precipitation can be extremely difficult k these circumstances

(due to high dust resistivity) and a fabric filter then could be considered. Its sizehowever would be excessive as the gas would be cooled by ambient air and thus resultin an increase in the quantity of gas to be treated. The filter medium, which isinvariably glass fibre for such applications, demands a low fikration velocity forsatisfactory operation - typically 0.5 -0.6 metres per minute and this also dictates alarge sized filter plant.

The waste air stream from a grate type of clinker cooler is very (h’y and the resistivityof the dusts is generally high. The gas temperature of this stream is typically about300*C but this can increase to 500°C during a kiln flush. To enable the use of a filterfitted with Nomex or polyester needle felt bags a method of cooling the gas is required.Gas was cooled in the past using water sprays but most modern installations incorporatean air to air heat exchanger. A cold air bleed may also be incorporated in the circuitfor emergency use during a kiln flush. The coarse nature of the dust permits a filtrationvelocity of about 1.5 m/min, thus making the filter relatively compact.

Experience with water spray systems on existing clinker cooler applications has not beenencouraging and where fabric filters have been used, temper~ture control by theautomatic introduction of fresh air has been opted for.

Fabric filters and electrostatic precipitators have both been used to de-dust cement millexhaust streams for many years. The trend is towards larger closed circuit millingoperations with separate mill and separator ventilation circuits. Fresh feed to the millis partially cooled by the coarse returns from the separator. This together withimproved mill ventilation results in less cooling water being required during the millingprocess. Hence most recent cement mill installations have opted for bag filters to de-dust the mill and separator circuits instead of precipitators.

The fabric filter finds its greatest application, in the cement manufacturing process,in the removal of dust from ambient air. Examples of these are at conveyor transferpoints, on rail wagon tipplers, de-dusting loading chutes and venting silos. All theseapplications can be successfully de-dusted with correctly sized filters. Problems havebeen encountered de-dusting clinker conveyors due to the passage into the filter ofglowing particies, which burn the filter elements. A satisfactory solution would seem

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<

FIGURE 9

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to be the installation of an inertial collector before theglowing particles before they enter the filter. Ceramicfor this application.

filter in order to remove thefibre filters are to be tested

9. RECENT DEVELOPMENTS

DCE Ltd and Neu Engineering Limited manufacture a rigid self-supporting elementwhich can also be retrofitted to an existing Dalamatic type filter or installed in newfilters. An element and the way it is installed in a filter is shown in Figure 9. Theseelements are moulded from sintered plastic granules and have a profiled outer surfacewhich is treated with a permeable coating of PTFE.

The duty of each module is greater than a similar sized fabric filter due to theincreased surface area developed by the profiling. The base filtration velocity for theseelements still remains at 1.5 m/min.

At present the filter medium is limited to a maximum operating temperature of 60°C.The manufacturers are currentiy looking at methods of raising this operatingtemperature.

There is a growing number of areas where sintered ceramic fibre filter elements mayhave applications within the cement indus~. These filter elements are suited to veryhigh temperature applications and therefore do not require protection in the same wayas a bag filter. Their disadvantages are primarily the high cost of the individualelements, the relatively small dimensions of the individual filter elements and the highcost of the resulting filter unit. Further developments in this area may change theeconomic viability of this type of filter for high temperature applications.

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Appendmi

3. Hood Design

Once the processes of identification andquantification have been carried o@ a dustcontrol engineer may plan his campaign bothfrom the engineering and economic viewpoint

Rarely can a particdar dust source becompletely eliminated, although the dustcontrol engineer and the process engineershould consider whetherany change ofproduction technique can minim- if noteliminate, the problem. The reduction of aparticular emission source by eithersuppression or containment is, in practice,often possible and usually repays investigation.

The next step is to design the exhaustenclosure. Formulae for hood design do exis4although experience counts for a great deal intheir application. The starting point for a hooddesign calculation is determining the emissionrate or velocity of the liberated d- From thisa capture velocity maybe decided upon whichwillalso be influenced by the type of dustFinaIfythe siting of the capture ~ from

whence the capture VdOcity is produced, mustalso be taken into consideration.

Unfortunatdy d] too often the economic andengineering irnpo~nce of the available*regardi~ the siting of CX&UXX hoods is eitherignored or completely misunderstood by-of those concerned in the specificationandpurchase of dust controI plant The fbllowingbrief excursion intothefieldofhtid~mayhelpinkktif@g the-m~

Muchoftheavaiktkdata -tothesitingofexhausthoods is based onw*atitiin the 1930’s by DaIlaWk and nearly 50 yeZWSlaterby Fletck By measuring contourVelocities in fiontofaninletm formula canbederivedforthe centreiineairflowrelatbmhip. From these formulae the ~onthecentre lineinfrontofa hood~expressed asapcmtage of thehoodfacevelocity Reference tdg.loshowstheterminoiogyused inthevarious formulae

FIGURE 10

Face areaX = W x ‘U Facevekityisthe average

Equivalentexeftedoverfaceofhood-%

diameter‘D*

Emissionveloc@

JXWXL47

w ~ conveyingvelocity=Q

\ ‘: ..:../ Areaof duct-... . k●,,. ---:-------

_,; ~u; ;,,.,.:... , -

source$~..-. ... . .. ...y.:”..‘:.“.... .....: ./ ....:-: “.:”;..’. lblume Q = ●A x face“..:<,....:...?.,:.“\j...

i-

Di&nce fromdustsourcecapture tohoodface=’XVeiocity‘v

Velocii

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Appendix z Cont.

The formuia of DallaVaile,(fig. 11), is a relativelysirnpie one. Although satisfactory where thehood mouth is eitk circuiar or square itshould not be used when the hood mouth is~guiar and where the aspect radio is anyother than one to one.

Cakuiation for the required volume of air

for round or square hinds, according toDallaVaile:-

Q = V(10X2 + A)

Where:Q = Quantity of airV = The capture velocityat the dust

X = The distance from hood mouth todust source

A = The open face area of the hood

Fig. 11 Formulaof DallaVaIle●

Fletcher’s formula, which is more compiexdoes however take into regard varying _ratios and use of his nomogram, (fig. 12),willgivea much more accurate resdtfor anygivenproblem.Howeverthe simpler DaUaVaUeequation canprovide the practical engineer with animmediate indication as to the likelyc-

an experienced dust contmi engineer canmake an accurate prediction as to the overallcoUection efficiency of the ckvice.The closerthe hood isto the dw generation Point themore economical the system is,,and generallyany capture hood that is sited more than 0.7diameters away from the dust source could beregarded as poorly positioned anduneconomic. Todemonstrate in real terms theinmiicatioriof this factoz the followingexample(fig.13) shows the difference in air tilumesrequired for the same collection problemfor two alternative distices between hoodand dust source.

The necessary air volume when a hood of400mm dia. is placed 320mm from adust source where a capture velocity of150m per minute is required is-

Q = V(10X2+ A)- DaUaWle= 150(10 x 0322+ rr x 022 )= 1723m3 per mm.

Kthesamehoodi snowm_ed200mm from the dust source and the__~@~_~e~IRAlmebecomes

Q=15010x022+rrx O~)!l= 79m per *

vekityofanexhaust~tiba~n [email protected] CakuMons allowingimportancedposition relativeto the dust SOUME.F- * ---

LOO~

,

10s ~

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 = Distance from face of hood to dust sourceA= Openareaofhoodface

-0.!0

-0.4

-030

-1.00

AsPEcT