Bag Filters

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3

5 ₅

BAG FILTERS

J RUSHWORTH

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1. 2. 3. 4. 5. 6. 7. 8. 9.

BAG FILTERS

the

fabric and the quantity of trapped dust is reached in a reasonably short time the effect

is 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 important

are:-●

●

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

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

Electrostatic Forces - which affect particles below 0.5 micron.

The early stages of filtration occur with the capture of individual particles by single 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 as additional sites for the capture of further particles and eventually chain like aggregates r-ult. As the process continues, a complete matrix, or cake, of dust particles is built up until finally particle capture is effected by true surface filtration, or sieving, and the function of the fabric, apart from acting as a support, becomes nominal. Following a cleaning action, further particles in the gas stream attach themselves to particles which have remained on the fibres and the cake building process recommences.

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Fibres used in the manufacture of fabrics for filtration are almost exclusively synthetic

and they are either woven or needle felted - see Figure 1. Woven fabrics are smoother and more easily cleaned than felts and sometimes, at low loads, no cleaning devices are

needed because the fabric is self cleaning. On the other hand they often camot be cleaned too vigorously because this would break down the entire dust cake and force the dust between the fibres so that the dust emission would be high. Needle felts are less permeable than woven fabrics, but they can be operated at considerably higher filtration velocities. The pores in needle felts are very small compared with woven fabrics, so dust penetration is low.

Generally, the filter elements, whether of woven or felted fabric, are cylindrical, but some 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 in

many ways including collapse of the filter element, mechanical shaking, reverse air

flow, reverse air pulse and reverse air jet. Any one, or combination of these methods may be employed but, as a generalisation, the reverse air pulse and reverse air jet are usually associated only with filters having needle felted elements.

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

flow, see Figure 3.

During the collapse of the filter ekment, or the shake or reverse air period, the gas flow 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 out of 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 the pressure drop across the others, and this factor must be taken into account at the design stage.

With reverse air pulse cleaning, moderately pressurised air from a secondary blower is introduced 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 the element for a time intwwai of about 0.1 second - see Figures 5 and 6. Cake release is accomplished by a combination of fabric deformation, due to the shock of air blast, and flow reversal. Both cleaning methods remove the dust with only a brief interruption in the gas flow and both invariably use needle felt fabrics.

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

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WOVEN

CLOTH

10 TIMES

F PARTICLES

NEEDLE

FELT

CROSS

SECTION

OF

WOVEN

AND FELTED

FILTER

FABRIC

Fig.

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f ‘4 :, .

;V

). .;. . .’ . . “,. h.. * .. .. _,. t y

‘

.-...

_{. ....,}

.. ..-

_..:.

1 ~! ... .,. . . .“. . -. . . .. # t COLLAPSING :AN

W

I

l!=

~~

AIR/CLEANED —

Ku

I

* *. . .. .* . . . ... ! # I %... -., . ‘. .=*: J., -. . ,~ . . . ..’ ..41 s. FAN CLOSED 4 COLECTED DUST

FILTERING

DIRTY GAS DIRTY GAS OUTLET IO COLLAPSING FAN OPEN + COLLECTED DUST

CLEANING

~LLAPSING FAN \ DIRTY \ AIR TO ~ +

OTHER

!

smoNs

y....p.

:,” , : ‘1 ,., ..<.:. >

FABRIC FILTER

WITH COLLAPSE

BAG

CLEANING

Fig.

2

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REVERSE All? FAN, BAG SHAKING /DEvlcE-OFF 99 -0 REVERSE AIR INLET cLOSED .. CLEANED GAS OUTLET OPEN $,. : ;$ ., + .. . t ? t. f I y :.. .. .. .. _. _. .. 4 ●< -.t .*... . _{.. :.}1. ... : .. - .. _,% “. ... .. .. _:4-1 ~: : _,.A ●. _.! _# -“ . ,“● < DIRTY GAS C&EANED GAS REVERSE AIR FAN \ BAG SHAKING DEVICE -ON \

Am

1-/ REVERSE AIR-INLET OPEN CLEANED GAS OUTLET CmSED ---- F .-7 .& :h .-. .-. DIRTY AIR TO ~)+ER + SECTIONS COLLECTED DUST

FILTERING

COLLECTED OUST

CLEANING

FABRIC

FILTER

WITH

SHAKE

AND REVERSE

AIR

CLEANING

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,REVERSE AIR

CLEANED GAS*

DIRTY GAS +

w

b

FILTER BAG WITH_ DUST LAYER (RLTER CAK= BUILDING UP BAG SUPPORT~

1!

!-. . . . FAN /TRAVELLING AIR TUBE - FILTER BE BEING CLEANED AIRW BRIEF LYREVERSED INFLATES BAG & O\ SLODGES DUST

COLLECTED DUST

FABRIC

FILTER

WITH

PULSE AIR CLEANING

AND

CYLINDRICAL

BAGS

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CLEANED GAS ~

DIRTY GAS e

FILTER BAG WITH DUST LAYER CFILTER CAKE) 8UILDING UP BAG SUPP9RT— DIAPHRAGM VALVES ~.H=~ ‘iR -— - (Co 100 R S.I.)

l--

JET TUBE INJECTI ?4G BURST OF COMPRESSED

-5

AIR INTO ‘FILTER BAG

1-

PILOT VALVES &/OR TIMERS FILTER BAG BEING CLEANED AIRmW BRIEFLY REVERSED INFLATES B=& DIS~DGES DUST 8 COLLECTED DUST

FABRIC FILER WITH PULSE

J=

CLEANING AND CYLINDRICAL BAGS

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01 RT Y

F LTER BAG —

BAG SUP POllT—

FILTER BAG WITH DUST LAYER (FILTER CAKE) BUILDING UP.

FILTER BAG BEING , CLEANED AIRFLOW BRIEFLY REVERSED lNFLA~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.

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I

_COMPLETE _{F[LTER [}

i

I

kLEANING CYCLE 1

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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Temoe

rature 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 are the choice of the correct type of f ibre and how it is woven into a fabric. These are normally chosen according to the type of dust to be filtered and the operating temperature and nature of the gas being treated. The maximum temperatures at which various filtration materials can be operated continuously are shown in Figure 8.

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

Conversely, excessively low temperatures can also influence the life of the fabric, since such 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 the cleaning device is unable to remove it. This rapidly leads to complete blinding of the fabric and the necessity for its replacement.

The chemical resistance of various filtration materials is also shown in Figure 8. The chemical resistances shown are based on dry gas conditions. When water vapour is present, 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 mechanism

limits 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 is}

known to be easy to filter.

An example of this would be clinker dust which is generally

coarsely sized. These values should be reduced by up to 20% for “difficult!? dusts. Fine

dusts such as coal dust, alkali-enriched

flue dust and additives

such as silica fume are

examples of difficult

dusts.

TABLE 1:

Base Values of Air to Cloth Ratio for Various Tvtws of Filter Plant

for “Normal” Dus@

Rang e of Base

TvDe of Fabric Filter

; Values of A/C

i.e. Method of Self-Cleaning

Protrietarv

Examnle

metres/minut~

Mechanical shaker

visco-Beth;

0.65 to 1.0

Spencer-Halstead

Mechanical shaker with low

Visco-Beth;

0.75 to 1.0

pressure reverse air

Norblo

Medium pressure reverse

air

SIM Luhq

1.2 Medium pressure pulsating

Luhr

1.8 reverse air

High pressure reverse

jet

(a) Envelope bags

DCE

1.5 (b) Cylindrical

bags <3m long

Airmasteq

MikroPul

(c) Cylindrical

bags >3m long

1.8 Cibel, AAF, Flakt, Joy,

First

:. 3m

GBE, etc

1.8*

3m

100*

>

I

* Value for illustration

only; depend heavily upon details of air purge system.

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5.2 Estimating Dedusting Air Flow Rates

The recommended reference on this subject is “Industrial Ventilation” published by the

American Conference of Government Hygienists.

Some guideline values are summarised

below;

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 different

applications within the cement manufacturing process. The base filtration velocities have also

been indicated for each application. Gore-Tex fabrics and their “lookalikes” appear to be able

to 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 the

base fabric, a normal needle felted medium.

These fabrics do have excellent dust release

properties 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 OF DUST/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 4.9

fabric

Clinker cooler waste air with PE or NX as design 1.65 5.4 heat 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 4.1 Coal mill (Epitropic or + 5% SS)

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

been used 1.5 4.9 Additives, extenders,

Limestone, Gypsum PP; PE 1.5 4.9 CAF2, SiO~, fime PP; PE 1.25 4.1 Cement/Raw mill 1.0 3.2 High effeciciency separator

filter

Cement/Raw mill vent filters 1.2 3.9 r

* _{Air to cloth ratio for DCE type filter.} _{Add up to approximately} _{20% for pulse jet filters with}

cylindrical bags < 3m long (see Table 1).

Key: PP = Polypropylene PE = Polyester DT = Dralon T NX = Nomex PEAV 600 = special fabric, do not speci@ without finther advice SS = 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 the original design data and comparing this with the current operating conditions. Several modifications may have been carried out over the years on the plant being de-dusted and these could have drastically changed the filter duty.

Increased emission levels are usually caused by broken filter bags. If the increased emission level has been indicated by a dust monitor and not visually, it would be worthwhile 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 dust

emission test be carried out to check the accuracy of the monitor.

In smaller filters broken bags are usuaily located by checking each individual bag. This would be a very arduous task however on a larger filter. For filters with long cylindrical bags suspended from a tubesheet, a broken bag can sometimes be detected by a pile of dust on top of the tube sheet next to the broken bag. It is therefore important to clean the tubesheet after each maintenance. For older filters with bags that are not supported by tubesheets the task can be more arduous. It is possible tO locate the compartment or compartments that have broken bags by selectively isolating each compartment in turn and noticing the change in dust emission, especially if a dust monitor has been installed.

Increased dust emissions can also be caused by leaks in the tubesheet or internal

chambers. Unless the crack or hole is relatively large the locations of these leaks are not always easy to find. Making use of fluorescent powder and a UV lamp can greatly assist in locating the leaks. A few kilograms of fluorescent powder are introduced in to the intake of the filter whilst it is in operation. The filter is then run for a few minutes to allow the powder to work its way through. The filter is then stopped and inspected internally with a W lamp.

Increased pressure drop across a filter is usually caused by blinded bags. If the pressure drop suddenly increases or reduces, a similar change on the exhaust fan current drawn may 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 bags usually result from problems with the cleaning mechanism. This could result from a loss

in compressed air pressure for pulse-jet filters. For product collecting filters on cement milIs it is normal to interiock the compressed air supply line pressure to mill operation. If the air pressure drops the mill is tripped out. Low air pressure apart from compressor fauits, can be caused by faulty water traps which have resulted in the line filters blocking. Excessive oil or water entrained in the air is often the cause of failure of 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 resulting in condensation on the bags, which can render the bag cleaning device ineffective. Poor gas distribution through a filter can also be detrimental to its operation with high flow areas 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 can result in rapid bag wear due to excessive impingement of dust on the bags. High gas velocities can cause attrition between individual bags also resulting in wear.

Short bag life can result from incorrect fabric choice for the application; high gas temperatures 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 blinding of a fabric. Maintaining gas temperatures below the rating of the filter fabric is also important to avoid it being overheated. These factors must be borne in mind when deciding whether or not a fabric filter should be used to de-dust the gases from any particular process. It is possible, though not necessarily practicable, to alter the condition 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 the

case with the exhaust streams from wet and semi-dry process kilns, an electrostatic precipitator would be the obvious first choice. A fabric filter could be used if supplementary heaters were installed in order to pre-warm the filter.

In the case of dry process, suspension preheater or precalciner kilns, the waste gases are naturally 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 would be necessary. This is best carried out by the evaporation of water into the hot gases in a conditioning tower. The increased moisture content of tie gas makes it more favorable to use electrostatic precipitation. A further factor to support this arises when use is made of the waste gases in the milling/drying circuit. Contact with the raw materials increases its moisture content and reduces the gas temperature.

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

The installation of large fabric filters entail lower capital costs than electrostatic precipitators (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 Alkali Inspectorate demand that the waste gases be exhausted to atmosphere via an exhaust stack 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 system facilitates the location of a faulty bag, whereas when a chimney iS used the task is more difficult. 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. This is to enable the whole exhaust stream to be measured.

A large quantity of water is required to cool the gases from a dI’Y process kiln (about

200g of water per kg of clinker) and in some parts of the world such quantities are not available. Electrostatic precipitation can be extremely difficult k these circumstances (due to high dust resistivity) and a fabric filter then could be considered. Its size however would be excessive as the gas would be cooled by ambient air and thus result in an increase in the quantity of gas to be treated. The filter medium, which is invariably glass fibre for such applications, demands a low fikration velocity for satisfactory operation - typically 0.5 -0.6 metres per minute and this also dictates a large sized filter plant.

The waste air stream from a grate type of clinker cooler is very (h’y and the resistivity of the dusts is generally high. The gas temperature of this stream is typically about 300*C but this can increase to 500°C during a kiln flush. To enable the use of a filter fitted 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 incorporate an air to air heat exchanger. A cold air bleed may also be incorporated in the circuit for emergency use during a kiln flush. The coarse nature of the dust permits a filtration velocity of about 1.5 m/min, thus making the filter relatively compact.

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

Fabric filters and electrostatic precipitators have both been used to de-dust cement mill exhaust streams for many years. The trend is towards larger closed circuit milling operations with separate mill and separator ventilation circuits. Fresh feed to the mill is partially cooled by the coarse returns from the separator. This together with improved mill ventilation results in less cooling water being required during the milling process. 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 transfer points, on rail wagon tipplers, de-dusting loading chutes and venting silos. All these applications can be successfully de-dusted with correctly sized filters. Problems have been encountered de-dusting clinker conveyors due to the passage into the filter of glowing 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 the glowing particles before they enter the filter. Ceramic for this application.

filter in order to remove the fibre filters are to be tested

9. RECENT DEVELOPMENTS

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

The duty of each module is greater than a similar sized fabric filter due to the increased surface area developed by the profiling. The base filtration velocity for these elements 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 operating temperature.

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

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Appendmi

3. Hood Design

Once the processes of identification and quantification have been carried o@ a dust control engineer may plan his campaign both from the engineering and economic viewpoint Rarely can a particdardustsource be

completely eliminated, although the dust control engineer and the process engineer should consider whether any changeof production technique can minim- if not eliminate, theproblem. The reduction of a particular emission source by either suppression or containment is, in practice,

often possible and usually repays investigation. The next step is to design the exhaust

enclosure. Formulae for hood design do exis4 although experience counts for a great deal in their application. The starting point for a hood design calculation is determining the emission

rate or velocity of the liberated d- From this a capture velocity maybe decided upon which willalso be influenced by the type of dust

FinaIfy the siting of the capture ~ from

whence the capture VdOcityis produced, must also be taken into consideration.

Unfortunatdy d] too often the economic and

engineering irnpo~nce of the available* regardi~ the siting of CX&UXX hoods iseither ignoredor completely misunderstood by-of those concerned in the specificationand

purchase of dust controI plant The fbllowing brief excursion intothefieldofhtid~

mayhelpinkktif@g the-m~

Muchoftheavaiktkdata -tothesiting ofexhausthoods is based onw*atiti

in the 1930’s by DaIlaWk and nearly 50 yeZWS later by Fletck By measuring contour

Velocities in fiontofaninletm formula can bederivedforthe centreiineairflow

relatbmhip. From these formulae the ~ onthecentre lineinfrontofa hood~

expressed asapcmtage of thehoodface velocity Reference

tdg.lo

showsthe

terminoiogyused inthevarious formulae

FIGURE 10

Face area X = W x ‘U Facevekityisthe average Equivalent exefted over faceofhood-% diameter‘D* Emission veloc@

J

XWXL 47

w

~

conveying

velocity

=Q

\ ‘: ..:../

Area

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

-Di&nce from dustsource

capture _{tohoodface=’X}

Veiocity

‘v

(23)

Appendix z

Cont.

The formuia of DallaVaile,(fig. 11), isa relatively

sirnpie one. Although satisfactory wherethe

hood mouth is eitk circuiar or square it should not be used when the hood mouth is

~guiar and where the aspect radio is any

other than one to one.

Cakuiation for the requiredvolume ofair

for

round or square hinds, according to

DallaVaile:-Q = V(10X2 + A) Where:

Q = Quantity of air

V = The capture velocity at the dust

X = The distance

from hood mouth to

dust source

A = The open face area of the hood

Fig. 11

Formula

of DallaVaIle

● Fletcher’s formula, which is

more compiex

does

however take into

regard varying _

ratios and use of his

nomogram, (fig. 12),will

give a much

more accurate resdtfor anygiven

problem.

Howeverthe simpler DaUaVaUe

equation can

provide the practical engineer with an

immediate indication as to the

likelyc-an experienced dust

contmi engineer can

make an accurate prediction as to

the

overall

coUection efficiency ofthe ckvice. Thecloser the hood is tothe dw

generation Point the

more

economical the system is,,and

generally

any capture hood that issited morethan 0.7 diameters away from the dustsource

could be

regarded as poorly positioned and

uneconomic. To demonstrate in real terms the

inmiicatioriof this factoz

the

followingexample

(fig.

13)shows the

difference in air tilumes

required for the same collection problem for two

alternative distices

between hood

and dust source.

The

necessary air volume when a hood of

400mm dia. is

placed

320mm from a

dust source where a

capture velocity

of

150m per minute is required

is-Q = V(10X2+ A) - DaUaWle

= 150(10 x 0322+ rr x 022 )

= 1723m3 per mm.

Kthesamehoodi

snowm_ed

200mm from the dust source and the

__~@~_~e~

IRAlmebecomes

Q=15010x022+rrx

O~)

!l

= 79m

per *

vekityofanexhaust~tiba~n

F@.

13- CakuMons allowingimportanced

position relativeto the dust SOUME.F- *

---LOO~ , 1 0s ~ 0.4- 03- 02- o.ls- 0.10- 0.07-. 0.06-O.M “ 0.03O.oa Om -0.015+ 0.01“ . .

Oms ~ X = Distance fromface of hood todust source

A= Openareaofhoodface -0.!0 -0.4 -030 -1.00 A s P E c T

Bag Filters

3

3

5

5

BAG FILTERS

J RUSHWORTH

BAG FILTERS

CONTENTS

the

WOVEN

CLOTH

10 TIMES

F PARTICLES

NEEDLE

FELT

CROSS

SECTION

OF

WOVEN

AND FELTED

FILTER

FABRIC

;V

‘

.-...

. ....,

.. ..-

..:.

W

I

l!=

~~

Ku

I

FILTERING

CLEANING

OTHER

!

smoNs

y....p.

FABRIC FILTER

WITH COLLAPSE

BAG

CLEANING

Fig.

2

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CLEANING

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CYLINDRICAL

BAGS

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FABRIC FILER WITH PULSE

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CLEANING AND CYLINDRICAL BAGS

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