Bag Filters

35

BAG FILTERS Return To T.O.C

J RUSHWORTH

BAG FILTERS

CONTENTS

1.

INTRODUCTION

2.

THE MECHANXSMOF PARTICLE CAPTURE

3.

CLEANING MEITIODS

4.

TEMPERATURE LIMITATIONS

5.

BAG FILTER SIZING 5.1 5.2

Filtration Velocity EstimatingDedustingAir Flow Rates

6.

CHOICE OF CORRECT FABRIC FOR APPLICATION

7.

TROUBLE SHOOTING

8.

COMMENTS ON APPLICATION

9.

RECENT DEVELOPMENTS

Appendix 1:

Hmd Design

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 manner that all the gas passes through the fabric whilst the dust impinges on the fibre of the fabric and is thereby retained. As the dust accumulates on the fabric a ‘cake’ is formed, which aids filtration by improving 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 gas flow 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 be cleaned to dislodge the cake. The pressure drop across the fabric will always be greater than the initial value, that is with new fabric, because some of the dust particles will become permanently lodged in the fabric. Provided steady state conditions between 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 micron.

●

Electrostatic

- which is the primary

Forces

collection

- which affect

particles

mechanism

for particles

below 0.5

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.

1

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 some manufacturers

3.

elements, whether of woven or felted fabric, are cylindrical, have adopted flat panel, or envelope elements.

but

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. 2

10 TIMES F PARTICLES

WOVEN

CLOTH

WOVEN

NEEDLE

FELT

CROSS SECTION OF AND FELTED FILTER FABRIC Fig.

3

~~ l!=

AIR/CLEANED

—

f

‘4

;V ).

.

.’ .

.; ..

.

h.. .. ,.

“,.

* ..

t

* *. . .. ..* .. ...

:,

!

.-...,. .. .....:‘.

#

I

t

y

~LLAPSING FAN

1...

~!

.,.

..

.“.

. -. .. #

..

COLLAPSING :AN

Ku

W I

DIRTY

DIRTY AIR TO

\

I

\ %... -.,

‘. J .. .

.

.

OTHER

smoNs

., -.

. .. .

+

.

.=*:

.’ ..41

!

. ,~

s.

y....p.

:,” , :

‘1 ,., ..

DIRTY GAS OUTLET IO COLLAPSING FAN CLOSED

FAN OPEN

+

4

COLECTED

COLLECTED

DUST

DUST

CLEANING

FILTERING

FABRIC FILTER WITH COLLAPSE Fig.

BAG CLEANING

2

4

REVERSE All? FAN,

BAG SHAKING /DEvlcE-OFF

BAG SHAKING DEVICE -ON \

REVERSE AIR FAN \

Am C&EANED 99

-0

1-

GAS

REVERSE AIR INLET cLOSED CLEANED GAS OUTLET OPEN

/ REVERSE AIRINLET OPEN CLEANED GAS OUTLET CmSED

.

t

,. : $ ;$ .. . ., + t. .. :.. . . . . . . .. f●< I y -. ? t 4 . . . 1. *.. .. :. ... : . .. . -“ ●

.“. . ~: .! .

.. ,% .. :4 -1 : ,. A #

,“ < ●

----

F .-7

.& :

h

..

.-

DIRTY AIR TO ~)+ER +

DIRTY GAS

SECTIONS

COLLECTED

DUST

COLLECTED

FILTERING

FABRIC SHAKE

OUST

CLEANING

FILTER

AND REVERSE Fig.

3

WITH AIR CLEANING

,REVERSE

AIR

FAN

/TRAVELLING

w

CLEANED GAS*

DIRTY

GAS

AIR

TUBE

+ b

FILTER BAG WITH_ DUST LAYER (RLTER CAK= BUILDING UP

1! - FILTER BE

BEING

CLEANED AIRW BRIEF LYREVERSED INFLATES BAG & O\ SLODGES DUST

BAG SUPPORT~

!-.

COLLECTED

FABRIC PULSE AIR CLEANING

.

..

DUST

FILTER

WITH

AND CYLINDRICAL F@

4

BAGS

DIAPHRAGM

VALVES

~.H=~ -

JET TUBE INJECTI ?4G BURST OF COMPRESSED AIR INTO ‘FILTER BAG

l-5

CLEANED GAS ~

DIRTY GAS

e

1-

FILTER BAG WITH DUST LAYER CFILTER CAKE) 8UILDING

‘iR (Co 100 R S.I.)

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

UP

BAG SUPP9RT—

-—

8 COLLECTED

DUST

FABRIC FILER WITH PULSE J= CLEANING AND CYLINDRICAL BAGS Fig.

5

-

01 RT Y

~

CLEANED

GAS

pox F LTER

--RT

BAG —

—JETTUBE

AIR VALVES & TIMERS

&

COUECTED

BAG

DUST

SUP POllT— f

FILTER BAG DUST

WITH

,

LAYER

(FILTER

CAKE)

“i

4

1 . . . :.

BUILDING FILTER

. ..

!.

..-...

-.-+.

UP. BAG BEING

CLEANED

AIRFLOW

BRIEFLY lNFLA~S

REVERSED BAG &

DKLODGES DUST.

:

:

:..-.

l.; ..,....-

J1’ ,

JET

‘..~> .-. .. -7..:. *

““””--..-.-”. /’

TUBE

INJECTING

BURST OF COMPRESSED

:’> ;’

‘.:%-

I

AIR INTO FILTER BAG ‘

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

i

I COMPLETE kLEANING

I

F[LTER [ CYCLE

1

I I

I PRESSURE DROP

~3RD.

I

SECTION

SECTION

-2ND.

fs7. SECTION

CLEANEO

CLEANED

CLEANED

TIME

SECTIONALISED

CONTINUOUSLY

RATED F[LTE R

PRESSURE DROP

TIME

CONTINUOUSLY

PRESSURE

WITH

FILTER

RATED

DROP

VARIATION

CAKE Fig.

FILTER

BUILD

UP.

7 9

Temoerature Mmitatiom and cknkal

Key to ChemicaI

Not very good

res istance of filter fabrics

Resistance:-

Excellent

Good

Fig. 8

10

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.

11

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 T vtws of Filter Plant for “Normal” Dus@

TvDe of Fabric Filter i.e. Method of Self-Cleaning Mechanical shaker

Protrietarv

Examnle

Rang e of Base ; Values of A/C metres/minut~

visco-Beth; Spencer-Halstead

0.65 to 1.0

Mechanical shaker with low pressure reverse air

Visco-Beth; Norblo

0.75 to 1.0

Medium pressure reverse air

SIM Luhq

1.2

Luhr

1.8

Medium pressure pulsating reverse air High pressure reverse jet (a) Envelope bags (b) Cylindrical bags 3m long First :. 3m Next 3m

DCE Airmasteq MikroPul Cibel, AAF, Flakt, Joy, GBE, etc

>

1.5 1.8 1.8* 100* I

* Value for illustration

only; depend heavily upon details of air purge system.

12’”

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;

6.

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.

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.

13

TABLE 2:

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

DUST/PROCESS

“BASE” VALUE OF AIR TO CLOTH RATIO*

FABRIC

(std/min)

(R/rein)

Cement transport systems

PP; PE

1.5

4.9

Cement raw materials

PE; NX

1.5

4.9

Dry: PE; moist: DT

1.25

4.1

Dry: PP or PE

1.25

4.1

PP; possibly DT; NX

1.25

4.1

PE or NX

1.5

4.9

NX or other high temp fabric

1.5

4.9

PE or NX as design

1.65

5.4

Whiting (CaCO~) Kiln BE Dust transport Enriched alkali precip-dust Clinker transport Clinker cooler waste air

Clinker cooler waste air with heat exchanger Furnace gases Raw meal transport

.—— — Glass NF; PTFE; Ryton

1.4; 1.5; 1.5?

PP; PE

1.4tol.5

4.6,4 .9,4.9 4.6 to 4.9

Coal PF or dry raw coal Coal mill

PE; DT; PEAV 600 (Epitropic or + 5% SS)

1.25

4.1

Kiln BE gases

Woven glass; ?Tefaire NX questionable, has been used

0.65 to 0.9; ?

2.1 to 2.95

1.5

4.9

PP; PE PP; PE

1.5 1.25

4.9 4.1

1.0

3.2

1.2

3.9

Additives, extenders, Limestone, Gypsum CAF2, SiO~, fime Cement/Raw mill High effeciciency separator filter Cement/Raw mill vent filters

r

*

Air to cloth ratio for DCE type filter. Add up to approximately 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

20% for pulse jet filters with

‘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 The filter is then run for a few to the intake of the filter whilst it is in operation. 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.

15

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; temperatures and chemical attack are also causes of premature failures.

8.

high gas

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 It is possible, though not necessarily practicable, to alter the particular process. 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 This is not the case in the U.S. A., where waste gases in U.K. and most of Europe. 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 which gave rise to the impression that high designed electrostatic precipitators, efficiency gas cleaning could not be achieved by electrostatic precipitators. The installation of large fabric filters entail lower capital precipitators (although running costs are higher).

costs

than

electrostatic

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 16

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’Yprocess 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 The trend is towards larger closed circuit milling exhaust streams for many years. operations with separate mill and separator ventilation circuits. Fresh feed to the mill This together with is partially cooled by the coarse returns from the separator. 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 dedust 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 A satisfactory solution would seem glowing particies, which burn the filter elements.

17

<

FIGURE 9

18

to be the installation of an inertial collector before the filter in order to remove the glowing particles before they enter the filter. Ceramic fibre filters are to be tested for this application.

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.

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 can a particdar dust source be completely eliminated, although the dust control engineer and the process engineer should consider whether any change of production technique can minimif not eliminate, the problem. The reduction of a

Rarely

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 dFrom this a capture velocity maybe decided upon which

whence the capture VdOcity is 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 is either ignored or completely misunderstood byof 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~

willalso be influenced by the type of dust FinaIfy the siting of the capture ~ from

expressed asapcmtage

of thehoodface

showsthe Reference tdg.lo terminoiogyused inthevarious formulae

velocity

FIGURE 10 Facevekityisthe average exeftedoverfaceofhood-%

Face areaX = W x ‘U Equivalent diameter ‘D* XWXL 47

J

~

w \ ‘: ..:../

-... . k ,,. --:------_,; ~u; ;,,. ,.:... , source$~... ..... y.:”.. ‘:.“..........: / . ..:-: “.:”;..’. “..: