Fabric filter

Rachmat Boedisantoso Jurusan Teknik Lingkungan FTSP – ITS

Kampus Sukolilo, Surabaya – 60111

TEKNOLOGI PENGENDALIAN

PENCEMAR UDARA

Advantages of Fabric Filters Very high collection efficiency

They can operate over a wide range of volumetric flow rates

The pressure drops are reasonably low.

Fabric Filter houses are modular in design, and can be pre-assembled at the factory

Fabric Filters (contd.) Disadvantages of Fabric Filters

Fabric Filters require a large floor area.

The fabric is damaged at high temperature.

Ordinary fabrics cannot handle corrosive gases.

Fabric Filters cannot handle moist gas streams

A fabric filtration unit is a potential fire hazard

Fabric Filters Principle

The filters retain particles larger than the mesh size

Air and most of the smaller particles flow through. Some of the smaller particles are retained due to interception and diffusion.

The retained particles cause a reduction in the mesh size.

The primary collection is on the layer of previously deposited particles.

Fabric Filtration merupakan alat kontrol

udara yang paling umum dipergunakan

Fabric filter menggunakan filter yang

terbuat dari nilon atau wol

Partikulat yang telah disisihkan/

terkumpul kemudian dibersihkan

dengan mekanisme pembersihan

tertentu.

Fabric filter juga disebut :

Baghouse

Fabric filter collectors,

Bag filters,

Fabric dust collectors,

Filter collectors,

Dust collectors,

Cloth collectors

Filter house.

keuntungan dan kerugian Fabric Filter :

Keuntungan penggunaan fabric filter adalah : Efisiensi sangat tinggi, bahkan untuk partikel yang halus

Dapat dipakai untuk berbagai macam debu

Dapat untuk volume gas yang besar

Dapat beroperasi pada pressure drop yang rendah

Sedangkan kerugiannya adalah : Memerlukan tempat luas

Bahan filter dapat rusak pada temperatur tinggi atau bahan asam

Tidak dapat beroperasi pada lingkungan yang lembab

Potensial kebakaran

Fabric filter terdiri atas :

inlet,

outlet,

filter bag,

hopper

Mekanisme pengumpulan Fabric Filter pada

umumnya ada tiga cara utama, yaitu :

Impaction, partikel memiliki gaya inersia yang terlalu besar untuk mengikuti aliran garis pada filter fiber sehingga tertumbuk pada permukaan filter

Interception, partikel mempunyai inersia yang sangat kecil (partikel yg lebih kecil). Partikel akan berada pada aliran viscous, bergerak melambat dan menyentuh barrier dan berhenti

Diffusion, partikel lebih kecil dari 1 mikron berada pada kisaran gerak Brown, sehingga terjadi gerakan random yang akhirnya terintersepsi dengan dust cake

Proses Filtrasi Fabric Filter terdapat dua jenis

desain yang dapat digunakan yaitu:

Interior Filtration, partikulat dikumpulkan pada bagian dalam dari bag filter. Gas yang mengandung partikulat memasuki fabric filter melalui bagian bawah dari kolektor dan diarahkan ke dalam bag dengan menggunakan diffuser vanes atau baffle dan juga cell plate.

Exterior Filtration, partikulat dikumpulkan pada bagian luar dari bag fliter. Proses penyaringan berlangsung dari luar bag filter ke dalam bag filter.

Jenis Proses filtrasi ( kiri : Interior Filtrasi, kanan : Eksterior

Filtrasi)

Terdapat beberapa cara pembersihan yang dapat

dipergunakan untuk menyisihkan partikulat yang menempel

pada permukaan bag filter, tiga cara yang paling sering

digunakan adalah :

(i). Shaking

Mechanical shaking menggunakan motor yang dihubungkan dengan bag

Energi yang diperlukan rendah

Gerakan dan kecepatan tergantung endapan debu

Arah gerakan Horisontal atau vertical

Gerakan di bagian atas frame tempat bag diletakkan

Aliran gas berhenti saat dilakukan proses pembersihan

Shaker baghouse umumnya menggunakan interior filtration

Diameter 15,2 – 45,7 cm ( 6-18 inch )

Panjang sampai 12,2 m ( 40 ft )

Lama pembersihan 30 dt – beberapa menit

Terdiri dari beberapa kompartemen

(ii). Reverse Air

Mekanisme yang paling sederhana

Aliran udara kotor dihentikan

Mengalirkan backwash air (udara bersih yang berlawanan arah)

Aliran udara bertekanan rendah

Debu akan jatuh ke hopper

Lama pembersihan 30 menit – beberapa jam

Durasi pembersihan 10 – 30 detik

Terdapat ring dengan jarak 10 – 46 cm

Reverse air baghouse berdiameter 20 – 46 cm, dan panjang 6,1 – 12,2 cm

Terdiri dari beberapa kompartemen

Gambar 7.14 menunjukkan typical reverse air

Typical Reverse Air Baghouse

(iii). Pulse Jet

Disebut juga pressure jet cleaning

40 – 50% baghouse baru di Amerika

Menggunakan high pressure jet dari udara

Sistem exterior filtration

Menimbulkan shock wave

Pulse jet baghouse berdiameter 10,2 – 16,2 cm

Panjang umumnya 2,4 – 3,7, tapi dapat mencapai 7,6 cm

Bahan bag house

Woven, terbuat dari benang, dipakai untuk

pembersihan energi rendah

Felted filter terdiri dari fiber yang dikompres

ke dalam mar dan dilekatkan pada woven Ada yang terbuat dari bahan alam seperti katun atau wol

Temperatur lebih kecil dari 212oF atau 100oC

Katun temperatur lebih kecil

Fiber sintesis

Nilon, orlon dan polyester tahan temperatur lebih tinggi

dan tahan terhadap bahan kimia

Polypropylen paling murah dan banyak digunakan

Nilon memiliki abrasive resistant yang paling tinggi

Polyester atau Dacron baik untuk menahan asam, alkali dan abrasi dan relatif murah

Nomex buatan Dupont

Membran material terbuat dari berbagai jenis fiber yang disusun membentuk membrane diantaranya adalah Gore-Tex membrane, jenis ini dapat mereduksi emisi dengan baik, pressure drop yang relatif rendah, umur bag yang meningkat dan ratio air-cloth yang lebih tinggi

Design of Fabric Filters The equation for fabric filters is based on

Darcy’s law for flow through porous media.

Fabric filtration can be represented by the following equation: S = Ke + Ksw Where,

S = filter drag, N-min/m3

Ke = extrapolated clean filter drag, N-min/m3

Ks = slope constant. Varies with the dust, gas and fabric, N-min/kg-m

W= Areal dust density = LVt, where

L = dust loading (g/m3), V = velocity (m/s)

Both Ke and Ks are determined empirically from pilot tests.

Fabric Filters

Δ P Total pressure drop

Δ Pf Pressure drop due to the fabric

Δ Pp Pressure drop due to the particulate layer

Δ Ps Pressure drop due to the bag house structure

Darcy’s equation

ΔPf Pressure drop N/m2

ΔPp Pressure drop N/m2

Df Depth of filter in the direction of flow (m)

Dp Depth of particulate layer in the direction of flow (m)

μ Gas viscosity kg/m-s

V superficial filtering velocity m/min

Kf, Kp Permeability (filter & particulate layer m2)

60 Conversion factor δ/min

V = Q/A

Q volumetric gas flow rate m3/min

A cloth area m2

Dust Layer

L Dust loading kg/m3

t time of operation min ρL Bulk density of the particulate layer kg/m3

ΔP = ΔPf + ΔPp

Filter Drag S = ΔP/V Areal dust density W = LVt S= k1+k2W

Parameter yang penting dan perlu

dipertimbangkan dalam merancang Fabric Filter

diantaranya adalah :

(i). Pressure Drop

Dinyatakan sebagai pressure drop per unit area sebgai fungsi dari karakteristik medium filter

Biasanya berkisar 2-4 inch

Dihitung dengan cara :

tVCkp

Vkp

ppp

fc

ff

cf

12

1

dimana : pf = pressure drop sepanjang FF

k1 = resistensi FF (inch air /menit atau cm/menit), merupakan fungsi dari karakteristik viscositas gas dan filter seperti ketebalan dan porositas (permeabilitas)

Vf = kecepatan filtrasi (ft/menit atau m/menit)

pf = pressure drop sepanjang cake dalam inch (cm)

k2 = resistensi dari cake (inch air /menit atau cm/menit

C1 = dust loading (lb/ft3 atau kg/m3) ditentukan secara experimen. Koefisien ini tergantung dari viscositas gas, densitas partikel dan porositas.

tVCkp

Vkp

ppp

fc

ff

cf

12

1

(ii). Kecepatan Penyaringan

Kecepatan penyaringan dinyatakan

sebagai :

kecepatan merupakan kecepatan

superficial filtering

A

Q

(iii). Performance Factor

Salah satu variable yang penting dalam mendesain baghouse adalah ratio air to cloth (A/C) atau ratio udara terhadap bahan filter

A/C menggambarkan berapa banyak gas kotor yang melewati permukaan filter dengan luas tertentu selama waktu tertentu.

Ratio yang tinggi berarti sejumlah besar udara yang melewati fabric

Satuan cm3/detik/cm2 atau ft3/menit/ft2

Tergantung dari mekanisme pembersihan, bahan filter dan partikel dust yang tersaring

Problem Estimate the net cloth area for a shaker

bag house that must filter 40,000 cfm of air with 10 grams of flour dust per cubic foot of air. Also specify the number of components to be used and calculate the total number of bags required if each bag is 8 feet long and 0.5 feet in diameter. The maximum filtering velocity for flour dust is 2.5 ft/min.

Solution Step 1: Calculate total area and number of

components required: A = Q/V

Step 2:

Calculate the area of each bag: A = Π(d)l

Step 3:

Calculate the total number of bags required.

Number of bags required = Total area / Area per bag = 1270 bags

30

2. Dividing Collection Devices

Filters and scrubbers do not drive the

particles to a wall, but rather divide the

flow into smaller parts where they can

collect the particles.

2.1 Surface Filters

A surface filter is a membrane (sheet steel,

cloth, wire mesh, or filter paper) with holes

smaller than the dimensions of the

particles to be retained.

31

However, one only needs to ponder the

mechanical problem of drilling holes of

0.1-µ diameter or of weaving a fabric with

threads separated by 0.1µ to see that

such filters are not easy to produce.

They are much too expensive and fragile

for use as high-volume industrial air

cleaners.

32

Although industrial air filths rarely have holes smaller than the smallest particles captured, they often act as if they did.

The reason is that, as fine particles are caught on the sides of the holes of a filter, they tend to bridge over the holes and make them smaller.

Thus as the amount of collected particles increases, the cake of collected material becomes the filter.

33

The particles collect on the front surface of

the growing cake.

For that reason this is called a surface

filter.

The flow through a simple filter is shown

schematically in Fig. 9.12.

35

If we follow the gas stream in Fig. 9.12

from point 1 to point 3 we see that the flow

is horizontal and has a small change in

velocity because the pressure drop,

causing the gas to expand, and because

the gas is leaving behind its contained

particles.

36

In most industrial filters, both for gases and liquids,

the flow velocity in the individual pores is so low that

the flow is laminar.

Therefore, we may use the well-known relations for

laminar flow of a fluid in a porous medium, which

indicate

Here, k is the permeability, a property of the bed.

(19)s

Q p k

A x

37

For a steady fluid flow through a filter cake supported

by a filter medium, there are two resistances to flow

in series, but the flow rate is the same through each

of them.

We find:

Solving for P2, we get

2 31 2

cake filter

-P=s

PP P k k

x x

2 1 3

cake filter

=s s

x xP P P

k k

38

Then solving for vs:

This equation describes the instantaneous flow rate

through a filter; it is analogous to Ohm’s law for two

resistors in series.

The Δx/k terms are called the cake resistance and

the cloth resistance.

1 3( ) (20)

[( / ) ( / ) ]s

cake filter filter

p p Qv

x k x k A

39

The resistance of the filter medium is usually

assumed to be a constant that is independent of time,

so (Δx/k)filter is replaced with a constant α.

If the filter cake is uniform, then its resistance is

proportional to its thickness:

1

1

cake

cake

cake

mass of cakex

area

volume of gas mass of solids removed

area volume of gas

40

Customarily we define: Here W is the volume of cake per volume of gas processed, which corresponds to a collection efficiency, η, of 1.0.

For most surface filters η=1.0, so the η is normally dropped in the equation. Thus Here V is the volume of gas cleaned.

1

cake

mass of solids removed volume of cakeW

volume of gas volume of gas processed

( ) and (21)cake

cake s

d xVx W W

A dt

41

Substituting Eq. (21) for the cake thickness in Eq. (20), we

find

For most industrial gas filtrations the filter is supplied by a

centrifugal blower at practically constant pressure, so (P1-

P3) is a constant, and Eq. (22) may be rearranged and

integrated to

1 3(P -P )Q 1 dV= = = (22)

A A dt [(V /kA+ ]s

W

2

1 3( ) (23)2

V W VP P t

A k A

42

For many filtrations the resistance α of the

filter medium is negligible compared with

the cake resistance, so the second term of

Eq. (23) may be dropped.

The two most widely used designs of

industrial surface filters are shown in Figs.

9.13 and 9.14 (next and second slides).

45

For the baghouse in Fig. 9.13 there must be some way

of removing the cake of particles that accumulates on

the filters.

Normally this is not done during gas-cleaning operations.

A weak flow of gas in the reverse direction may also be

added to help dislodge the cake, thus deflating the bags.

Often metal rings are sewn into filter bags at regular

intervals so that they will only partly collapse when the

flow in reversed.

46

Because it cannot filter gas while it is being cleaned, a

shake-deflate baghouse cannot serve as the sole

pollution control device for a source that produces a

continuous flow of dirty gas.

Typically, for a major continuous source like a power

plant, about five baghouses will be used in parallel,

with four operating as gas cleaners during the time

that the other one is being shaken and cleaned.

Each baghouse might operate for two hours and then

be cleaned for 10 minutes.

47

The other widely used baghouse design,

called a pulse-jet filter, is shown in Fig.

9.14.

In it the flow during filtration is inward

through the bags, which are similar to the

bags in Fig. 9.13 except their ends open at

the top.

The bags are supported by internal wire

cages to prevent their collapse.

48

The bags are cleaned by intermittent jets

of compressed air that flow into the inside

of the bag to blow the cake off.

Often these baghouses are cleaned while

they are in service.

49

Example 15

The shake-deflate baghouse on a power

station has six compartments, each with

112 bags that are 8 in. in diameter and 22

ft long, for an active area of 46 ft2 per bag.

The gas being cleaned has a flow rate of

86,240 ft3/min.

The pressure drop through a freshly

cleaned baghouse is estimated to be 0.5

in. H2O.

50

The bags are operated until the pressure

drop is 3 in. H2O, at which time they are

taken out of service and cleaned.

The cleaning frequency is once per hour.

The incoming gas has a particle loading of

13 grams/ft3.

51

The collection efficiency is 99%, and the

filter cake is estimated to be 50% solids,

with the balance being voids.

Estimate how thick the cake is when the

gags are taken out of service for cleaning.

What is the permeability, k, of the cake?

52

Solution: The average velocity coming to the filter surface:

The 5 is used here because one of the six

compartment is always out of service for cleaning.

vs is commonly referred to as the air-to-cloth ratio,

face velocity, or superficial velocity.

3

2

86240 /3.35 1.02

(5)(112)(46 )s

Q ft min ft m

A ft min min

53

If the filter remains in service for 1 hour before

cleaning and vs is constant, the 1 square foot of bag

will collect the following mass of particles:

3

2 2

60 13 3.35 0.99 (1 )

7000

0.369 1.80

s

m gr lbm ft minc t h

A ft gr min h

lbm kg

ft m

54

The thickness of the cake collected in 1 hour is

Taking

2

3 3 3

3

/ 0.369 /

(2 / )(0.5)(62.4 / / )

5.9 10 0.071 . 1.8

m A lbm ft

g cm lbm cm ft g

ft in mm

0, we can solve Eq. (20) for k, filter

x

k

5 2

2

2 2

12 2 13 2

(3.35 / min)(0.071 /12 .)(0.018 )(2.09 10 / / )( / 60 )

( ) (3 . )(5.202 / / . )

7.96 10 7.40 10

s x ft ft in cp lbf s ft cp min sk

p in H O lbf ft in H O

ft m

55

Compare this with values found in ground water flow:

The calculated permeability of this material is roughly

the same as that of a highly permeable sandstone. #

12 2

11 2(7.96 10 ) 0.75

1.06 10

darcyk ft darcies

ft

56

This calculation shows that the collected is

about 1.8 mm thick.

If the cleaning were perfect, this would be

the cake thickness.

However, it is hard to clean the bags

completely, and in power plant operation it

is common for the average cake thickness

on the bags to be up to 10 times this

amount.

57

One of the advantages of the pulse-jet

design is that it cleans the bags more

thoroughly, allowing a higher vs, at the

cost of a somewhat shortened bag life.

Fig. 9.15 (next slide) is a set of typical

results from tests of collection efficiency

for this kind of filter.

59

If the superficial velocity increases, the

efficiency falls; for a superficial velocity of

3.35 m/min the outlet concentration is

about 20 percent of the inlet concentration.

The particles that pass through such a

filter do not pass through the cake but

through pinholes, which are regions where

the cake did not establish properly (Fig.

9.16, next slide).

61

The pinholes are apparently about 100 µ

in diameter, much too large for a single

particle to block because there are rarely

100-µ particles in the streams being

treated.

When the superficial velocity is high, more

pinholes form.

62

Example 16

Estimate the velocity through a pinhole in

a filter with a pressure drop of 3 in. of

water.

Assuming that this is the pressure drop

corresponding to the curve for 0.39 m/min

on Fig. 9.15, that the steady-state

penetration at that velocity is 0.001, and

that the pinholes have a diameter of 100 µ,

estimate how many pinholes per unit area

there are in the cake.

63

Solution: The flow through the pinhole is best described by Bernoulli’s

equation, from which we find the average velocity:

Here the (area ∙velocity) of the pinholes must be 0.001 times the

(area ∙velocity) of the rest of the cake. Hence

1/ 21/ 2

2

3 2

2

2 2(3 ) 249 0.61

(1.20 / )

21.5 /

pinholes

P in H O Pa kgC

kg m in H O Pa m s

m s

27

2

0.001 0.001 (0.39 / )3.0 10

(21.5 / )(60 / )

pinholes s

cake pinholes

A m min m

A m s s min m

64

Each pinhole has an area of A =

(100 10-6 m)2 (π/4) = 7.85 10-9 m2, so there

must be

The calculated velocity through a pinhole is (21.6 x

60) /0.39 = 3300 times the velocity through the cake.

72

9 2

3.0 1038 pinholes/m #

7.85 10 m

65

2.2 Depth Filter

Filters do not form a coherent cake on the

surface, but instead collect particles

throughout the entire filter body are called

depth filters.

The examples with which the student is

probably familiar are the filters on filter-

tipped cigarettes and the lint filters on

many home furnaces.

66

In both of these a mass of randomly

oriented fibers (not woven to form a single

surface) collects particles as the gas

passes through it.

In Fig. 9.17 (next slide), we see a particle-

laden gas flowing toward a target, which

we may think of as a cylindrical fiber in a

filter.

68

To determine whether a particle bumps

into the target or flows around it, we can

compute the relative velocity between

particle and gas using the appropriate

equivalents of Stokes’ law.

That task was first undertaken by

Langmuir and Blodgett, and Fig. 9.18 (next

slide) conveniently summarizes the

mathematical solutions for the small

particles of interest in air pollution work.

70

To see how they obtained Fig. 9.18,

consider a single particle in a turning part

of the gas stream as shown in Fig. 9.19

(next slide).

72

In Fig. 9.19, the appropriate velocity to use in Stokes’ law is not the overall stream velocity but rather the

difference in y-directed velocity between the particle

and the gas stream.

Generally, the gas stream will have a larger velocity

in this direction than the particle, so

3 ( )y drag y gas y particleF D

73

The resisting (inertial) force of the particle is

If there are no electrostatic, magnetic, or other forces

acting, then these two forces are equal and opposite,

so

3

6

y particle

y inertial

dF ma D

dt

2

18 ( )y particle y gas y particled

dt D

74

Rearrange,

Δt is the time available for the y-directed forces to

move the particle around the garget, which must be

proportional to the time it takes the main gas flow to

go past the target.

2

18 (24)

( )

y particle

y gas y particle

d t

D

75

This time = Db/v, where Db is the diameter of the

barrier, so we may substitute in Eq. (24), finding

Ns is the separation number, which appears on the

horizontal axis in Fig. 9.18.

It is equal to the diameter of the barrier divided by

the stokes stopping distance.

2

12

18 1 (25)

( )

y y particle b

yy gas y particle s

d D

D N

76

Some authors call Ns the impaction

parameter or inertial parameter.

In Eq. (25) we can see that if the integral

on the left is a large number (a low value

of Ns), then there is plenty of time and

force for the flow to move the particle

around the target and the target efficiency

(ηt) will be low.

77

Example 17

A single, cylindrical fiber 10 µ in diameter

is placed perpendicular to a gas stream

that is moving at 1 m/s.

The gas stream contains particles that are

1 µ in diameter and the particle

concentration is 1 mg/m3.

What is the rate of collection of particles

on the fiber?

78

Solution: If all the particles that start moving directly toward the

fiber hit it, then the collection rate would be equal to

the volumetric flow rate approaching the fiber times

the concentration of particles:

If we catch them all, we will collect 10-8 g/s for every

meter of fiber length.

5 3 3

8

maximum possible rate = (1 / )(10 )(10 / )

10 / /

bD c m s m g m

g m s

79

The actual amount caught will be this number times

the target efficiency.

The separation number is

From Fig. 9.18, we see that for cylinders this value of

Ns corresponds to a target efficiency of about 0.42,

so we would expect to collect about 0.42 10-8

g/m/s. #

3 6 2

5 5

(2000 / )(10 ) (1 / )0.617

(18)(1.8 10 / / )(10 )s

kg m m m sN

kg m s m

80

Example 18

A filter consists of a row of parallel fibers across a flow, as described in Example 17, with the center-to-center spacing of the fibers equal to five fiber diameters.

What collection efficiency will the filter have for the particles?

Assume that the fibers are far enough apart that each one behaves as if it were in an infinite fluid, uninfluenced by the other fibers.

81

Solution:

From Example 17, ηt = 42% for a single

fiber.

If the fibers are spaced five fiber diameters

apart, then the open area is 80%, and the

blocked area is 20%.

So, Collection efficiency = (target efficiency) (percentage blocked)

= 42%20% = 8.4% #

82

Example 19

A filter consists of 100 rows of parallel

fibers as described in Example 18,

arranged in series.

They are spaced far enough apart that the

flow field becomes completely uniform

between one row and the next.

What is the collection efficiency of the

entire filter.

83

Solution:

ηoverall = 1 - poverall = 1 - (pindividual)n

= 1 - (1-0.084)100 = 0.9998 #

These three examples show, in idealized

form, what goes on within depth filters.

Most such filters do not have an orderly

array of parallel fibers; the filter medium

consists of a tangled jumble of fibers in a

random orientation, making up a thick mat.

84

In depth filters that diffusion leads to small

particle collection in addition to that

computed above by impaction.

Friedlander developed a theoretical

equation, with constants determined by

experiment, for the case of diffusion

collection of particles from a gas steam

flowing past a cylinder under

circumstances where impaction was

negligible.

85

Most of the published data could be represented by

where all the terms are as defined previously, Di is

the diffusivity and ν is the kinematic viscosity.

The first term on the right is for diffusion collection,

where as the second is for collection by noninertial

contact (interception)

2/3 2 1/ 2

1/ 2 3/ 21/ 6 1/ 2 1/ 2

6 3 (26)i

t

b b

D D

D D

86

Example 20

Repeat Example 17 for particles having a diameter of 0.1 µ. Take into account impactions, diffusion, and interception.

Solution: In this case Ns is (0.1)2 = 0.01 times the previous value, or 0.0062, for which, from Fig. 9.18 we can read ηt ≈ 0.

From Fig. 8.1, we can read that the diffusivity is about 6 10-6 cm2/s = 6 10-

10 m2/s.

87

So

The diffusion term is (0.0086 / 0.00025) = 34.4 times

the interception term.

Table 9.3 (next slide) shows the effect of changes in

velocity and particle diameter on the collection

mechanisms.

10 2 2/3 7 2 1/ 2

5 2 5 1/ 2 1/ 2 5 2 1/ 2 5 3/ 2

6(6 10 / ) 3(10 ) (1 / )

(1.49 10 / )(10 ) (1 / ) (1.49 10 / ) (10 )

0.0086 0.00025 0.0088 0.9% #

t

m s m m s

m s m m s m s m

89

There is some particle size at which there

is a minimum collection efficiency.

Typically, this size is in the range 0.1 to 1

µ, which is the size most likely to be

deposited in the human lung.

90

2.3 Filter Media

For shake-deflate baghouses, the filter

bags are made of tightly woven fibers

(surface filter), much like those in a pair of

jeans.

Pulse-jet baghouses use high strength

felted fabrics, so that they act partly as

depth filters and partly as surface filters.

91

Filter Fabrics are made of cotton, wool,

glass fibers, and a variety of synthetic

fibers.

Cotton and wool cannot be used above

180 and 200oF, respectively, without rapid

deterioration, whereas glass can be used

to 500oF (and short-time excursions to

550oF).

92

In addition the fibers must be resistant to

acids or alkalis if these are present in the

gas stream or the particles as well as to

flexing wear caused by the repeated

cleaning.

Typical bag service life is 3 to 5 years.

Sumber:

Control of Primary Particulates by Hsin Chu, Professor, Dept. of Environmental Engineering, National Cheng Kung University, Taiwan

Technology for Air Pollution Control by NN