fabric filter

Fabric filter

Particulate control equipment

FABRIC FILTER

MINIMUM PARTICLE

SIZE : less than 1 micron

% OF EFFICIENCY Bigger

than 99 %

ADVANTAGES

Dry collection possible

Decrease of performance is

noticeble

Collection of small particles

possible

High efficiencies possible

FABRIC FILTER

ADVANTAGES

Very high collection efficiencies even for very

small particles

Operate on a wide variety of dust types

Modular in design, and modules can be

preassembled at the factory

Can operate over an extremely wide range of

volumetric flow rates

FABRIC FILTER

DISADVANTAGES

Sensitivity to filtering

velocity

High temperature gases

must be cooled to 100

to 450C

Affected by relative

humidity

(condensation)

Susceptibility of fabric to

chemical attack

FABRIC FILTER

DISADVANTAGES

They require large floor areas

Fabrics can be harmed by high temperatures or

corrosive chemicals

They cannot operate in moist environment;

fabric can become blinded

They have potential for fire or explosion

Fabric Filter –Theory

∆P = total pressure drop

∆Pf = pressure drop due

to fabric

∆Pp = pressure drop due

to particulate layer

∆Ps = pressure drop due

to baghouse structure

∆Ps usually is low and

can be ignored

The particulate layer is a

very efficient filter but as

might be expected, it

increases the resistance to

gas flow. The pressure drop

through a baghouse at a

given gas flow rate is given

by

(1)


∆P = pressure drop N/m2

D =Depth (in the direction of

flow) of the filter and the

particulate layer

µ=Gas viscosity

V=Superficial filtering

velocity,m/min

K= Permeability of the filter

and the particulate layer m2

60 = conversion factor, s/min

From darcy’s equation for

fluid flow through porous

media, equations can be

written individually for the

fabric and the particulate

layer

(2)

(3)


As the filter operates and the layer of dust

grow, Dp increases. For a constant

filtering velocity and a constant mass

concentration of dust (often referred to

dust loading), Dp should increase linerly

with time

Superficial filtering velocity,V also

known as the air/cloth ratio, is equal

to the volumetric gas flow rate

divided by the cloth area

V= Q/A

Q = volumetric gas flow rate, m3/min

A = cloth area, m2

L = dust loading (kg/m3)

t = time of operation (min)

ρL = bulk density of the

particulate layer ( kg/m3)

(4)


and adding Eq.(2), Eq. (1) becomes Eq.(5);

Substituting Eq.(4) into Eq.(3)

(5)


K1 = (9)

K2 = (10)

Divide by V and define the filter drag S and the

areal dust density W as:

S = P/ V (6)

W = LVt (7)

S = filter drag, N-min/m3 or Pa-min/m

W = areal dust density, kg/m2 of fabric

S = K1 + K2 W (8)


S = K1 + K2 W (8)

Example 1 Based on the following test for a clean fabric, predict the design pressure drop in a baghouse after 70 minutes of operation with L = 5.0 g/m3 and V = 0.9 m/min.

Test Data

Time , min P, pa Constant

0 150 V = 09 m/min

5 380 L = 5.0 g/m3

10 505

20 610

30 690

60 990

Solution First, we use the test data to generate a plot of filter drag versus areal dust density. The data to be plotted are:-

S = P/V, Pa-min/m W = LVt, g/m2

167 0

422 22.5

561 45

678 90

767 135

1100 270

Plot these data and obtain the following Figure

W= 315 g/m2

S= 1205 Pa-min/m

∆P=1085 Pa

Recall this!!

S = K1 + K2 W (8)

DESIGN CONSIDERATIONS

Table 1 Maximum Filtering Velocities for Various Dusts in Shaker or Reverse Air Baghouse

Dusts Maximum Filtering

Velocity, cfm/ft2 or

ft/min

Activated Charcoal, Carbon Black, Detergent, Metal

Fumes

1.50

Aluminum Oxide, Carbon, Fertilizer, Graphite, Iron

core, Lime, Paint Pigment, Fly Ash, Dyes,

2.0

Aluminum, Clay, Coke, Charcoal, Cocoa, Lead Oxide,

Mica, Soap, Sugar, Talc

2.25

Bauxite, Ceramics, Chrome Ore, Feldspar, Flour,

Flint, Glass, Gypsum, Plastic, Cement

2.50

Asbestos, Limestone, Quartz, Silica 2.75

Cork, Feeds and Grain, Marble, Oyster, Shell, Salt 3.0-3.25

Leather, Paper, Tobacco, Wood 3.50

Table 2 Temperature and Chemical Resistance of Some Common Industrial Fabrics

Fabric Recommended

Maximum

Temperature, 0F

Chemical Resistance

Acid Base

Dynel 160 Good Good

Cotton 180 Poor Good

Wool 200 Good Poor

Nylon 200 Poor Good

Polypropylene 200 Excellent Excellent

Orlon 260 Good Fair

Dacron 275 Good Fair

Nomex 400 Fair Good

Teflon 400 Excellent Excellent

Glass 550 Good Good

Table 3 Number of Compartments as a Function of Net Cloth Area

Net Cloth Area, ft2 Number of Compartments

1-4000 2

4000-12000 3

12000-25000 4-5

25000-40000 6-7

40000-60000 8-10

60000-80000 11-13

80000-110000 14-16

110000-150000 17-20

>150000 >20

Example 2 Estimate the net cloth area for a shaker bghouse that must filter 40,00 cfm of air with 10 grains of flour dust per cubic foot air. Also specify the number of compartments to be used and calculate the total number of bags required if each bag is 8 feet long and 6 inches in diameter

fabric filter

Documents