Environmental Engineering FOURTH EDITION

394 ENVIRONMEWALENGINEEFSNG

used in industrial applications, although they may be sensitive to high temperatures
and humidity.
The basic mechanism of dust removal in fabric filters is thought to be similar to
the action of sand filters in water quality management, as discussed in Chap. 7. Dust
particles adhere to the fabric because of surface force that result in entrapment. Particles
are brought into contact with the fabric by impingement or Brownian diffusion. The
removal mechanism cannot be simply sieving, since fabric filters commonly have an
air space-to-fiber ratio of 1-to-1.
As particles adhere to the fabric, the removal efficiency increases, but so does the
pressure drop. The pressure drop is the sum of the pressure drop owing to the fabric
and the drop owing to the caked or adhered particles, and is expressed as

(20.12)

where

A P = overall pressure drop in meters of water,

v = superficial gas velocity through the fabric (in ds),

p = gas viscosity (in P),

x = thickness of filter (f) and particle layer (p) (in m), and

K = filter (f) and particle layer (p) permeability.

The value of K must be obtained by experiment for each type of fabric and type of

dust to be collected.

Wet Collectors

The spray tower or scrubber pictured in Figs. 20-4 and 20-8 can remove larger particles

effectively. More efficient scrubbers promote the contact between air and water by

violent action in a narrow throat section into which the water is introduced. Generally,

the more violent the encounter, hence the smaller the gas bubbles or water droplets, the

more effective the scrubbing. Aventuri scrubber, as shown in Fig. 20-9, is a frequently

used high-energy wet collector. Gas flow is constricted through a venturi throat section

and water is introduced as high-pressure streams perpendicular to the gas flow. The

venturi scrubber is essentially 100% efficient in removing particles >5 pm in diameter.

The pressure drop is estimated by an empirical equation

AP = v2L x (20.13)

where

A P = pressure drop across the venturi in centimeters of water,

v = gas velocity in the throat (in cds), and

L = water-to-gas volume ratio (in urn3).