Encyclopedia of Environmental Science and Engineering, Volume I and II

838 PARTICULATE REMOVAL

fields with low dust concentrations can tolerate higher volt-

age without excessive sparking than can inlet fields. Further

electrical sectionalization within fields is often desirable.

A spark discharge momentarily hurts the performance of an

entire electrical section, and small sections obviously limit

the extent of the disturbance.

Electrodes are rapped with either pneumatic or solenoid

rappers. Variable frequency and intensity controls are usu-

ally provided so that rapping may be optimized. Ideally that

dust layer should be dislodged before becoming so thick as

to interfere with electrical performance but after becoming

thick enough to drop off in a coherent clump of particles.

Rapping to an appreciable extent redisperses collected par-

ticles back into the gas stream, and a puff of dust is often

seen in effluent gas when electrodes are rapped.

Precipitator Performance Parameters Precipitator col-

lection efficiency is commonly expressed by the Deutsch^35

equation:

h
 1 exp  ,

AW

Q

⎛

⎝⎜

⎞

⎠⎟

(4)

where

h 
 collection efficiency

A 
 plate electrode area

Q 
 volumetric gas flow rate

W 
 effective migration velocity rate of the dust.

In practice the migration velocity is an empirical constant

which depends on the application and dust and gas properties.

Typical migration velocities are given in Table 1. Although

a considerable range of values is shown for some applica-

tions, a large part of such variation can be attributed to known

causes and effects and does not represent uncertainty.

White^26 has derived a theoretical equation for migration

velocity

W

EEdopp

=

4 pm

,

where

E o 
 charging field gradient, stat V/cm

E p 
 collecting field gradient, stat V/cm

d p 
 particle diameter, cm

m 
 gas viscosity, poise.

Values of W measured on commercial installations are

lower by a factor of 2–5 than the theoretical value above.

Although the theoretical W is of little use for qualitative

FIGURE 8 Perspective view of electrostatic precipitator showing main fea-

tures of construction. (Courtesy Research Cottrell.)

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