Environmental Engineering FOURTH EDITION

Air Pollution Control 397

The collection efficiency of an electrostatic precipitator may be estimated by an
empirical equation

-AUd

R = 1 - exp (a>, (20.14)

where

A = total area of the collecting surface of the collection electrodes (m2),

Q = flow rate of gas through the pipe (m3/s), and

Ud = drift velocity (ds).

The drift velocity is the velocity of the particles toward the collecting electrode, and
may be cdculated theoretically by equating the electrostatic force on the charged
particle in the electrical field with the drag force as the particle moves through the
gas. The drift velocity is analogous to the terminal settling velocity, as given, for
example, in Eq. (18.9), except that in the latter case the force acting in opposi-
tion to the drag force is gravitational rather than electrostatic. Drift velocity may be
estimated by

ud = 0.5d, (20.15)

where d is the particle size (in pm). Drift velocities are usually between 0.03 and
0.2 ds.
As the dust layer builds up on the collecting electrode, the collection efficiency
may decrease, particularly if the collection electrode is the inside of a cylindrical pipe.
Moreover, some dust has a highly resistive surface, does not discharge against the
collection electrode, and sticks to the electrode. Heated or water-flushed electrodes
may solve this difficulty. Electrostatic precipitators are very efficient collectors of very
fine particles. However, the amount of dust collected is directly proportional to the
current drawn, so that the electrical energy used by an electrostatic precipitator can be
substantial, with resulting high operating cost.
Figure 20-1 1 shows the effectiveness of an electrostatic precipitator in controlling
emissions from a power plant. The large white boxes in the foreground are the elec-
trostatic precipitators. The one on the right, leading to the two stacks on the right, has
been turned off to show the effectiveness by comparison with the almost undetectable
emission from the stacks on the left.

Comparison of Particulate Control Devices

Figure 20-12 shows the approximate collection efficiencies, as functions of particle
size, for the devices discussed. Costs of collection also vary widely.