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192 Chung-Shin J. Yuan and Thomas T. Shen

6. Changing raw materials. Changes in the raw materials used in a plant process can have a
profound effect on particulate resistivity. The basic factors that govern particulate conduc-
tivity can be used as guidelines in finding better raw materials from the viewpoint of
improving precipitation.
7. Graded-resistance electrodes. Making the electrodes of semiconductor material, which will
counteract the adverse effects of the high-resistivity deposits, alters the basic electrical
properties of the collecting electrodes.
It must be mentioned in passing that low particulate resistivity can also be a problem,
4
as was pointed out in Section 2.4.3. If the particulate resistivity is below 10 Ω-cm, the
collected particles are so conductive that their charges leak to ground faster than they
are replenished by the corona. With no charge to hold them, the particles are either
reentrained in the exit gas or they pick up positive charges and are repelled.
Nonuniform gas flow through a precipitator lowers performance in two ways. First,
uneven treatment of the gas lowers collection efficiency in the high-gas-velocity zones
to a degree not compensated for in the low-velocity zones. Second, particles already
captured may be blown off the plate surfaces in high-gas-velocity regions and be lost
from the precipitator. The second loss predominates where gas flow is especially bad.
Techniques available for controlling and correcting gas flow patterns include the use of
guide vanes to change gas flow direction, flue transitions to couple flues of different
sizes and shapes, and various types of diffusion screen and device to reduce turbulence.
The remainder of the fundamental problems can usually be corrected and even avoid-
ed by sound engineering design and judicious selection of precipitator components, as
discussed in Section 3.

5.2. Mechanical Problems

Mechanical problems are associated with (1) poor alignment of electrodes and sec-
tionalization design, (2) vibrating or swinging discharge wires, (3) bowed or distorted
collecting plates, (4) excessive dust deposits on electrodes, (5) full or overflowing with
collected dust in hoppers, (6) air leakage in hoppers, gas ducts, or shell, and (7) dust piles
in connecting gas ducts. A sound maintenance program based on routine measurements
and on-site observations is most effective and highly recommended.
5.3. Operational Problems

Operational difficulties are associated with (1) process changes, (2) poor electrical
settings, (3) mismatched power supply to load, (4) failure to empty hoppers, (5) over-
loading precipitator equipment by excessive gas flow rate and/or particle loading in gas
stream, and (6) upsets in operation of the furnace or process equipment to which the
precipitator is connected. To overcome these difficulties, it is essential to have a set of
simple but complete operation instructions or standard operation procedure (SOP) for
the electrostatic precipitator.
5.4. Chemical Problems

A large number of physical components of the electrostatic precipitator are exposed
to the potential attack of corrosive atmospheres, mainly acidic gases and moisture con-
tent in the gas stream. Critical zones of the precipitator that are most vulnerable to metal