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

very fine tungsten wires 5–10 mils in diameter with which everyone is familiar. The thin
wires operated at very low voltages (12-kV ionizer and 6-kV collector) and utilized cur-
rents of positive polarity. The compact size and lower cost for the collector were
achieved by using light aluminum plates spaced about 0.25 in. apart. These basic design
elements were incorporated in the “Precipitation” first marketed by Westinghouse in the
late 1930s. In general, the removal efficiencies of modern electrostatic precipitators can
approach 99.9% or higher (4). However, if not properly designed and/or operated, small
changes in the properties of particles/droplets or the gas stream can significantly affect
the removal efficiency of the electrostatic precipitators (5,6).
The electrical mechanisms for the precipitation of particles or droplets are provided
by discharge electrodes, which charge the particles or droplets in a corona discharge and
create the electrostatic field that causes the charged particles or droplets to migrate toward
the collecting electrodes. The essential components of the electrode system consist of one
or more discharge electrodes of relatively small diameter (such as wires) as well as col-
lecting electrodes (such as plates or tubes). In general, the discharge electrodes are of
negative polarity, whereas the collecting discharges are at ground potential and considered
positive polarity.
Electrostatic precipitation differs fundamentally from the fabric filtration and scrubbing
processes in that the separation forces are electrical and are applied directly to the parti-
cles/droplets themselves, rather than indirectly through the gas stream. The electrical
process has the inherent capability of capturing submicron particles or droplets at high
efficiency with relatively low energy consumption and small pressure drop through the
gas cleaning system. In comparison with other commercial particulate control devices,
electrostatic precipitators have the following advantages and disadvantages (7):
A. Advantages
1. High removal efficiency of fine particles/droplets
2. Handling of large gas volumes with low pressure drop
3. Collection of either dry powder materials or wet fumes/mists
4. Sustenance of a wide range of gas temperature up to approx 700°C
5. Low operating costs, except at very high removal efficiencies
B. Disadvantages
1. High capital costs
2. Unable to collect gaseous pollutants
3. Large space requirement
4. Inflexibility of operating conditions
5. Variation of removal efficiency with particle/droplet properties (e.g., resistivity of
particles/droplets)
This chapter is intended to serve as a guide to the understanding of electrostatic pre-
cipitation. It covers principles of operation, types of precipitator, design methodology,
major field of application, limitations, and future developments. The reader is referred to
refs. 4–13 for further reading on the subject of electrostatic precipitation.

2. PRINCIPLES OF OPERATION
Compared to other particulate control devices, electrostatic precipitators are as elegant
as they are efficient. Instead of performing work on the entire gas stream in the cleaning