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Electrostatistic Precipitation 157

Fig. 3. Ionization of particles/droplets during the corona discharge.

charged in a short time (0.1 s or less). Typically, 1-µm particles/droplets will carry about
300 electron charges, whereas a 10-µm particle will carry about 30,000 electron charges
(12). The charged particles/droplets, in turn, being under the influence of the high
potential difference maintained between the discharge and collecting electrodes, are
attracted to the collecting electrodes and thus are separated from the gas stream. Solid
particles build up a layer on the collecting surface, from which the accumulated
deposit has to be periodically removed by rapping or flushing, and are allowed to col-
lect into a hopper. Liquid droplets form a film on the collecting surface, which then
drips off into a sump. Single-stage precipitators have proved to be universally appli-
cable in the cleaning of contaminated industrial gases, and two-stage precipitators are
generally used for domestic and commercial indoor air cleaning, especially when low
ozone generation is essential.
In the following subsections, some of the fundamental aspects of precipitator opera-
tion, such as corona discharge, electrical field, particle charging, and particle collection,
are analyzed.

2.1. Corona Discharge
The high-voltage direct corona utilized in the ionization stage of electrostatic precip-
itation is a stable self-maintaining gas discharge between a discharge electrode and a
collecting electrode. As the potential difference between the discharge and collecting
electrodes is raised, the gas in the vicinity of the more sharply curved electrode breaks
down at a voltage less than the spark-breakdown value for the gap length in question.
This incomplete breakdown, known as the corona, appears in air as a highly active region
of glow, extending into the gas a short distance beyond the discharge electrode.
Ionization processes of the corona discharge are confined to or near this glow region.
Most of the ionization is produced by free electrons, which then migrate and attach to
gas molecules, forming ionized gas molecules. The ionization process of corona discharge
is illustrated in Fig. 3.