Page 177 - Air Pollution Control Engineering

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

Fig. 2. Types of electrostatic precipitators: (A) single-stage flat surface type; (B) single-stage
tubular type; (C) two-stage type.

Fig. 2B). In both forms of single-stage precipitator, the ionization and the collection of
particles/droplets are achieved in a single stage; that is to say, the corona discharge
and precipitating field extend over the full length of the apparatus. The two-stage pre-
cipitators differ in the sense that the ionization of particles/droplets is carried out in
the first stage confined to the region around the corona discharge wires, followed by
particle collection in the second stage, which provides an electrostatic field whereby
the previously charged particles are migrated onto the surface of collecting electrodes
(see Fig. 2C).
A gas stream with suspended particles/droplets is passed between the parallel plates or
through the cylinders. Assuming that a sufficient potential difference exists between the
discharge and collecting electrodes, a corona will form around the wires. As a result, large
numbers of negative and positive ions are formed in the corona zone near the wires. With
the discharge electrodes at negative polarity, the negative ions are attracted to the wires.
The particles/droplets moving with the gas stream in passing through the interelectrode
space are subjected to intense bombardment by the negative ions and become highly