Page 193 - Air Pollution Control Engineering

04_chap_Wang.qxd 05/05/2004 1:15 pm Page 172
172 Chung-Shin J. Yuan and Thomas T. Shen

Fig. 6. Components of a single-stage electrostatic precipitator of the flat surface type.

Flat–plate ESPs differ from plate–wire types in that the electrons consist of flat plates
rather than wires. A number of smaller precipitators use flat plates instead of wires.
These plates increases the average electric field used to collect particles, and they pro-
vide increased surface collection area, relative to plate wires. A flat-plate ESP operates
with little or no corona (a region of gaseous ions), which leads to high rapping losses,
particularly if the emission stream velocity is high. These ESPs perform well with
small, high-resistivity particles, provided the velocity is low.
Tubular ESPs are the oldest type and the least common. Tubular ESPs are typically
used in sulfuric acid plants, Coe oven byproduct gas cleaning (tar removal), and iron and
steel sinter plants. The tube is usually a circular, square, or hexagonal honeycomb with
gas flowing lengthwise through the system. The tubular ESP is most commonly applied
where the particles are wet or sticky.
The two-stage ESP is a series device where the first unit is responsible for ionization
and the second is responsible for collection. This results in more time for particle chang-
ing and economical construction for smaller (less or equal 50,000 acfm) applications.
Two-stage units are often used to collect oil mists, smokes, fumes, and other sticky par-
ticulates because there is little electrical for to hold the collected particles onto the plates.
Ionizing wet scrubbers (IWSs) also may be used as a particulate control device. An
IWS combines the principle of wet electrostatic particle charging with packed-bed scrub-
bing into a two-stage collection system. A constant DC voltage is applied to the ionizing