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Electrostatistic Precipitation 171

Nonuniform gas velocity can result in excessively high gas velocity through some
sections of the electrostatic precipitator. The excessive turbulence from such high
velocities produces re-entrainment of the collected particles on the electrodes owing to
the scouring action of the gas. To prevent such erosion, various special designs of col-
lecting electrodes are used. The objective in all of these designs is to provide quiescent
zones to prevent or reduce erosion. The various designs increase collection efficiency
by (1) providing baffles to shield deposited particles from the re-entraining forces of
the gas stream, (2) providing catch pockets that convey precipitated particles into a
quiescent gas zone behind the collecting electrode, and (3) minimizing protrusions
from the plate surface in order to raise sparkover voltage. Furthermore, gas flows
through the hoppers can sweep collected particles back into the gas stream. This can
be minimized by installing baffles in the hoppers to reduce the circulation of gas
bypassing the electrodes through the hoppers.
The frequency and intensity of the rapping cycle have an important effect on collection
efficiency because the collected particles may falls as much as 12 m (40 ft) through a
transverse gas stream before reaching the hoppers. High collection efficiency requires
that the particles, when rapped loose from the collecting plate, should fall as coarse
aggregates so that they are not redispersed into the gas stream. This is achieved by
frequent, gentle rapping. Rapping cycles are determined experimentally after the elec-
trostatic precipitator is placed in operation. Typically, a rapping frequency of one impact
per minute may be used.

3. DESIGN METHODOLOGY AND CONSIDERATIONS
Electrostatic precipitator design involves (1) the determination of precipitator size
and electrical energization equipment required to achieve a given level of collection
efficiency, (2) the selection of the electrode systems, (3) the design of a gas flow sys-
tem to provide acceptable gas flow quality, (4) the structural design of the precipitator
housing, and (5) the selection of means to remove the collected particles. The overall
design must result in a completely integrated system. The essential components and par-
tial cross-sectional views of a typical single-stage electrostatic precipitator of the flat
surface type are given in Fig. 6.
Over the past 40 yr, significant improvements have been made in the design and con-
struction of electrostatic precipitator (EPS) components; however, in terms of design
practice, the present methodology is still based on empirical relations. The values for
current design variables were obtained mostly from experiences with similar ESP appli-
cations. Unfortunately, the records of these accumulated field experiences, often regarded
as proprietary, are unavailable to the public. Therefore, the designer will face many
decisions for which there are no clear-cut solutions.
In a plate–wire ESP, gases flow between parallel plates of sheet metal and high-
voltage electrodes. The electrodes consist of long weighted wires hanging between
the plates and supported by rigid frames. The gases must pass through the wires as
they traverse the ESP unit. This configuration allows many parallel lanes of flow and
is well suited for handling large volumes of gas. The cleaning and power supplies for
this type are often sectioned, to improve performance. The plate–wire ESP is the most
popular type.