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Electrostatistic Precipitation 173

section, which the emission stream passes through before introduction to the scrubbing
section. The electrostatic plates in the ionizing section are continually flushed with water
to prevent resistive layer buildup. The cleaned gas exiting the ionizing section is further
scrubbed in a packed-bed section. Unlike dry ESPs, IWSs are fairly insensitive to particle
resistivity. For best performance of IWSs, monitoring of plate voltage and packed-
bed-scrubbing water is recommended.
A rigorous design of a given ESP system can become quite complex, as it normally
includes consideration of electrical operating points (voltages and currents), particle
charging, particle collection, sneakage, and rapping re-entrainment. The most important
variable considered in the design of an ESP is the specific collection plate area assum-
ing that the ESP is already provided with an optimum level of secondary voltage and
current. Secondary voltage or current is the voltage or current level at the plates them-
selves, and this voltage and current are responsible for the electric field. The collection
plate area is a function of the desired collection efficiency gas stream flow rate and
particle drift velocity.
Pretreatment of the emission stream temperature should be within 50–100°F above
the stream dew point. If the emission stream temperature does not fall within the stated
range, pretreatment (i.e., emission stream preheat or cooling) is necessary. The primary
characteristics affecting ESP sizing are drift velocity of the particles and flow rate.
Therefore, after selecting a temperature for the emission stream, the new stream flow
rate must be calculated. The calculation method depends on the type of pretreatment
performed. The use of pretreatment mechanical dust collectors may also be appropriate.
In the emission stream (20–30 µm), pretreatment with mechanical dust collectors is
typically performed.

3.1. Precipitator Size
Although there are many variations in the details of determining the size of an ESP
to handle a given volumetric flow of gas, the Deutsch–Anderson equation or its modi-
fied form is generally used. Other design approaches are the use of tests in a pilot-scale
electrostatic precipitator to arrive at the design conditions or theoretical analysis to
extrapolate known conditions to those corresponding to the new requirements.
The Deutsch–Anderson equation provides the basis for the development of quantitative
relationship (i.e., η, w, A, and Q) in spite of the fact that other variables and conditions
must be included. These variables are discussed later. In engineering design practice,
however, the modified Deutsch–Anderson equation based on the empirical data has
been found to be practical for developing approximate solutions, which are sufficiently
accurate for determining the size of an ESP. Sometimes, the overall shape and size of
the ESP is governed by the space available, particularly in retrofit installations. The
ranges of design variables for ESPs (12,13,19) are summarized in Table 1. The values
of these variables vary with particulate and gas properties, with gas flow, and with required
collection efficiency. The typical values of migration velocity for various applications
are listed in Table 2.
The quantitative relationships of migration velocity, collecting plate area, gas flow
rate, and collection efficiency, as indicated in the Deutsch–Anderson equation can be
best illustrated by the following simple examples. It should be noted, however, that the