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Electrostatistic Precipitation 187

of dust from the hoppers with regular frequency is also crucial for avoiding dust bridging
and hopper collapsing.

3.10. Instrumentation
Instrumentation is of major importance in electrostatic precipitation and falls under
the following two categories: (1) process instrumentation and (2) instrumentation for
electrical variables. Process instrumentation provides the measurement of process
variables such as gas flow rate, gas temperature, relative humidity, and gas pressure.
Variations in these process conditions can affect precipitator performance and it is
therefore necessary to monitor them during normal operation. Conventional instru-
ments such as Pitot-tube meters, thermocouples, hygrometers, and manometers are
used for this purpose. Various analytical instruments may also monitor specific com-
pounds in the gas stream. Other process instrumentation consists of sensors to measure
the dust level in the collection hoppers and to detect the intensity of the rappers for
rapping control.
Electrical variables that are measured are high voltage, current, and sparkover rate
for the discharge electrodes and readings for the rectifier equipment. Kilovoltmeters and
conventional milliammeters provide information whereby current to the discharge elec-
trodes may be set to provide the maximum voltage. Direct-reading sparkover-rate
meters are used to obtain the optimum sparkover rate for a given precipitator.
Oscilloscopes (CRT) are especially useful for studying sparking characteristics and
for troubleshooting electrical faults. Furthermore, oscilloscopes aid in the monitoring of
current and voltage waveforms. The optimum voltage wave shape is one that has a bal-
ance between the peak voltage and the average current or voltage, because the charging
field is determined by the peak voltage and the collecting field is a function of the average
current or voltage.

4. APPLICATIONS
Electrostatic precipitators have been used not only for collecting solid particles and
liquid droplets to comply with air pollution control regulations, but also for removing
particles in office buildings and stores and in manufacturing and process operations,
in which particle-free air is essential. Precipitators have also been used in industrial
processes to recover valuable materials such as copper, lead, or gold in the fluidized
catalyst process and soda ash in Kraft paper mills (9,12). Other applications pertain
to purifying fuel and chemical process gases for quality improvement, collecting par-
tially condensable vapors for chemical product or byproduct recovery, and separating
contaminant gases and vapors from gas streams by sorption on solid particles for later
removal (9,12,13). The major fields for the application of electrostatic precipitator are
summarized in the following subsections.

4.1. Electric Power Industry
Electrostatic precipitators in the electric power industry are used principally for col-
lection of solid particles from coal-fired power plants. The application constitutes the
largest single use of precipitators in the United States—about 75% of the total applica-
tion in terms of gas volume treated (19,25). The ash content of the coals being burned