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Electrostatistic Precipitation 183

Fig. 11. Collection efficiency as a function of corona power ratio. (From ref. 23.)

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values of energy requirement range from 100 to 1000 W/(m /s of gas flow) (see Table
1). To ensure continuous maximum collection efficiency, automatic voltage regulators
are generally used for adjusting voltages automatically, even under widely varying
operating conditions.
Modern power packs are equipped with a system of automatic voltage regulation to
maintain an optimum, average precipitator sparking rate, usually in the range of 5 or 10
to about 75 sparkovers per minute, according to the application and dust concentration
proceeding from outlet to inlet (27). Because maximum collection efficiency depends
predominantly on maximum voltage, power packs should have substantial overcapacity
both with regard to voltage as well as current.
Operating power consumption in an ESP mainly comes from corona power and
pressure drop, with corona power being the main source. The corona power can be
approximated by
P = I V (49)
c avg
c
where P is the corona power, I is the corona current, and V is the average voltage.
c c avg
Even though voltages in ESPs are very high, the current flow as a result of gas ion
migration is low. Thus, the power consumption is not unreasonably high. The effective
migration velocity of particles can be related to the corona power:
w = k' P c (50)
A
where w is the migration velocity of particles, k′ is simply an adjustable constant
(0.5–0.7), and A is the surface area of the collecting plates. The ratio of P to A (P /A)
c c
is also known as the power density. Although the power density often increases gradually
from the inlet of the gas flow to the outlet, the overall power density is a fairly stable and