Encyclopedia of Environmental Science and Engineering, Volume I and II

PARTICULATE REMOVAL 837

An excellent monograph on practical aspects of pre-

cipitator specification, operation, testing, maintenance, and

troubleshooting has been recently published by Katz.^33

Precipitator Operation Electrostatic precipitators have

been constructed in a great many configurations. Contemporary

commercial practice is fairly standard, however, and only

this will be considered here. A typical precipitator, shown in

Figure 8, consists of an array of parallel vertical plates, spaced

about 9 or 10 in. spacing. Rows of wires hang vertically

between the plates. These wires, called discharge electrodes,

are maintained at a voltage of 30–60 kV with respect to the

grounded plates. The very high electric field gradient near the

wire electrode produces a corona discharge generating a high

concentration of ions, which in turn charge the dust particles

in the flowing gas stream. Under influence of the voltage

gradient particles migrate to the plates, also called collector

electrodes, where the charge is largely dissipated. A residual

charge retains particles on the plates, and they are periodically

dislodged by rapping. When dislodged, the particles fall in

clumps into collection hoppers beneath the electrodes.

Gas flow across a flat plate surface tends to re-entrain

collected dust particles causing an appreciable debit to col-

lection efficiency. Various types of baffles are therefore used

to shield the plate surface and provide a stagnant collection

zone. The usual baffles are vertical and are perpendicular to

the plate surface. They are typically 20 in. apart and extend

1.5 in. from the plate surface. Experimental and theoretical

studies have shown that with such baffles re-entrainment of

typical dusts by gas flow at the plate surface is not signifi-

cant at gas velocities below 6 to 8 ft/sec.^34 This is well above

recommended precipitator gas velocity.

The power supply for a precipitator consists of a high

voltage step-up transformer and a rectifier. Rectifiers of

various types have been used over the years, but solid state

silicon rectifiers are almost universal in new installations.

Unfiltered double half wave rectification is usually used.

Full wave rectification is preferable for certain high current

applications, and a simple change in connections usually

suffices to change between the two wave forms.

Particle collection is enhanced by high voltage and current,

but excessive voltage causes undesirable arcing or sparking

between the electrodes. An arc is an ionized short circuit which

diverts most current away from charging and collecting parti-

cles and also causes an undesirable voltage drop. Impingement

of the arc on dust collected on the plate electrode can cause

reentrainment. Optimum overall collection efficiency usually

occurs when power input is adjusted to a spark rate of about

50–100 discharges/min. Modern precipitator control systems

will automatically adjust voltage to maintain a preset spark-

ing rate. Saturable core reactor control came into use in the

late 1950’s and a decade later this was being supplanted by

entirely solid state (thyristor, i.e. silicon controlled rectifier or

“S.C.R.”) controls. SCR control has appreciably lower power

losses and much faster response characteristics than the satu-

rable core reactor and is able to operate at appreciably higher

voltage and current without excessive sparking.

Large precipitators are usually sectionalized into several

chambers in parallel and (within chambers) several electri-

cal fields in series with respect to gas flow. Chambers can

usually be independently isolated from gas flow so that

one chamber may be shut down for maintenance while the

others remain on stream. Use of several electrical fields in

series permits optimum voltage control in each field. Outlet

FIGURE 6 Typical manifolding arrangement of multicellular

cyclones. (Courtesy: Wheelabrator Air Pollution Control.)

100

80

60

40

20

0

510 152025

PARTICLE DIAMETER, MICRONS

COLLECTION EFFICIENCY, %

Efficiency Of

Single Element

Efficiency Of

Multiple Array

FIGURE 7 Performance debit for manifolding of multicones.

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