Encyclopedia of Environmental Science and Engineering, Volume I and II

282

ELECTROSTATIC PRECIPITATION

This article deals with high efficiency (99.5%) particu-

late removal techniques often required of modern central sta-

tion power plants. The reader is also referred to the article

“Particulate Removal” for a discussion of control methods

including those used when more moderate conditions apply.

Electric power companies are required to analyze

proposals for, and subsequently to purchase, electrostatic

precipitators based on cost and performance.

The basic design factors which determine the collection

efficiency are the collecting plate area, the velocity of the gas,

the time that the gases are in contact with the discharge wires

and collecting plates, and the electrical system supplying the

useful power to the flue gas. It is the differences in these fac-

tors in the manufacturers’ proposals that give the engineer the

most trouble in choosing the precipitator that will continually

produce the required efficiency. The amount of useful power,

and therefore the collection efficiency, is primarily determined

by the number of active high tension electrical bus sections

into which the precipitator is divided (see Figure 1).

The collection efficiency of a precipitator is closely

related to the useful amount of electrical power than can

be supplied to the precipitator, the greater the useful power,

the higher the efficiency. If we imagine a precipitator with

all the discharge wires being supplied by one power source

through a single cable, the highest voltage that could be

maintained between the wires and the collecting plates

would be limited by the first wire to spark excessively. The

reason that one wire may spark excessively before another

is due to many factors including uneven distribution of the

gas and dust as they enter the precipitator, uneven build up

of ash on the wires and plates, mechanical misalignment of

the wires or plates and the fact that the collection process

produces a different amount of ash in the gas at the entrance

and discharge end of the precipitator. Even if all the wires

spark, at the same voltage, there is an appreciable loss in

efficiency due to lowered voltage in the wires operating in

parallel because the excessive sparks from one wire affect

all the others.

From this it is evident that the ideal precipitator would be

one in which each wire has its own stabilization control and

power source, but this, of course, would not be economically

feasible. Somewhere between these extremes is the practical

number of power sources or electrical bus sections that will

continually produce the desired efficiency.

Figure 1 shows the efficiency curve which may be used in

preparing specification and predicting actual operating efficiency.

This relationship between efficiency and active bus sections has

been referred to by White^2 as the “Ramsdell Equation.”

An active bus section refers to a separately energized

precipitator section where a transient electrical disturbance

in a given section is not reflected in any other section. This

condition exists when either one section is energized by

a single rectifier or when two sections are energized by a

double half wave rectifier.

A design criterion or an equation for the physical sizing of

precipitation is required. A curve based on Con Edison’s own

experience and that of other utilities is presented in Figure 2.

BUS SECTIONS PER 100,000 CMF

B

1

50

60

70

80

90

95

96

97

98

99

99.5

2345

COLLECTION EFFICIENCY, Ep, PERCENT

ELECTROSTATIC PRECIPITATOR

COLLECTION EFFICIENCY

Ep = 1–e–RB R = 1.3*

* CON EDISON

1.0% SULPHUR

300°F

FIGURE 1

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