456
MODERN GASWORKS PRACTICE
ELECTRICAL PRECIPITATION
Electrical precipitation is the process for removing suspended particles from a gas by electrifying the particles statically, and then separating them from the gas by attraction to a surface charged with an opposite potential. A discharge of positive electricity manifests itself as a brush reaching out from the discharging surface, the suspended particles being electrified, while the gas itself is unaffected.
The principle introduced has been borrowed from methods employed in connection with smelter furnaces, where it has been customary to bring about precipitation of solid suspended particles escaping with the gases by means of a direct current high-tension electrical discharge. Investigations have shown tliat under certain conditions it is possible to tlirow down the suspended tarry vesicles in crude coal gas under identical conditions.
The installations, or treaters, that have been developed com-mercially are of two principal types, the pipe treater and the plate treater, the general action of both types being similar.
In operation, insulated wires are suspended down the centres of the pipes in the pipe treater and between the plates in the plate treater.
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I
From Exhauster
To Secondary Condenser
Fig. 283.—Electrical Discharge Apparatus.
The most common diameter of pipe, or distance between plates, is about 12 inches. The potential that must be maiirtained for such distances varies with the Chemical composition of the gas and with the temperature, from, say, 50,000 to 100,000 volts.
As the gas passes through the pipes or between the plates, suspended particles, eitlier liquid, or solid, are electaified by the discharge from the wires. The negatively elec-
trified particles are then. repelled from the negative wire and attracted to the surface of the plate or pipe tliat is charged positively.
The apparatus as employed by Professor White, of Michigan University, consists essentialfy of specially constructed electrodes suspended within a gas-tight chamber (Fig. 283). The chamber is composed of a cast-iron. inverted U-tube, constructed from 8-inch pipe covered on the outside with a jacketing of feit. Each of the arms of the U-tube is 9 feet long, and these form precipitating chambers. An electrode is suspended from the apex of each arm, and is made up from two cast-iron discs connected together by a light piece of gas pipe, while extremely fine discharge wires are afterwards stretched from disc to disc. The discs are 4 inches in diameter,