THE CONDENSATION OF COAL GAS 455
through the spray, and assists in keeping free the openings in the screens. Around. the top portion of the central chamber is fitted a series of screens by means of which any excess of tar is removed, the gas then passing out by way of the annular chamber,
being cooled slightly by contact with the outer stell, but at the same time prosm-ving a higher and fairly even temperature within the central chamber. The lower part of
the central chamber forms a tar receiver from which the warm tar may, if so desired, be pump ed in continuous circulation throughout the apparatus.
The Fisher-Ficldes ex-tractor, Fig. 282, is an adaptation of the princip le employed in the removal of dust from hot producer gas. The hot stream of gas is passed through a rapidly moving film of clean gas, whereby the suspended particles are caused to impinge against a plate, being separ-ated by this process and limning to a special collection
chamber.
The gas enters at tlie bottom of the extractor and passes up a
coned passage at the top of which it meets a film of gas issuing at a high velocity from a narrow slot stretching round the periphery of a pressure cham-ber. The tar particles are deflected against the side of the main casing at an
angle of about 45 degrees, and they rapidly form a film which itself assists in arresting further particles. The best position for the apparatus is just prior to the exhausters,
a connection being taken from the exhauster outlet to the pressure chamber, and the gas pressure being reduced to 12 inches of water. At Tottenham gasworks tlie folio wing results1 were obtained from a series of tests :—
Test. Gas passed. Cubic feet. Extracted Gallons. Extracted per million cubic feet.
A. 186,000 7 35-0 gallons.
B. 750,000 22 29-0
C. 5,000,000 191 38-0
D. 3,500,000 72 20-6
E. 5,500,000 249 45-0
Average . 33-5 gallons.
1 Gas Journal, CXXXIX, p. 20.