Zschocke washers have been used almost entirely in Germany for wet primary cleaning. These consist of cylindrical or square steel towers fitted with a series of wooden grids or hurdles placed at suitable intervals within the apparatus. These hurdles are arranged in such a manner that the water, which is sprayed in at the top of the tower, is broken up into very fine streams, which drip down between the grids and meet the gas coming up, the gas being introduced at the bottom of the tower. The intimate contact so obtained wets down the dust, which is carried with the water to the bottom. These Zschocke towers are usually water-sealed and cone-shaped at the bottom, and the latest type has a siphon arrangement; in either case, the dust is readily removed from the bottom of the apparatus.

"Zschocke towers have been found sufficient to cool and clean the gas to the proper degree for use in hot-blast stoves, under boilers, and for similar purposes. A fan washer, into which water is introduced, is frequently used as an auxiliary to the Zschocke towers for primary cleaning, especially when the scrubbing capacity of the towers is small.

"A water separator, equipped with internal baffles, is usually located beyond the washer to allow separation of the entrained water.

"Zschocke washers are used considerably in the United States, and some additional systems have also been developed here for the wet separation of dust; for instance, the Duquesne spray tower and the Steinbart spray tower. The basic principle of these spray towers consists of the creation of a rain or spray by means of suitably arranged nozzles, and the gas is cleaned and cooled in passing through this spray.

Duquesne Spray Tower

The Duquesne tower consists of a shell about 80 ft. high by 12 ft. in diameter. As shown in Fig. 191, the tower contains five sets of double screens, the sets being spaced 6 ft. 10 in. apart. Under the first set of screens are distributed seven nozzles, the feed water for which is controlled by a valve outside the tower. Under the fifth set of screens, seven similar nozzles, also controlled by a valve outside the tower, are distributed, just above the range of the lower nozzles.

"The controlling valves have a revolving core which successively blocks off the openings to the different nozzles, thereby temporarily stopping the flow of water and creating an area of low pressure directly above the nozzle. When the core has passed, the flow of water resumes through this nozzle and sprays the gas which has reached this point. The core is revolved electrically, at the rate of about 15 revolutions per minute, and a 5-h.p. motor is ample to operate four valves, which are sufficient for two towers.

"The screens, which are placed above the nozzles, break up the water into fine drops, permitting intimate contact of the gas and the water.

Section on Line 9-9. Gas Inlet. Fig. 191. Duquesne spray tower.

"In the operation of these towers at Duquesne, the gas rises through the scrubber at the rate of 4 ft. per second, and the water at the rate of 60 ft. per second with a head of 35 lb. main pressure. The gas is cooled down very effectively, the temperature of the outgoing gas being only from 5° to 6° Fahr. above the temperature of the incoming water, while the moisture content averages only about 0.5 grain per cubic foot above the saturation point at the temperature of the outgoing gas.

Bian Gas Washer

"The Bian gas washer, as shown in Figs. 192 and 193, consists of a stationary horizontal steel cylinder through which the gas passes from one end to the other. Inside the cylinder, there slowly revolves a shaft which carries a number of vertical disks consisting of wire netting of fine mesh. The diameter of these disks is very slightly less than the inside diameter of the cylinder, and this arrangement necessitates the gas passing through the openings in the screens as it travels through the apparatus. The screens, to the extent of nearly half their diameter, dip into water contained in a trough upon which the open bottom of the cylinder rests, and as the shaft revolves, the part of the screens which has been immersed rises from the water with the meshes covered with thin films of water, thus allowing thorough contact with the gas as it passes through the perforations.

Outlet. Figs. 192, 193. Bian gas washer.