The dust-laden air impinges on the surface of the water, and the major portion of the dust is trapped by being driven actually into the water. The dust particles too light to be brought into contact with the water are compelled to pass through the coke screen and are there arrested. If necessary, the scheme can be made to deal with finer dusts by having the coke constantly sprayed with water. The dust is reclaimed from the tank in the form of sludge or mud through the door or valve provided for that purpose.
Fig. 7.—Sturtevant Wet Filter.
Another type of air filter which might be developed in connection with the pneumatic handling of material consists of a very slowly revolving drum or cylinder, which is fitted with a continuous corrugated tape running spirally from the centre to the extreme edge of the casing. The space between the corrugated sheets is very small, and as a stream of water is continually running over and around the divisions, the air passing through the very tortuous path provided is bound to come into contact with these wet surfaces and give up its dust or other contamination, which is washed off when it arrives at the bottom of the cylinder. Naturally, this or any other wet filter would not be used where the recovery of dust in dry form was desired.
Another form of wet filter consists of a chamber of suitable proportions (according to the amount of air to be cleansed), fitted with racks in which are placed strips of glass at an angle of 45° to the flow of air, and at 90° to one another. The glass strips have a serrated or prismatic face, and the air carries the dust forward into the angles of the glass. A very fine water spray keeps the glass moist and eventually trickles down to the drain channels, washing the glass in its course.
The development of apparatus for air-washing has received considerable attention during recent years, owing to the necessity of having pure, dust-free air for ventilating turbo-generators, etc., and no difficulty should present itself in obtaining satisfactory results for pneumatic conveying plants, except in cases when the collection and retention of dust is required. In these cases the bag or fabric sheet filter is the only type available.
CHAPTER III
DETAILS OF PLANT—(Contd.)
DISCHARGERS, PIPE LINES AND SUCTION NOZZLES
Dischargers. The advocates of “blowing” material, instead of “sucking” it through the pipe, often lay great stress on the alleged difficulties of extracting the material from the system without allowing air leakage. This, however, has been overcome successfully by several designers and is not the serious difficulty so frequently suggested, providing that the material is suitable for this means of treatment. The only exception to this is, when the high velocity at which the material enters the discharger—say, 45 to 50 ft. per sec.—causes it to pack or bind so that it will not fall by gravity into the rotary valves and then out into the storage bins.
The function of the discharger is to cause the incoming material to lose its velocity and to fall into a compartment which can eventually be discharged after it has been moved from the low pressure to which the chamber itself is subjected.
This is best accomplished by the use of a rotary valve, somewhat similar to a paddle-wheel, which is revolved slowly but continuously. This wheel can be placed either vertically or horizontally at the bottom of the receiving chamber; the material enters the large chamber above, loses its velocity, drops by gravity, is caught in the box formed by the revolving paddle-wheel, and gradually is carried forward out of the chamber, eventually passing over an aperture through which it again gravitates to the bunker, silo, or other container. Probably the horizontal type of rotary valve is preferable because, owing to the increased surface exposed to the vacuum, the “suction” effect assists in holding the valve up to its seat.