Bag Filters. An alternative type of air filter is the ordinary bag filter, which consists of a number of closely woven fabric “stockings” in a cast iron container. The air is led into the casing so that it passes down through the inside of the tubes, through the fabric, where it deposits the greater proportion of the dust, and then out to the atmosphere.
Naturally, after working a certain period, the fabric becomes choked with deposited dust, and it is then necessary to dislodge the dust by shaking the “stocking” somewhat violently. This is usually carried out by a mechanically operated vibrating apparatus, but cleaning is done more effectively and time is saved if the air is shut off from the filter while the dust is dislodged. This is done most conveniently by having two similar filters installed, working in parallel; then, when cleaning becomes necessary, all the air is passed through each filter in turn, whilst the other is cleaned.
The bags or tubes hang vertically in the casing, and as the air is brought in at the top and discharged in a downward vertical direction, it naturally discharges the heavier material straight into the bottom of the casing, owing to the inertia of the heavy particles and the high velocity of the incoming air.
Fig. 5.—Automatic Bag Filter.
Showing filter bags through the open access doors
Fig. 5 illustrates a Sturtevant automatic bag filter. In this case the air is brought in at the bottom. The cleaning of the bags is effected automatically. At frequent intervals and in rotation, each section of bags is cut off automatically from the supply of dust-laden air by closing the outlet valve of that section. At the same time, that section is opened to the atmosphere at the top, causing a reverse current of air downwards. The bags are then agitated automatically and the dust adhering to the fabric drops, and is blown into the discharge hopper below. The opening and closing of the valves is accomplished by a simple mechanism driven by the pulley shown at the top of the illustration. Where the amount of dust to be handled is large and continuous it can be extracted from the hopper by a worm or screw conveyor, or, as in some other patterns, by a rotary valve placed in the bottom of the hopper.
Fig. 6.—Mollers’ Air Filter.
Mollers’ Air Filter. This apparatus consists of separate fabric pockets mounted on a frame. The pockets are rectangular and taper towards the top. Referring to Fig. 6, A is the frame on which the fabric is stretched, and B is the fabric pocket. As many of these units as necessary are mounted side by side in an adjustable frame, each unit having fixing and tension bolts. This arrangement permits a filter of any desired capacity to be built up quickly, and facilitates repairs and cleaning of filter bags.
Wet Filters. The type of filtering apparatus used for dealing with poisonous material, emery dust from grinding wheels, sand from sand-blasting apparatus, etc., is usually of the wet type, of which Fig. 7 is a good example. This apparatus consists of a tank having a wire shelf on which a layer of coke is supported. The tank is partially filled with water, the level of which is regulated by an overflow.