Dry filters are usually made in standard units of definite rated capacity and resistance to the passage of air. At the time the installation is designed, ample filter area must be included so that the air velocity passing through the filters is not excessive; otherwise, proper filtration will not be secured. The proper area is secured by mounting the proper number of filters on an iron frame. Adry filter (Fig. 3-7) may be provided with permanent filtering material from which the dirt and dust must be removed pneumatically or by vibration. Or, it may be provided with an inexpensive filter medium that can be dispensed with when loaded with dirt and replaced with a new slab.
A dry-type filter is known as a dry plate. The cells are composed of a number of perforated aluminum plates, arranged in series and coated with a fireproof filament. Air drawn through the cell is split into thin streams by the perforations so that the dust particles are thrown against the filament by the change in the direction of the air between the plates.
The filament tentacles hold the dust particles and serve to purify the air. Dust particles are deposited on the intervening flat surfaces. When the plate is loaded with dirt and requires cleaning, this may be accomplished in two ways.
In moderate-sized installations, the operator removes the cells from the sectional frame, rests one edge on the table or floor, and gently raps the opposite side with the open hand, causing the collected dirt to drop out and restoring the efficiency of the filter. Larger installations (Fig. 3-8), where a great number of cells are used, or where an abnormally high dust load is encountered, use a mechanical vibrator in which the cell is placed and rapped for about a minute to free it of dust. Under average dust conditions, cells must be cleaned or replaced about twice a year.
