229. Types of Screens.—The definitions of some types of screens as proposed by the American Public Health Association follow: A bar screen is composed of parallel bars or rods. A mesh screen is composed of a fabric, usually wire. A grating consists of 2 sets of parallel bars in the same plane in sets intersecting at right angles. A band screen consists of an endless perforated band or belt which passes over upper and lower rollers. A perforated plate screen is made of an endless band of perforated plates similar to a band screen. A wing screen has radial vanes uniformly spaced which rotate on a horizontal axis. A disc screen consists of a circular perforated disc with or without a central truncated cone of similar material mounted in the center. The Reinsch Wurl screen is the best known type of disc screen. A cage screen[[136]] consists of a rectangular box made up of parallel bars with the upstream side of the box or cage omitted. Allen[[137]] gives the following definitions: A drum screen is a cylinder or cone of perforated plates or wire mesh which rotates on a horizontal axis. A shovel vane screen is similar to a wing screen with semicircular wings and a different method of removing the screenings. Examples of a band screen, a wing screen, a shovel vane screen, a drum screen and a disc screen are shown in Fig. 150. A bar screen is shown in Fig. 151 and a cage screen is shown in Fig. 152.
Fig. 151.—Sketch of a Bar Screen.
Fig. 152.—Sketch of a Cage Screen.
Screens can be classed as fixed, movable, or moving. Fixed screens are permanently set in position and must be cleaned by rakes or teeth that are pulled between the bars. Movable screens are stationary when in operation, but are lifted from the sewage for the purpose of cleaning. Moving screens are in continuous motion when in operation and are cleaned while in motion. Fixed bar screens may be set either vertical, inclined, or horizontal.
Movable screens with a cage or box at the bottom are sometimes used. The box should be of solid material to prevent the forcing of screenings through it when the screen is being raised for cleaning. A mesh screen should be used only under special circumstances because of the difficulty in cleaning. Screens which must be raised from the sewage for cleaning should be arranged in pairs in order that one may be working when the other is being cleaned. Movable screens are undesirable for small plants because the labor involved in raising and lowering is greater than in cleaning with a rake and the screens are more likely to be neglected. In a large plant rakes operated by hand are too small for cleaning the screens. A fixed screen is sometimes used with moving teeth fastened to endless chains. The teeth pass between the parallel bars and comb out the screenings. If the screen chamber in a small plant is too deep for accessibility a movable cage or box screen may be desirable.
Moving screens are generally of fine mesh or perforated plates. They are kept moving in order to allow continuous cleaning. They are cleaned by brushes or by jets of air, water, or steam.
230. Sizes of Openings.—The area or size of the opening of a screen is dependent upon the character of the sewage to be treated and upon the object to be attained.
Large screens, with openings between 1½ inches and 6 inches are used to protect centrifugal pumps, tanks, automatic dosing devices, conduits, and gate valves from large objects such as pieces of timber, dead animals, etc., which are found in sewage. The quantity of material removed is variable, and is usually small.