Air Strainer

a device for removing dust from the air supplied to room spaces in ventilation and air conditioning systems or from the air used in manufacturing processes (for example, in the production of oxygen), in gas turbines, or in internal-combustion engines.

Air strainers are made in three classes according to efficiency (filtration capacity). First-class air strainers virtu-ally completely collect dust particles of all sizes (“absolute” strainers), second-class strainers efficiently collect dust particles larger than 1 micron, and third-class strainers remove particles larger than 10 microns. There are many types of air strainers, differing both in the design of the filtering element and in the materials used. Fiber, oil, and sponge air strainers, in which the dust particles are collected when they come in contact with the surfaces of the pores in the filtering material (bed), are the most commonly used.

Fiber strainers have porous filter beds of varying thickness made of fibers that are usually bound together with adhesives. In the fiber-roll air strainer, rolls of filtering material are mounted on reels in the top part of the strainer and re-wound onto reels to the lower part as the material fills with dust. The used materials are discarded; in certain cases they may be washed or cleaned pneumatically. In oil strainers the filter bed consists of metal gauze or perforated plates or rings that are moistened with mineral oil; they may be cellular or self-cleaning. In the latter type the filter bed is a strainer belt that runs continuously through an oil bath, which removes the dust particles. In sponge-type strainers the filter bed consists of spongy polyurethane foam or sponge rubber. In order to increase filter capacity, these materials are treated by a process that helps to open the pores; the filter bed is regenerated pneumatically or by washing. Electric (electrostatic) strainers, usually of the two-zone type, are also used: in the first (ionization) zone, minute dust particles receive a charge as a result of collisions with air ions whose flows are formed by means of wire corona electrodes; in the second (precipitation) zone, the charged dust particles are deposited on plate electrodes as a result of coulomb electric forces. The dust is then removed by periodic washing.