FILTRATION
Air filtration is an important part of the central air handling system and is usually considered part of the ventilation system. Two basic types of filters are available: mechanical filters and electrostatic precipitation filters (also called electronic air cleaners). Mechanical filters are subdivided into standard and high efficiency.
Filters are selected according to the degree of cleanliness required, the amount and size of particles to be removed, and acceptable maintenance requirements. High-efficiency particulate air (HEPA) mechanical filters (Fig. 16) do not release the collected particles and therefore can be used for clean rooms and areas where toxic particles are released. HEPA filters significantly increase system pressure drop, which must be considered when selecting the fan. Figure 17 shows other mechanical filters.
Other types of mechanical filters include strainers, viscous coated filters, and diffusion filters. Straining removes particles that are larger than the spaces in the mesh of a metal filter and are often used as prefilters for electrostatic filters. In viscous coated filters, the particles passing through the filter fibers collide with the fibers and are held on the fiber surface. Diffusion removes fine particles by using the turbulence present in the air stream to drive particles to the fibers of the filter surface.
An electrostatic filter (Fig. 18) provides a low pressure drop but often requires a mechanical prefilter to collect large particles and a mechanical after-filter to collect agglomerated particles that may be blown off the electrostatic filter. An electrostatic filter electrically charges particles passing through an ionizing field and collects the charged particles on plates with an opposite electrical charge. The plates may be coated with an adhesive.
