Mechanical filters

Mechanical filters used for air cleaning can actually be further classified into two major types.

1. Flat or panel filters generally consist either of a low packing density of coarse glass fibers, animal hair, vegetable fibers, or synthetic fibers often coated with a viscous substance (e.g., oil) to act as an adhesive for particulate material, or slit and expanded aluminum. (A flat filter in use in many homes is the typical furnace filter installed in central heating and/or air-conditioning systems.) Flat filters may efficiently collect large particles, but remove only a small percentage of respirable size particles.
   
   Flat filters may also be made of "electret" media, consisting of a permanently-charged plastic film or fiber. Particles in the air are attracted to the charged material.
   
2. Pleated or extended surface filters generally attain greater efficiency for capture of respirable size particles than flat filters. Their greater surface area allows the use of smaller fibers and an increase in packing density of the filter without a large drop in air flow rate.

Some newer systems on the market are referred to as "hybrid" devices. They contain two or more of the particle removal devices discussed below. For example, one or more types of mechanical filters may be combined with an electrostatic precipitator or an ion generator.