Air Filter Types and Application ( HVAC )
Air filters of many types and materials of construction have been designed, manufactured, and applied to meet a wide variety of industrial and commercial requirements for clean air (e.g., industry makes full use of all filter types). Commercially available filters are divided into three distinct categories based on how they operate to remove suspended particulate matter from the air passing through them. The largest category, often referred to as ventilation or heating, and air conditioning (HVAC) filters, is composed of highly porous beds of resin-bonded glass or plastic fibers with diameters ranging from 1 to 40 micrometers (m). The fibers act as targets for collecting airborne dust. As their name indicates, HVAC filters are widely used for air cleaning in mechanical ventilation systems. They are almost all single-use, disposable items, and are used in all sectors of industry, including as pre-filters that reduce the amount of coarse dust reaching more efficient filters located downstream.
Typical HVAC filters:
Fiberglass Filter: Typically manufactured using medium to heavy cardboard frame with layered fiberglass reinforced with a metal mesh for support from collapse. Used in most HVAC filtration applications.
Polyester Filter Media Pad: Polyester offers higher resistance and dust holding capabilities than most fiberglass filters. Frequently used to replace fiberglass filters, which can fragment and send fibers into the air stream. Used in most HVAC filtration applications.
Electrostatic Filter: These filters create a self-generated charge helping to collect dust particles. These filters are typically manufactured using multiple layers of polypropylene media or similar within a galvanized frame or cardboard frame.
A second category also is comprised of single-use, disposable filters called HEPA filters. By definition, a HEPA filter is a throwaway, extended-medium, dry-type filter with: (1) a minimum particle removal efficiency of no less than 99.97 percent for 0.3-m particles, (2) a maximum resistance, when clean, of 1.0 inches water gauge (in.wg) when operated at 1,000 cfm, and (3) a rigid casing that extends the full depth of the medium [Note: Filters of different flows and resistances are allowable by the AG-1 Code. 2 A filter of identical construction and appearance, but having a filtering medium with a retention of 99.9995 percent for 0.1 m particles, is referred to as an ultra-low penetration aerosol filter (ULPA). The filtering medium of HEPA filters is thinner and more compressed, and contains smaller diameter fibers than HVAC filters. HEPA filters are widely used throughout industry.
A third category of commercial air filters is known as industrial cleanable cloth filters. As the designation indicates, these filters have built-in mechanisms for periodically cleaning the filtering surfaces of accumulated dust. Unlike the first two types, industrial cleanable cloth filters rely on building a thick layer of dust on the surface of the cloth to provide a high-efficiency filtering medium. This type of filter is used in industry involving high concentrations of coarse mineral dusts.
Further, this third category includes special types of particulate filters for chemical and combustion operations. These include deep beds of sand in graded granular sizes, deep beds of glass fibers, and stainless steel membranes formed from compressed and sintered granules or fibers. Stainless steel membrane filters operate like industrial cleanable cloth filters in that they depend on a dust layer for high-efficiency particle removal and must be cleaned periodically, usually by reverse compressed air jets.