Key Components of Air Distribution Systems (Filters)

September 18, 2023

Air filters are used to prevent airborne particulates from entering and recirculating within a building, and to protect fans, coils, other downstream equipment and the occupants. Most contaminants in the air stream are generated internally from occupants by items such as clothing, paper dust, and copier toner. External contaminants also contribute particulates to the indoor environment. For instance, engine exhaust contains a fine soot-like substance. Filters have various levels of filtration and are classified by ASHRAE Standard 52.2 by MERV (Minimum Efficiency Reporting Value).

Fibrous medium panel: The filter medium has a viscous surface and particulates strike the medium and stick. These range from the old standard of 30% (MERV 4) to 99.99% HEPA filters (MERV 15 and up). Sizes vary from 12″ square to 24″ square.

Renewable fibrous roll filters: These filters typically use MERV 4 filter material on rolls. A differential-pressure sensor activates a small motor that rolls the filter across the air stream in the air handler. This action does not replace the filter across the entire air stream but only enough to lower the pressure drop back to an acceptable level. This creates a low (less than design) velocity in the dirty portion and a higher (above the design) velocity through the clean portion.

Electronic air cleaners: These use electronic precipitation to filter the air. Particulates are positively charged as they pass through an ionization section. These positively charged particulates then pass through a collection section with positively DC-charged plates. The particulates are attracted to the plates and adhere. These air cleaners produce ozone, though rarely in concentrations that is a health hazard, when properly maintained.

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