Air filters are commonly installed in the outdoor air intake ducts of the air-conditioning system and often in the recirculating air ducts as well.
Air filters are logically mounted ahead of heating or cooling coils and other air-conditioning equipment in the system to prevent dust from entering. Filters should be installed so that the face area is at right angles to the airflow wherever possible (see Fig. 3-13). In most cases, failure of an air-filter installation can be traced to faulty installation, improper maintenance, or both.
The American Society of Heating and Ventilating Engineers gives the most important requirements for a satisfactory and efficiently functioning air-filter installation as follows:
? The filter must be of ample size for air it is expected to handle. An overload of 10 to 15 percent is regarded as the maximum allowable. When air volume is subject to increase, a larger filter should be installed.
? The filter must be suited to the operating conditions, such as degree of air cleanliness required, amount of dust in the entering air, type of duty, allowable pressure drop, operating temperatures, and maintenance facilities.
? The filter type should be the most economical for the specific application. The first cost of the installation should be balanced against depreciation as well as expense and convenience of maintenance.
The following recommendations apply to filters and washers installed with central fan systems:
? Duct connections to and from the filter should change their size or shape gradually to ensure even air distribution over the entire filter area.
? Sufficient space should be provided in front as well as behind the filter to make it accessible for inspection and service. A distance of 2 ft may be regarded as the minimum.
? Access doors of convenient size should be provided in the sheet-metal connections leading to and from the filters.
? All doors on the clean-air side should be lined with felt to prevent infiltration of unclean air. All connections and seams of the sheet-metal ducts on the clean-air side should be airtight.
? Electric lights should be installed in the chamber in front of and behind the air filter.
? Air washers should be installed between the tempering and heating coils to protect them from extreme cold in winter.
? Filters installed close to air inlets should be protected from the weather by suitable louvers, in front of which a large mesh wire screen should be provided.
? Filters should have permanent indicators to give a warning when the filter resistance reaches too high a value.
