Air Filters
Conventional air filters are commonly used in forced warm-air furnaces to trap and remove airborne particles and other contaminants from the air (see Figure 12-31). These filters are generally placed in the return air duct at a point just before the air supply enters the furnace or in the outdoor air intake ducts. In air conditioners, filters are properly placed ahead of heating or cooling coils and other air- conditioning equipment in the system to protect them from dust and dirt. Air filters are not used in gravity warm-air furnaces because they
cleaned when they become loaded with foreign matter. If this action is not taken, the efficiency of the furnace or air conditioner will be greatly reduced. As the dirt builds up on the filter, it increases the resistance of the passage of the air.
In a new house, the first set of filters may become clogged after a short time due to the presence of dust and dirt in the air created by the building operation. Check the filters after the first month of operation.
In an older house in which a winter air-conditioning system has been installed, the dust and dirt that accompanies the dismantling of the old heating system may also clog the first set of filters in a short time. The new filters should also be checked after about a month of furnace operation.
If new rugs or carpets have been installed in the house, consider- able lint will be given off at first. Under such conditions, replace- ment or cleaning of the filters will be necessary.
Dry Air Filters
A dry air filter consists of a dry filtering medium such as cloth, porous paper, pads of loosely held cellulose fiber, wool felt, or some similar material held together in a lightweight metal or wire frame.
Both washable and disposable (throwaway) dry air filters are used in forced warm-air furnaces. Disposable filters are constructed of inexpensive materials and are designed to be discarded after one use. The frame is frequently a combination of cardboard and wire. Washable filters usually have metal frames. Various cleaning methods have been recommended, such as air jet, water jet, steam jet, or washing in kerosene and dipping in oil. The latter method may serve both to clean the filter and to add the necessary adhesive.
Viscous Air Filters
A viscous air filter (or viscous-impingement air filter) contains a filtering material consisting of coarse fibers coated with a sticky sub- stance. This sticky substance catches the dust and dirt as the air passes through the mat. Viscous air filters can be reconditioned by washing them and recoating their surface with fresh liquid.
An oil or grease, sometimes referred to as the adhesive or saturant, is used as the viscous substance in these filters. The arrangement of the filter mat is such that the airstream is broken up into many small airstreams, and these are caused to abruptly change direction a number of times in order to throw the dust and dirt par- ticles by centrifugal force against the adhesive.
The method used for cleaning a viscous filter will depend on the filter and the dust and dirt particles trapped by it. Most dry dust and dirt particles, as well as lint, can often be removed by rapping the filter frame.
Filter Installation and Maintenance
Access to filters must be provided through a service panel in the furnace. Inspection and replacement of the filter by the user must be made possible without the use of special tools. When a new furnace is installed, care must be taken to provide sufficient clearance to the filter service panel. The furnace manufacturer will usually specify the minimum clearance in the installation instructions. An additional set of filter instructions should be attached to the filter service panel.
Always replace a disposable filter with one having the same dimensional size. The filter dimensions are printed on the filter frame. Always replace a filter with one of the same size.
The mat-type filter is commonly used in a gas-fired downflow furnace. It consists of a removable metal cage located in the blower cabinet of the furnace. The metal cage contains the filter material, which is either cut to size and prepackaged or available in rolls for measuring and cutting on-site. When installing a mat-type filter, make sure the side that collects the dust and other airborne contaminants (that is, the oily side) is facing upward into the cold-air duct.
Box-type filters are commonly installed in the top of the blower or in a slot in the cold-air return. Some of these filters have an arrow on the top of the frame. The arrow points to the direction of the airflow and the filter should always be installed with the arrow facing the blower. Most box-type filters have one side that is covered with wire mesh. The wire mesh should face the blower (see Figure 12-31).
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