1.  Field of the Invention
This invention relates to filter elements for removing dust and other solid particles from air in a circulating air system, and more particularly to an improved reusable filter element which is intended to last for the life expectancy of a circulating air system such as a residential or commercial heating and air conditioning system with which it is used. 
2.  Description of the Prior Art
Reusable air filter elements for use in circulating air systems have been known for many years and numerous filter elements of this type are commercially available.  The known reusable filter elements have not met widespread acceptance, however, particularly for use in residential and light commercial applications, despite their obvious potential economic advantages, and the single use or throw away filter element continues to be used in the vast majority of such systems. 
One difficulty with the known prior art reusable filters is that, when such units were made sturdy enough to withstand repeated and aggressive cleanings, as by high pressure water streams, the filtering media were frequently ineffective in removing fine dust particles.  To avoid this defect, multiple layers of filtering medium, reinforced with metal supporting grids have been used, but this construction can result in increased flow resistance to the air passing through the filtering unit with a consequent reduction in efficiency of the overall heating and/or air conditioning system. 
One prior art reusable air filter element is disclosed in U.S. Pat.  No.  2,724,457 and employs a plurality of layers of electrostatic fabric woven from yarns of vinyl material such as vinyl chloride or a polyvinylidene such as seran retained in a generally rectangular frame by suitable means such as diagonally extending rods or by a grid or screen on the front and back of the filter and with at least one screen disposed between layers of the superimposed filter fabric to provide additional air pockets.  In one embodiment, the filter screen has crimps or spaced projections in its central portion to cause the center of the filter assembly to bow outward.  The layers of filter medium are all of the same vinyl fabric which is woven with irregularly shaped projections or pockets to disturb the flow of air through the filter and facilitate trapping of the dust particles by the electrostatic material. 
U.S. Pat.  No.  4,518,402 also discloses a reusable air filter employing a self-charging electrostatic filter fabric with at least one stage of the filter being three dimensional and folded over on itself to retain its filtering characteristics and provide a biasing force retaining other filter media in spaced relation.  Expanded metal screens are provided on the front and back of the assembly and ties are provided to hold the adjacent filter elements outwardly in contact with the retaining screens. 
Another known reusable filter comprises a single layer of open cell foam material supported in a frame by a metallic grill contacting one surface of the foam sheet and a removable retaining bar assembly on the other side whereby the foam sheet may be removed from the frame to facilitate cleaning by washing operation. 
To the best of applicant's knowledge, none of the prior art filters described above have met wide scale acceptance.  In the case of the known filters employing multiple fabric layers superimposed one upon the other, when sufficient layers of fabric are used to effectively filter fine dust particles and the like from air, flow resistance becomes unacceptable.  Similarly, with respect to the open cell foam material, flow resistance, particularly after accumulation of dust particles, results in severe energy loss and loss in efficiency of the circulating air system. 
It is an object of the present invention to overcome the deficiencies of the known prior art reusable filters by providing a simple, efficient filter structure which will effectively remove dust and other solid particles from an air stream while providing minimum resistance to air flow through the filter. 
Another object is to provide such a filter system which can be easily and repeatedly cleaned without damage to the filter. 
Another object of the invention is to provide such a filter constructed of materials which will not be damaged by atmospheres, including salt atmospheres, normally encountered in residential and light commercial heating and air conditioning systems.