1.  Field of the Invention
The present invention pertains to porous media type air filter units having filter elements with full bore flow tubes including planar filter panels and oval shaped filter elements and multiple jet reverse flow cleaning systems for the filter elements wherein a substantial volume of jet cleaning air and entrained ambient air is used to flush the filter elements to remove accumulated dust particles. 
2.  Description of the Prior Art
The configuration of conventional air filter elements, being of substantially cylindrical annular pleated paper or cloth bag or being of V-shaped segmented bore, dual panel type structures has resulted in the development of reverse air flow cleaning systems wherein a jet nozzle is disposed in or somewhat downstream of the inner clean air chamber defined by the filter element and aimed in the opposite direction to that of the clean air flow through the filter element itself.  The single jet nozzle is typically adapted to provide a short burst or pulse of pressure air to create a pressure wave which flows through the filter element in reverse direction to dislodge or remove the dust which has accumulated on the surface of the element itself. 
The conventional design of reverse flow cleaning systems relies on the concept of delivering a relatively high pressure pulse of pressure air in the range of 80-100 psig and of very short duration.  However, with high pressure air pulse type reverse flow cleaning systems a relatively inefficient process is carried out and the reverse flow pressure wave unduly stresses the filter media.  In order to overcome the problems with prior art systems certain types of barrier filter elements such as the cylindrical porous media pleated paper type have been grossly derated as to their filtering capacity to prevent early filter element deterioration and failure due to the reverse pulse jet action.  One prior art type of filter cleaning system disclosed in Frey U.S. Pat.  No.  4,218,227 pleated paper porous media elements and requires derating of the cylindrical element for most applications to a maximum capacity of 1-3 cubic feet per minute of filter air flow per square foot of filter surface area. 
In the system of Frey U.S. Pat.  No.  4,218,227, the air discharging from the cylindrical bore of the pleated filter at a velocity of 500 to 1,000 feet per minute depending upon the filter media face velocities, which range from 1-3 feet per minute.  The velocity pressur due to the exiting flow stream is about 0.015 to 0.150 inches of water.  The losses associated with such velocity pressure are very low and are desirable.  The reverse flushing flow tube or venturi of the system of the '227 patent, which is mounted over the discharge end of the filter bore, is by necessity reduced in diameter relative to the filter bore in order to accommodate the eductor action for the single jet reverse flushing process.  This flow tube reduction tends to reaccelerate the filter bore discharge air stream, thus generating an undesirable energy loss.  Advances in self-cleaning air filter technology permit media face velocities in excess of the 1-3 feet per minute of the system of the '227 patent.  However, media face velocities in excess of 3 feet per minute leads to undesirably high exit velocity from the bore of the filter element, which leads to higher energy losses in current self-cleaning filter systems.  system disclosed in Copley U.S. Pat.  No.  4,359,330, provides for a cross draft flow stream passing perpendicular to the pulse jet discharge and across a zone of the filter media exterior.  The purpose of the cross draft, which flows parallel to the media pleat folds, is to facilitate movement of loosened particles from the face of the panel filter. 
Copley obviously has added energy through inclusion of the cross draft flow stream which in turn augments the energy from the pulse jet shock wave.  It is noted however that this system as well as other cleaning enhancement means disclosed by '330, provide for the filter pleat folds to be perpendicular to the energy discharge from the pulse jet.  This jet to pleat fold tends to remove particles from one outward facing pleat and subsequently impinge the particle into the next opposing outward facing pleat. 
Obviously, the planar panel, energy augmented, pulse jet cleaning system is somewhat complex relative to the straight-forward pulse jet cleaning disclosed by Frey.  The lack of pleat fold orientation relative to the pulse jet flow stream would add to cleaning problems. 
Another problem associated with prior art air filter systems and of the type having reverse flow filter cleaning is the amount of space occupied by the air filter elements and by the reverse flow jet nozzle and associated structure, particularly as the size of the filter element itself increases.  Since the cabinet structrue around the filter element and the element itself are desirably kept at a minimum size for a given filtering capacity, prior art pulse type air jet cleaning systems have occupied a disproportionately large amount of space in the filter cabinet structure which is not used to any other advantage. 
The capabilities of prior art reverse jet filter cleaning apparatus has also limited the configuration of the filter element and the filter flow area for a given filtering capacity and the flow area of the clean air discharge conduit or flow tube associated with the filter element.  For example, known types of reverse jet filter cleaning systems have been confined to use with filter elements which are substantially cylindrical tubular structures since the pulse delivered by a single jet nozzle will not conform to the shape of an irregular or noncircular filter cross-sectional shape and produce effective cleaning of the entire filter surface.  Such is the example of U.S. Pat.  No.  4,359,330, wherein a flushing jet diffuse is placed over the rather square discharge flow tube, wherein the pulse air from the single jet is diffused to optimize its efficiency in releasing particulate matter from the filter.  Copley offers an alternate means wherein the single conical pulse jet is released through a somewhat square operative into the flow tube.  Obviously, both measures present problems relative to flow restrictions and the ability to encompass the flow tube opening with the air pulse.  Perhaps the filter cleaning energy augmentation systems disclosed by Copley are directly related to their problems. 
The aforementioned problems in the art of porous media air filters including reverse jet cleaning systems therefor, together with the recognition of the need to provide a filter structure which has a maximum filtered air flow area, provides for a minimum filtered air pressure drop for a given filtering requirement, and a more thorough reverse flow cleaning system which will minimize stress on the filter media, has led to the development of the present invention.