Abstract:
A deep format filter element has a peripheral frame with outwardly protruding mounting flanges. The mounting flanges simulate the periphery of a shallow format filter element to allow the deep format filter element to replace the shallow format filter element in many situations.

Description:
This is a continuation of U.S. patent application Ser. No. 11/065,753, filed Feb. 25, 2005, now U.S. Pat.No. 7,090,713, which is a continuation of U.S. patent application Ser. No.10/140,405 filed May 6,2002, now U.S. Pat. No.6,875,250. 

   BACKGROUND OF THE INVENTION 
   It is customary to filter the air provided to occupied spaces by heating, ventilating, and air conditioning (HVAC) equipment. A typical HVAC system for a residence for example, has a fan that while operating draws air present within the occupied space through a return air intake opening and into a plenum or air duct leading to a furnace or air conditioner (generically, air processor) for reprocessing. 
   HVAC systems frequently cause the reprocessed air to pass through a filter to remove particulate contamination. One convenient and effective way to do this is to filter the air as it enters the return air intake opening in the plenum or duct leading to the air processor. This prevents dirty air from reaching heat exchanger surfaces. 
   The filter may be a simple mechanical filter with a disposable or renewable element, or may be electronic. The following description pertains to mechanical filters that collect these particles on or within the filter material through which the filtered air passes. 
   It is helpful at this point to define terms applying to mechanical air filters that will be frequently used in the description to follow. The medium of an air filter is the actual material that performs the filtering function. The air filter element is the entire disposable unit including the medium and any support structure that is installed in a filter housing and is discarded after the medium material has become clogged with air contaminants. 
   In residential systems, the medium is usually formed in a nominally one inch (2.5 cm.) thick rectangular shape. The filter element usually includes a frame forming the periphery of the filter element as a support structure. The medium is typically either a woven glass fiber mat, or pleated paper or other sheet-type medium material. The breadth and width dimensions of these filter elements vary to conform to the dimensions of the opening in which the element is to be installed, but typically are each on the order of two feet (61 cm.). 
   In a common design, the frame comprises flexible cardboard edging having a U-shaped cross section enclosing the medium&#39;s edges and a small portion of the medium&#39;s periphery adjacent the edges. The frame provides stiffness for the filter element and seals the edges against most air leakage around the filter element. This filter format will be referred to hereafter as a shallow filter element or shallow format filter. 
   The return air intake openings in which these filters are installed typically have annular or inwardly projecting sheet metal or plastic flanges around the entire interior periphery of the opening. The flanges&#39; outer surfaces all lie in a common plane and are set back from the opening a short distance. 
   The filter&#39;s frame is pressed against the flange&#39;s outer surface by force from a grille cover having an internal ridge bearing against the filter frame&#39;s outer surface to thereby create a tight seal between the outer intake flange surfaces and the inwardly facing filter frame surface. This tight seal forces most all of the air entering the plenum to pass through the filter element medium. 
   As one would expect, different types of air filters have different efficiencies. “Efficiency” in this context refers to the percentage of the total number of particles in the air stream within a given size range entering the filter that the filter element can catch. The efficiency of filters varies with different particle size ranges. For example, a high efficiency filter medium can catch a significant percentage of particles whose size is on the order of 0.3 micron, where a low efficiency medium catches relatively few of them. 
   There is also the consideration of overall efficiency as opposed to filter medium efficiency. Overall efficiency takes into account the air leakage around a filter element mounted in its housing. Leaking air is of course completely unfiltered. Its particle load pollutes the stream of filtered air, resulting in an overall efficiency lower than the medium efficiency. 
   But efficiency is not the only measure of medium quality. It is also important that a filter not create a large pressure drop in the air passing through it. A large pressure drop requires a more powerful fan to force the required air volume through it. And if the pressure drop is too great, the medium will deflect and perhaps even burst or tear as the load of trapped debris obstructs the air passages through the medium. 
   The amount of pressure drop presented by a particular medium depends largely on the number of voids or openings per unit area of the medium, on the average minimum cross section area of the pores, and of course on the total area of the medium through which the air flows. To a lesser extent, pressure drop is also dependent on the medium thickness, in the same manner that a long duct creates more resistance to air flow through than does a short duct, other things being equal. 
   Obviously, as a filter element loads up with debris during use, its pressure drop increases. This leads into a further consideration for filters, that of carrying capacity and filter element life. “Carrying capacity” refers to the number of particles the filter element can catch or hold per unit area projected to the air stream before clogging up to a point where the ability to remove particles is impaired and/or the pressure drop across the filter element becomes unacceptable. (“Dust-holding” capacity is an industry term that we intend to be substantially equivalent to carrying capacity.) Other things being equal, carrying capacity is directly related to total medium area. The capacity of mat filters which trap some of the particles within their volume may also depend to some extent on their thickness. Carrying capacity is one factor in determining the life of the element and thus the cost of filtering the air. 
   Filter technology advances have led to improvements in each of these characteristics. Nevertheless, it is still true that there are tradeoffs between efficiency, pressure drop, and carrying capacity. For example, as a filter medium becomes more efficient, its pressure drop typically increases because the individual passages through the medium become smaller, other things being equal. Of course, it may be possible to add more passages per unit area, but this is not a trivial problem. 
   Increasing the filter efficiency will often reduce the carrying capacity of the titter element. Often, higher efficiency produces a higher initial pressure drop. Thus as the filter element loads up with particles, the pressure drop reaches an unacceptable level more quickly. 
   An easy way to minimize pressure drop and maximize capacity is to increase total medium area. This fact has led to the development of the pleated filters mentioned. These pleated filters are made from a long strip of sheet filter medium which is folded back and forth on itself accordion-fashion to form a series of pleats. So long as the adjacent pleat panels do not touch each other the air can easily flow through the individual pleats. 
   In order to maintain spacing of adjacent pleats from each other under the force created by the normal pressure drop across the medium, it is possible to insert combs on the downstream side of the medium that have individual teeth between each pair of adjacent pleat panels. The teeth prevent adjacent pleats from collapsing against each other. 
   Improved filter media have been developed whose pressure drop and carrying capacity is superior to that of shallow format mat and pleated filters. These media typically have relatively deep pleats (4-5 in. or 10-12.5 cm.) to provide a relatively large medium area providing lower pressure drop and better carrying capacity. These deep pleat elements are intended for use in return air ducts having intake openings capable of receiving such filter elements. 
   In one design the filter elements collapse into a relatively small volume for shipping. They have relatively rigid cardboard or plastic end strips or panels that detachably mate with reusable side panels to form a reasonably rigid rectangular filter element. See U.S. Pat. No. 5,840,094 issued on Nov. 24, 1998 to Osendorf, et al. (&#39;094) for an example of such a collapsible filter medium which can be assembled into a deep format pleated filter element using a pair of special side panels. The filter element assembly is mounted in the return air intake, placing the filter element directly in the return air stream. 
   Collapsible filter media have the distinct advantage of compactness during shipping. But the time and effort required for assembling collapsible filter media for use is one disadvantage of them. The many pleats each require a tooth of the comb, whose insertion between each pair of pleats is time-consuming. And overall filtering efficiency suffers because of difficulty in providing a total air seal between the intake flanges of the return air duct and the cardboard sides of the filter element. 
   BRIEF DESCRIPTION OF THE INVENTION 
   We have developed a deep format filter element structure that requires no assembly and has a frame for supporting the filter element made of cardboard or other thin, flexible sheet-type material. This deep format filter element can replace a shallow format filter element in the same way that the filter element of the &#39;094 patent can do so. 
   The term “cardboard” hereafter includes not only for example the thin stock called “beverage board” in the industry and used for soda can boxes, but also includes corrugated cardboard, thin plastic, or even thin metal stock. The preferred beverage board material thickness for the filter element frame is on the order of 0.024 in. (0.60 mm.). The distinct characteristics of the “cardboard” material comprising the filter element&#39;s frame are that the material can be easily bent into 90° angles along the proper lines and can be securely assembled using easily-applied bonding agents. Scoring of the bend lines on the outside of the bend often makes accurate bending of cardboard easier. 
   The invention is a deep filter element having a filter medium with a plurality of edge planes defining a medium periphery and first and second opposite facing face planes through which air to be filtered can pass. A frame encloses the medium periphery. The frame has a plurality of body sections. Each body section has a wall in generally facing relationship to one of the medium edge planes. 
   At least first and second body sections each have a mounting flange projecting from the body section wall and away from the medium edge plane. Each mounting flange has a rectangular cross section defined by first and second side panels and an end panel. Each mounting flange simulates a part of the periphery of a shallow filter element. 
   The frame comprises at least a first cardboard sheet forming individual sides of the frame and sections of the flange. The terms “inside fold” and “outside fold” in the description of the invention refer to the relationship of the fold to adjacent surfaces of the cardboard sheet. An outside fold is a fold in the cardboard sheet that forms an included angle facing generally away from the filter element. An inside fold is a fold in the cardboard sheet that is not an outside fold. 
   For each mounting flange the first cardboard sheet includes at least two inside folds defining edges of the mounting flange&#39;s end panel and first and second side panels, and least one outside fold defining an edge of the second side panel of the mounting flange. In one preferred embodiment each of the frame sides has a flange projecting therefrom. 
   In this embodiment, the first side panels of each of the mounting flanges are generally coplanar and the second side panels of the mounting flanges are generally coplanar. The term “generally” is used here and throughout the description to mean “functionally” and “approximately”. Since the structural material here is relatively thin and flexible, normal loading and stress will often deflect the material to shapes that do not have the exact structural relationship stated. 
   In a further embodiment, each body section has a first fastening tab defined by the outside fold and end edge. The end edge forms an edge of the first fastening tab. The first fastening tab is bonded with glue or other type of fastening means to another surface of the body section. 
   For efficient assembly and improved rigidity of the assembled frame, it is advantageous to form each body section from a second cardboard sheet in addition to the first cardboard sheet. The first and second cardboard sheets are scored and folded to form different panels of the body section. The second cardboard sheet is glued to the first cardboard sheet to stabilize them into the desired shape of the body section. 
   A number of design variants exist that make the various panels as parts of one or the other of the first and second cardboard sheets. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a perspective view of a prior art shallow filter element. 
       FIG. 2  is an exploded perspective of an installation employing the filter element of the invention. 
       FIGS. 3-6  are cross-sections of the peripheries of filter elements having different versions of the structure of the invention. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   The prior art shallow format filter  10  shown in  FIG. 1  has a frame  11  surrounding a filter element  12 . Frame  11  is usually made from some type of relatively light, thin material such as beverage board. Frame  11  includes a flange  15  extending the length of each side of the filter element  12 . Braces  13  lend rigidity to frame  11 . In many cases, format of braces  13  will be substantially more complex than that shown to provide mechanical support for the relatively flexible material comprising frame  11 . 
   Filter element  10  is designed to fit into a return air intake opening  22  of duct  20  as shown in  FIG. 2 . Opening  22  may be constructed to have any combination of many different heights and widths These heights and widths are somewhat standardized however, so that perhaps 10 to 20 different sizes of filter elements  10  suffice to match most openings  22 . 
   Four duct walls  23  define opening  22 . Interior sealing flanges  25  project perpendicularly from each of the four duct walls  23 . Sealing flanges  25  are arranged in a coplanar fashion with each other. When filter element  10  is mounted for use in the opening  22  defined by the four duct walls  23  and the sealing flanges  25 , flanges  25  come into close contact with filter element frame  11 . 
   A door  30  is mounted on hinges  27  to swing between the open position shown and a closed position where door  30  covers opening  22  with an interior surface facing into the duct  20 . Door  30  has an internal door flange  38  near the periphery of door  30  and projecting perpendicularly from the interior door  30  surface. Openings  32  allow air to flow unimpeded through door  30 . A latch  35  holds door  30  in the closed position while allowing door  30  to be easily opened. 
   A filter element  11  suitable for duct  20  and mounted in opening  22  is held in slight compression between flanges  25  and  38 . This compression, along with the drag force arising from the air drawn through filter element  10  provides adequate mating between flanges  25  and  38  to resist air leakage between flanges  25  and frame  11 . 
   The invention is embodied in features of an improved deep format filter element  40 . Filter element  40  includes a pleated medium  46  and a filter frame  41 . While medium  46  need not be pleated, the advantage that a deep medium  46  has is most significantly available in today&#39;s technology from a pleated structure. Medium  46  has a generally orthogonal peripheral or surface shape defined by four rectangular edge planes  47  (one of which is shown on edge in  FIG. 3 ), and first and second rectangular face planes. Combs (not shown) may be placed on the downstream side of medium  46  to keep individual pleats from touching each other. Air to be filtered by medium  46  passes through the face planes as indicated by the wavy arrows  49 . 
   Frame  41  comprises the four body sections  52 , etc. and braces  48  and  49 , all of which may comprise sheet cardboard such as beverage board. At least two non-adjoining body sections  52 , etc. (and preferably all four) each include a mounting flange  60 ,  60   a , etc. integral with the adjacent body section. Braces  48  are typically integral with a side panel of mounting flanges  60 ,  60   a , etc. Braces  48  help to hold medium  46  within frame  41  and to lend rigidity to frame  41 . Braces  48  can have many different configurations to prevent flexing or bending of the individual frame sections  52 , etc. and the mounting flanges  60 ,  60   a , etc. The configuration for braces  48  shown in  FIG. 2  is simply exemplary, and less complex than that of a present commercial version. 
   Similar braces  49  on the side of filter element  40  facing away from the viewer are only shown on edge in  FIGS. 3 and 4 . Braces  49  are integral with a frame flange  101  (see  FIGS. 3-6 ). Braces  48  and  49  in addition to providing necessary rigidity for filter element  40  should also be configured to provide little resistance to air flowing through medium  46  and to allow maximum exposure of medium  46  to the air stream. 
   Body sections  52 , etc. enclose the edge planes  47  of medium  46 . Each body section  52 , etc. includes walls  54 , etc. facing the medium  46  edge planes  47 . The mounting flange  60 ,  60   a , etc. extends away from the adjacent edge plane  47  and projects above the adjacent wall  54 , etc. Mounting flanges  60   a ,  60   b , etc. are intended to simulate the frame  11  of the shallow format filter element  10  shown in  FIG. 1 . 
   Each body section  52 , etc. includes a frame flange  43  unitary with one side of mounting flange  60  and extending to overlap the adjacent face plane of medium  46 . Extending flange panel  43  to overlap medium  46  is a convenient way to enhance the overall stiffness of filter element  40 . A similar frame flange  101  is shown only on edge in  FIGS. 3-6 . 
   Walls  54 , etc. define a structure that will fit inside the intake flanges  25  and project into opening  22  of duct  20 . Similarly, flanges  60 ,  60   a , etc. must fit within duct walls  23  and form facing contact with flanges  25 . Duct  20  must be configured to permit filter element  40  to project past flanges  25  and into duct  20  without interference. 
   As suggested by the dotted alignment lines, filter element  40  fits into the intake of duct  20 . Walls  54  slip within the opening defined by the interior edges of flanges  25 . The outside-facing surfaces of flanges  25  seal against the facing surfaces of flanges  60 ,  60   a , etc. when filter element  40  is installed properly. Door  30  can be swung on hinges  27  to cover and retain filter element  40  in the intake of duct  20 . An air stream symbolized by wavy arrows  49  flows through openings  32  and the medium  46 , removing any contaminants that medium  46  is capable of removing. 
   The construction of frame  41  is shown in greater detail in the section view  3 - 3  of  FIG. 3  taken from  FIG. 2 .  FIGS. 4-6  are similar views of  FIG. 2  that show variant structures for frame  41 .  FIG. 3  shows most of the important characteristics of the invention.  FIGS. 4-6  show the variety of construction details that are possible in implementing the features of the invention. 
   Frame  41  can be constructed from beverage board or other light cardboard with thickness in the range of 0.020 to 0.030 in. (0.050 to 0.075 cm.). The thickness should be chosen to allow filter element  50  to maintain its shape during handling and to assist medium  46  as needed to resist air pressure forces while in use. Size of filter element  40  as well as characteristics of the available materials affects the thickness of the cardboard to be used. Note the earlier definition of sheet “cardboard”. As used herein. “cardboard” is defined relatively broadly to deal with the likely changes in technology or consumer preferences in the years to come. 
   The details of construction for four different variants of frames  41  are shown in  FIGS. 3-6 . Each of these variants has first and second cardboard sheets  70  and  71  forming at least a first body section  52  of frame  41 . First and second sheets  70  and  71  may be partially unitary or integral with one or more similar sheets forming parts of other body sections  52 , etc. 
   Individual planar sections of a cardboard sheet  70  or  71  between two folds or between a fold and an edge will be generally referred to as panels. Panels run longitudinally along a body section  52 , etc. and are defined along the long dimensions by two folds or one fold and an edge of the cardboard sheet  70  or  71 . For consistency, the same reference numbers have been given to an outer panel  73  and an inner panel  74  that cooperate to form a wall  54  in each of  FIGS. 3-6  regardless of which cardboard sheet  70  or  71  they are a part. Panels  73  and  74  are bonded together to form wall  54 , which is functionally identical in each of  FIGS. 3-6 . 
   Cardboard sheets  70  and  71  are first cut from sheet cardboard to the required shape and scored to assist folding. The sheets are then folded to form the individual panels of the cross section shape shown in  FIGS. 3-6 . 
   The external dimensions for each variation of body section  52 , etc. in  FIGS. 3-6  are similar. The structure here does not demand great dimensional precision. The dimensions are shown on  FIG. 5  only because more room is available than on  FIG. 3  for example. In each of  FIGS. 3-6 , W f  and W d  will typically be in the range of 1 in. (2.5 cm.) but of course can have a variety of convenient dimensions. W u  will typically range between 3 and 5 in. (7.5 to 12.5 cm.). 
   In  FIGS. 3-6 , the reader will note a small gap or space between adjacent cardboard panels as for example between cardboard panels  73  and  74  in each of  FIGS. 3-6 . This gap represents glue or other attachment means for fastening the adjacent cardboard panels to each other. In fact, these gaps can even represent staples for fastening adjacent sections to each other if this is found to be more efficient. 
   A typical assembly process includes cutting each sheet into the desired shape and then scoring according to established principles to assist bending. Then individual surfaces of the panels are coated with a layer of appropriate glue as needed to form the bond for fastening to the adjacent panel. So for example, panel  73  may be coated with glue to fasten panel  73  to panel  74 . 
   The glue should have suitable strength and harden quickly enough to avoid delay in further assembly and packaging of individual filter elements  40 . On the other hand the hardening time of the glue should be adequate to allow complete assembly before adhesion deteriorates. The glue can also be a contact type possibly requiring a coating on both surfaces to be bonded to each other. Glue can also be used to stabilize the individual pleats of medium  46  within frame  41 . All this is well known in the technology of cardboard products, and more particularly, in carton design and assembly. 
     FIGS. 3-6  show frame section  52  cross-sections that have structural variants, but that are functionally identical. Considering the specific variant shown in  FIG. 3  as representative, each frame section  52  includes the mounting flange  60 , frame flanges  43  and  101 , and wall  54 . The term “fold” will be used as the equivalent of “fold line” hereafter. 
   The first cardboard sheet  70  is scored on the appropriate side along each fold line, typically opposite the fold, and then folded at the fold lines. Fold  83  forms a first side of both side panel  86  and end panel  80  of mounting flange  60 . Side panel  86  and frame flange  43  are coplanar and together form a single larger panel. Fold  83  is an inside fold since the inside included cross section angle formed by panels  86  and  80  generally faces toward the side plane of medium  46 . 
   Fold  63  forms one side of both side panel  66  and a second side of end panel  80 , and is also an inside fold. Fold  72  is an outside fold that defines the second side panel  66  and outer panel  73 . Panel  73  forms a fastening tab in  FIG. 3  (and in  FIG. 5  as well) that is bonded to panel  74  in the position shown to collectively form wall  54 . Panel  73  may extend to fold  98 . 
   The second cardboard sheet  71  is scored on the appropriate side along each fold line, and then is folded to form folds  88  and  98 . Fold  88  forms one side of each of panels  74  and  90 . Panel  90  is a fastening tab that is bonded to the inner surface of flange panel  43 . 
   Fold  98  defines the second side of inner panel  74 , and the one side of the panel forming frame flange  101 . Inner panel  74  was mentioned earlier as bonded to panel  73  to collectively form wall  54 . Frame flange  101  overlaps a face plane of medium  46  to add stiffness to filter element  40 . 
   Braces  48  and  49  are integral with frame flanges  43  and  101  respectively. Typically, braces  48  and each of the first cardboard sheets  70  are cut from the same larger sheet of cardboard. The corners are formed with tabs that extend from at least one of the panels that form mounting flanges  60 , walls  54 , and frame flanges  43  and  101 . Several of these panels are folded 90° as needed and are bonded to the similar panel of the adjacent cardboard sheet  70  and  71 . 
   Forming these panels and then fastening them as described to the various other panels allows a filter element  40  to be formed from an appropriately sized medium  46  and an appropriately cut sheet of cardboard stock. The cardboard material forms all of the elements of frame  41 . 
   At least some of the portions of medium  46  that contact surfaces of cardboard sheets  70  and  71  can be bonded to the adjacent surfaces of those cardboard sheets. Such additional bonding lends more rigidity to filter element  40 . This construction has more than adequate rigidity and mechanical strength for a discardable filter element  40 . 
     FIG. 4  shows a frame section  52  where all of the various panels are a part of cardboard sheet  70  except for two panels  74  and  90  formed by second cardboard sheet  71 . Panels  74  and  90  are defined by fold  98  and form fastening tabs that are bonded or otherwise attached to the adjacent surfaces of panels  73  and  43  to provide the structural integrity for the frame  40 . 
     FIG. 5  shows another construction variant for a body section  52 . Panels of cardboard sheet  71  form both frame flanges  43  and  101 . A panel  43   a  that is part of panel  86  and sheet  70  is bonded to and is a part of frame flange  43 .  FIG. 5  is the actual commercial embodiment presently in use. Extending panel  73  to frame flange  101  may provide desirable added stiffness for wall  54 . 
     FIG. 6  shows a final variant for frame section  52 . Cardboard sheet  71  forms frame flange  43  only. A panel  43   a  that is part of panel  86  and sheet  70  is bonded to and is a part of frame flange  43 . 
   The invention taught by the above description has a frame  41  formed only of cardboard, and yet has adequate rigidity and mechanical strength to support a deep filter medium  46  when mounted in a typical intake  22  designed for a shallow filter medium such as medium  10 . The variant to be chosen depends on considerations that are beyond the scope of this description. Other variants that provide similar functionality are possible as well. All of these variants are within the scope of the invention.