Patent Publication Number: US-2007095037-A1

Title: Supplementary air filter and retainer

Description:
BACKGROUND OF THE INVENTION  
      The invention relates generally to air filters for forced-air heating, ventilation and air conditioning (HVAC) systems and, more particularly, to supplementary gas adsorption or absorption filters, such as filters including activated carbon, for removing odors and chemicals from indoor air.  
      Forced-air HVAC systems conventionally include air filters. Most common are air particulate filters including a filter medium, which typically is either a woven glass fiber mat, or pleated paper or other sheet-type medium material. In residential systems, a filter element is formed in a rectangular shape nominally one inch in thickness, and includes, in addition to the filter medium, a paperboard frame forming the periphery of the filter element as a support structure. Such filter elements for residential systems are available in a variety of more or less standard sizes, having dimensions of the order of two feet. HVAC systems thus include one or more filter mounts for receiving and holding the air filter element, located typically, but not necessarily, at the air intake opening of an air handler including a blower.  
      For removing odors and chemicals from indoor air, filter elements are known which include a gas adsorbent or absorbent material, such as activated carbon, bonded to and supported by a substrate such as a porous (open cell) foam material, or, particularly in the case of a pleated filter, a sheet-like substrate of non-woven fibers.  
      To avoid prematurely clogging the activated carbon filter, combination filters are also known, which include an upstream air particulate filter and a downstream activated carbon filter.  
     SUMMARY OF THE INVENTION  
      In one aspect, a retainer for a supplementary filter medium is provided for use within a filter mount within a forced-air heating and/or ventilation and/or air conditioning system in combination with a conventional air particulate filter element of the type which is rectangular and which has an upstream side and a downstream side. The retainer includes a frame-like planar flange having an upstream side and a downstream side, and having outer edges defining a rectangle sized to match the rectangular conventional air filter element, the frame-like planar flange defining a central opening for air flow; and a retaining structure for the supplementary filter medium extending from the downstream side of the frame-like planar flange adjacent the opening. The upstream side of the frame-like planar flange is positionable facing the downstream side of the conventional air particulate filter element within the filter mount.  
      In another aspect, a supplementary filter is provided for use within a filter mount within a forced-air heating and/or ventilation and/or air conditioning system in combination with a conventional air particulate filter element of the type which is rectangular which has an upstream side and a downstream side. The supplementary filter includes a supplementary filter medium formed to fit within a hexahedral volume defined by two opposed rectangular major sides and four edge sides establishing a thickness of the hexahedral volume, the supplementary filter medium having an upstream side and a downstream side; and a retainer for the supplementary filter medium. The retainer in turn includes a frame-like planar flange having an upstream side and a downstream side, and having outer edges defining a rectangle sized to match the rectangular air particulate filter element, the frame-like planar flange defining a central opening for air flow, and a retaining structure for the supplementary filter medium extending from the downstream side of the frame-like planar flange adjacent the opening. The upstream side of the supplementary filter medium and the upstream side of the frame-like planar flange are positionable facing the downstream side of the conventional air particulate filter element with the supplementary filter medium retained by the retainer within the filter mount.  
      In yet another aspect, a supplementary filter is provided for use within a filter mount within a forced-air heating and/or ventilation and/or air conditioning system in combination with a conventional air particulate filter element of the type which is rectangular and which has an upstream side and a downstream side. The supplementary filter includes a supplementary filter element including a supplementary filter medium formed to fit within a hexahedral volume defined by two opposed rectangular major sides and four edge sides establishing a thickness of the hexahedral volume, and a frame forming the periphery of the supplementary filter element as a support structure, the supplementary filter element having an upstream side and a downstream side; and a frame-like planar flange having an upstream side and a downstream side and attached to the frame generally on the upstream side of the supplementary filter element, the frame-like planar flange having outer edges defining a rectangle sized to match the rectangular air particulate filter element, and the frame-like planar flange defining a central opening for air flow. The upstream side of the supplementary filter element and the upstream side of the frame-like planar flange are positionable facing the downstream side of the conventional air particulate filter element with the supplementary filter element retained by the frame-like planar flange within the filter mount.  
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       FIG. 1  is an exploded three-dimensional view of a supplementary filter medium and a retainer therefor in combination with a conventional air particulate filter element in a filter mount within a representative part of an HVAC system, the retainer including a retaining grid;  
       FIG. 2  is a three-dimensional view of the supplementary filter medium and retainer therefor of  FIG. 1  in isolation, viewed from the upstream side thereof;  
       FIG. 3  is a three-dimensional view of the supplementary filter medium and retainer therefor of  FIG. 1  in isolation, viewed from the downstream side thereof;  
       FIG. 4  is an exploded three-dimensional view of a supplementary filter medium and a retainer therefor similar to that of  FIG. 1 , but showing a frame forming the periphery of the supplementary filter medium as a support structure to define a supplementary filter element, and inwardly-projecting flanges instead of a retaining grid to retain the supplementary filter element;  
       FIG. 5  is an exploded three-dimensional view of a supplementary filter medium and a retainer therefor nearly identical to that of  FIG. 4 , but showing a supplementary filter medium comprising a pleated sheet of material;  
       FIG. 6  is a three-dimensional view of a retainer including a retaining grid and a slotted opening, and a supplementary filter medium being inserted into position through the slotted opening, as an alternative to the retainer and supplementary filter medium of  FIG. 1 ;  
       FIG. 7  is a three-dimensional view of a retainer including a retaining grid and a slotted opening, and a supplementary filter element including a pleated supplementary filter medium being inserted into position through the slotted opening, as an alternative to the retainer and supplementary filter element of  FIG. 3 ;  
       FIG. 8  is a side elevational view, with a portion cut away to show an internal cross section, of a supplementary filter medium and a retainer therefor in combination with a conventional air particulate filter element in a filter mount within a representative part of an HVAC system, as an alternative to the HVAC system of  FIG. 1 ; and  
       FIG. 9  is a cross-sectional view of one embodiment of a supplementary filter including a supplementary filter element and an attached frame-like planar flange; and  
       FIG. 10  is a cross-sectional view of another embodiment of a supplementary filter including a supplementary filter element and an attached frame-like planar flange. 
    
    
     DETAILED DESCRIPTION  
      As described in detail hereinbelow, embodiments of the invention facilitate the use of supplementary filters, such as activated carbon filters for removing odors, in combination with conventional air particulate filters in an existing HVAC system, without modification to the HVAC system.  
      Referring first to the exploded three-dimensional view of  FIG. 1 , a structure  20  is representative of a forced-air HVAC system, and includes a representation  22  of a filter mount  22  of an HVAC system. The representative structure  20  including the filter mount  22  may be the air return portion of an air handler including heating and/or cooling elements (not shown), or may be the end of an air return duct remotely located from an air handler, as examples. A typical return air intake opening in which an air filter is installed has inwardly-projecting sheet metal or plastic flanges around the entire periphery of the opening. The flanges lie in a common plane and are set back from the opening a distance corresponding to the thickness of a conventional air particulate filter. In any event, air flow is drawn into the representative structure  20 .  
      Although the acronym HVAC conventionally refers to heating, ventilation and air conditioning system, it will be appreciated that not all such forced-air systems which employ filters include all three elements. Thus, a system may provide forced-air heating, with or without outside ventilation, but no air conditioning. Likewise, a system may provide air conditioning, with or without outside air ventilation, but no heating. Accordingly, it will be appreciated that embodiments of the invention are employed within forced-air heating and/or ventilation and/or air conditioning systems.  
      The  FIG. 1  filter mount  22  is designed to hold and retain a conventional air particulate filter element  24  of the type which is rectangular and nominally one inch in thickness. The conventional air particulate filter element  24  shown in  FIG. 1  is representative and includes a filter medium  26  in the form of a glass fiber mat  26  and a paperboard frame  28  which is U-shaped in cross section and which forms the periphery of the filter element  24  as a support structure. The filter element  24  has an upstream side  30  and a downstream side  32 . During operation, the filter medium  26  removes particulates from an air stream drawn into the representative structure  20 . In the exemplary filter element  24  of  FIG. 1 , in order to retain the glass fiber mat filter medium  26 , support grids are provided on the upstream and downstream sides  30  and  32 . In  FIG. 1 , a metal support grid  34  on the upstream side  30  is visible, in the form of a thin metal sheet  34  with circular apertures  36  through which air flow passes, as is well known. The support grids exemplified by the support grid  34  are adhesively attached to the paperboard frame  28 .  
      Positioned immediately downstream of the conventional air particulate filter element  24  is a supplementary filter medium  40  for removing odors from the air stream drawn into the representative structure  20 . Such a supplementary filter medium  40  typically comprises activated carbon, which functions to adsorb or absorb odors from the air. In the embodiment of  FIG. 1 , the supplementary filter medium  40  comprises a porous (open cell) foam support structure, with activated carbon particles bonded to surfaces of the foam material. Such so-called activated carbon filters are commercially available.  
      The supplementary filter medium  40  is similar in shape to the conventional air particulate filter element  24 , and thus is formed to fit within a hexahedral volume defined by two opposed major sides  42  and  44  of the filter medium  40  and four edge sides  46 ,  48 ,  50  and  52  establishing a thickness of the hexahedral volume. Accordingly, the supplementary filter medium  40  comprises a porous body within the hexahedral volume. The major side  42  corresponds to an upstream side  42  of the supplementary filter medium  40 , and the major side  44  corresponds to the downstream side  44  of the supplementary filter medium  40 .  
      For retaining the supplementary filter medium  40  in place, a retainer  60  is provided. The retainer  60  includes a frame-like planar flange  62  having an upstream side  64  and a downstream side  66 . The frame-like planar flange  62  has four outer edges  68 ,  70 ,  72  and  74  defining a rectangle sized to match the rectangular conventional air particulate filter element  24 , and the flange  62  defines a central opening  76  for air flow. The retainer  60  includes a retaining structure, generally designated  78 , for the supplementary filter medium  40 . The retaining structure  78  extends from the downstream side  66  of the frame-like planar flange  62  adjacent the opening  76 .  
      Although in the  FIG. 1  embodiment the supplementary filter medium  40  has a thickness comparable to the thickness of the conventional air particulate filter element  24 , there is no particular requirement that the supplementary filter medium  40  have the same thickness. By way of example and without limitation, the supplementary filter medium  40  can range in thickness from one-quarter inch to four inches, selected in view of air flow and odor removal characteristics desired. The rectangular dimensions of the supplementary filter medium  40  are slightly smaller than the rectangular dimensions of the conventional air particulate filter element  24 , since the rectangular dimensions of the supplementary filter medium  40  generally correspond to the size of the central opening  76  defined by the planar flange  62 .  
      The upstream side  64  of the frame-like planar flange  62  is positionable facing the downstream side  32  of the conventional air particulate filter element  24  when positioned or installed within the filter mount  22 . During use, the conventional air particulate filter element  24  and the supplementary filter medium  40  and retainer  60  in combination are assembled in a sandwich-like configuration having rectangular dimensions substantially identical to that of the conventional air particulate filter element  24 , but an increased thickness which is accommodated within available space within the HVAC system represented by the structure  20 . However, it is the combined thickness of the conventional air particulate filter element  24  and the planar flange  62  only of the retainer  60  which is required to be accommodated by the filter mount  22 , and not the entire thickness of the supplementary filter medium  40 . In most HVAC systems designed to hold a conventional air particulate filter element nominally one inch in thickness, there is a sufficient dimensional tolerance which, in addition, accommodates the thickness of the frame-like planar flange  62 . Most of the retaining structure  78  remains clear of the filter mount  22 .  
      Accordingly, it will be appreciated that the retainer  60  allows the supplementary filter medium  40  to be used in combination with existing conventional air particulate filter elements such as the filter element  24  within a standard HVAC system represented by the structure  22 , without any modifications to the filter mount  22  portion of the HVAC system.  
      In the embodiment of  FIG. 1 , the retaining structure  78  more particularly includes four sidewall segments  82 ,  84 ,  86  and  88  extending perpendicularly to the frame-like planar flange  62  adjacent the opening  76 . The sidewall segments  82 ,  84 ,  86  and  88  include corresponding upstream edges  90 ,  92 ,  94  and  96  attached to the frame-like planar flange  62 , and corresponding distal downstream edges  98 ,  100 ,  102  and  104 .  
      The  FIG. 1  retaining structure  78  additionally includes a retaining grid  106  attached to the distal downstream edges  98 ,  100 ,  102  and  104  of at least some of the sidewall segments  82 ,  84 ,  86  and  88 . The sidewall segments  82 ,  84 ,  86  and  88  and the retaining grid  106  accordingly together define at least a partial cavity-like structure for receiving and retaining the supplementary filter medium  40  in position against downstream forces. No retainer is required for the upstream side  42  of the supplementary filter medium  40  because, when assembled in the sandwich-like configuration, the supplementary filter medium  40  is retained in position against possible upstream forces (such as gravity at times when there is no air flow) by the downstream side  32  of the conventional air particulate filter element  24 .  
       FIGS. 2 and 3  show what may be viewed as or termed a supplementary filter  120  for use within the filter mount  22  of the forced-air heating and/or ventilation and/or air conditioning system represented by the structure  20 . More particularly,  FIG. 2  is a three-dimensional view of the supplementary filter medium  40  and retainer  60  therefor of  FIG. 1  in isolation, viewed from the upstream side thereof, that is, from the upstream side  42  of the filter medium and the upstream side  64  of the frame-like planar flange  62 .  FIG. 3  is a corresponding three-dimensional view of the supplementary filter medium  40  and the retainer  60  therefor of  FIG. 1  in isolation, viewed from the downstream side thereof, in particular, from the downstream side  44  of the filter medium  40  and the downstream side  66  of the frame-like planar flange  62 .  
       FIG. 4  is an exploded three-dimensional view similar to that of  FIG. 1 , but illustrating another embodiment of the invention. Conventional elements in  FIG. 4  are identical to those of  FIG. 1  and accordingly are not again described in detail. Those conventional elements in particular are the structure  20  representative of a forced-air HVAC system, including a representation  22  of a filter mount  22  of an HVAC system; and the conventional air particulate filter element  24 .  
      Positioned immediately downstream of the conventional air particulate filter element  24  in  FIG. 4  is a supplementary filter element  130  for removing odors from the air stream drawn into the representative structure  20 . Except for the filter medium, the supplementary filter element  130  is similar in construction to the conventional air particulate filter element  24 . More particularly, the supplementary filter element  130  includes a supplementary filter medium  132  which, like the supplementary filter medium  40  of  FIGS. 1-3 , typically comprises activated carbon, which functions to adsorb or absorb odors from an air stream drawn into the representative structure  20 . In the embodiment of  FIG. 4 , the supplementary filter medium  132  comprises a porous (open cell) foam support structure, with activated carbon filters bonded to surfaces of the foam material. The supplementary filter element  130  has an upstream side  134  and a downstream side  136 .  
      In addition to the supplementary filter medium  132 , the  FIG. 4  supplementary filter element  130  includes a paperboard frame  138  which forms the periphery of the supplementary filter element  130  as a support structure. The paperboard frame  138  is U-shaped in cross section, and is similar in construction to the paperboard frame  28  of the conventional air particulate filter element  24 . Again, the supplementary filter element  130  is similar in shape to the conventional air particulate filter element  24 , but with slightly smaller rectangular dimensions. In addition to the upstream side  134  and downstream side  136 , the supplementary filter element  130  has four edge sides  140 ,  142 ,  144  and  146 , establishing the thickness of the supplementary filter element  130 .  
      In order to retain the supplementary filter medium  132 , support grids, shown as representative metal support grids, are provided on the upstream and downstream sides  134  and  136 , and attached, such as by adhesive, to the paperboard frame  138 , similar to the metal support grid  34  of the conventional air particulate filter element  24 . In  FIG. 4 , a metal support grid  148  on the upstream side  134  is visible, in the form of a thin metal sheet  148  with circular apertures  150  through which air flow passes.  
      For retaining the supplementary filter element  130  including the supplementary filter medium  132  in place, a retainer  160  is provided. In the  FIG. 4  embodiment, as in the retainer  60  of  FIG. 1 , the retainer  160  includes a frame-like planar flange  162  having an upstream side  164  and a downstream side  166 . The frame-like planar flange  162  again has four outer edges  168 ,  170 ,  172  and  174  defining a rectangle sized to match the rectangular conventional air particulate filter element  24 , and the flange  162  defines a central opening  176  for air flow. The retainer  160  includes a retaining structure, generally designated  178 , for the supplementary filter element  130  including the supplementary filter medium  132 . The retaining structure  178  extends from the downstream side  166  of the frame-like planar flange  162  adjacent the opening  176 .  
      Although the supplementary filter element  130  has a thickness comparable to the thickness of the conventional air particulate filter element  24 , there is no particular requirement that the supplementary filter element  130  have the same thickness. The rectangular dimensions of the supplementary filter element  130  are slightly smaller than the rectangular dimensions of the conventional air particulate filter element  24 , since the rectangular dimensions of the supplementary filter element  130  generally correspond to the size of the central opening  176  defined by the planar flange  162 .  
      In  FIG. 4 , the upstream side  164  of the frame-like planar flange  162  again is positionable facing the downstream side  32  of the conventional air particulate filter element  24  when positioned or installed within the filter mount  22 . During use, the conventional air particulate filter element  24  and the supplementary filter element  130  and retainer  160  in combination are assembled in a sandwich-like configuration having rectangular dimensions substantially identical to that of the conventional air particulate filter element  24 , but an increased thickness which is accommodated within available space within the HVAC system represented by the structure  20 . However, it is the combined thickness of the conventional air particulate filter element  24  and the planar flange  162  only of the retainer  160  which is required to be accommodated by the filter mount  22 , and not the entire thickness of the supplementary filter element  130 . In most HVAC systems designed to hold a conventional air particulate filter element nominally one inch in thickness, there is a sufficient dimensional tolerance to, in addition, accommodate the thickness of the frame-like planar flange  162 . Most of the retaining structure  178  remains clear of the filter mount  22 .  
      In the embodiment of  FIG. 4 , the retaining structure  178  more particularly includes four sidewall segments  182 ,  184 ,  186  and  188  extending perpendicularly to the frame-like planar flange  162  adjacent the opening  176 . The sidewall segments  182 ,  184 ,  186  and  188  include corresponding upstream edges  190 ,  192 ,  194  and  196  attached to the frame-like planar flange  162 , and corresponding distal downstream edges  198 ,  200 ,  202  and  204 .  
      Instead of the retaining grid  106  of the embodiment of  FIG. 1 , the  FIG. 4  retaining structure  178  additionally includes inwardly-projecting flanges  210 ,  212 ,  214  and  216  attached to the distal downstream edges  198 ,  200 ,  202  and  204  of at least some of the sidewall segments  182 ,  184 ,  186  and  188 . The sidewall segments  182 ,  184 ,  186  and  188  and the inwardly-projecting flanges  210 ,  212 ,  214  and  216  together define at least a partial cavity-like structure for receiving and retaining the supplementary filter element  130  including the supplementary filter medium  132  in position. The paperboard frame  138  of the supplementary filter element  130 , and the metal support grids, such as the metal support grid  148 , provide sufficient rigidity for the supplementary filter element such that the retaining grid  106  as part of the retaining structure  78  of  FIG. 1  is not required.  
       FIG. 5  illustrates yet another embodiment of the invention, nearly identical to that of  FIG. 4 . The  FIG. 5  embodiment, instead of the supplementary filter element  130  of  FIG. 4  including the supplementary filter medium  132  comprising a porous (open cell) foam support structure, employs a supplementary filter element  220  including a supplementary filter medium  222  comprising a pleated sheet  222  of material.  
      Again, conventional elements in  FIG. 5  are identical to those of  FIG. 1 , and accordingly are not described in detail. Those conventional elements in particular are the structure  20  representative of a forced-air HVAC system, including a representation  22  of a filter mount  22  of an HVAC system; and the conventional air particulate filter element  24 .  
      The supplementary filter element  220  including the supplementary filter medium  222  is positioned immediately downstream of the conventional air particulate filter element  24  in  FIG. 5 , and functions to remove odors from the air stream drawn into the representative structure  20 . The supplementary filter medium  222  comprising a pleated sheet  222  of material is in part supported by a paperboard frame  224  which forms the periphery of the supplementary filter element  220  as a support structure. Again, the paperboard frame  224  is U-shaped in cross section, and is similar in construction to the paperboard frame  28  of the conventional air particulate filter element  24 . The supplementary filter medium  222  more particularly is made of activated carbon particles bonded to a substrate of non-woven fibers. The pleated configuration, in addition to providing additional surface area, enhances the structural rigidity of the supplementary filter element  220 . The supplementary filter element  220  is conventional in construction, and similar filter elements are commercially available, except that the rectangular dimensions required for the supplementary filter element  220  in embodiments of the invention are slightly smaller than standard filter element sizes. One example of a similar commercially-available filter element is an AmerSorb™ P activated carbon pleated panel filter sold by Precision Filtration Products, P.O. Box 218, Pennsburg, Pa. 18073 (www.precisionfiltration.com). Such activated carbon pleated panel filters are available in one-inch, two-inch, and four-inch thicknesses.  
      Again, the supplementary filter element  220  has an upstream side  226  and a downstream side  228 , as well as four edge sides  230 ,  232 ,  234  and  236  establishing the thickness of the supplementary filter element  220 .  
      For retaining the supplementary filter element  220  in place, in  FIG. 5  the retainer  160  is employed as described hereinabove in detail with reference to  FIG. 4 . The retainer  160  includes the frame-like planar flange  162 , as well as the retaining structure  178  in turn including inwardly-projecting flanges  210 ,  212 ,  214  and  216  attached to the distal downstream edges  198 ,  200 ,  202  and  204  of at least some of the sidewall segments  182 ,  184 ,  186  and  188 .  
      With reference now to  FIG. 6 , illustrated is a supplementary filter medium retainer  240  embodying the invention including a retaining structure generally designated  242 , which defines an insertion slot  244  for a supplementary filter medium  246  for removing odors from an air stream drawer through the filter medium  246 . The retainer  240  with filter medium  246  in place can be employed as an alternative to the retainer  60  and supplementary filter medium  40  of  FIG. 1 , as an alternative to the retainer  160  and supplementary filter element  130  of  FIG. 4 , or as an alternative to the retainer  160  and supplementary filter element  220  of  FIG. 5  within the representative HVAC structure  20 . The insertion slot  244  allows the supplementary filter medium  246  to be replaced, without opening up the sandwich-like construction of the retainer  240  against the conventional air particulate filter element  24 .  
      As in the  FIG. 1  embodiment, the retainer  240  of  FIG. 6  includes a frame-like planar flange  248  having an upstream side  250  and a downstream side  252 . The frame-like planar flange  248  has four outer edges  254 ,  256 ,  258  and  260  defining a rectangle sized to match the rectangular conventional air particulate filter element  24 , and the flange  248  defines a central opening  262  for air flow. The retaining structure  242  extends from the downstream side  252  of the frame-like planar flange  248  adjacent the opening  262 .  
      In the embodiment of  FIG. 6 , the retaining structure  242  includes three sidewall segments  264 ,  266  and  268  extending perpendicularly to the frame-like planar flange  248  adjacent the opening  262 . The sidewall segments  264 ,  266  and  268  include corresponding upstream edges  270 ,  272  and  274  attached to the frame-like planar flange  248 , and corresponding distal downstream edges  276 ,  278  and  280 . There is no fourth sidewall segment, and the insertion slot  244  is defined at the location of the absent fourth sidewall segment. A retaining grid  282  is attached to the distal downstream edges  276 ,  278  and  280  of at least some of the sidewall segments  264 ,  266  and  268 . In a manner comparable to the  FIG. 1  embodiment, the sidewall segments  264 ,  266  and  268  and the retaining grid  282  together define at least a partial cavity-like structure for receiving and retaining the supplementary filter medium  246  in place against downstream forces. The  FIG. 6  retaining structure  242  differs from the  FIG. 1  retaining structure  78  primarily in that the insertion slot  244  is included.  
       FIG. 7  illustrates another supplementary filter medium retainer  290  embodying the invention, for use in combination with the more rigid supplementary filter element  220  comprising the pleated supplementary filter medium  222  described hereinabove with reference to  FIG. 5 , as an example. The  FIG. 7  retainer  290  includes a retaining structure generally designated  292 , which defines an insertion slot  294  for the supplementary filter element  220  including the pleated secondary filter medium  222 . The retainer  290  with supplementary filter element  220  in place can be employed as an alternative to the retainer  60  and supplementary filter medium  40  of  FIG. 1 , as an alternative to the retainer  160  and supplementary filter element  130  of  FIG. 4 , as an alternative to the retainer  160  and supplementary filter element  220  of  FIG. 5 , or as an alternative to the retainer  240  and supplementary filter medium  246  of  FIG. 6  within the representative HVAC structure  20 . The insertion slot  294  allows the supplementary filter element  220  to be replaced, without opening up the sandwich-like construction of the retainer  290  against the conventional air particulate filter element  24 .  
      The retainer  290  of  FIG. 7  includes a frame-like planar flange  298  having an upstream side  300  and a downstream side  302 . The frame-like planar flange  298  has four outer edges  304 ,  306 ,  308  and  310  defining a rectangle sized to match the rectangular conventional air particulate filter element  24 , and the flange  298  defines a central opening  312  for air flow. The retaining structure  292  extends from the downstream side  302  of the frame-like planar flange  298  adjacent the opening  312 .  
      The  FIG. 7  retaining structure  292  includes three sidewall segments  314 ,  316  and  318  extending perpendicularly to the frame-like planar flange  298  adjacent the opening  312 . The sidewall segments  314 ,  316  and  318  include corresponding upstream edges  320 ,  322  and  324  attached to the frame-like planar flange  298 , and corresponding distal downstream edges  326 ,  328  and  330 . There is no fourth sidewall segment, and the insertion slot  294  is defined at the location of the absent fourth sidewall segment. Instead of the retaining grid  282  of the embodiment of  FIG. 6 , the  FIG. 7  retaining structure  292  additionally includes inwardly-projecting flanges  332 ,  334  and  336  attached to the distal downstream edges  326 ,  328  and  330  of at least some of the sidewall segments  314 ,  316  and  318 . In a manner comparable to that of the retaining structure  178  of  FIGS. 4 and 5 , in  FIG. 7  the sidewall segments  314 ,  316  and  318  and the inwardly-projecting flanges  332 ,  334  and  336  together define at least a partial cavity-like structure for receiving and retaining the supplementary filter element  220  including the pleated supplementary filter medium  220  in position. To further support the filter element, a support bar  338  extends along the insertion slot  294 .  
      Referring now to  FIG. 8 , another structure  360  is representative of a forced-air HVAC system.  FIG. 8  in particular illustrates how embodiments of the invention may be employed within an HVAC system structure  360  which has a slotted opening  362  which is sufficiently wide for insertion of a conventional air particulate filter element  364 , but not wide enough to accommodate the combined thickness of the conventional air particulate filter element  364  and a supplementary filter medium  366  when assembled together in a sandwich-like configuration. The upper part of  FIG. 8  is a side elevational view, which is broken away to show a cross section in the lower part of  FIG. 8 .  
      The  FIG. 8  structure  360  includes a round air return duct  370 , an air filter housing  372 , and an air handler represented at  374  including heating and/or cooling elements (not shown). Air flow from the air return duct is drawn into the air handler  374 . The air filter housing  372  accordingly in part serves as a transition between the air return duct  370  and the air handler  374 .  
      The air filter housing  372  more particularly includes a side panel  376  within which the slotted opening  362  is formed. A hinged access door  378  is shown in its open position in  FIG. 8 . A filter mount  380  is provided within the air filter housing  372 , in the form of upper and lower U-channels  382  and  384 . In  FIG. 8 , the legs of the upper U-channel  382  are hidden behind the side panel  376 , and only a portion of the upper U-channel  382  is visible through the open slotted opening  362 . The lower U-channel  384  is shown in cross-section. Upstream and downstream legs  386  and  388  of the lower U-channel  384  are spaced a distance apart sufficient to accommodate a conventional air particulate filter element which is nominally one inch in thickness. However, typically there is a sufficient dimensional tolerance which, in addition, can accommodate the thickness of a frame-like planar flange which is part of embodiments of the invention, such as the frame-like planar flange  192 .  
      The particular conventional air particulate filter element  364  represented in  FIG. 8  by way of example includes a filter medium  392  in the form of a glass fiber mat  392  and a paperboard frame  394  which is U-shaped in cross section and which forms the periphery of the filter element  364  as a support structure. A conventional air particulate filter element employing a pleated filter (not shown) may as well be employed. In order to retain the glass fiber mat filter medium  392 , support grids  396  and  398  are provided on upstream and downstream sides  400  and  402  of the conventional air particulate filter element  364 .  
      The supplementary filter medium  366 , by way of example, is identical to the supplementary filter medium  222  described hereinabove with reference to  FIG. 5 , and is part of the supplementary filter element  220  described hereinabove in detail with reference to  FIG. 5 . Also shown in  FIG. 8  is the retainer  160 , described hereinabove in detail with reference to  FIG. 4 .  
      Although the  FIG. 8  slotted opening  362  is not wide enough to accommodate the combined thickness of the conventional air particulate filter element  364  and the supplementary filter element  366  when assembled together in a sandwich-like configuration, the two can separately be inserted through the slotted opening  362 . During installation, the retainer  160  with the supplementary filter element  220  in place is inserted through the slotted opening  362 , and is then shifted in a downstream direction (to the right in the  FIG. 8  orientation) until the downstream side  166  of the frame-like planar flange  162  contacts the downstream legs of the U-channels  382  and  384 , the downstream leg  388  of the lower U-channel being visible in  FIG. 8 . Sufficient clearance remains so that the conventional air particulate filter element  364  can be inserted through the same slotted opening  362 , and the access door  378  closed. The process is reversed for removal.  
      Referring next to  FIG. 9 , a supplementary filter  410  embodying the invention includes a supplementary filter element  412  essentially identical in construction to the supplementary filter element  220  of  FIG. 5 , including a supplementary filter medium  412  comprising a pleated sheet with activated carbon for removing odors. The supplementary filter element  412  has an upstream side  416  and a downstream side  418 . The supplementary filter element  412  includes as a support structure a paperboard frame  420  which is U-shaped in cross section.  
      A frame-like planar flange  430  has an upstream side  432  and a downstream side  434 , and is adhesively attached to the paperboard frame  420 , as indicated by an adhesive layer  436 . For additional structural strength, support members  438  and  440  are provided, likewise secured by adhesive layers  442  and  444 . The frame-like planar flange  430 , as well as the support members  438  and  440 , may be made either of paperboard of plastic.  
      The pleated supplementary filter medium  414  is representative of other forms of supplementary filter medium as described hereinabove, and comprises a gas-adsorption or absorption filter. The supplementary filter medium  414  is thus representative of a supplementary filter medium in the form of a porous body within the hexahedral volume such as the supplementary filter medium  40  of  FIG. 1 .  
      The supplementary filter  410  of  FIG. 9  differs from embodiments of the invention described hereinabove in that the frame-like planar flange  430  is permanently attached to the supplementary filter element  412 . Thus the supplementary filter  410  is sold as a unit, and is intended to be disposed when the activated carbon is no longer effective. In use, however, the frame-like planar flange  430  functions in the same manner as in the embodiments described hereinabove, and the supplementary filter  410  is employed in combination with existing conventional air particulate filter elements such as the filter element  24  in either the HVAC system represented by the structure  20  of  FIGS. 1, 4  and  5 , or the HVAC system represented by the structure  360  of  FIG. 8  as examples.  
      Thus, the upstream side  416  of the supplementary filter element  410  and the upstream side  430  of the frame-like planar flange  430  are positionable facing the downstream side of the conventional air particulate filter element, with the supplementary filter element  412  retained by the frame-like planar flange  430  within a filter mount.  
      Referring finally to  FIG. 10 , illustrated is yet another embodiment of a supplementary filter  450 , differing from the supplementary filter  410  of  FIG. 9  only in that the support members  438  and  440  are not included. In some cases, sufficient structural rigidity is provided simply by adhesively attaching the frame-like planar flange  430  to the paperboard frame  420 .  
      While specific embodiments of the invention have been illustrated and described herein, it is realized that numerous modifications and changes will occur to those skilled in the art. It is therefore to be understood that the appended claims are intended to cover all such modifications and changes as fall within the true spirit and scope of the invention.