Patent Publication Number: US-7909918-B2

Title: Air filtration system

Description:
BACKGROUND OF THE INVENTION 
     The filtration of air in interior or indoor spaces has become important to maintain and improve human health and to keep interior spaces and furnishings more clean than heretofore obtainable. An effective type of electrically energized air filtration system is known as an Intense Field Dielectric (IFD) filtration system. U.S. Pat. No. 6,749,669 issued Jun. 15, 2004 to Griffiths et al is directed to an IFD type filtration system. U.S. Patent Application Publication No. US 2007/0039472 A1, published Feb. 22, 2007 by Bias, et al. and assigned to the assignee of the present invention discloses and claims several improvements in IFD type filtration systems. U.S. patent application Ser. No. 11/516,263 filed Sep. 6, 2006, by Woodruff, et al. and also assigned to the assignee of the present invention discloses and claims additional improvements in IFD air filtration systems. The above-referenced patent, patent application publication and pending patent application are each incorporated herein by reference. 
     Additional improvements have been sought in IFD type air filtration systems including the need to provide a system which can be mounted on an interior wall or ceiling at the inlet to a return air duct for an air conditioning apparatus which is circulating air to an enclosed space. Remotely locating an IFD air filtration system with respect to the air conditioning apparatus with which it is associated presents certain problems in mounting the system, providing certain control features and providing suitable power to the system. By providing an IFD air filtration system which can be mounted remote from air conditioning apparatus, greater flexibility in providing an overall efficient and desirable air conditioning system is obtained and it is to these ends that the present invention has been developed. 
     SUMMARY OF THE INVENTION 
     The present invention provides an improved air filtration system, particularly a system of the so-called Intense Field Dielectric (IFD) type. In particular, the invention provides an IFD type air filtration system which is adapted for mounting remote from air conditioning apparatus with which the filtration system is associated. More particularly, the invention provides an air filtration system which is adapted to be mounted at the inlet to a return air duct leading to the air conditioning system or apparatus with which the filter is associated. 
     In accordance with one important aspect of the present invention, an air filtration system is provided which includes a chassis or frame and a door hinged to the frame and comprising an air inlet grille, the door supporting certain components of the air filtration system including a field charging unit and a main, electrically charged, air filter unit which removes particulates and the like which have received an electrical charge from the field charging unit. A so-called prefilter unit may or may not be disposed on the door. The above-mentioned components are conveniently supported on the grille or door and at least the pre-filter unit, if used, and the main filter unit are easily removed from the grille or door for cleaning, repair or replacement. 
     In accordance with another aspect of the present invention an IFD type air filtration system is provided which includes a control system enclosure mounted on a frame which is adapted for mounting on a wall or ceiling and supported by structural members of the wall or ceiling. The control system is electrically connected to a field charging unit and a main filter unit when the grille or door is in a closed or working position with respect to the chassis or frame and the control system is disconnected from the field charging unit and the main filter unit when the door or grille is moved to an open non working position. The control system and an enclosure therefor includes contact elements which engage corresponding contact elements on the field charging unit and the main filter unit when the grille or door is in the closed position. 
     Several features facilitate servicing the filtration system of the invention including a conveniently actuatable latch for latching the main filter unit in its working position, an arrangement to assure that the main filter unit is correctly positioned and supported on the system, an arrangement of hinges and latches which facilitate moving the grille or door between open and closed positions and removal of the grille or door from the frame. A control box or enclosure for the filtration system is mounted on the system frame or chassis and includes conveniently positioned human actuatable controls and indicators, as well as other elements which will be appreciated by those skilled in the art. 
     Still further in accordance with the invention, an improved arrangement of electrical contact elements is provided wherein the contact elements are supported by the system control box or enclosure for engagement with components which require electrical power for operation. 
     The present invention still further provides an air filtration system which includes an improved interlock, comprising a control feature which de-energizes the electrically operated filter components when the grille or door is moved from a closed working position to an open non working position. A user interface or control panel is advantageously mounted on the control box or enclosure and which is easily accessible by a user of the air filtration system upon moving the grille or door to its open position, although the system may be started and stopped by actuating a push button on the face of the door without moving the door to an open position. 
     The invention yet further provides an air filtration system adapted for interpositioning in a duct or cabinet of an air conditioning system and which enjoys many of the advantages of the embodiment of the system which is adapted for mounting remote from an air conditioning apparatus. 
     Certain features of the invention have been described generally hereinabove and those skilled in the art will recognize the advantages and superior features of the invention heretofore described as well as other important aspects thereof, upon reading the detailed description which follows in conjunction with the drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a perspective view of an air filtration system in accordance with the invention mounted at the inlet of a return air duct for an air conditioning apparatus; 
         FIG. 2  is a perspective view of the filtration system showing  FIG. 1  in a so called open position of a door or grille of the system; 
         FIG. 3  is perspective view similar to  FIG. 2  illustrating additional components of the filtration system; 
         FIG. 4  is a detail section view taken generally along the line  4 - 4  of  FIG. 3 ; 
         FIG. 4A  is a detail section view taken along the same line as  FIG. 4  showing an alternate embodiment of a latch mechanism. 
         FIG. 4B  is a detail perspective view of a charging pin for the field charging unit; 
         FIG. 5  is a detail view illustrating certain features of the system door and how components of the system are supported thereby; 
         FIG. 6  is a perspective view of the primary or main electrically charged filter unit in accordance with the invention; 
         FIG. 7  is a perspective view similar in some respects to  FIGS. 1 and 3  with the field charging unit and filter units removed for illustration of their support structure and the door hinges; 
         FIG. 8  is a detail perspective view of one of the door hinges shown in  FIG. 7 ; 
         FIG. 9  is a detail view illustrating one of the latch mechanisms used to hold the door and grille in a closed position; 
         FIG. 9A  is a detail view of an alternate embodiment of a latch mechanism for the door and grille; 
         FIG. 9B  is another detail view of the latch mechanism shown in  FIG. 9A ; 
         FIG. 10  is a perspective view of the control box or enclosure for the system of the present invention; 
         FIG. 11  is a detail front elevation of a user interface or control panel; 
         FIG. 12  is a detail cutaway view showing the relationship between a user actuatable system start and stop switch and lighting therefor; 
         FIG. 13  is an exploded perspective view of the control box or enclosure for the control system for the present invention; 
         FIG. 14  is a detail exploded perspective view of one of the contact element support brackets or covers for supporting two of the four required contact elements for the system of the invention; 
         FIG. 15  is detail composite section view taken generally along the line  15 - 15  of  FIG. 13 ; 
         FIG. 16  is another detail perspective view of the control system enclosure illustrating certain features thereof; 
         FIG. 17  is a schematic block diagram of the control system for the air filtration system of the invention; 
         FIG. 18  is a perspective view of another embodiment of an air filtration system in accordance with the invention; and 
         FIG. 19  is a perspective view of the system shown in  FIG. 18  taken from a perspective opposite that of  FIG. 18 . 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     In the description which follows like parts are marked throughout the specification and drawings with the same reference numerals, respectively. Certain features of the invention may be shown in somewhat generalized or schematic form in the interest of clarity and conciseness. 
     Referring to  FIGS. 1 and 2 , there is illustrated an improved air filtration system in accordance with the invention and generally designated by the numeral  20 . The air filtration system  20  is adapted to mount on a wall or ceiling including, for example, a vertical wall  22 , FIG.  1 , whereby a suitable opening is provided in the wall for receiving a chassis or frame of the system and generally designated by the numeral  24  in  FIGS. 2 and 3 . Those skilled in the art will recognize that the filtration system  20  may also be mounted in other positions including in a ceiling or a non-vertical wall or a panel or within cabinetry or the like, for example. 
     The filtration system  20  is advantageously disposed at the inlet of a return air duct  21 ,  FIG. 1 , and the direction of airflow through the components of system is as indicated by the arrows  20   a  in  FIGS. 1 ,  4  and  5 . As shown in  FIGS. 2 and 3 , the frame or chassis  24  is preferably of a rectangular perimeter type including a perimeter flange  26  joined to top and bottom walls  24   a  and  24   b  and opposed sidewalls  24   c  and  24   d  and defining a substantial rectangular opening  25 ,  FIG. 2 . Chassis or frame  24  is preferably dimensioned to fit between wall supporting structural members or so called studs  27 ,  FIG. 3 . Suitable cross members, not shown, extending between the studs  27  may be provided to engage and support the top and bottom walls  24   a  and  24   b  of frame  24 . The frame  24  may be suitably secured to the wall  22  at the structural members  27  using conventional fasteners extending through the walls  24   c  and  24   d  as well as the top and bottom walls  24   a  and  24   b.    
     Referring further to  FIGS. 2 and 3 , the air filtration system  20  includes a door  32  which may be integrally formed with or separable from a grille  30  having a substantial number of closely spaced apart generally horizontally extending louvers  31 ,  FIGS. 1 and 4 , formed thereon and allowing air to flow therethrough. Alternatively, the louvers  31  may extend vertically or be replaced by other means forming openings, such as spaced apart holes. Door  32  is suitably hinged to the frame  24  at flange  26  by spaced apart hinges  29 , see  FIG. 7 , for movement between a closed working position shown in  FIG. 1  and the non working door open positions shown in  FIGS. 2 ,  3  and  7 , for example. 
     Door  32  includes a rectangular opening  33  formed therein. Grille  30  may be secured to door  32  by suitable snap fit features, not shown, or by conventional fasteners, if desired. Door  32  supports and may be integrally formed with opposed spaced apart parallel somewhat channel shaped support members, or brackets,  34  and  35 ,  FIGS. 2 ,  3  and  7 , which are adapted to support three major filter components including a main, electrically powered, filter unit  36 , an electrically powered element generally known as a field charging unit  38 , and a pre-filter  40 , which may be a conventional impingement type air filter. Moreover, the filter unit  36  and the field charging unit  38  may be constructed substantially like the corresponding filter unit and field charging unit disclosed in U.S. Patent Application Publication No. US 2007/0039472 A1. The filter unit  36  and the field charging unit  38  may also embody features of the corresponding elements disclosed in U.S. Pat. No. 6,749,669. 
     Although the field charging unit  38  is constructed substantially like that described in Patent Application Publication No. US2007/0039472 A1, certain improvements have been developed with respect to certain components of the field charging unit. As described also in the aforementioned Patent Application Publication, the field charging unit  38  includes a frame  38   a  having a handle  38   c  formed thereon. Frame  38   a  supports a so called earth plate  38   d ,  FIG. 4 , and provided with plural spaced apart openings  38   e , one shown in  FIG. 4 . An array of conductor pins  38   f , one shown in  FIG. 4 , is mounted on frame  38   a  disposed such that a pin  38   f  is aligned with each of the openings or holes  38   e.    
     Conductor pins  38   f  are preferably formed of a tungsten composition and are secured to respectively elongated conductor bars  38   g , a portion of one shown in  FIG. 4B . Each pin  38   f  may be secured to a conductor bar  38   g  by short clamp member  38   h  by suitable means, such as spot welding operations, for example. Still further, it is desirable to minimize electrical arcing by and corrosion of the conductors of the field charging unit by encapsulating the conductor bars  38   g  with a suitable coating, such as a high voltage insulating polymer, for example, and indicated by numeral  39  in  FIG. 4B . As further shown in  FIG. 4B , a portion of one of the grid members of a supporting grid  38   l  is illustrated and which is operable to support the aforementioned spaced apart pins  38   f  generally in the manner described in the aforementioned Patent Application Publication. Alternatively, pins  38   f  may be embedded in a conductive polymer member taking the place of components  38   g  and  38   h  and encapsulated by a nonconductive polymer structure taking the place of parts  38   l  and  39 , for example. 
     Referring briefly to  FIGS. 7 and 8 , the hinges  29  are adapted to provide for at least partially counterbalancing the weight of the door  32  or providing sufficient friction to enable opening of the door  32  without the weight of the door impairing operations to service the components supported on the door, such as filter units  36  and  40  and charging unit  38 . This is particularly advantageous for filter units  20  which are mounted in a ceiling so that when the door  32  is opened it does not drop rapidly to the fully open position. In this regard, each of the hinges  29  includes a first hinge part  29   a  adapted to be suitably secured to the perimeter flange  26  of frame  24  and a second hinge part  29   b  suitably secured to the door  32 . A hinge pin part  29   c ,  FIG. 8 , projects from hinge part  29   a  into a suitable bore in hinge part  29   b  and is slidably retained therein by a digitally actuatable latch part  29   d . Actuation of the respective latch parts  29   d  enable separation of the hinge parts  29   b  from hinge parts  29   a  so that the door  32  may be moved with respect to the frame  24  by sliding the door to the left, viewing  FIG. 7 , whereby the hinge parts  29   a  and  29   b  become disengaged for complete removal of the door  32  from the frame  24 , when desired. Alternatively, the hinges  29  may be oriented to allow door  32  to be disconnected from frame  24  by moving the door to the right, viewing  FIG. 7 . As mentioned previously, hinges  29  may be provided such that when the hinge parts  29   b  are rotated relative to the hinge parts  29   a  sufficient friction or a counterbalancing force is exerted by each hinge to retard or control movement of the door  32 . Hinges  29  may be of a type commercially available, such as from the Reell Precision Manufacturing Corporation, St. Paul, Minn. 
     Door  32  is maintained in a so called closed position with respect to chassis or frame  24  by respective digitally actuatable latches  23 ,  FIGS. 1 and 9 . Grille  30  is provided with spaced apart openings or recesses  30   r , see  FIG. 9 , by way of example, at which a digitally actuatable latch member  23   a  is disposed and may be depressed downward, viewing  FIG. 9 , to engage and move a latch member  23   b  so that a latch finger  23   c  secured thereto disengages from a latch hook  24  suitably mounted on frame  24 . Latch member  23   b  is biased into the position for engagement of the finger  23   c  with the hook member  24   l  by a suitable coil spring  23   d  supported by door  32 .  FIG. 9  is exemplary of both of the latches  23  and in response to digital actuation of both latches the door  32  may be pivoted with respect to the frame  24  between closed and open positions. 
     Referring to  FIGS. 9A and 9B , an alternate embodiment of a latch mechanism for maintaining the door  32  in a closed position is illustrated. The latch mechanism illustrated in  FIGS. 9A and 9B  is similar in some respects to the latch  23  but includes a modified digitally actuatable latch member  23   f  engageable with a latch member  23   g  including a finger  23   h . Latch member  23   g  includes spaced apart downwardly projecting legs  23   j  disposed for sliding engagement with the door  32 . A torsion coil spring  23   k  is mounted on a hub  23   l  of latch member  23   g  and includes opposed tines  23   m  and  23   n  engageable with door  32  to bias the latch member into the latching position shown in  FIG. 9A . However, in response to digital actuation of latch member  23   f , latch member  23   g  is moved downwardly, viewing  FIGS. 9A and 9B , against the bias of spring  23   k  to allow the door to be released from its closed position with respect to the chassis or frame  24 . 
     The main filter unit  36  is characterized by a generally rectangular perimeter frame  36   a , see  FIG. 6 , supporting stacked filter elements like those disclosed in Patent Application Publication No. US 2007/0039472 A1 and designated by numeral  36   b  in  FIGS. 2 ,  3  and  6 . Frame  36   a  of filter unit  36  includes a handle part  36   c  for use in moving the filter unit into and out of the slot formed by the frame members  34  and  35 , said slot being formed by flanges  34   b  and  34   c , see  FIG. 4 , and  35   b  and  35   c , see  FIGS. 5 , and  7  also. A deflectable latch member  36   d ,  FIG. 4 , includes a latch projection  36   e  engageable with a flange or wall forming the handle  38   c  of field charging unit  38 , as shown. Accordingly, filter unit  36  may be retained on the door  32  by latching the filter unit to the field charging unit  38  which is suitably retained in a slot formed by the members  34  and  35  and delimited by the flanges  34   c ,  34   d ,  35   c  and  35   d , see  FIGS. 4 ,  5  and  7 . 
     Referring briefly to  FIG. 4A , an alternate embodiment of a latch for latching the filter unit  36  in its working position engaged with the field charging unit  38  is illustrated. As shown in  FIG. 4A , a digitally actuatable latch member  36   k  includes a stepped latch part  36   l  and a digitally actuatable or engageable part  36   m  which parts extend in opposite directions from a web part  36   n . The base of web part  36   n  includes a boss  36   p  which is supported for pivotal movement about a pivot pin  36   q  and is biased into the latching position shown by a torsion coil spring  36   r . Accordingly, latch member  36   k  engages the flange forming handle  38   c  of field charging unit  38  in generally the same manner as provided for by the latch  36   d ,  36   e.    
     As shown in  FIGS. 5 and 6 , filter unit frame  36  includes spaced apart elongated projections  36   f  extending along one side of the frame and operable to receive a boss  35   f ,  FIG. 5 , slidably therebetween so that the filter unit  36  may not be placed between the support members or brackets  34  and  35  incorrectly. Frame  36   a  is thus dimensioned such that it may not be placed inverted with respect to door  32 , thanks to the parallel projections  36   f  and boss  35   f . Viewing  FIG. 3 , spaced apart electrical contact members  41   a  and  41   b  are provided on filter unit frame  36   a  and are polarity sensitive. Accordingly, the filter unit  36  is required to be oriented in a pre-determined position, as shown in the drawings, and should not be reversed or placed between the members  34  and  35  upside down. The aforementioned pre-filter  40  is also operable to be disposed between the brackets  34  and  35  and suitably supported thereby between flanges  34   d  and  34   e , and between the flanges  35   d  and  35   e ,  FIGS. 4 , and  5 . 
     As shown in  FIGS. 2 and 3 , field charging unit  38  is also provided with spaced apart electrical contact members  43   a  and  43   b  which are polarity sensitive and are disposed, together with the contact members  41   a  and  41   b , in predetermined positions for engagement with corresponding contact members disposed on a control box or enclosure, generally designated by the numeral  42 , see  FIGS. 2 ,  3  and  10 , for example. Control enclosure  42  is mounted on frame  24  between sidewalls  24   c  and  24   d  and preferably contiguous with wall  24   a . Suitable brackets  44   a  and  44   b ,  FIG. 10 , are supported on or formed integral with opposed sidewalls  24   c  and  24   d  for supporting control enclosure  42 , as shown. Each of brackets  44   a  and  44   b  may be provided with suitable detent means  45 , one shown, for engagement with a corresponding projection on control enclosure  42  to assist in supporting the control enclosure when removable fasteners  46 , for example, are being secured to or removed from the control enclosure. 
     Control enclosure  42  includes spaced apart electrical contact elements  48   a ,  48   b ,  50   a  and  50   b  suitably supported thereon as will be explained in further detail herein. Contact elements  48   a ,  48   b ,  50   a  and  50   b  are elastically deflectable and are engageable with corresponding contact elements on the filter unit  36  and the field charging unit  38 . Accordingly, when the door  30  is moved to a closed position, as shown in  FIG. 1 , contact element  48   a  engages contact member  41   a ,  FIG. 3 , contact element  50   a  engages contact member  43   a , contact element  48   b  engages contact member  41   b , and contact element  50   b  engages contact member  43   b . As indicated in  FIG. 10 , the contact elements  48   a ,  50   a ,  48   b  and  50   b  are marked with the appropriate polarity markings. Thus, when door  32  is moved between the position shown in  FIG. 1  and a door open position, electric power to the filter unit  36  and the field charging unit  38  is interrupted. Still further, as will be described hereinbelow, electrical power is also cut off with respect to the contact elements  48   a ,  48   b ,  50   a  and  50   b  mounted on the control box or enclosure  42  so that these contact elements are de-energized. 
     Referring now to  FIGS. 13 ,  14  and  15 , control enclosure  42  includes opposed sections defining spaced apart enclosure spaces  51  and  52  for receiving certain electrical components of the controls for the system  20 , including a circuit board  53  and a power supply or transformer  54 . Suitable heat sinks, not shown, may be disposed in space  51  also for maintaining a suitable operating temperature for power supply  54 . A suitable cover  42   a  is operable to cover the enclosure  42  including the spaces  51  and  52 . Enclosure  42  includes an intermediate recessed portion  42   b  forming a chase,  FIG. 13 , to provide clearance for handle portions  36   c  and  38   c ,  FIG. 3 , of the filter unit  36  and field charging unit  38  when the door  32  is moved to a closed position. Enclosure  42  may be formed in multiple parts suitably secured together by intermediate section  42   b.    
     Contact elements  48   a ,  48   b ,  50   a  and  50   b  are supported on control enclosure  42  in a manner as illustrated in  FIGS. 13 through 15 . By way of example, viewing  FIG. 15 , contact elements or members  48   b  and  50   b  are suitably supported on enclosure  42  by respective bosses  42   f , one shown, formed integral with an enclosure bottom wall  42   g ,  FIG. 15 , whereby contact member  50   b  is shown supported by a boss  42   f , by way of example, and projecting through a slot  42   h  in wall  42   g  to assume the working position illustrated in  FIG. 10 . Contact elements  48   b ,  50   a  and  48   a  are supported in a manner virtually identical to that illustrated in  FIG. 15 . Each pair of contact elements  48   a ,  50   a  and  48   b ,  50   b  is also protected by respective removable covers  56   a , and  56   b ,  FIGS. 13 through 15 . Covers  56   a  and  56   b  are mirror image parts or are otherwise configured to prevent misplacement and are each retained in engagement with enclosure  42  by suitable fasteners, such as machine screws  58 , one shown in  FIG. 15  for cover  56   a.    
     As shown by example in  FIG. 15 , covers  56   a  and  56   b  are each also provided with a top wall  60  and spaced apart depending endwalls  61 ,  62  and  63 ,  64 , which fit over upstanding fins  42   j  as shown. This arrangement of the covers  56   a  and  56   b  and the fins  42   j  provides isolation of electrical charges imposed on the contact elements  48   a ,  50   a ,  48   b  and  50   b  from other control components.  FIG. 15  is a composite section view illustrating the manner in which each of the contact elements  48   a ,  48   b ,  50   a  and  50   b  is supported on the enclosure  42  and how these elements are covered by the contact element covers  56   a  and  56   b . Suitable clearance holes  42   l  may be provided in the enclosure  42  for receiving fasteners which may be used to secure the frame  24  to a suitable cross member, not shown, forming part of wall framing for receiving and supporting the frame  24 . 
     As shown in  FIG. 16 , control enclosure  42  includes a suitable electrical connector part  42   k  supported thereon for connection to a source of electrical power not shown for the filter unit  20 . Respective, spaced apart integrally molded hooks  42   m  may be provided on enclosure  42 , as illustrated in  FIG. 16 , for training electrical conductors along one side of the enclosure  42  to minimize the chance of such conductors dangling in the flow path of air flowing through the return air duct  21  when the filtration system  20  is installed in its working position shown in  FIG. 1 . 
     Referring now to  FIGS. 10 and 11 , the control enclosure  42  is adapted to support user operable control switches including a main power on-off switch  62  and additional switches  64 ,  66  and  68  which project through suitable openings in enclosure wall portion  42   s . Switches  64 ,  66  and  68  are, for example, operable for inputting selected control operating features, such as for selecting a minimum or maximum high voltage condition, a reminder to service or change the pre-filter  40  and a reminder to service the filter unit  36 , for example. A seven segment visual display  70  is also provided for use in selecting the above-mentioned parameters. Still further, a visual indicator  72  is provided to indicate the status of the filtration system  20 , that is, on versus off and/or a need to service the system  20 . The switches  62 ,  64 ,  66 ,  68 , the display  70  and the indicator  72  may be mounted on a so called daughter control board  74 ,  FIGS. 13 and 16 , supported within the enclosure  42  and adjacent wall portion  42   s . Suitable elastomeric buttons, not shown, may cover the switches  62 ,  64 ,  66  and  68  to prevent unwanted contact with control board  74 . As shown in  FIGS. 1 and 12 , an actuator  76  for the switch  62  is supported by the grille  30 , as indicated, and includes an arm  76   a  engageable with the switch  62 , see  FIG. 12 , when door  32  is in the closed working position. Actuator  76  is preferably formed of a somewhat resilient electrically nonconductive material. As further shown in  FIG. 12 , switch actuator  76  includes a translucent curvilinear rim part  76   b  which is operable to transmit light from the visual indicator  72  therethrough for observation by a user of the filtration system  20  without opening the door  32 . Accordingly, a user of the filtration system  20  may energize and de-energize the system without opening door  32 , observe whether power is being supplied to the system and/or observe if there is a need to service the system. 
     Referring further to  FIG. 11 , the control system for the filtration system  20  includes a so called interlock which de-energizes the system when the door  32  is moved from a closed position, as shown in  FIG. 1 , to an open position. A preferred embodiment of the interlock includes a reed switch  80 ,  FIGS. 11 and 17 , preferably mounted adjacent to or on the daughter control board  74  or on or adjacent to circuit board  53 . The reed switch  80  is in proximity to a magnet  82 ,  FIG. 3 , when the door  32  is in the closed position, which magnet may be mounted on the door  32 . Accordingly, when the door  32  is moved from its closed position, as shown in  FIG. 1 , to an open position, as shown in  FIGS. 2 and 3 , magnet  82  moves out of range of the reed switch  80  thereby causing the reed switch to open and interrupt power to the filtration system  20 . Alternatively, the magnet  82  may be mounted on the grille  30  in the same general location such that, when door  32  or the grille alone is moved, reed switch  82  will open. 
     Referring now to  FIG. 17 , there is illustrated a schematic block diagram of a control system  84  for the filtration system  20 , including the circuit board  53 , the power supply  54 , and the daughter board  74 . Visual indicator  72  is shown mounted on board  74  in the diagram of  FIG. 17 , as well as switch  62 , the display  70  and the switches  64 ,  66  and  68 . Power for the control system  84  is typically received from an external 24 volt AC transformer connected to or associated with a unit of air conditioning equipment with which the return air duct  21  is associated. A power source, such as a transformer  88 , is indicated schematically in  FIG. 17 . The control system  84  is also adapted to receive control signals from a thermostat or other control circuit  90  associated with the aforementioned air conditioning apparatus such that, when a call for “heat”, or “cooling” or “fan only” is generated by the control circuit  90 , signals are sent via conductors  92  or  94  to a conditioning circuit  96  which is connected to a suitable microprocessor  98  forming part of control system  84 , as shown in  FIG. 17 . 
     Microprocessor  98  is connected to daughter circuit board  74  via a suitable signal conditioning and transmission interface circuit  100 . Power input through the control circuit  84  is by way of transformer  88  and a fused conductor  89  and conductor  91  connected to power supply  54  by way of the aforementioned interlock which includes the reed switch  80  as shown in  FIG. 17 . As shown in  FIG. 17 , microprocessor  98  also receives input signals from a zero crossing sensor  102 , an input voltage monitor  104 , a high voltage power supply input current monitor  106 , a high voltage monitor  108 , a high voltage setting circuit  110  and a programming header  112 . Regulated high voltage power is supplied by power supply unit  54  to the field charging unit  38  and the filter unit  36  by suitable electrical connections including the contact elements associated with the respective units and with the control box or enclosure  42 , as described above. The control system  84  may be similar in some respects to the control system described in U.S. Patent Application Publication No. US2007/0039462 A1 by Helt et al., published Feb. 22, 2007, and assigned to the assignee of the present invention. The subject matter of Patent Application Publication No. US2007/0039462 A1 is also incorporated herein by reference. 
     Referring now to  FIGS. 18 and 19 , another preferred embodiment of an air filtration system in accordance with the invention is illustrated and generally designated by the numeral  120 . The filtration system  120  is similar in many respects to the system described in Patent Application Publications US2007/0039462 A1 and US2007/0039472 A1 identified hereinabove. The system  120  includes a support frame  122  characterized as a substantially u-shaped sheet metal member having a somewhat shallow channel shaped cross section and comprising a bottom wall  124  and opposed sidewalls  126  and  128 . Sidewalls  126  and  128  are provided with suitable elongated parallel guiderails for supporting a primary or main filter unit  130  corresponding generally to the filter unit  36 , a field charging unit  132  corresponding generally to the field charging unit  38  and a suitable pre-filter  134  corresponding to the pre-filter  40 . Accordingly, the filter unit  130 , the field charging unit  132 , and the pre-filter  134  may be supported by the frame  122  in a manner substantially like that of the filter system  20 . Frame  122  includes a perimeter frame member  138  suitably secured to the sidewalls  126  and  128  and adapted to support a door assembly  140  for retaining the filter unit  130 , the field charging unit  132  and the pre-filter  134  in assembly with the frame  122 . 
     Door assembly  140  includes a base plate  142   FIG. 19 , including spaced apart retainer tabs  144  and rotatable latches  146  for releasably connecting the door to the perimeter frame  138 . Spaced apart sets of electrical contact elements  150 ,  152  and  154  cooperate with the filter unit  130  and the field charging unit  132  in substantially the same manner as described in the aforementioned Patent Application Publications which are incorporated herein by reference. Door assembly  140  also comprises an enclosure for a control circuit and components thereof substantially like that illustrated in  FIG. 17  or like that disclosed in Patent Application Publication US2007/0039462 A1. The filter system  120  is advantageous for retrofitting an improved filter system in accordance with the invention to ductwork and in locations other than an inlet to a return air duct, for example. 
     Preferred embodiments of the invention have been described herein in sufficient detail, it is believed, to enable one skilled in the art to practice the invention. Conventional engineering materials and practices may be used to construct the embodiments of the invention, except as otherwise described herein or in the documents which are incorporated herein by reference. Although preferred embodiments of the invention have been described in detail, those skilled in the art will also recognize that various substitutions and modifications may be made without departing from the scope and spirit of the appended claims.