Abstract:
An air duct system includes a conical fabric filter disposed within a cylindrical air duct. In some embodiments, both the filter and the air duct are inflatable. A fabric collar and a pair of zippers not only allow the filter to be readily removed for cleaning, but also allow the air duct system to continue operating with the filter removed. Pleats can provide the filter with more surface area, and the pleats can be interconnected in an alternating pattern to inhibit the filter from over-inflating.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
   This application is a continuation-in-part application of U.S. patent application Ser. No. 09/916,987, filed on Jul. 27, 2001, now U.S. Pat. No. 6,626,754. 

   FIELD OF THE DISCLOSURE 
   The subject disclosure generally pertains to air filters and, more particularly, to a fabric filter for use inside an air duct. 
   BACKGROUND OF THE DISCLOSURE 
   Fans or blowers are used along with ductwork to circulate air through a room or area of a building. The blower typically draws air from within the room through a return air duct and then forces the air back into the room through a supply air duct. To heat or cool the air, the blower may also force or draw the air across a heat exchanger. 
   To help prevent dust from accumulating on the heat exchanger, blower, and ductwork, often a conventional filter is installed at the downstream end of the return air duct. Finer, less porous filters are used where dust removal is more critical, such as in so called clean rooms or in buildings having occupants with dust-related allergies. Unfortunately, fine filters usually create a higher pressure drop that reduces the amount of airflow. To minimize the pressure drop, an effective cross-sectional area of the filter can be increased in various ways, such as by adding pleats to the filter, installing the filter at an angle relative to the duct, or by forming the filter as an elongated bag that extends lengthwise into an air duct. 
   Some examples of filters that are elongated along the direction of airflow are disclosed in U.S. Pat. Nos. 2,853,154; 3,151,962; 3,195,296; 3,204,391; 3,204,392; 3,396,517; and 3,538,686. When mounting such filters within a return air duct, upstream of the blower, a significant distance is needed between the blower and where the filter attaches to the duct, simply due to the length of the filter. In many cases, this can be difficult or impossible to do, because of bends or elbows in the ductwork. Also, much of the ductwork is usually inaccessible, as it is often installed within the walls of the building or between the floor and ceiling. Filters in a return air duct are therefore typically installed immediately adjacent the blower, which may prohibit the use of an elongated filter or at least significantly limit its length. 
   On the other hand, if an elongated air filter were installed in the supply air duct, the filter would do little in preventing dust from accumulating on the blower and the heat exchanger, because dust often originates in the room. With a filter installed in the supply air duct, dust from the room could pass across the blower and heat exchanger before ever reaching the filter. 
   Moreover, if elongated filters of current designs were installed within a generally cylindrical duct having a pliable fabric wall, the non-conical shape of the filter may cause the fabric of the duct to flutter, due to uneven patterns of airflow velocity. If the cross-sectional area of airflow between the exterior of an elongated filter and the interior of the cylindrical fabric duct is not circumferentially uniform, as could be the case with a flat-sided filter within a cylindrical duct, localized areas of higher velocity may exist. Also, abrupt changes in velocity along the length of a fabric duct may also cause the fabric to flutter. 
   SUMMARY OF THE DISCLOSURE 
   In some embodiments, an air duct system includes a conical filter disposed within a cylindrical duct. 
   In some embodiments, an air duct system includes an inflatable conical filter with pleats. 
   In some embodiments, the pleats are interconnected in an alternating pattern of connection points to inhibit the filter from billowing excessively outward. 
   In some embodiments, an air duct system includes a blower and a heat exchanger interposed between an upstream pre-filter and a downstream conical filter, which is less porous. 
   In some embodiments, an inflatable fabric filter is disposed within an inflatable fabric air duct. 
   In some embodiments, the fabric wall of the air duct is air permeable. 
   In some embodiments, the integrity of a fabric air duct can be maintained regardless of whether the elongated filter is attached to the duct. 
   In some embodiments, a zipper removably attaches an elongated filter to a fabric air duct. 
   In some embodiments, a plurality of conical filters have the same length to diameter ratio even though the filters are of different diameters for various diameter air ducts. 
   In some embodiments, a releasable circumferential connector removably attaches the elongated filter to the fabric air duct and is manufactured from extruded plastic pieces having interlocking ridges. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a cutaway view of an air duct system with a fabric air duct and a conical fabric filter. 
       FIG. 2  is a cutaway view of an air duct system with a relatively rigid air duct and a conical fabric filter. 
       FIG. 3  is similar to  FIG. 1 , but with the fabric duct and filter deflated. 
       FIG. 4  is a perspective view of the filter used in the air duct system of FIG.  1 . 
       FIG. 5  is a closer up view of the supply air duct and conical filter of FIG.  1 . 
       FIG. 6  is similar to  FIG. 5 , but with the filter removed and two sections of the supply air duct zipped together. 
       FIG. 7  is similar to  FIG. 4 , but showing a fabric conical filter that is pleated. 
       FIG. 8  is a cross-sectional view taken along line  8 — 8  of FIG.  7 . 
       FIG. 9  shows one of a plurality of conical air filters. 
       FIG. 10  is similar to  FIG. 9 , but showing a larger filter with the same length to diameter ratio. 
       FIG. 11  is a side view of a supply air duct and conical filter depicting an alternative form of releasable circumferential connector. 
       FIG. 12  is a sectional view of the releasable circumferential connector of  FIG. 11 , taken along line  12 — 12  of FIG.  11 . 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENT 
   An air handling system  10  of  FIG. 1  is used to heat or cool an area  12  of a building  14 . To do this, system  10  includes a blower  16 ; a heat exchanger  18 ; a pre-filter  20 ; a finer, less porous inflatable filter  22 ; a supply air duct  24 ; and a return air duct  26 . Heat exchanger  18  is schematically illustrated to represent any device for heating or cooling air, such as by electrical resistance or by heat transfer with another fluid, such as refrigerant, water, or glycol. A housing  28  can enclose one or more of the components of system  10 . 
   In operation, blower  16  draws air  30  from area  12 , through return air duct  26  and across pre-filter  20 , with pre-filter  20  being any conventional filter known to those skilled in the art. Pre-filter  20  can be used to capture the larger dust particles in the air that might otherwise accumulate on heat exchanger  18  and blower  16 . Pre-filter  20  also helps prevent large dust particles from quickly plugging up the less porous filter  22  in supply air duct  24 . 
   After the air passes through pre-filter  20 , blower  16  draws the air across heat exchanger  18 . Blower  16  then discharges the air through inflatable filter  22 , through supply air duct  24 , and into area  12  through the pores or other openings in supply duct  24 . Filter  22 , being relatively fine, can be used to remove smaller dust particles that were able to pass through pre-filter  20 . In some embodiments, the fabric material of filter  22  is provided by 3M of St. Paul, Minn., and has a standard particle removal efficiency of 80 to 90%, at 150 to 300 cfm/ft 2 , with a static pressure drop of 0.2 inches of water. 
   Conical fabric filters, such as filter  22 , can be installed within various types of ducts. The supply air duct can be made of sheet metal or some other relatively rigid material, as is the case of conical filter  22 ′ in supply air duct  32  of  FIG. 2 , or can be made of a pliable fabric  34 , as is the case of duct  24 . With a metal air duct, air registers  36  provide one or more openings for air to discharge into area  12 . As an alternative or in addition to registers  36 , the fabric of air duct  24  may be air-permeable and/or be provided with cutouts or discharge openings  38  that deliver air to area  12 . Examples of fabric air duct  24  are disclosed in U.S. Pat. Nos. 5,655,963 and 5,769,708, which are specifically incorporated by reference herein. 
   In the example of  FIG. 1 , the fabric wall of duct  24  has a generally cylindrical or tubular shape when inflated by the discharge pressure of blower  16 . However, when the heating or cooling demand of area  12  has been satisfied, blower  16  may turn off, which deflates filter  22  and leaves the fabric walls of duct  24  hanging relatively limp, as shown in FIG.  3 . Some fabric air ducts have a rigid frame that helps hold the fabric walls of the duct in a generally tubular shape even when the blower is not running. Such frame-supported ducts are also well within the scope of the disclosure. 
   Filter  22  can be installed within an air duct (metal or fabric, supply or return) in various ways. In a currently preferred embodiment, a collar  40 , made of fabric or some other material, couples filter  22  to a first segment  24   a  and a second segment  24   b  of fabric air duct  24 . Referring further to  FIG. 4 , fabric rim  42  at a base  44  of filter  22  is sewn or otherwise attached to the interior of collar  40 . Collar  40  includes two half-zippers  46  and  48  that removably interlock with mating half-zippers  50  and  52  on supply air duct  24 , as shown in FIG.  5 . Half-zippers  46  and  50  comprise a first zipper  54 , and half-zippers  48  and  52  comprise a second zipper  56 . Zippers  54  and  56  allow filter  22  to be temporarily removed from duct  24  for filter cleaning or replacement. If filter  22  is removed for an extended period, half-zippers  50  and  52  may be zipped together to re-establish a continuous supply air duct, as shown in FIG.  6 . 
   To minimize the pressure drop created by filter  22  and to extend the period between filter cleanings, filter  22  is elongated to provide a large surface area through which the air may pass. This is accomplished by having filter  22 , when inflated, be of a generally conical shape (i.e., most of its contour or outer envelope fits the shape of a cone). In some embodiments, filter  22  is in the shape of a cone (i.e., substantially all of its contour or outer envelope fits that of a cone). 
   To help prevent the fabric walls of duct  24   b  from fluttering, the velocity and flow direction of the air between the exterior of filter  22  and the interior of duct  24   b  is kept as smooth as reasonably possible. This can be achieved by installing a conical filter within a cylindrical duct to create an airflow path whose annular cross-sectional area increases gradually from an upstream to a downstream end of filter  22 . 
   To provide a conical filter with more surface area, a filter  58  can have a pleated fabric wall, as shown in  FIGS. 7 and 8 . The pleats run generally lengthwise with each pleat being connected to its two adjacent pleats in an alternating pattern of discrete points. For example, a central pleat  60  lies between a first pleat  62  and a second pleat  64 . Central pleat  60  has a central peak  60 ′ that zigzags between an adjacent first peak  62 ′ and a second peak  64 ′ of pleats  62  and  64 , respectively. Central peak  60 ′ is attached to first peak  62 ′ at points  66 ,  68  and  70 . Central peak  60 ′ is also attached to second peak  64 ′ at points  72 ,  74  and  76 . The alternating pattern of connection points inhibits the blower&#39;s discharge air pressure from flattening the pleats and restrains filter  58  to a generally conical shape. 
   To provide a plurality of conical filters that provide the same flow rate for a given area of filter material regardless of the duct&#39;s diameter, each filter&#39;s length to diameter ratio is the same. For example, in  FIG. 9 , a filter  78  in a first duct  80  has a diameter  82  of 24 inches, as measured along a base  84  of conical filter  78 , and has a length  94  of 120 inches, as measured from a center  86  of base  84  to an apex  88  of filter  78 . Similarly, in  FIG. 10 , a filter  90  in a larger duct  92  has a diameter  96  of 48 inches and a length  98  of 240 inches, whereby both filters  78  and  90  have a length to diameter ratio of five (120/24=5, and 240/48=5). 
   In an alternative embodiment, depicted in  FIG. 11 , the filter  22  may be coupled to the air duct  24  using releasable circumferential connectors provided in a form different from the conventional metal zippers  46 ,  48 ,  50 ,  52 . The first releasable circumferential connector  100  may extend from the collar  40  to connect the filter  22  to the first segment (not shown) of the air duct  24 , while the second releasable circumferential connector  102  may also extend from the collar  40  and be used to connect the second segment  24   b  to the filter  22 . As used herein, “releasable circumferential connector” is understood to mean any type of fastener extending substantially around the entire circumference of the duct  24  and which may be readily pulled apart by a user either with or without the use of a slide as used in a zipper or the like. Frictional engagement of interlocking protrusions as described below holds such a releasable circumferential connector together. 
   The cross-sectional view depicted in  FIG. 12  shows the releasable circumferential connector  102  as having interlocking strips  103  and  104 , while releasable circumferential connector  100  is shown having a strip  106  adapted to interlock with a strip (not shown) provided on the first segment of the air duct. Each strip  103 ,  104 , and  106  includes a base  108  from which protrusions  110  extend. Each protrusion  110  includes a stem  112  and a head  114 , the head  114  being wider than the stem  112 , thereby creating a shoulder  116 . Each protrusion  110  is separated by a void  118 . Any number of protrusions  110  can be provided, including two or four, with a corresponding number of voids  118  being created therebetween. 
   By providing such structure, the strips  103 ,  104 , and  106  can be connected by orienting them in opposing fashion, and applying compressive force, as with a thumb and forefinger, for example. In so doing, it can be seen that the protrusions  110  of the strip  103  extend into the voids  118  of the other strip  104  with each head  114  interlocking against one of the shoulders  116  of the opposing strip. The compressive force causes the protrusions  110  to laterally deflect to a degree sufficient to allow for entry of the opposing protrusions  106 . Manufacturing the strips  103 ,  104 , and  106  from a resilient material facilitates such movement, with polypropylene being one suitable example material. More specifically, the resilience of the material enables one strip to be stretched sufficiently to overlie the other, whereupon the two can be compressed together. 
   Although the invention is described with reference to a preferred embodiment, it should be appreciated by those skilled in the art that various modifications are well within the scope of the invention. Therefore, the scope of the invention is to be determined by reference to the claims that follow.