CROSS-REFERENCE TO RELATED APPLICATION 
   This is a continuation-in-part application of U.S. Ser. No. 09/694,715, filed on Oct. 23, 2000, now U.S. Pat. No. 6,558,250. 


   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The subject invention generally pertains to fabric air ducts and more specifically to a damper for such a duct. 
   2. Description of Related Art 
   In HVAC systems (heating, ventilating, air conditioning), conditioned supply air discharged from a blower is often conveyed to various rooms or areas within a building by way of ductwork. Conventional sheet metal ductwork may include a main header duct that receives the forced air from the blower and distributes the air onto several branch ducts. The branch ducts, in turn, include one or more discharge registers that deliver the air to the various designated areas. 
   To ensure that each branch duct receives an appropriate volume of air to adequately condition or ventilate each room or area, airflow control dampers are often installed within the branch ducts, upstream of the ducts&#39; discharge registers. Partially closing a damper prevents its respective branch duct from starving other branch ducts of their supply of air. The various dampers are adjusted until the supply air to each of the branches is properly apportioned, which is a process known as balancing the airflow. 
   In addition to dampers disposed within the ducts, in some cases, additional dampers are installed at each discharge register. The dampers at the discharge registers allow more individualized control of airflow through each register or allow a register to be shut off completely. The occupants of the building typically adjust the individual dampers at each register, while the other dampers within the ducts are thermostatically controlled or manually adjusted and set when the HVAC system is first installed. 
   Balancing the airflow is readily accomplished when the ductwork, dampers and registers are all made of relatively rigid sheet metal; however, in many cases, air ducts are made of fabric. Fabric ducts typically have a flexible fabric wall that is porous and/or includes additional holes along its length for evenly dispersing air, from within the duct, to the areas being conditioned or ventilated. An example of such a duct is a DUCTSOX by the Frommelt Safety Products Corporation of Milwaukee, Wis. Fabric ducts are often suspended from a horizontal cable or track by way of several hangers distributed along the length of the duct. Fabric is often preferred over sheet metal when cleanliness, even air dispersion, condensation control, or appearance is a significant concern. Unfortunately, using conventional metal dampers within fabric ducts creates some problems. 
   First, the pliability of fabric may inhibit the duct from effectively supporting the weight of a metal damper without excessive distortion or sagging of the duct. Second, the supply air blower turning on and off to meet the conditioning demand of the building causes a fabric duct to alternately inflate and deflate. When the duct is deflated, a metal damper may create an unsightly bulge in the duct. 
   Fabric ducts are also affected by problems during the initial operation of the duct. Unlike metal ducts, fabric ducts maintain their inflated shaped only when they are receiving airflow from the blower. When there is no airflow, the fabric duct is in a collapsed state because there is no static air pressure in the fabric duct. The fabric duct also experiences a shrinkage in that its distal length is shorted somewhat as the duct is in a recoiled position, in comparison to its length when fully inflated. From the shrunken and collapsed position, when the airflow is initiated, the blower feeds a large stream of air that must eventually erect the entire fabric duct. The airflow is typically quite high and as it fills the fabric duct the most distal end of the duct, farthest away from the blower, pops out into the erect position. A large popping sound results. Not only is the popping sound annoying to personnel nearby, the violent fabric duct movement that causes the sound may cause wear over time. 
   SUMMARY OF THE INVENTION 
   In accordance with an example, provided is an air duct assembly including a first duct comprising a fabric; a second duct comprising a fabric; and a fabric flow restriction having a first flow resistance over a first region and a second flow resistance different than the first flow resistance over a second region, the fabric flow restriction being interposed between the first duct and the second duct. 
   In accordance with another example, provided, for use in a fabric duct, is a fabric flow restriction including a sleeve; a releasable fastener attached to the sleeve and adapted to fasten the sleeve to the fabric duct for the communication of airflow between the sleeve and the fabric duct; and a fabric flow restriction having a resistance that varies with radius across the fabric flow restriction. 
   In accordance with another example, provided is a method of conveying a volume of air including conveying the air through a first fabric duct; and conveying the air through a fabric flow restriction that is upstream of the first fabric duct, wherein the fabric flow restriction has a first flow resistance over a first region and a second flow resistance different than the first flow resistance over a second region. 


   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a front view with a partial cut-away showing a fabric air duct assembly that includes fabric flow restrictions. 
       FIG. 2  is a bottom view of FIG.  1 . 
       FIG. 3  is a closer view of the cut-away portion of FIG.  1 . 
       FIG. 4  is a cross-sectional view taken along line  4 — 4  of FIG.  2 . 
       FIG. 5  is similar to  FIG. 4  but of another flow restriction. 
       FIG. 6  illustrates manipulating a fabric flow restriction to vary the volume of airflow therethrough. 
       FIG. 7  is a cross-sectional view taken along line  7 — 7  of FIG.  2 . 
       FIG. 8  is similar to  FIG. 3 , but with another mesh added to the flow restriction. 
       FIG. 9  shows one configuration of how the two meshes shown in  FIG. 8  can overlap one another. 
       FIG. 10  shows another configuration of how the two meshes shown in  FIG. 8  can overlap one another. 
       FIG. 11  is a side view of a fabric flow restriction disposed inside and situated between an upstream fabric duct and a downstream fabric duct. 
       FIG. 12  is similar to  FIG. 11 , but with the flow restriction providing greater flow resistance. 
       FIG. 13  is a side view of another fabric flow restriction disposed inside and situated between an upstream fabric duct and a downstream fabric duct. 
       FIG. 14  is similar to  FIG. 13 , but with the flow restriction providing greater flow resistance. 
       FIG. 15  is a side view of another fabric flow restriction interposed between an upstream fabric duct and a downstream fabric duct. 
       FIG. 16  is similar to  FIG. 15 , but with the flow restriction providing less flow resistance. 
       FIG. 17  is similar to  FIG. 15 , but with a fabric shroud covering the flow restriction. 
       FIG. 18  is a view similar to that of  FIG. 4  but of another fabric flow restriction that may be used in a duct assembly. 
       FIG. 19  is a view similar to that of  FIG. 18  but of another fabric flow restriction. 
       FIG. 20  illustrates a fabric flow restriction formed of two overlaid meshes. 
       FIG. 21  shows another configuration of how the two meshes shown in  FIG. 20  can be overlaid. 
       FIG. 22  is an illustration of a portion of a fabric air duct assembly that includes a fabric flow restriction. 
   


   DESCRIPTION OF THE PREFERRED EMBODIMENT 
   An air duct assembly  10 , shown in  FIGS. 1 and 2 , includes several fabric ducts  12 ,  14 ,  16 ,  18  and  20  that are suspended within a building  22  by series of connectors  24 , which hang from one or more elongated support members  26 , such as a track or a taut cable. Several fabric flow restrictions  28 ,  30 , and  32  help balance the volume of airflow through the ducts. An air handler  34 , such as a fan or blower disposed within an enclosure, forces air  36  into a main duct  12 . In this example, duct  12  consists of fabric; however, it could also be made of sheet metal or of another material. The term, “fabric,” refers to any pliable sheet of material that may or may not be air permeable or porous. Examples of a fabric include, but are not limited to, woven or knit cloth, flexible plastic sheeting that is not necessarily woven, plastic impregnated cloth, fiber reinforced plastic, and various combinations thereof. 
   Main duct  12  feeds air  36  into two branch ducts  14  and  18 , which in turn feed air  36  into two other branch ducts  16  and  20  respectively. Thus, ducts  14  and  16  are in series-flow relationship to each other, and so are ducts  18  and  20 . Duct  14  is in parallel-flow relationship with ducts  18  and  20  and so is duct  16 . The term, “parallel-flow” refers to airflow being split between two different paths. Forced air  36  from air handler  34  or another source inflates each of the fabric ducts to a tubular shape, as shown. Porosity and/or other openings in the ducts&#39; fabric allow the air within ducts  14 ,  16 ,  18  and  20  to disperse into a room or area that is being ventilated or otherwise conditioned by air  36 . In some cases, main duct  12  may be air-permeable to disperse some air into part of the building as well. 
   Flow restrictions  28 ,  30  and  32  each have a flow resistance that has been individually set to apportion the airflow being discharged through the fabric wall of each of ducts  14 ,  16 ,  18  and  20 . The term, “flow resistance” is a measure of a restriction&#39;s ability to create a pressure drop for a given volume of airflow through the restriction. Thus, for a given volume of airflow, a higher pressure drop is created by a restriction having a higher flow resistance. Likewise, for a given pressure drop across a restriction, a lower volume of airflow is conveyed through a restriction having a higher flow resistance. The flow resistance of a flow restriction can be set or adjusted by a variety of methods, depending on the structural design of the restriction. 
   For restriction  28 , for example, flow resistance is created by an air-permeable fabric mesh  38  whose periphery is sewn or otherwise attached to a fabric sleeve  40 , as shown in FIG.  3 . To provide relatively low flow resistance, mesh  38  can be relatively course, as shown in FIG.  4 . To provide greater flow resistance, an alternate, finer mesh  42  with more openings  44  per square-inch can be used, as shown in FIG.  5 . Greater flow resistance can also be provided by a mesh having the same quantity or even less openings than mesh  38 , but with openings that are smaller than those of mesh  38 . 
   To manipulate or adjust the flow resistance, sleeve  40  is provided with a releasable fastener  46  at each end to releasably attach to ducts  12  and  14 . This allows restriction  28  to be readily replaced by another restriction  28 ′ having a more desirable flow resistance, as shown in FIG.  6 . Fastener  46  has been schematically illustrated to encompass a variety of releasable fasteners including, but not limited to, a zipper; a touch-and-hold fastener, such as VELCRO; and snaps. 
   Flow resistance can also be adjusted by varying the size of a patch  48  that overlays a fabric mesh  50  of a flow restriction, such as restriction  32 , as shown in FIG.  7 . Here, patch  48  is of a fabric that is less air-permeable than mesh  50 , which thus further restricts airflow. Patch  48  can be attached to mesh  50  by a variety of fasteners including, but not limited to, safety pins  52 , snaps, touch-and-hold fasteners, adhesive, etc. Cutting or folding of patch  48  can be used to adjust its size or effective area. 
   In another embodiment, shown in  FIGS. 8 ,  9  and  10 , adjustable flow resistance is provided by overlaying a second fabric mesh  54  over mesh  38  to create a flow restriction  28 ″. Placing the two meshes  38  and  54  in rotational registry, as shown in  FIG. 9 , aligns the respective openings of meshes  38  and  54  to provide restriction  28 ″ with one level of flow resistance. Rotating mesh  38  relative to mesh  54 , as shown in  FIG. 10 , then provides restriction  28 ″ with more flow resistance. Although, meshes  38  and  54  may be concentrically aligned to each other,  FIGS. 9 and 10  show them slightly offset to more clearly illustrate the rotational orientation of each mesh  38  and  54 . 
   For another flow restriction  56 , shown in  FIGS. 11 and 12 , adjustable flow resistance is provided by varying the tightness of a cinch  58 . Here, restriction  56  includes an annular fabric web  60  whose perimeter is sewn or otherwise attached to a fabric duct  62 . A constrictable elongated member  64 , such as a string, cable, strap, etc., feeds through a sleeve  66  that lines a central opening  68  of web  60 . Drawing member  64  tighter constricts opening  68 , which increases the flow resistance of restriction  56 , and thus reduces the airflow to a downstream fabric duct  70 , as shown in FIG.  12 . Loosening member  64 , as shown in  FIG. 11 , widens opening  68  to provide less flow resistance. Once achieving a desired flow resistance, member  64  can be held in place by some type of conventional fastener or even by a simple knot. Access to member  64  can be provided by a closable access opening through duct  62  or  70 , or a pull-ring  72  can be provided on the exterior of the ducts by feeding member  64  through a small hole in duct  70 . The fabric of web  60  can be porous or impermeable to air, depending the desired range of flow restriction. 
   In another flow restriction  74 , similar to restriction  56  and shown in  FIGS. 13 and 14 , adjustable flow resistance is provided by varying the tightness of a cinch  76  about an inner diameter of an annular fabric web  78 ; however, flow resistance decreases with the tightness of cinch  76 . When a constrictable elongated member  80  of cinch  76  is loose, as shown in  FIG. 14 , overlapping fabric flaps  82  extending from web  78  tend to close upon themselves to resist airflow from an upstream fabric duct  84  to a downstream fabric duct  86 . Upon tightening member  80 , as shown in  FIG. 13 , flaps  82  tend to pucker, which creates a central opening  88  in restriction  74  that reduces flow resistance. Flow restriction  74  can be created by adapting the structure disclosed in U.S. Pat. No. 5,655,963, which is specifically incorporated by reference herein. 
   Adjustable flow resistance can also be provided by simply wrapping a constrictable member  90  about the exterior of a continuous fabric duct  92 , thereby creating an upstream duct  92 ′ and a downstream duct  92 ″ with a fabric flow restriction  94  between the two, as shown in  FIGS. 15 and 16 . Tightening member  90  chokes off air  36  flowing from duct  92 ′ to duct  92 ″, as shown in FIG.  15 . Loosening member  90 , as shown in  FIG. 16 , reduces the air resistance. Threading member  90  through loops  96  attached to ducts  92 ′ and  92 ″ can help keep member  90  in position. A knot  98  or some other type of fastener can be used to hold member  90  at its proper constriction. 
   To improve the appearance of ducts  92 ′ and  92 ″, a tubular fabric shroud  100  can be added to cover flow restriction  94 . Shroud  100  can be attached to ducts  92 ′ and/or  92 ″ by a conventional fastener, examples of which include, but are not limited to, a zipper, touch-and-hold fastener, clips, snaps, buttons, adhesive, and a sewn seam. Access to member  90  can be provided by having at least one end  102  or  104  of shroud  100  removably attached or unattached to duct  92 ′ or  92 ″. Access to member  90  can also be provided by moving a pull-ring  106  to the exterior of shroud  100  by feeding member  90  through a small hole in shroud  100  or by feeding it through a small gap between shroud  100  and duct  92 ′ or  92 ″. 
   To address the problem of popping experienced by some fabric ducts,  FIGS. 18-21  provided example flow restrictions having variable resistance across the flow restriction.  FIG. 18  shows a fabric flow restriction  200  that may be used in a fabric duct in place of the flow restrictions shown above. The flow restriction  200  includes a first region  202  having a first flow resistance and a second region  204  having a second flow resistance. The first region  202  is mesh in the example of  FIG. 18 , and the second region  204  is an open cavity. Upon start-up, airflow from an upstream blower passes through the second region  204  at a higher rate than the first region  202 , causing a varying air flow, in cross-section, in the downstream duct. The illustrated restriction  200  may include a stability member at an inner edge  206  to reduce wear. The stability member may be formed of a mesh fabric folded onto itself or a flexible member. 
   The fabric flow restriction  200  has a variable resistance that varies with radius from a central axis  208  to an outer edge  210 . The variable resistance may exhibit a step-wise variability, like that shown in FIG.  18 . The variable resistance may take on any desired variability pattern, including a continuously, radially varying resistance measured from the central axis  208 . In addition to a step-wise pattern, other example resistance versus radius patterns include parabolic and Gaussian patterns. Further still, the variable resistance flow restriction of  FIG. 18  may be replaced with a uniform resistance restriction that has a sufficiently high flow resistance to prevent popping, but a sufficiently low resistance to allow normal operation of the fabric duct. 
     FIG. 19  shows a flow restriction  300  similar to the restriction  200 . The restriction  300  includes a first region  302  and a second region  304 , in place of the opening  204 . The second region  304  is formed of a mesh having a higher porosity and lower resistance than the mesh forming the first region  302 . Holes  306  in the region  304  are larger than holes  308  spanning region  302 . The differences in the porosity between the first region  302  and the second region  304  may be chosen based on the size, cross-sectional shape and length of the downstream fabric duct connected thereto. 
   The first region  302  and the second region  304  may be connected together through a fastener, such as VELCRO, a zipper, a tie, or a series of snaps. Alternatively, the regions  302  and  304  may be fused or bonded together or formed on a single mesh sheet that has been exposed to different perforations for each of the two regions  302  and  304 . 
     FIGS. 20 and 21  show yet another example restriction. A fabric flow restriction  400  having a variable resistance is formed of a first mesh sheet  402  and a second mesh sheet  404 . In the illustrated example, the first mesh sheet  402  has an opening over a first region  406 . In the aligned position of  FIG. 20 , the restriction  400  has the same resistance at all radial positions. When the second mesh  404  is rotated relative to the position of  FIG. 20 , the meshes  402  and  404  combine to form a first region  408  (partially shown) having a lower porosity, and thus higher resistance, than a second region  410 . 
   The fabric flow restrictions  200 ,  300 , and  400  may be attached to a fabric duct, using the techniques described above. By way of example,  FIG. 22  shows the restriction  200  attached to a sleeve  450  having a first, releasable fastener  452  and a second, releasable fastener  454  for fastening the sleeve  450  to a fabric duct  456  and a fabric duct  458 , respectively. Suitable releasable fasteners are described herein. 
   In the illustrated configuration, the flow restriction  400  may receive a substantially uniform pressure airflow or laminar airflow from the duct  456  and convert that airflow into a radius dependent airflow  460  at the entrance of the duct  458 , resulting in an airflow pressure near a central axis  462  thereof being higher than the air flow pressure at an outermost radius  464  of the duct  458 . This has the effect of reducing the popping effect at the end of the duct since the restriction  400  has reduced the popping potential of the advancing air (also known as the static regain potential) by introducing a programmed, defined pressure drop in that advancing air. Without the restriction  400 , the end of the duct would be subject to the entire static regain potential, but the pressure drop provided by restriction  400  prevents this from happening. At the same time, the variable nature of the restriction  400  creates a radius dependent airflow that maintains some (albeit reduced) airflow at the periphery of the duct as compared to the center. This helps prevent the airflow as restricted by restriction  400  from becoming turbulent and causing a fluttering of the duct walls. Reduction of popping is thus provided without the drawback of turbulent flow. The flow restrictions  200 ,  300  and  400  share this functionally. 
   Any of the restrictions shown in  FIGS. 18-21  may be used in such a sleeve and fastener configuration as shown in FIG.  22 . Furthermore, the configuration of  FIG. 22 , while shown with two fasteners, may instead have a single fastener or no fastener. The restrictions may be formed integrally with a fabric duct, for example, on an inner surface of the duct. 
   The flow restrictions  200 ,  300 , and  400  may be disposed at various locations along a fabric duct. It is preferred, however, to position the flow restriction upstream of the distal-most end of the downstream duct, where popping is most likely to occur. By way of example, for a duct having a length, L, as measured from the point of entrance of the blower&#39;s airflow into the duct, the flow restriction may be positioned from between 0 to 0.9 L downstream of that point of entrance, leaving approximately no less than least 10% of the duct downstream of the fabric flow restriction. Generally, however, the flow restrictions may be positioned at any location within a duct assembly to provide a large restriction upon blower start-up and a relatively low restriction during normal operation of the fabric duct. 
   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.
A fabric flow restriction and method for conveying a volume of air through a fabric duct prevents the violet popping that may occur with a fabric duct upon start-up. The fabric ducts are air permeable and/or include discharge openings that evenly disperse supply air from within the duct to a room being heated, cooled, ventilated, or otherwise conditioned by the air. The ducts are typically in a collapsed positioned prior to blower start-up. With the initial airflow, the ducts quickly fill with air and may make a popping sound at their distal ends as the airflow fills the entire fabric duct. The flow restrictions disclosed may be formed of a flexible fabric that has an airflow resistance that varies with radius across the flexible fabric. The flow restrictions may have a first region and a second region each with different resistances, for example. Varying the resistance across the fabric flow restriction reduces or eliminates the popping condition. The condition may also be eliminated by using a uniform resistance flow restriction that has a high resistance during start-up and a low resistance during normal operation.