Pliable air duct with dust and condensation repellency

A fabric air duct with main discharge openings includes additional, much smaller openings that help ventilate the surface of the duct. Ventilating the surface of the duct with a slight yet even amount of airflow helps inhibit the accumulation of condensate and dust on the surface of the duct. In some embodiments, the duct includes a pliable sheet consisting of a rather porous fabric base material. To achieve an appropriately low level of airflow, a plastic coating on the fabric reduces, but does not eliminate the fabric's porosity. A calendering process then reduces the porosity even further. In some embodiments, the calendering process occurs before the plastic coating process. In other embodiments, the pliable sheet is substantially air impermeable, except for its main discharge openings. The sheet is then perforated with numerous smaller openings to achieve the desired amount of surface ventilation. In yet other embodiments, an emorizing or sueding process is used to abrade or nap the surface of a porous or non-porous base material to create a pliable sheet having a desired amount of porosity. From any of these processes, air ducts of various shapes may be formed, including air ducts that are circular, ½ round, and ¼ round in shape, as well as air ducts having a non-uniform cross-sectional shape across their lengths.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The subject invention generally pertains to pliable air ducts and more specifically to the air permeability of 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 ductwork is typically formed of sheet metal and is often installed above ceilings for convenience and aesthetics. But in warehouses, manufacturing plants and many other buildings, the ducts are suspended from the roof of the building and are thus exposed. This not only creates a poor appearance in many cases, but can create other problems as well.

For example, temperature differentials between an air duct and the air on either side of the duct wall can create condensation on both the interior and exterior of the duct. The presence of condensed moisture on the interior of the duct may form mold or bacteria that the duct then passes onto the room or other areas being supplied with the conditioned air. If an exposed sheet metal duct conveys relatively cool air, condensation can form on the exterior of the duct. The condensate may then drip onto the floor, inventory, and personnel below. The consequences of the dripping can range anywhere from a minor irritation to a dangerously slippery floor for the personnel, or complete destruction of the products it may drip on (especially in food-processing facilities).

Further, metal ducts with localized discharge registers have been known to create uncomfortable drafts and unbalanced localized heating or cooling within the building. In many food-processing facilities where the target temperature is 42 degrees Fahrenheit, a cold draft can be especially uncomfortable and perhaps unhealthy.

Many of the above problems associated with exposed metal ducts are overcome by the use of fabric ducts, such as DUCTSOX fabric ducts by Frommelt Safety Products Corporation of Milwaukee, Wis. Such ducts typically have a fabric wall that is air-permeable to broadly and evenly disperse the air into the room being conditioned or ventilated. If greater airflow in needed in certain areas, the fabric duct can be provided with additional discharge openings, such as air registers or cutouts in the fabric.

The porosity of conventional fabric can pass a substantial amount of air, which can be desirable in many applications where the airflow through the pores of the fabric is used primarily for evenly dispersing air into a room. However, some applications require airflow that is more directed toward certain areas of a room. In such cases, it may be desirable to have relatively large discharge openings provide most of the air airflow, while the pores of the fabric provide only enough airflow to inhibit dust and condensation from accumulating on the outer surface of the fabric material.

Unfortunately, it can be difficult to acquire an air duct material whose porosity provides an appropriately small amount of airflow, such as 2 cfm (two cubic feet per minute of air across one square-foot of material subject to a 0.02 psi air pressure differential). Standard fabric materials have been found to pass 40 cfm or more. Such materials have been calendered in an attempt to reduce the materials porosity. Although calendering conventional fabric does reduce its porosity temporarily, much of the effect is lost after the material is washed. Thus, simply calendering just any porous fabric is not a permanent solution to the problem.

SUMMARY OF THE INVENTION

An air duct consists of an air permeable material that passes air therethrough at a flow rate that is substantially less than what the air duct discharges through other larger openings.

In some embodiments, an air duct is made of a porous fabric that is plastic coated to reduce, but not eliminate, the fabric's porosity.

In some embodiments, an air duct includes a pliable sheet that includes a porous fabric base. The sheet is coated with a plastic that renders the sheet substantially impermeable to air. The sheet is provided with discharge openings for supplying air to a room, and is perforated with much smaller openings that help inhibit the formation of condensation or inhibit the accumulation of dust.

In some embodiments, an air duct with primary discharge openings and much smaller pores or perforations is made of a fabric with anti-microbial properties.

In some embodiments, an air duct is made of a plastic coated porous fabric that is calendered to reduce the fabric's porosity.

In some embodiments, the an air duct is made of a fabric sheet having numerous minute pores or perforations that convey only one to four CFM/ft2(cubic feet per minute per square-foot of material) when a 0.02 psia pressure differential exists across the sheet.

In some embodiments, an air duct material is perforated by displacing material rather than by removing a significant portion of it. Displacing material not only helps reinforce the periphery of each perforation, but also helps reduce the amount of scrap during the perforating process.

In some embodiments, an air duct includes a fabric sheet having a base material of polyester for strength and porosity, and having an acrylic or polyurethane coating to reduce or eliminate the base material's porosity.

In other examples, an air duct comprises a pliable sheet configured to convey air, wherein the pliable sheet has a porosity formed by an emorizing or sueding process, wherein the pliable sheet includes a plurality of discharge openings that each provide a first area and wherein the porosity of the pliable sheet provides a plurality of second areas, with the first area being greater than the second open area.

In some examples, an air duct, comprises a pliable sheet configured to convey air, wherein the pliable sheet includes a fabric base material that is porous and a plastic coating on the fabric base material that reduces the porosity of the fabric base material yet leaves the pliable sheet porous, wherein the pliable sheet includes a plurality of discharge openings that provide a first open area and the porosity of the pliable sheet provides a second open area with the first open area being greater than the second open area.

In some yet other examples, a method of creating an air duct comprises applying pressure to a pliable sheet having a porosity below a desired porosity to increase the porosity of the pliable sheet to the desired porosity; and configuring the pliable sheet to convey air.

In some examples, a method of creating an air duct comprises applying pressure to a pliable sheet to decrease a porosity of the pliable sheet; after the application of the pressure, applying a plastic coating to the pliable sheet; and configuring the pliable sheet to convey air.

DESCRIPTION OF THE PREFERRED EMBODIMENT

An air duct10, shown inFIG. 1, consists of a pliable sheet12in a tubular shape. Duct10is adapted to be suspended overhead to convey forced air14from a blower to specific rooms or desired areas within a building. Depending on the application, the air may be for ventilation purposes only, or may be conditioned by heat, cooling, filtering, humidifying, dehumidification, and various combinations thereof.

Most of the air delivered to the rooms comes from discharge openings16in duct10, as indicated by airflow arrows18. Openings16can assume a variety of forms including, but not limited to cutouts, discharge registers, and screens.

To help inhibit condensation or dust from accumulating on the surface of duct10, the fabric wall between discharge openings16is provided with another set of much smaller openings20. Openings20allow the fabric wall of the duct to breathe in the areas between discharge openings16. A slight current of air22passing outward through the duct wall may help keep dust from settling on the exterior of the duct. But also, when duct10is conveying cool air, a small current of cool air passing through the duct's fabric wall tends to keep the warmer room air slightly away from the exterior surface of duct10. Thus, moisture in the warmer room air is less likely to condense on the surface of duct10.

The actual size, number, and spacing of smaller openings20can vary; however, there does appear to be an optimum design range. The relative open areas of openings16and20should allow about ten percent (preferably less than more) of supply air14to pass through smaller openings20and about ninety percent through discharge openings16. Sheet12should preferably pass one to four CFM/ft2with a 0.02-psi pressure differential across sheet12(i.e., 0.02 pounds per square inch of air pressure should force one to four cubic feet of air across a one square-foot of sheet material every minute). Higher airflow rates through smaller openings20reduce the amount of air that discharge openings16can direct to specific areas, while lower airflow rates are less effective at reducing condensation or dust. In some cases, positive results are achieved when openings16are able to pass more than twenty times as much air as smaller openings20. Moreover, the distribution of openings20should be sufficiently dense to provide an even flow of air through sheet12. To avoid having smaller openings20direct too much airflow in any particular direction, openings20are preferably distributed over nearly the full circumference or perimeter of duct10.

To achieve appropriate airflow characteristics, sheet12may consist of a fabric base material24with a plastic coating26, as shown inFIG. 2. In some embodiments, fabric24is a porous woven material, such as polyester. Coating26, such as an acrylic or polyurethane coating, is applied to fabric base24to reduce but not eliminate the porosity of sheet12. If the resulting openings20′ are too large, as shown inFIG. 2, compressing or calendering sheet12can reduce their size. Compressing sheet12forces coating26into openings20′ until the open area of openings20′ are reduced as indicated by openings20ofFIG. 3. Coating26tends to maintain the desired size of openings20′ even after the material is washed.

The process of producing sheet12is schematically illustrated inFIG. 4. Applying coating26is schematically illustrated to encompass conventional coating processes that are well known to those skilled in the art. Reducing the size of openings20′ by compression can be achieved by a calendering process where sheet12is compressed between two rollers28and30. In some cases, applying heat32to at least one of the rollers softens coating26, which may help in permanently reducing the size of openings20′. Once openings20are of an appropriate size, sheet12can be formed into a tubular shape34. In some cases, coating26and/or fabric material24provides appreciable antimicrobial properties as determined by standard tests, such as AATCC Method100(where AATCC stands for the American Association of Textile Chemist and Colorists, of Research Triangle Park, N.C.). Coating26and/or fabric material24can also render sheet12flame retardant, whereby sheet12is self-extinguishing.

In an alternate embodiment, shown inFIGS. 5–8, a duct36includes a pliable sheet38that may begin as a porous fabric base material40. The fabric base material40is then sealed with a plastic coating26′, which substantially eliminates the porosity of sheet38, as shown inFIG. 6. To allow sheet38to breathe, a tool42perforates sheet38to create numerous perforations44that are significantly smaller than discharge openings16′, as shown inFIGS. 5 and 8. In some embodiments, tool42is a needle that creates perforations44by displacing material, rather than by just removing material. In this way, built-up material46forms around the periphery of each perforation44, with the volume of material46being generally equal to the void of each perforation44. Such a process reduces scrap and at the same time may avoid weakening a perforation's circumference.

Just as with the embodiment ofFIGS. 1–4, the size, number, shape and spacing of perforations44ofFIGS. 5–8can vary. However, in preferred embodiments, perforations44have an open span48or effective diameter of less than 0.1 inches and are distributed at a spacing50that is greater than a nominal thickness52of sheet38but less than 0.5 inches. The term, “effective diameter” equals the square-root of a hole's open area times two and divided by the square-root of one divided by pi (effective diameter=2(A/3.14)0.5). In some cases, desirable results may be achieved when the effective diameter of perforations44is less than thickness52, and perforations44have a distribution of 100 to 2000 perforations per square-inch.

A technique for producing a pliable sheet, alternative to that shown inFIG. 4, is shown inFIG. 9. The technique applies pressure to a pliable sheet100, which may be porous or non-porous prior to pressure application. The applied pressure may create porosity in the pliable sheet100by an emorization process whereby the outer surfaces of the pliable sheet100are abraded. For example, the pliable sheet100may include an initially porous, fabric base102treated with the plastic coating26to completely remove the porosity of the fabric base102, as shown inFIG. 10. The coating26may alternatively leave the coated pliable sheet100partially porous, similar to the illustrations inFIGS. 2 and 3.

The pliable sheet100is compressed between two rollers104and106, which have abrasion surfaces,108and110, respectively. The abrasion surfaces108,110have protrusions112compressing against the surface of the pliable sheet100to create the desired level of porosity. Thus, the process ofFIG. 9may form porosity in a pliable sheet that has no porosity prior to the process, or it may increase porosity in a pliable sheet that has a porosity below a desired porosity. Desirable porosity levels may very and include those described hereinabove.

FIG. 9may also represent a sueding process, as described below with respect toFIG. 12.

The abrasion surfaces108and110may be formed of sandpaper or other rough surfaces, such as a surface coated with industrial diamond particles. The protrusions112may be periodic or aperiodic. In an example, the protrusions112cover the entire surface108and110.

FIG. 11Aillustrates the pliable sheet100, with the fabric base layer102and plastic coating layers113aand113b.The plastic coating layers113aand113bresult from application of the coating26. The layer113brepresents a portion of the coating26that has diffused or crept into the fabric base layer102. The remaining layer113ais exposed on an outer surface of sheet100.FIG. 11Billustrates the pliable sheet100after it has been abraded by the rollers104,106. In the example ofFIG. 11B, the layer113ahas been completely removed by the emorizing or sueding process ofFIG. 9, and the fabric base layer102and the imbedded coating layer113acreate the desired porosity. Alternatively, the emorizing or sueding process may abrade the pliable sheet100to create porosity therein—for example, if the pliable sheet100is non-porous or if the porosity of the pliable sheet100is to be increased beyond that of the its normal porosity.

Although the pliable sheet100is illustrated inFIG. 9as being exposed to a plastic coating process that reduces the porosity of an already porous sheet, this process is optional. The plastic coating process also may be eliminated, for example, when the pliable sheet100is formed of flexible non-porous material. A further alternative technique includes applying heat116to the rollers104,106, as described above. Another technique includes abrading a single surface of the sheet100. Further still, the pliable sheet100may be exposed to an additional porosity creating process, such as a perforation process, either before or after compression. Indeed, the emorizing process illustrated inFIG. 9may be replaced with a sueding or napping process by using a roller120(FIG. 12) having a series of hooks or angled teeth122that may latch into a pliable sheet during roller compression, where this latching may catch on the pliable sheet and nap the surface thereof to create or increase porosity. In any of these examples, the porous pliable sheet formed by the rollers104,106may be configured into a tubular pliable sheet124to convey air. Preferably, the tubular pliable sheet124would include discharge openings, like the discharge openings16inFIG. 1.

Another suitable process is a calendering process like that ofFIG. 4, but with the plastic coating process coming after the calendering process.FIG. 13illustrates a pliable sheet150that is compressed between two rollers152and154. The sheet150may or may not be porous. The rollers152and154calender the sheet150to reduce its porosity, for example. Heat156may be applied during calendaring, as described above. After calendering, a plastic coating is applied by a wedge158that functions somewhat like a squeegee evenly distributing the plastic coating across the top surface of the sheet150. By calendering first, the top surface of the pliable sheet150may be smoothened to allow for a more even application of the plastic coating. The calenderized sheet150is formed into a tubular pliable sheet160in the illustrated example, although alternatively it may be applied to another process, such as a porosity-increasing process like that ofFIG. 9or a perforation process. As with the tubular pliable sheet124, in an example, the tubular sheet160includes discharge openings.

While the tubular pliable sheets124and160are shown having a uniform cross-sectional shape that is circular, the sheets124and160may be formed into other cross-sectional shapes and may be uniform or non-uniform across their tubular length.FIGS. 14–16show some such example air ducts. InFIG. 14, an air duct200is formed having a first flat surface202, a second flat surface204, and a curved surface206, collectively forming a shape having a ¼ round cross-section. The air duct200may be used, for example, as a duct running along a corner formed by two orthogonal support members, such as a wall and ceiling. The surfaces202,204, and206may all be porous surfaces formed through a perforation, emorizing, sueding, or calenderization process. Alternatively, some of the surfaces202,204, and206may have varying porosities including no porosity. To flatten surface202, the surface202is affixed to a track208, which may be sown in the duct200or mounted thereto, for example, through a button, latch, glue, or VELCRO mounting. A second track210is shown (in phantom) extending along wall204. In operation, the two tracks208,210are mounted against orthogonal support members, such as a wall and ceiling and the duct200takes a ¼ round shape. Screw or bolt fasteners may be used to mount to the support member, for example. The duct200may include discharge openings, not shown.

FIG. 15shows a similar air duct in the form of ½ round cross sectional air duct300having a top surface302and a curved surface304. The duct300may be formed by any of the techniques described herein and may include two tracks306,308that may be mounted on a support member, such as a ceiling, to make the top surface302taught, a support306for a connector308. The duct300may further include discharge openings, not shown.

FIGS. 14 and 15show ducts with a uniform cross-sectional area and shape over a tubular length. Alternatively, an air duct may have a non-uniform area or shape. An example air duct400is shown inFIG. 16. The air duct400has a tapered profile, where discharge openings402and indentations404, formed by the process ofFIGS. 9,12, or13, extend the length of a tapered region406. The tapered region406is capped by an end408. The duct400is by way of example only, however—the cross-sectional shape may change across the tubular length of an duct, for example, by having a circular shape at one cross-sectional position and a ¼ round or ½ round shape at another. Other cross-sectional profiles may be achieved.

Although the invention is described with reference to a preferred embodiment, it should be appreciated by those of ordinary skill in the art that various modifications are well within the scope of the invention. For example, the pliable sheets described herein do not have to include a fabric base. The sheets could simply be a pliable air impermeable plastic sheet that is perforated with micro-perforations or pinholes to achieve desired flow characteristics. And in the case of the emorizing and sueding techniques described with reference toFIGS. 9 and 12, as well as the calendering process ofFIG. 13, the pliable sheets fed to the pressuring step need not be porous at all. Therefore, the scope of the invention is to be determined by reference to the claims that follow.