Abstract:
A durable, semi-rigid, flexible duct including a pair of coaxial sleeves, namely an inner sleeve and an outer sleeve disposed parallel to and about the inner sleeve and a resilient wound element disposed between the sleeves. Each of the inner sleeve and the outer sleeve constitutes an aluminum foil ribbon. The wound element imparts corrugations to the two sleeves, such that the duct is extendible between a compacted configuration suitable for storage and for shipping and an extended configuration suitable for installation in a gas transport arrangement. Closely and evenly-spaced ridges that are situated in between the corrugations, add rigidity and durability to the duct. Both the inner sleeve and the outer sleeve are of a predetermined thickness rendering the duct substantially rigid when in an extended configuration and enabling the duct to maintain its substantial rigidity upon extension from a compacted configuration.

Description:
REFERENCE TO CO-PENDING APPLICATIONS 
     The present application is a continuation-in-part of U.S. patent application Ser. No. 12/645,517 entitled “Semi-Rigid Flexible Duct”, filed Dec. 23, 2009, which is a continuation-in-part of U.S. patent application Ser. No. 11/717,411 entitled “Semi-Rigid Flexible Duct”, filed Mar. 13, 2007 now abandoned, which is a continuation-in-part of U.S. patent application Ser. No. 11/389,623, entitled “Flexible Semi-Rigid Clothes Dryer Duct”, filed Mar. 24, 2006 now abandoned, the contents of which are incorporated herein by reference. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to ducts, particularly semi-rigid flexible ducts. 
     BACKGROUND OF THE INVENTION 
     Ducts are used for different purposes, including for the conveyance of air, such as in ventilation, heating and cooling systems, or for conveying away exhaust gas from clothes dryers or other similar machines, as well as for carrying electrical cables and wiring, or other utilities. 
     When used for air conditioning or ventilation systems, such as within suspended ceilings, particularly in industrial and office premises, the ducts are circular and must be supported, as they have little self-support. 
     A further, very well known use of ducts is an exhaust vent for clothes dryers, in which the duct is fabricated of a resilient wire helix covered with vinyl or aluminum tubing. Both type of ducts lack structural integrity, while the vinyl-covered duct is not flame resistant. The lack of structural integrity of these ducts typically results in sagging and crinking thereof, causing the duct, over time, to become lined with lint from the clothes dried in the dryer, posing a fire hazard. In the United States alone, thousands of fires associated with clothes dryers occur, causing deaths and injuries and millions of dollars in damages. It is generally recommended by clothes dryer manufacturers not to use vinyl ducts such as these for dryer exhaust transition ducts. 
     Representative of the prior art in ventilation systems is U.S. Pat. No. 5,281,187 to Whitney, for a “Unitary Vent and Duct Assembly” which discloses a “semi-rigid flexible duct” for a ventilation system installed with a suspended ceiling structure. The duct taught in this patent is actually a solid aluminum tube which is corrugated or “accordion-folded” so that it can be compressed or compacted for storage or shipping. The corrugated aluminum tube duct taught therein, is meant to be coupled to a duct assembly of which it is an integral part, which is intended only for installation within a framed section of a suspended or dropped ceiling. However, once such a tube has been compressed and then re-extended for installation, it is unlikely to maintain its rigidity, depending on the thickness of the aluminum. A tube of this type maintains its rigidity by virtue of its being fabricated of solid metal, is heavy and expensive and can be unwieldy to install. The corrugated aluminum, when extended after compression, has significant ridges and other obtrusive topographical features along its interior due to the corrugations, which cause frictional resistance to the air flow within, a further disadvantage. 
     Corrugated aluminum is also employed for the exhaust vent of clothes dryers, as, for example, in U.S. Pat. Nos. 5,121,948, 5,133,579, and 5,145,217, which solve the above-described problem of insufficient rigidity, but by using totally rigid segments. Even though the aluminum tubing itself is clearly fire resistant, the ridges and other internal topographical features similar to those mentioned hereinabove with respect to the Whitney patent, also cause frictional resistance to the air flow within, permitting accumulation of lint, which, as stated hereinabove, presents a fire hazard. 
     U.S. Pat. No. 5,526,849, included herein by reference, to Gray for a “Flexible Duct” discloses a duct and a method for manufacture thereof. The duct disclosed therein is formed of plastic tapes wound on a rotating mandrel with a wire resilient helix and a yarn helix therebetween. The duct so produced, while flame resistant, has rigidity limited to that provided by the wire helix. The additional yarn helix complicates the manufacturing process and adds to the internal topographical features of the duct, increasing friction and the possibility of lint accumulation therein, as described above. 
     The shape of ducts also has significance. Relatively heavy, rectangular metal ducts, formed of heavy gauge sheet metal, are often used for heating and cooling systems in industrial and office premises. A rectangular cross-sectional shape is desired due to the possibility of placing the duct against a support surface, and thereby utilizing available space more efficiently than a circular duct. These ducts are limited, however, in length, due to their relatively heavy weight and rigidity, as well as to transportation considerations. Accordingly, several lengths of these ducts may need to be joined together on site in order to provide adequate lengths of duct. It will be appreciated that they also require sufficiently strong hangers and other mechanical supports to be provided so as to provide adequate support. Furthermore, specially made corner portions must be provided to take account of bends. 
     A further consideration that must be taken into account when installing exhaust ducts for directing exhaust gases from machines, is the fact that the exhaust vents (or in the case of air conditioning units, the cool air supply vents) often have a square or rectangular shape, requiring somewhat unorthodox adaptive connections to conventional round ducts. 
     During manufacturing of conventional round ducts, a problem has been encountered with wrapping of semi-rigid materials, such as a thin aluminum sheet, about a rotating mandrel. While the semi-rigid materials provide flexibility to the resulting duct produced in this fashion, the manufacturing process is complicated due to the fact that semi-rigid materials may tear under the tension applied during the wrapping procedure. The solution to this problem, until now, has been to avoid the use of thin aluminum sheets in constructing semi-rigid ducts, and to rely on heavier, more rigid materials, which do not lend themselves to flexibility, and are unwieldy to install, as mentioned above in relation to U.S. Pat. No. 5,281,187 to Whitney. 
     A “SEMI-RIGID DUCT” product number A045/9 5-FT, manufactured by “Deflecto Corporation”, is known, manufactured without the use of wire or glue, and has the disadvantage of not being rigid enough, so if it gets crushed, it will obstruct the airflow and there will be a lint buildup which is a fire hazard. 
     In our previous application, U.S. patent application Ser. No. 12/645,517, filed by the present inventors, a method and duct product is shown having only aluminum layers, which overcome the above-mentioned disadvantages. Even with this improvement, heavy boxes or bottles such as detergents and fabric softeners often fall off the dryer and land on the duct, crushing it as a result. 
     Therefore, it would be desirable to manufacture a duct that is more rigid and durable, which will withstand heavy impact forces, which might crush the duct and reduce its diameter. 
     SUMMARY OF THE INVENTION 
     The present invention seeks to provide a durable semi-rigid, multi-purpose flexible duct that is fire resistant and that is lighter in weight and less expensive than those used in the prior art, while maintaining rigidity and structural integrity, even after having been compressed to a compacted configuration for shipping and storage and then re-extended for installation. 
     Further, the duct has minimal internal topographical features or structure, even when re-extended after having been compressed to a compacted configuration for shipping and storage. 
     The present inventive duct is more rigid and maintains its structure and is not affected by heavy impact forces. Further, the inventive duct does not lose diameter size after being re-extended following compression, thus providing maximal airflow and minimal lint accumulation. 
     A further aim of the present invention is to provide a semi-rigid, multi-purpose flexible duct having a cross-sectional configuration which may be round, square or rectangular according to need, and which may be used for such diverse uses as gas transport, for example in air conditioning systems or as a gas dryer duct; and the enclosure of utility pipes and cables in an isolated and low-fire-hazard environment. 
     The present invention further seeks to provide a method for manufacturing such a duct that is simple, fast, and efficient. 
     In a preferred embodiment there is provided a duct, which includes a pair of coaxial sleeves including an inner sleeve and an outer sleeve, each constituting a thin aluminum sheet used as the construction material for the duct, provided as an aluminum foil ribbon with sufficient thickness to provide flexibility and withstand the tension developed during the wrapping procedure about a rotating mandrel. 
     There is thus provided, a semi-rigid, flexible duct, which, in accordance with the present invention, may be used for gas transport, such as in cooling or heating systems or for machine exhausts, including but not limited to clothes dryers. It may further be used for enclosing utility lines, such as water, gas, electricity, and telecommunications, particularly when the duct is employed in its rectangular configuration. The duct of the present invention, when formed so as to have a rectangular cross-section, may easily be disposed between two leaves of a hollow wall construction, beneath a suspended wooden or other floor, and within a suspended ceiling, so as to provide an efficient, lightweight yet secure, and easily installable manner of passing utility lines behind, beneath or below building elements. 
     In a preferred embodiment there is provided a durable semi-rigid duct, comprising a pair of aluminum foil ribbons wrapped to form a pair of coaxial sleeves, having an inner sleeve and an outer sleeve disposed parallel to and about the inner sleeve, and a resilient helical element disposed between them; 
     wherein each of the inner sleeve and the outer sleeve have metallic properties; 
     wherein the helical element imparts helical corrugations to the inner sleeve and the outer sleeve, said inner and outer sleeves being formed with a plurality of closely and evenly-spaced ridges embossed in between a pair of said corrugations, such that the duct is axially extendible between a compacted configuration suitable for storage and for shipping and an extended configuration; 
     and wherein the inner sleeve and the outer sleeve are of a predetermined thickness rendering the duct substantially rigid when in the extended configuration, and enabling the duct to maintain its substantial rigidity upon extension from the compacted configuration, and 
     wherein said closely and evenly-spaced ridges impart additional rigidity to said duct, causing said duct to be resilient and withstand heavy impact forces, which might crush the duct and reduce its diameter. 
     An advantage of the above-mentioned embodiment of the present invention is that due to its rigidity and structural integrity, there is a reduction in the tendency of the duct to accumulate lint, thereby reducing fire hazards. 
     A further advantage of the above-mentioned embodiment of the invention is that unlike the prior art flexible ducts, such as mentioned in U.S. Pat. No. 5,526,849 (see Background), the elimination of a plastic layer from the duct construction further reduces fire hazards. 
     In the preferred embodiment, the thickness of the inner sleeve and of the outer sleeve is in the range of 24 to 35 microns. 
     In the preferred embodiment, the duct contains closely and evenly-spaced ridges additional to the ridges already formed by the spiral internal wire. The fine ridges add strength to the duct, and resilience, so that when it is compressed and then extended, there is no loss of the inner diameter, unlike in the previous inventive duct. 
     The inventive duct is designed to be connectable to various types of equipment, by use of a specially-designed threadable connector ring. 
     Because the ridges are multiple and in close proximity to one another, they serve as threads that provide a screw connection. This enables a threaded connecting ring to be screwed onto the duct, having matching threads, on one side thereon, and on the other to be connected to the dryer via a snap-on adaptor. Until now, a dryer duct would be connected to a dryer by using adhesive tape or the like, which does not provide a reliable connection. Also, the screwed-on connector provides easy maintenance of the duct, since it eases the operation of disconnecting and re-connecting it to the dryer. 
     The connector ring can also be utilized for the exhaust on the other end of the duct, connecting it to the wall. The connector ring can then be connected to one part of a “draft blocker” that is connected to an opening in the wall, and the second part of the “draft blocker” is connected to the other side of the opening in the wall. The second part has a flap that opens only when air is blowing through the wall, and not when it is blowing in the other direction. Therefore, air from the outside cannot penetrate into the duct. 
     Another use for the connector ring is connecting it to a “lint trap”, which is used when there in no access for the duct to the outside. The “lint trap” provides housing for the lint that is released from the duct, so as not to allow the lint to scatter around the vicinity of the dryer. The “lint trap” can be hung on the wall, or placed on the floor. 
     In a preferred method of manufacturing a durable semi-rigid flexible duct, the method comprises the steps of: 
     a) providing a mandrel of preselected diameter for fabricating a duct therearound, said mandrel having a plurality of individuals rollers, at least one of which has circumferential grooves formed on the distal end thereof; 
     b) providing a first continuous aluminum ribbon of predetermined thickness to form a first continuous tape; 
     c) providing a second continuous aluminum ribbon of predetermined thickness to form a second continuous tape; 
     d) wrapping the first continuous tape with a predetermined overlap around the mandrel to form an inner sleeve; 
     e) winding a wire around the inner sleeve; and 
     f) wrapping the second continuous tape with a predetermined overlap around the inner sleeve and the wire winding to form an outer sleeve disposed parallel to and about the inner sleeve, thereby to form a duct. 
     g) embossing ridges on said aluminum ribbon by engaging a ridged break-down wheel, supported by a mounting arm external to said mandrel, with a modified roller, while said aluminum ribbon is in between them. 
     When a predetermined length of the duct is in the extended configuration and is disposed horizontally and supported at a first end thereof, the duct is fabricated to bend under the influence of gravitational force such that a second unsupported end thereof is lower than the first supported end by no more than a predetermined percentage of the predetermined length. Further, when a predetermined length of the duct is in the extended configuration and is disposed horizontally and supported at both ends thereof, the duct is fabricated to bend under the influence of gravitational force such that the central portion thereof is also lower than the level of the supported ends by no more than a predetermined percentage of the predetermined length, which, for a 2 meter length of a duct with a diameter of approximately 10 centimeters, will be up to 3 millimeters for an extended duct that was not compacted and also for a duct that was extended from the compacted configuration. 
     Additionally, the resilient helical element is fabricated of a metal having spring-like resilience, such as a wound galvanized wire of a diameter in the range 0.9 to 1.3 millimeters. 
     Further, in accordance with a preferred embodiment of the invention, the resilient helical element is aligned with the inner wound wrapping so that the wound galvanized wire is approximately centered over the overlap of the inner helical wrapping of the inner sleeve and the outer helical wrapping of the outer sleeve is aligned with the resilient helical element so that the overlap of the outer helical wrapping of the outer sleeve is approximately centered over the spaces between the wires of the wound galvanized wire of the resilient helical element. 
     The duct may serve as a gas transport duct or as a duct for enclosing utility supply lines, and has a cross-sectional configuration which may be circular or polygonal, such as square or rectangular. 
     In accordance with a preferred embodiment of the present invention, the method further includes in step f) of winding, the additional step of imparting to at least a portion of the duct, a polygonal cross-sectional configuration, such as square or rectangular. 
     Thus, the present invention advantageously provides a semi-rigid, multi-purpose flexible duct that is resilient, fire resistant and that is lighter in weight and less expensive than those used in the prior art. 
     Further advantages of the invention will become apparent from the following drawings and description. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present invention will be more fully understood and appreciated from the following detailed description, taken in conjunction with the drawings, in which: 
         FIG. 1  is a side view of a portion of a prior art duct having a circular cross-sectional configuration; 
         FIG. 2   a  shows an end view of the circular configuration of the inventive duct; 
         FIG. 2   b  is a cross-sectional view taken along section lines A-A of  FIG. 2   a , showing a length-wise portion of the duct, constructed and operative in accordance with a preferred embodiment of the present invention; 
         FIG. 2   c  shows an enlarged view of a portion B of the duct of  FIG. 2   b.    
         FIG. 3  shows a circular cross sectional view of a portion of a duct constructed and operated in accordance with a first embodiment of the present invention; 
         FIG. 4  shows a top view of a mandrel construction used in manufacturing the duct of  FIG. 2 ; 
         FIG. 5  shows a front view of the mandrel of  FIG. 4 , being fed by wire and aluminum foil tape used in the manufacturing process; 
         FIG. 6  shows a wire-feed system for tension control of the wire fed to the mandrel; 
         FIG. 7  shows an aluminum tape-feed system for tension control of the tape fed to the mandrel; 
         FIG. 8  shows a perspective view of a mandrel during the manufacturing process of the duct of  FIG. 2 ; 
         FIG. 9  is a schematic oblique view of a segment of a duct that has been compressed; 
         FIG. 10  is a schematic representation of the vertical sag of the unsupported center of a segment of duct such as that of  FIG. 2  supported at its ends; 
         FIG. 11  is a schematic representation of the vertical displacement from the horizontal of the unsupported end of a segment of duct such as that of  FIG. 2  supported at its other end; 
         FIG. 12  is a schematic view of a duct, constructed and operative in accordance with an embodiment of the present invention, which is installed as an exhaust transition duct of a clothes dryer; 
         FIG. 13  shows a perspective and side view of the duct of  FIG. 2 , with a screwed on adaptor ring on both ends; 
         FIG. 14   a  shows a perspective view of the duct connector ring, showing the side which connects to the duct of  FIG. 2 ; 
         FIG. 14   b  shows a perspective view of the duct connector ring, showing the side which connects to an adaptor ring; 
         FIG. 14   c  shows a perspective view of an adaptor ring for connecting to a dryer; 
         FIG. 14   d  shows a perspective view of the duct connector ring connected to the adaptor ring for dryer connection; 
         FIG. 15  shows the duct of  FIG. 2  connected to a dryer via the duct connector ring and adaptor ring; 
         FIG. 16   a  shows a lint trap; 
         FIG. 16   b  shows the top part of the lint trap; 
         FIG. 16   c  shows the bottom part of the lint trap; 
         FIG. 16   d  shows the lint trap connected to the duct of  FIG. 2 ; 
         FIG. 17   a  is a perspective view of a draft blocker when the dryer is turned off; 
         FIG. 17   b  is a perspective view of a draft blocker when the dryer is turned on; 
         FIG. 18  is a perspective view of a duct connected to a partition via a draft blocker; 
         FIG. 19  is a perspective view of a duct connected to a wall via a draft blocker connected to a solid tube; 
         FIG. 20  is a perspective view of a duct connected to a wall via the adaptor ring and dryer connector connected to a solid tube; 
         FIG. 21   a  is a schematic diagram of apparatus for imparting a selected polygonal cross-sectional configuration to a circular duct; 
         FIG. 21   b  is an enlarged schematic representation of the apparatus of  FIG. 21   a ; and 
         FIG. 21   c  is an end view of the apparatus illustrated in  FIG. 21   b.    
         FIG. 22   a  is a perspective view of a square duct having a round two-piece connector on both its ends; 
         FIG. 22   b  is a side view of the duct of  FIG. 22   a ; and 
         FIG. 23  is a pictorial representation of a compound duct; 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring now to the drawings, there is shown, in  FIG. 1 , a side view of a segment of a prior art duct, referred to as  25 . Duct  25  is of a two-layer cylindrical construction having an axis  32  and corrugations  34 , and may be used for gas transport or for enclosing utility lines. 
     Referring now to  FIGS. 2   a - 2   b , there are shown end and side views, respectively, of duct  30 , constructed in accordance with the principles of the present invention, and as per  FIG. 2   a  duct  30  is round. 
     As shown in  FIG. 2   b  duct  30  has corrugations  34 , and at least two additional closely and evenly-spaced circumferential ridges  28  between two corrugations  34 , for adding strength and resilience to the duct  30 . 
     Referring now to  FIG. 2   c  there is shown an enlargement of a portion  26  of duct  30 , illustrating the corrugations  34  and ridges  28 . 
     In accordance with the present invention, the specific description below of cylindrical duct  30  applies equally to non-cylindrical ducts, such as square duct  100  ( FIGS. 22   a - 22   b ) as well as variations thereof, all as described hereinbelow. 
     By way of clarification, the term “helical,” and variations thereof, derives from the description of the manufacture of the ducts of the invention, and relates to the act of winding various elements in a spiral or helix. In the embodiments of the invention in which the duct remains cylindrical, the helical windings clearly remain helical. In those polygonal embodiments of the invention however, the windings, while not being strictly helical, retain a general square-helical arrangement, and may be referred to as such, although mainly they are referred to merely as “windings” or “wound.” 
     Referring now to  FIG. 3 , duct  30  has inner and outer sleeves, referenced  35  and  37 , respectively, which are coaxial, each preferably being formed of a wound helical wrapping of a single-layer aluminum ribbon provided as a tape,  36   a  and  36   b , bonded together with adhesive layers  38   a  and  38   b , each layer of adhesive on a ribbon layer, respectively. Coaxially wound around inner sleeve  35  is a wound helical wire  40 , preferably galvanized wire, disposed between inner sleeve  35  and outer sleeve  37  encapsulated between two layers of adhesive,  38   a ,  38   b , thereby bonding layers  35  and  37  to helical wire  40  and to each other. Aluminum ribbon  36   b  is helically wound around a mandrel  42  (see  FIG. 4 , discussed hereinbelow), to form inner sleeve  35 . 
     Referring now to  FIG. 4 , the construction of mandrel  42  is shown, comprising a plurality of rollers  44 . Rollers  44  are all individually rotatable, and each is mounted on a fixed plate  43  at an angle  46  with respect to the plate  43 . Each individual roller  44  has formed therein a set of grooves  48  for accepting the wire  40  which forms the basis for the spiral format of the flexible duct  30 . These grooves  48  are precision-shaped and are precision-spaced apart in order to accept the predetermined flow of wire  40 , and this flow is established by the angle  46  of the roller. Typically the angle  46  is adjusted to establish the correct flow of both wire and ribbon. The wire  40  is fed from a spool onto the mandrel  42 , and the mandrel  42  is designed such that each individual roller  44  is mounted thereon at a particular angle  46 , to provide a flow effect which enables the tape  36  ( FIG. 5 ) to be fed onto the mandrel  42  and to be taken off in a helical form. Thus, the wire feed becomes a spiral form for the length of the duct  30  being drawn off the mandrel  42  in an automatic fashion. 
     Referring now to  FIGS. 5 and 8 , there are shown, respectively, a front view and perspective view of mandrel  42  in the midst of the process of fabricating a duct  30 . The size of the duct  30  being fabricated is determined by mandrel  42  which is rotated about its longitudinal axis  56 . Inner single-layer aluminum tape  36   b  is helically wound with a predetermined overlap  33  around mandrel  42  as it turns to produce the single-layer inner sleeve  35  of duct  30  as a first step in forming duct  30 . Galvanized wire  40  is helically wound around the single-layer inner sleeve  35  of duct  30  as mandrel  42  turns with the single-layer inner sleeve  35  formed thereupon. Outer single-layer aluminum tape  36   b  is helically wound with a predetermined overlap  31  around the inner sleeve  35  of duct  30  with galvanized wire  40  wound thereabout as mandrel  42  turns with the single-layer inner sleeve  35  and the wire  40  wound thereupon to produce the single-layer outer sleeve  37  of duct  30 . 
     For the purpose of creating closely and evenly-spaced ridges  28 , one of the rollers  44   a  of mandrel  42  is modified by adding circumferential grooves  39  to the distal end of the roller, to create modified roller  44   b . A mounting arm  41 , external to the mandrel  42 , holds a ridged break-down wheel  45 , having projecting ridges thereof, which mesh with the circumferential grooves  39  of roller  44   b  by engagement. The design of break-down wheel  45  is such that the projecting ridges thereof form at least two closely and evenly-spaced ridges  28 , between a pair of corrugations  34 , on the aluminum tape  36 . During the setup process the wheel  45  is adjustably moved into position (arrow E) so that its ridges engage the grooves  39  formed at the distal end of the modified roller  44   b . Once this position is established it is maintained during production. The aluminum tape  36  passes in between the roller  44   b  and wheel  45 , as it advances along the rollers of the mandrel  42 , thus the grooves and ridges of the roller  44   b  and wheel  45 , respectively, are embossed on aluminum tape  36 , so as to form the closely and evenly-spaced ridges  28  of the duct  30 . 
     Referring now to  FIG. 6 , a wire-feed system  50  for tension control of the wire  40  fed to the mandrel  42  is shown. The tensioning of the wire  40  is provided by equipment placed on the automatic wire-feed system  50  which incorporates a load cell  52  that controls an electronic brake  54  which controls the flow of wire  40  onto the mandrel  42 , supplied by the wire feed supply spool  56 . The equipment for tensioning also includes a plurality of tension pulleys  58 . 
     Referring now to  FIG. 7 , there is shown an aluminum tape-feed system  60  for tension control of the tape  36  fed to the mandrel  42 . A load cell  65  and electronic brake  63  are provided to control rotation of the spool  68  of aluminum tape  36 , thereby providing it with tension. With the correct control of the spool  68  rotation, to provide a constant tape tension, a proper feed and feed rate are achieved for automatically winding the tape  36  onto the mandrel  42  which is rotated at a sufficient speed to provide an automatic flow for efficient production of the flexible duct  30 . A glue applicator  66  is mounted on the system  60  as well for applying glue to the aluminum tape  36  so the two layers of tape  36  will bond to the wire  40  and to each other, when creating the duct  30 . 
     The tension of the two aluminum ribbons  36   a ,  36   b  must be identical and constant at all times, otherwise the ribbon will tear. Also, the tension of the wire  40  must be constant and equal to the tension of the two aluminum ribbons  36   a ,  36   b . The tension typically required for fabricating a duct  30  ranges between 65 kgf-70 kgf. The rollers  44  of the mandrel  42  are mounted to plate  43 , and are designed so as to provide a flexible spring-like action which absorbs any irregularities in the tension settings or any irregularities associated with the tape and wire materials being used. 
     Duct  30  is manufactured fully extended by a continuous process, further described hereinbelow, and is then cut to a desired length. The corrugations  34  imparted thereto by wound helical wire  40 , and the closely and evenly-spaced ridges  28  imparted by the ridged break-down wheel  45 , allow duct  30  to be axially compressed into a compact configuration convenient for storage or shipping. When duct  30  is compressed, as shown in  FIG. 3 , aluminum layers  36   b  and  36   a  naturally fold between the ridges (referenced  34  in  FIG. 2 ) created by wound helical wire  40 . For example, a 2.4 meter length of 10 centimeter diameter duct fabricated in accordance with the present invention can be compressed to a length of approximately 15 centimeters, which is comparable to the compression of simple prior art ducts described hereinabove that do not have the advantages and improvements of the present invention. 
     A particular advantage of the unique, multilayered construction of the present invention is that duct  30  maintains its rigidity and structural integrity and functions like a totally rigid duct even after having been compressed to its compact configuration and re-extended to its original length. 
     Referring now to  FIG. 9 , there is shown a compressed segment of the duct  30 . The ability to compress the duct after it has been manufactured is advantageous for purposes of storing and shipping. Furthermore, the duct  30  retains its shape after compression so once it is extended it returns to its original duct shape, retaining its substantial rigidity. 
     Referring now to  FIG. 10 , there is shown, schematically, the vertical sag c of the unsupported center  101  of a horizontal segment of duct  30  spanning between two supports  99  a distance L apart. For example, for a length of duct that has been returned to its extended configuration after having been compressed, a 1.5 meter horizontal span of 10 centimeter diameter duct with no support in its center will substantially maintain its rigid shape and sag (c) in the unsupported center by no more than 1 millimeter, while a similar 2 meter horizontal span of 10 centimeter diameter duct will sag in the unsupported center by no more than 3 millimeters. For a length of duct  30  that has not been compressed, a 1.5 meter horizontal span of 10 centimeter diameter duct that has no support in its center will maintain its rigid shape with negligible sag (c), while a 2 meter horizontal span of 10 centimeter diameter duct will sag in the unsupported center by no more than 3 millimeters. 
     Referring now to  FIG. 11 , there is shown, schematically, the vertical displacement y from the horizontal of one unsupported end  96  of a horizontal segment of duct  30  of length L, as a result of bending due to gravity, when the other end  98  has support  99 . Similarly, a vertically deployed segment of the duct of the present invention will maintain its rigidity, and not sag or collapse, even when returned to its extended configuration after having been compressed. As will be clear to those familiar with the art, these features represent a major improvement over the prior art, including solid aluminum corrugated tubes such as those employed in the invention of the Whitney patent (U.S. Pat. No. 5,281,187) discussed hereinabove. 
     Another advantage of the unique multilayered construction of the present invention is that when it is fully extended after compression, the inward-facing surface of the aluminum layer  36   b  of the inner sleeve  35  is substantially smooth and featureless except for the helical corrugations imparted by wire winding  40 , and the closely and evenly-spaced ridges  28  imparted by the break-down wheel  45  and roller  44   b  of mandrel  42 . This reduces frictional resistance to air flow within the duct, and, for clothes dryer exhaust transition ducts, significantly impedes the accumulation of lint inside the duct, thereby greatly reducing the fire hazard cited hereinabove with respect to the prior art. 
     Referring now to  FIG. 12 , there is shown a schematic view of a duct  30 , constructed and operative in accordance with an embodiment of the present invention, installed as an exhaust transition duct of a clothes dryer  78 . Duct  30  is connected to dryer exhaust port  80  and has a vertical segment  82  and two right angle bends  84  connecting it to an outside exhaust port  86 , thereby allowing it to vent the exhaust gases of clothes dryer  78 . The features of the present invention discussed hereinabove, notably the rigidity and structural integrity and the reduced tendency to accumulate lint are particularly advantageous in applications such as this. 
     The advantageous properties of the duct of the present invention result both from its unique construction described hereinabove and from the method of manufacture thereof. 
     Referring now to  FIGS. 13   a - 13   b , there is shown a duct  30 , hanging vertically as shown in  FIG. 12 , having a duct connector ring  70 , on both its ends. 
     Referring now to  FIG. 14   a , there is shown the duct connector ring  70  on its side  70   a  that connects to duct  30 . Side  70   a  of threaded ring  70  is screwed onto duct  30 , via tooth-like projections  64 , where the duct has matching threads created by the closely and evenly-spaced ridges  28 . 
     Referring now to  FIG. 14   b , there is shown the duct connector ring  70  on its side  70   b  that snaps on to an adaptor ring  72  ( FIG. 14   c ); 
     Referring now to  FIG. 14   c , there is shown adaptor ring  72 , having two sides, side  72   a , which connects to connector ring  70 , and side  72   b , the appliance connection side. Side  72   b  connects to a dryer or other appliances described hereinbelow. Side  70   b  of duct connector ring  70  has an annular ridge  69  matching an annular groove  71  of side  72   a  of adaptor  72 , so that when adaptor ring  72  and connector ring  70  are brought together, they connect via a snap-on connection between the annular ridge  69  and annular groove  71 . Side  72   a  of adaptor  72  has a plurality of slits  73  allowing flexibility to side  72   a.    
     Referring now to  FIG. 14   d , there is shown a two-piece connector  75 , comprised of duct connector ring  70  and adaptor ring  72 , connected to each other. 
     Referring now to  FIG. 15 , there is shown a dryer  78  with two-piece connector  75 , connected via the adaptor ring  72  to dryer exhaust port  80 , and threaded on to duct  30  via the duct connector ring  70 . 
     Referring now to  FIGS. 16   a - 16   d , there is shown a lint trap  85 , comprised of top part  81  and bottom part  83 , having an integrally formed adaptor ring  72 , including annular groove  71 , on the top part  81 , for the purpose of connecting to duct connector ring  70  threaded onto duct  30 . The lint trap  85  is used when the duct  30  has no access to the outside, for the purpose of providing housing for the lint that is released from duct  30 , so as not to allow the lint to scatter around the vicinity of the dryer. The lint trap  85  can be hung on the wall, or placed on the floor. 
     Referring now to  FIGS. 17   a - 17   b , there is shown a draft blocker  87  which is utilized for the exhaust on the end of duct  30 , connecting it to the wall. The draft blocker  87  has an inlet section  87   a , functioning as adaptor ring  72  for connecting to connector ring  70  threaded onto duct  30 . The outlet section  87   b  on the other side of the draft blocker  87  is inserted through an opening in the wall. A flange  94  surrounding the draft blocker  87  is present for the purpose of hanging the draft blocker onto the wall. When the dryer  78  is turned on, a flap  89  situated vertically inside the blocker  87  blocking the air flow, is opened and thus allows the air to pass through in one direction, so not to allow air from the outside to penetrate the duct. When the dryer is stopped the flap closes, and no air will pass through. 
     Referring now to  FIG. 18 , there is shown a duct  30  threaded onto duct connector ring  70 , connected to draft blocker  87  inserted through a thin partition  91 . 
     Referring now to  FIG. 19 , there is shown the duct  30  and draft blocker  87  of  FIG. 18 , inserted through a wall  92  via a long solid pipe  93 , inserted on one end into the outlet section  87   b  of draft blocker  87 , since the outlet section  87   b  is not long enough to extend all the way through a wall  92 . 
     Referring now to  FIG. 20 , there is shown the duct  30  threaded onto two-piece connector  75 , connected to solid pipe  93  via the appliance connection side  72   b . There is no draft blocker  87  involved in this option, thus air from the outside is allowed to enter the duct. 
     Referring now to  FIGS. 21   a - 21   c , there is shown the polygonal ducts of the present embodiment which may be manufactured in substantially the same manner as shown and described hereinabove in conjunction with  FIGS. 5 and 8 . In the present embodiment however, the cylindrical duct which results from the hitherto described method of manufacture is converted, either wholly or partially, into a polygonal duct, preferably square or rectangular. 
     Conversion of a length of cylindrical duct  30  may be achieved by mounting a length thereof onto an expanding metal profile  126 , having an external shape adapted to expand to the shape and size desired. Once the duct  30  is mounted onto profile  126 , the profile is operated as known in the art, so as to expand against the interior surface of the round duct, thereby to deform it into a predetermined shape. As seen in the drawings, it may also be desired to complement the outward deformation forces applied from the interior of the duct by the expanding metal profile  126 , by external deformation forces, such as may be provided by trolley  128 . Trolley  128  comprises a chassis  130 , onto which are mounted a plurality of cylindrical wheels  132  which, as seen in  FIG. 23C , define, together with wheels  132 , internal right-angled profiles  134 . As trolley  128  travels along the profile  126  and then engages duct  30 , the duct is stretched both from the interior by profile  126 , and is also squeezed between the profile  126  and the inward-facing right-angled profiles of trolley  128 , thereby to impart to the duct a desired polygonal shape. In the present example, this shape is rectangular, but this is by way of example only, as it could be any desired shape, whether rectangular, or any other type of polygon. In accordance with an alternative embodiment of the invention, there may be provided an additional trolley in order to properly form the bottom corners of the polygonal duct. 
     Referring now to  FIGS. 22   a - 22   b , there are shown a perspective view and a side view, respectively, of a square duct  100  having a round two-piece connector  75  on both its ends; 
     Referring now to  FIG. 23 , there is seen a portion of a compound duct  125  which has both a cylindrical portion, referenced  30 ′, substantially as shown and described above in conjunction with  FIG. 2 ; and a square or rectangular portion, referenced  100 ′, substantially as shown and described above in conjunction with  FIGS. 22   a - 22   b . The two differently shaped portions are connected via a transition portion  122 . Typically, compound duct  125  is primarily cylindrical, and has a rectangular end portion so as to facilitate connection of the duct to the outlet ports of different types of gas emitting machines, wherein the outlet ports are square or rectangular. Use of the illustrated duct clearly avoids the necessity of unorthodox and sometimes unsafe connections, in order to connect a square or rectangular machine outlet to a cylindrical duct. The compound duct  125  may be formed as described above in conjunction with  FIGS. 21   a - 21   c , or by any other suitable method. 
     Clearly, also in accordance with the present invention, and referring also to  FIG. 23 , in the event that a cylindrical duct is to remain cylindrical but with a square or rectangular end only, such as for connection purposes to the outlet of a gas emitting machine, this will be done by mounting only that portion of the duct desired to be transformed, onto the expanding profile, thereby to obtain a rectangular or square portion, referenced  100 ′ in  FIG. 23 . 
     It will further be appreciated by persons skilled in the art that the scope of the present invention is not limited by what has been specifically shown and described hereinabove, merely by way of example. Rather, the scope of the present invention is defined solely by the claims, which follow.