Abstract:
A semi-rigid, flexible, duct for gas transport and for clothes dryer exhaust transition, and a method for manufacture thereof, including a pair of coaxial sleeves, 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 includes a first aluminum layer and a second polyester layer, wherein the helical element imparts helical corrugations to the sleeves such that the duct is axially extendible between a compacted configuration suitable for storage and shipping and an extended configuration suitable for installation in a gas transport arrangement, and wherein all the layers of the sleeves are of a thickness predetermined to together render the duct substantially rigid when in the extended configuration and to together enable the duct to maintain its substantial rigidity upon extension from the compacted configuration.

Description:
FIELD OF THE INVENTION  
       [0001]     The present invention relates generally to vents and ducts for gas transport and, more particularly, to ducts of the type commonly installed as exhaust transition ducts for household and commercial clothes dryers and as air ducts in heating, ventilation, and air conditioning (HVAC) systems.  
       BACKGROUND OF THE INVENTION  
       [0002]     Air ducts for ventilation systems are well known. They are typically used to direct air flow for heating and air conditioning systems. Another common application is for the exhaust vent of clothes dryers.  
         [0003]     A very typical and common exhaust vent for clothes dryers is fabricated of a resilient wire helix which is covered with vinyl tubing, which lacks structural integrity and is generally not flame resistant or with aluminum tubing, which lacks structural integrity. The lack of structural integrity typically results in sagging and crinking of the duct. Ducts of these types also tend, over time, to become lined with lint from the clothes dried in the dryer, posing a fire hazard. According to the Consumer Products Safety Commission, there are over 15,000 fires annually associated with clothes dryers, causing deaths and injuries and some $90 million in damages. It is generally recommended by clothes dryer manufacturers not to use vinyl ducts such as these for dryer exhaust transition ducts.  
         [0004]     Representative of the prior art in ventilation systems is U.S. Pat. No. 5,281,187, included herein by reference, 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 may not be likely to maintain its rigidity, depending on the thickness of the aluminum. A tube of this type that will maintain its rigidity, by virtue of its being fabricated of solid metal, will be heavy and expensive and can be unwieldy to install. The corrugated aluminum, when extended after compression, will have significant ridges and other obtrusive topographical features along its interior due to the corrugations, which will cause frictional resistance to the air flow within, a further disadvantage.  
         [0005]     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 also solve the above-described problem of insufficient rigidity by using totally rigid segments. Even though the aluminum tubing itself is obviously fire resistant, the ridges and other internal topographical features similar to those mentioned hereinabove with respect to the Whitney patent, also cause fictional resistance to the air flow within, permitting accumulation of lint, which, as stated hereinabove, presents a fire hazard.  
         [0006]     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.  
       SUMMARY OF THE INVENTION  
       [0007]     The present invention seeks to provide a flexible duct for a ventilation system, and particularly for clothes dryer exhaust, 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 should have minimal internal topographical features or structure, even after having been compressed to a compacted configuration for shipping and storage and then re-extended for installation. The present invention further seeks to provide a method for manufacturing such a duct that is simple, fast, and efficient.  
         [0008]     There is thus provided, a semi-rigid, flexible, duct for gas transport and in particular, to serve as a clothes dryer exhaust transition duct, having an axis, including a pair of coaxial sleeves, including 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 includes a first layer having metallic properties and one or both of which further include a second, plastic layer bonded to the first layer having metallic properties;     wherein the helical element imparts helical corrugations to the inner sleeve and the outer sleeve, such that the duct is axially extendible between a compacted configuration suitable for storage and for shipping and an extended configuration suitable for installation in a gas transport arrangement;     and wherein all the layers of both the inner sleeve and the outer sleeve are of a thickness predetermined to together render the duct substantially rigid when in the extended configuration and to together enable the duct to maintain its substantial rigidity upon extension from the compacted configuration.        
 
         [0012]     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 less than 1 centimeter for an extended duct that was not compacted and less than 5 centimeters for a duct that was extended from the compacted configuration. Additionally, when the duct is in the extended configuration after having been compressed to the compacted configuration, the inward-facing surface of the first layer having metallic properties of the inner sleeve is substantially smooth and featureless except for the helical corrugations.  
         [0013]     Further, both the inner sleeve and the outer sleeve include a first layer having metallic properties and a second, plastic layer, forming thereby, respectively, an inner two-layer laminate and an outer two-layer laminate, which are fabricated of fire-resistant and puncture-resistant materials. In all of the two-layer laminates, the layers are bonded together with a fire-retardant adhesive and the inner two-layer laminate is also bonded to the outer two-layer laminate with a fire-retardant adhesive. Additionally, the first layers having metallic properties of the inner two-layer laminate and the outer two-layer laminate are fabricated of aluminum ribbon of predetermined thicknesses and the second, plastic layers of the inner two-layer laminate and the outer two-layer laminate are fabricated of polyester ribbon of predetermined thicknesses, respectively bonded together to form thereby, respectively, an inner two-layer laminated tape of predetermined thickness and an outer two-layer laminated tape of predetermined thickness, and wherein the inner two-layer laminate is an inner helical wrapping with a predetermined overlap of the inner two-layer laminated tape and the outer two-layer laminate is an outer helical wrapping with a predetermined overlap of the outer two-layer laminated tape. Further, in the inner sleeve, the second plastic layer is disposed parallel to and about the first layer having metallic properties and in the outer sleeve, the first layer having metallic properties is disposed parallel to and about the second plastic layer. The first layer having metallic properties of the inner two-layer laminate is fabricated of aluminum ribbon of a thickness in the range of 6 to 12 microns, and the first layer having metallic properties of the outer two-layer laminate is fabricated of aluminum ribbon of a thickness in the range of 24 to 35 microns. The second plastic layers of both the outer and inner two-layer laminates are fabricated of polyester ribbon of a thickness in the range of 10 to 14 microns.  
         [0014]     Additionally, the resilient helical element is fabricated of a metal having spring-like resilience, such as a coiled bronze-coated steel wire of a diameter in the range of 0.9 to 1.3 millimeters.  
         [0015]     Further, in accordance with a preferred embodiment of the invention, the resilient helical element is aligned with the inner helical wrapping so that the coiled bronze-coated steel wire is approximately centered over the overlap of the inner helical wrapping of the inner two-layer laminated tape and the outer helical wrapping of the outer two-layer laminated tape is aligned with the resilient helical element so that the overlap of the outer helical wrapping of the outer two-layer laminated tape is approximately centered over the spaces between the wires of the coiled bronze-coated steel wire of the resilient helical element.  
         [0016]     In accordance with a further embodiment of the invention, the duct also includes an insulating sheath fabricated of fiberglass, disposed parallel to and about the outer sleeve, and an enclosing jacket disposed parallel thereto and thereabout. The enclosing jacket is a multi-layer jacket including a tubular, plastic inner wrapping and a two-layer laminate outer wrapping, including a plastic inner layer and an outer layer having metallic properties, bonded together with a fire-retardant adhesive, disposed parallel and about the tubular, plastic inner wrapping and bonded thereto with a fire-retardant adhesive. The plastic inner wrapping is fabricated of polyester ribbon of predetermined thickness, and the plastic inner layer of the two-layer laminate outer wrapping is fabricated of polyester ribbon of predetermined thickness and the outer layer having metallic properties of the two-layer laminate outer wrapping is fabricated of aluminum ribbon of predetermined thickness. The insulating sheath is fabricated of fiberglass of a thickness in the range of 25 to 50 millimeters. The plastic inner wrapping is fabricated of polyester ribbon of a thickness in the range of 10 to 14 microns. The plastic inner layer of the two-layer laminate outer wrapping is fabricated of polyester ribbon of a thickness in the range of 10 to 14 microns, and the outer layer having metallic properties of the two-layer laminate outer wrapping is fabricated of aluminum ribbon of a thickness in the range of 6 to 9 microns.  
         [0017]     There is further provided, in accordance with the present invention, a method for manufacturing a semi-rigid, flexible, duct of a preselected diameter for gas transport, including the steps of: 
        a) providing a mandrel of preselected diameter for fabricating a duct therearound;     b) combining a first aluminum continuous ribbon of predetermined thickness in the range of 6 to 12 microns with a first polyester continuous ribbon of predetermined thickness in the range of 10 to 14 microns to form a first two-layer laminated continuous tape;     c) combining a second aluminum continuous ribbon of predetermined thickness in the range of 24 to 35 microns with a second polyester continuous ribbon of predetermined thickness in the range of 10 to 14 microns to form a second two-layer laminated continuous tape;     d) helically wrapping the first two-layer laminated continuous tape with a predetermined overlap around the mandrel with the first aluminum ribbon facing inward toward the mandrel and the first polyester ribbon facing outward with respect to the mandrel to form an inner two-layer sleeve;     e) helically coiling a bronze-coated steel wire of a thickness in the range of 0.9 to 1.3 millimeters around the inner two-layer sleeve; and     f) helically wrapping the second two-layer laminated continuous tape with a predetermined overlap around the inner two-layer sleeve and the bronze-coated steel wire coil with the second polyester ribbon facing inward toward the mandrel and the second aluminum ribbon facing outward with respect to the mandrel to form an outer two-layer sleeve disposed parallel to and about the inner two-layer sleeve.        
 
         [0024]     Additionally, the step b) of combining a first aluminum ribbon includes the sub-step of applying a fire-retardant adhesive between the first aluminum ribbon and the first polyester ribbon to bond them together; and the step of c) combining a second aluminum ribbon includes the sub-step of applying a fire-retardant adhesive between the second aluminum ribbon and the second polyester ribbon to bond them together. Further, the step of b) combining a first aluminum ribbon further includes the sub-step of coating the polyester face of the first two-layer laminated continuous tape with a fire-retardant adhesive; the step c) of combining a second aluminum ribbon further includes the sub-step of coating the polyester face of the second two-layer laminated continuous tape with a fire-retardant adhesive; and in the step d) of helically wrapping the second two-layer laminated continuous tape, the outer two-layer sleeve is bonded to the inner two-layer sleeve with the bronze-coated steel wire helically coiled therebetween.  
         [0025]     Additionally in accordance with the method of the present invention, the step e) of helically coiling a bronze-coated steel wire includes the sub-step of aligning the coiled wire with the overlap in the wrapping of the inner two-layer sleeve so that the coiled wire is approximately centered over the overlap in the wrapping of the inner two-layer sleeve, and the step f) of helically wrapping the second continuous two-layer laminated tape includes the sub-step of aligning the wrapping of the second continuous two-layer laminated tape so that the overlap in the wrapping of the outer two-layer sleeve is approximately centered over the spaces between the coils of wire.  
         [0026]     Further in accordance with the method of the present invention, the steps d), e), and f) of helically wrapping the first two-layer laminated continuous tape, helically coiling the bronze-coated steel wire, and helically wrapping the second two-layer laminated continuous tape are performed by rotating the mandrel as the first two-layer laminated continuous tape, the bronze-coated steel wire, and the second two-layer laminated continuous tape are respectively deposited thereupon; and the steps d), e), and f) of helically wrapping the first two-layer laminated continuous tape, helically coiling the bronze-coated steel wire, and helically wrapping the second two-layer laminated continuous tape are performed continuously and simultaneously with predetermined phase differences, with respect to the rotation of the mandrel, therebetween. Namely, the steps d) and e) of helically wrapping the first two-layer laminated continuous tape and helically coiling the bronze-coated steel wire are performed continuously and simultaneously with a phase difference of 360 degrees, with respect to the rotation of the mandrel, therebetween; and the steps e) and f) of coiling the bronze-coated steel wire and helically wrapping the second two-layer laminated continuous tape are performed continuously and simultaneously with a phase difference of 120 degrees, with respect to the rotation of the mandrel, therebetween.  
         [0027]     In accordance with an additional embodiment of the present invention, the method further includes, after the step f) of helically wrapping the second two-layer laminated tape, the steps of: 
        g) sheathing the outer two-layer sleeve with a fiberglass insulating sheath of a thickness in the range of 25 to 50 millimeters, disposed parallel thereto and thereabout; and     h) enveloping the insulating sheath with an enclosing jacket. 
 
 Additionally, the step h) of enveloping includes the following sub-steps: 
       
 
         [0030]     1) providing a mandrel of preselected diameter for fabricating the enclosing jacket therearound; 
        2) combining a polyester continuous ribbon of predetermined thickness in the range of 10 to 14 microns with an aluminum continuous ribbon of predetermined thickness in the range of 6 to 9 microns to form a two-layer laminated continuous tape;     3) helically wrapping a polyester continuous ribbon of predetermined thickness in the range of 10 to 14 microns around the mandrel to form an inner plastic sleeve; and     4) helically wrapping the two-layer laminated continuous tape around the inner plastic sleeve with the polyester ribbon facing inward toward the mandrel and the aluminum ribbon facing outward with respect to the mandrel to form an outer two-layer sleeve disposed parallel to and about the inner plastic sleeve.        
 
         [0034]     The sub-step 2) of combining includes the sub-sub-step of applying a fire-retardant adhesive between the polyester ribbon and the aluminum ribbon to bond them together, and the sub-step 3) of helically wrapping a polyester ribbon includes the sub-sub-step of coating the outer face of the inner plastic sleeve with a fire-retardant adhesive to bond it to the two-layer laminated tape.  
         [0035]     Additionally, the sub-steps 3) and 4) of helically wrapping a polyester ribbon and helically wrapping the two-layer laminated tape are performed by rotating the mandrel as the polyester ribbon and the two-layer laminated tape are respectively deposited thereupon. Further, the sub-steps 3) and 4) of helically wrapping a polyester ribbon and helically wrapping the two-layer laminated tape are performed continuously and simultaneously with a predetermined phase difference, namely, of 360 degrees, with respect to the rotation of the mandrel, therebetween. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0036]     The present invention will be more fully understood and appreciated from the following detailed description, taken in conjunction with the drawings, in which:  
         [0037]      FIG. 1  is a side view of a segment of a duct, constructed and operative in accordance with an embodiment of the present invention;  
         [0038]      FIG. 2  is a schematic axial cross-sectional view of the duct of  FIG. 1 ;  
         [0039]      FIG. 3  is a schematic oblique view of a segment of a duct that has been compressed;  
         [0040]      FIG. 4  is a schematic oblique view of a duct similar to that shown in  FIG. 1 , further including an insulating sheath, constructed and operative in accordance with a further embodiment of the present invention;  
         [0041]      FIG. 5  is a schematic axial cross-sectional view of the duct of  FIG. 4 ;  
         [0042]      FIG. 6  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;  
         [0043]      FIG. 7  is a schematic axial view of a duct such as that of  FIG. 1  being fabricated according to the method of the present invention;  
         [0044]      FIG. 8  is an enlarged detailed schematic cross-sectional view of a portion of the wall of a duct such as that of  FIG. 1 ;  
         [0045]      FIG. 9  is a schematic axial view of an enclosing jacket such as that of  FIG. 5  being fabricated according to the method of the present invention;  
         [0046]      FIG. 10  is a schematic representation of the vertical sag of the unsupported center of a segment of duct such as that of  FIG. 1  supported at its ends;  
         [0047]      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. 1  supported at its other end; and  
         [0048]      FIG. 12  is a schematic representation of the fabrication of an insulated duct such as that of  FIG. 5 .  
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0049]     Referring now to the drawings, there are shown, in  FIG. 1 , a side view of a segment of a duct, referred to generally as  100 , constructed and operative in accordance with a preferred embodiment of the present invention, and a schematic axial cross-sectional view thereof in  FIG. 2 . Duct  100 , which is intended for use in a gas transport arrangement, is cylindrical, having an axis  150 , and is of multi-layer construction, as shown in detail in  FIG. 2 . Duct  100  has inner and outer sleeves, referenced  220  and  230 , respectively, which are coaxial and are of a laminate construction, each preferably being formed of a helical wrapping of a two-layer laminated tape formed of two layers of ribbon,  222 ,  224 , and  232 ,  234 , respectively, bonded together with an adhesive layer  240 ,  280 . Inner sleeve  220  has an internal layer of aluminum ribbon  222  and an external layer of polyester ribbon  224  bonded together with adhesive layer  240  to form a two-layer laminated tape which is helically wrapped around a mandrel ( 710 , see  FIG. 7 , discussed hereinbelow) to form inner sleeve  220 . Coaxially coiled around inner sleeve  220  is a helical wire  250 , preferably of bronze-coated steel, disposed and encapsulated between inner sleeve  220  and outer sleeve  230  with a layer of adhesive  260 . Outer sleeve  230  is fabricated in a manner similar to inner sleeve  220 , but wherein, the helically wrapped two-layer laminated tape has an internal layer of polyester ribbon  234  and an external layer of aluminum ribbon  232 , bonded together with adhesive layer  280 . The helically coiled bronze-coated steel wire  250  imparts helical corrugations  160  to duct  100 , as can be seen in  FIG. 1 .  
         [0050]     Polyester ribbon layers  224  and  234  are both heat resistant and fire retardant and further are made thick enough to contribute to the rigidity and structural integrity of duct  100  together with aluminum ribbon layers  222  and  232 , which, being metallic, are fireproof as well. The adhesive employed in adhesive layers  240 ,  260 , and  280  is also heat resistant and fire retardant. It should be noted that polyester ribbon layers  224  and  234  are also puncture resistant, which is a further advantage of the duct  100  of the present invention.  
         [0051]     Duct  100  is manufactured fully extended by a continuous process, further described hereinbelow, and is then cut to a desired length. The corrugations  160  imparted thereto by helical wire  250  allow duct  100  to be axially compressed into a compact configuration convenient for storage or shipping. When duct  100  is compressed, as shown in  FIG. 3 , aluminum layers  222  and  232  and polyester layers  224  and  234  naturally fold between the ridges (referenced  160  in  FIG. 1 ) created by helical wire  250 . 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.  
         [0052]     A particular advantage of the unique, multilayered construction of the present invention is that duct  100  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. 10 , there is shown, schematically, the vertical sag c of the unsupported center  210  of a horizontal segment of duct  200  spanning between two supports  215  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 in the unsupported center by no more than 1 centimeter, while a similar 2 meter horizontal span of 10 centimeter diameter duct will sag in the unsupported center by no more than 5 centimeters. For a length of duct  100  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, while a 2 meter horizontal span of 10 centimeter diameter duct will sag in the unsupported center by no more than 1 centimeter. Referring now to  FIG. 11 , there is shown, schematically, the vertical displacement y from the horizontal of one unsupported end  290  of a horizontal segment of duct  200  of length L, as a result of bending due to gravity, when the other end  295  has support  215 . 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.  
         [0053]     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  222  of the inner sleeve  220  is substantially smooth and featureless except for the helical corrugations imparted by wire helix  250 . 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.  
         [0054]     Referring again to  FIG. 2 , in a preferred embodiment of the present invention in a typical product of the invention, duct  100  may have the following exemplary dimensions. The two-layer laminated tape of inner sleeve  220  has an inner aluminum ribbon layer  222  that is 7 microns thick and a polyester ribbon layer  224  that is 12 microns thick, so that, with the adhesive  240 , inner sleeve  220  has a thickness of 21 microns. The wire helix  250  is 0.9 mm diameter bronze-coated steel wire. The two-layer laminated tape of outer sleeve  230  has an outer aluminum ribbon layer  232  that is 25 microns thick and a polyester ribbon layer  234  that is 12 microns thick, so that, with the adhesive  280 , outer sleeve  230  has a thickness of 39 microns. The use of the thinner (7 microns) of aluminum ribbon layer  222  in inner sleeve  220  contributes to the above-mentioned smoothness of the inner surface of duct  100 . It should be noted that the above-mentioned dimensions are typical and are exemplary of a preferred embodiment of the present invention, and that the present invention is not limited thereto. It should further be noted that, with suitable dimensions for the other layers of the duct of the present invention, either polyester layer  224  of inner sleeve  220  or polyester layer  234  of outer sleeve  230  may be omitted without loss of the improvements in rigidity of the present invention, albeit at a cost of additional thickness of aluminum, resulting in additional weight and expense. As such, either of these alternative configurations should be considered as being included in the present invention, as well as alternative dimensions of the layers that can still provide the desired performance of duct  100 . Similarly, metallic layers or plastic layers fabricated of materials having properties comparable to those of the aluminum and polyester layers described hereinabove should also be considered as being included in the present invention.  
         [0055]     Referring now to  FIG. 4 . there is shown a schematic oblique view of a segment of a duct, referred to generally as  400 , A schematic axial cross-sectional view of duct  400  is shown in  FIG. 5 . As shown in  FIG. 5 , duct  400  is similar to that shown in  FIG. 1 , but also includes an insulating layer  470  disposed parallel to and about outer sleeve  430  constructed and operative in accordance with a further preferred embodiment of the present invention. Additionally, insulating layer  470  has an enclosing jacket serving as a vapor barrier, referred to generally as  490 , disposed thereabout. Insulating layer  470  is typically fabricated of fiberglass, which provides the desired insulation and is fire resistant. Enclosing jacket  490  is formed of an inner helical wrapping of polyester ribbon  484 , bonded with a layer of heat and fire retardant adhesive  485  and an outer helical wrapping of a two-layer laminated tape having an inner layer of polyester ribbon  494  and an outer layer of aluminum ribbon  492  bonded together by a heat resistant and fire retardant adhesive  495 .  
         [0056]     In a preferred embodiment of the present invention, insulating layer  470  and enclosing jacket  490  of duct  400  have the following dimensions. Depending on the application, insulating layer  470  typically may be either 25 or 50 millimeters in thickness. The wrapping of polyester ribbon  484  is 12 microns thick. The two-layer laminated tape of the outer helical wrapping has an inner polyester ribbon layer  494  that is 12 microns thick and an outer aluminum ribbon layer  492  that is 7 microns thick, so that, with the adhesive  495 , outer helical wrapping has a thickness of 21 microns. It should be noted that the above-mentioned dimensions are typical and are exemplary of a preferred embodiment of the present invention, and that the present invention is not limited thereto.  
         [0057]     Enclosing jacket  490  is manufactured by a continuous process, similar to that of duct  100 , and is then cut to a desired length. Duct  400  is assembled from an insulating layer  470  cut to the desired length and an enclosing jacket  490  cut to the desired length, which are drawn onto a segment of uninsulated duct, similar to duct  100 , cut to the desired length.  
         [0058]     Referring now to  FIG. 6 , there is shown a schematic view of a duct  600 , constructed and operative in accordance with an embodiment of the present invention, installed as an exhaust transition duct of a clothes dryer  650 . Duct  600  is connected to dryer exhaust port  640  and has a vertical segment  660  and two right angle bends  670  connecting it to an outside exhaust port  680 , thereby allowing it to vent the exhaust gases of clothes dryer  650 . 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.  
         [0059]     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  FIG. 7 , there is shown a schematic axial view of a duct, referred to generally as  700 , in accordance with the present invention being fabricated according to the method of the present invention. The size of the duct  700  being fabricated is determined by mandrel  710  which is rotated about its longitudinal axis  715 . Inner two-layer laminate tape  720  is helically wrapped with a predetermined overlap  828  ( FIG. 8 ) around mandrel  710  as it turns to produce the two-layer inner sleeve of duct  700  as a first step in forming duct  700 . Bronzed-coated steel wire  730  is helically coiled around the two-layer inner sleeve of duct  700  as mandrel  710  turns with the two-layer inner sleeve formed thereupon. Outer two-layer laminate tape  740  is helically wrapped with a predetermined overlap  848  ( FIG. 8 ) around the two-layer inner sleeve of duct  700  with bronzed-coated steel wire  730  coiled thereupon as mandrel  710  turns with the two-layer inner sleeve and the wire coil formed thereupon to produce the two-layer outer sleeve of duct  700 .  
         [0060]     Referring now to  FIG. 8 , there is shown an enlarged detailed schematic cross-sectional view of a portion of the wall of a duct, referred to generally as  800 , constructed in accordance with the present invention, being fabricated according to the method of the present invention. Inner two-layer laminate tape, referred to generally as  820 , is formed by combining an aluminum ribbon  822  with a polyester ribbon  824  by applying a fire-retardant adhesive  826  therebetween to bond them together. Similarly, outer two-layer laminate tape, referred to generally as  840 , is formed by combining a polyester ribbon  844  with an aluminum ribbon  842  by applying a fire-retardant adhesive  846  therebetween to bond them together. It should be noted that inner two-layer laminate tape  820  and outer two-layer laminate tape  840  are both prepared prior to their being helically wrapped around mandrel  710  ( FIG. 7 ) to fabricate duct  800 , and that inner two-layer laminate tape  820  is wrapped around the mandrel with the aluminum ribbon  822  side inward toward the mandrel and outer two-layer laminate tape  840  is wrapped around the mandrel with the polyester ribbon  844  side inward toward the mandrel. It should further be noted that inner two-layer laminate tape  820  and outer two-layer laminate tape  840  are each respectively helically wrapped with a predetermined partial overlap,  828  and  848  respectively, so that successive wrappings produce continuous inner and outer two-layer sleeves. Additionally, it should be noted that the wires of wire coil  830  are aligned approximately centered above the overlap  828  in inner two-layer laminate tape  820 , and the overlap  848  in outer two-layer laminate tape  840  is aligned approximately centered above the spaces between the wires of wire coil  830 , which has been found to enhance the strength and rigidity of duct  800 . Prior to inner two-layer laminate tape  820  and outer two-layer laminate tape  840  being helically wrapped around the mandrel to fabricate duct  800 , the outer, polyester ribbon  824  side of inner two-layer laminate tape  820  and the inner, polyester ribbon  844  side of outer two-layer laminate tape  840  are coated with a fire-retardant adhesive, such as with a rolling adhesive applicator, thereby allowing them to be bonded together with an adhesive layer  836  which also encapsulates bronzed-coated steel wire coil  830  there between, when all are wound around mandrel  710  ( FIG. 7 ) to fabricate duct  800 .  
         [0061]     Returning now to  FIG. 7 , it can be seen that both inner two-layer laminate tape  720  and outer two-layer laminate tape  740 , as well as bronzed-coated steel wire  730 , are all continuously and simultaneously wrapped and coiled, respectively, around mandrel  710  as it rotates. The wrappings and the coiling, while occurring simultaneously, are performed with predetermined phase differences, with respect to the rotation of mandrel  710 , between them. Thus duct  700  is fabricated in one continuous operation. In an exemplary preferred embodiment of the present invention, the phase difference between the wrapping of inner two-layer laminate tape  720  and the coiling of bronzed-coated steel wire  730  is 360 degrees or one complete rotation of mandrel  710 , and the phase difference between the coiling of bronzed-coated steel wire  730  and the wrapping of outer two-layer laminate tape  740  is 120 degrees or one third of a complete rotation of mandrel  710  about axis  715 .  
         [0062]     For the insulated duct  400  of  FIGS. 4 and 5 , enclosing jacket  490  is fabricated by a process analogous to that used to fabricate duct  700  described hereinabove. Referring now to  FIG. 9 , there is shown a schematic axial view of an enclosing jacket, referred to generally as  900 , in accordance with the present invention being fabricated according to the method of the present invention. A two-layer laminate tape  940  with an inner polyester ribbon layer and an outer aluminum ribbon layer bonded with a fire-retardant adhesive is formed. A continuous inner plastic sleeve is produced by helically wrapping a polyester ribbon  920  around a rotating mandrel  910  of the desired diameter, and a continuous outer two-layer sleeve is produced by helically wrapping the two-layer laminate tape  940  around the inner plastic sleeve as the mandrel rotates, with a fire-retardant adhesive layer applied therebetween. Further as described hereinabove, enclosing jacket  900  is produced in one continuous operation, with continuous inner plastic sleeve and outer two-layer sleeve both wrapped around mandrel  910  continuously and simultaneously, with only a specific phase difference, with respect to the rotation of mandrel  910 , between them. In a preferred embodiment of the present invention, the phase difference between the wrapping of the inner plastic sleeve and that of the outer two-layer sleeve is 360 degrees or one complete rotation of mandrel  910  about axis  915 . In additional embodiments of the present invention, an additional tape of open-mesh laid fiberglass scrim may be wrapped between polyester ribbon  920  and two-layer laminate tape  940  in enclosing jacket  900  (not pictured).  
         [0063]     To produce insulated duct  400 , a piece of continuously produced uninsulated duct  700  is cut to the desired length, and a piece of continuously produced enclosing jacket  490  is cut to the desired length. As shown schematically in  FIG. 12 , the desired length piece of enclosing jacket  490 , together with an insulating fiberglass sheath  470  of the desired length and suitable inner and outer diameters, are drawn over the desired length piece of uninsulated duct  700  to produce the insulated duct  400  shown in  FIGS. 4 and 5 .  
         [0064]     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.