Collapsible duct element, system and method of assembly

Abstract

The invention relates to a duct element 10 comprising a sheet of flexible plastics material folded about a plurality of longitudinal fold lines 12 to delimit successive perimeter wall panels 16, 18, 2

Description

The present invention relates to a duct for channelling gases between zones of a building employing a Heating Ventilation and Air Conditioning (HVAC) system. Particularly, though not exclusively, the invention relates to a ventilation duct element assembled from folded corrugated flexible plastic sheeting which is deployable at the point of use from a collapsed or flat-pack configuration. Rigid air ducting systems have been in existence for many years. Typically, they are formed into conduit components of substantially rectangular cross-section from tubular metallic elements, e.g. stainless steel, galvanised steel, or aluminium. A number of differently shaped conduit sections are required to route ducting around obstacles and to connect to other component parts of an HVAC system. It is also known to use flexible ducting components within awkward or restricted spaces. This may employ materials such a polyurethane attached to a galvanised steel spiral presenting a circular cross-section which can be cut to a desired length. However, conventional flexible ducting can be costly due to the amount of waste material created during the forming process. Furthermore, once installed, flexible ducting can be difficult to connect. Ideally, it should be maintained in a taut configuration to prevent air leakage and associated efficiency problems. Moreover, flexible ducting components are considered to be a consumable item, often lacking sufficient insulation properties, and so they may be unsuitable for some applications. The inventor of the present invention has previously proposed an alternative ventilation duct assembly cut from a three-dimensional block of elastomeric material. This is disclosed within international (PCT) publication No. WO2017 / 037466A1 assigned to Unibrak Limited (GB). The inventor of the present invention has now devised an alternative flexible ventilation duct element, system, and method of manufacture which overcomes or ameliorates some the problems associated with prior art ventilation ducts and / or provides an alternative or improvement to the ventilation duct assembly disclosed in the aforementioned international patent publication. According to a first aspect of the present invention there is provided a duct element comprising a sheet of flexible plastics material folded about a plurality of longitudinal fold lines to delimit successive perimeter wall panels of a unitaiy hollow gas-circulation conduit; a first wall panel of the conduit being connected to another wall panel to define a rectilinear annular perimeter extending between two opposed longitudinally separated open ends; and wherein said conduit is collapsible about said fold lines such that said successive perimeter wall panels are movable into a substantially parallel primary overlapping configuration. It should be appreciated that for the purpose of the present disclosure, and unless the context requires otherwise, the term "longitudinal” is to be understood as defining the direction extending between the separated open ends of the duct element, i.e. the direction of gas travel within the duct element. It does not necessarily follow that the duct element is longer in the longitudinal direction than it is in a transverse direction. It will be further appreciated that when the walls are collapsed, the hollow opening extending through the conduit is substantially eliminated. This serves to reduce the overall volume of the conduit which reduces space requirements and associated costs for storage and transportation purposes. Optionally, the sheet of flexible plastic material further comprises one or more transverse fold lines to subdivide the corresponding perimeter wall panel(s) in the longitudinal direction. Optionally, said conduit is additionally collapsible about said one or more transverse fold lines such that said perimeter wall panels are movable into a substantially parallel primary overlapping configuration via said longitudinal folds; and then into a substantially parallel secondary overlapping configuration via said transverse folds. Optionally, each one of said successive perimeter wall panels is rectangular. Alternatively, one or more of said successive perimeter wall panels is a non-rectangular quadrilateral. Alternatively, one or more of said successive perimeter wall panels is polygonal with more than four sides. It will be appreciated that the presence of primary and secondary overlapping configurations facilitated by mutually perpendicular fold lines allows for the assembly of a diverse range of complexly shaped three-dimensional duct element structures, particularly when irregularly shaped wall panels are included within the sheet of flexible plastics material. Optionally, the sheet of flexible plastics material is corrugated or fluted. Optionally, the corrugations or flutes extend between two outer walls to provide a doublewalled, air-filled structure. It will be appreciated that constructing the duct element from a light-weight corrugated plastic material - e.g. Correx® - further serves to reduce costs associated with storage and transportation whilst improving thermal / insulation properties of the duct element. Optionally, said first wall panel of the conduit is connected to said another wall panel via an overlap formed between the respective panels. Optionally, the overlap is facilitated by a flange extension extending entirely or partly along a longitudinal edge of one or both of said panels. Optionally, the flange extension extends entirely or partly across one or both of said panels in a transverse direction. Optionally, the flange extension is defined by a folded tab portion. Optionally, the or each flange extension of one panel is affixed to the other panel to define said rectilinear annular perimeter of the hollow gas-circulation conduit. It will be appreciated that each flange extension may be affixed by various means, including but not limited to any one or more of adhesive bonding, heat sealing, welding, and taping. Optionally, the sheet of flexible plastics material is provided with weakened regions to facilitate the formation of said plurality of fold lines Optionally, the sheet of flexible plastics material is provided with weakened regions to facilitate the formation of said flange (s). Optionally, the weakened regions are formed by cuts, score lines, or perforations formed in a surface of the sheet of flexible plastics material. Alternatively, the weakened regions are formed by deformations or creases applied to the surface of the sheet of flexible plastics material. It will be appreciated that the corrugated or fluted nature of the sheet of plastic material may be locally deformed to reduce or eliminate the volume of air within its double-walled structure. For example, the sheet of plastic material may be routed to remove one or more flutes or corrugations to make it easier to form right-angled folds and / or to reduce the degree of resiliency tending to resist the formation of said folds. Alternatively, a modified paper creasing machine may be used to form creases coinciding with fold lines. As a further alternative, cold welding or pressure welding processes may be used to locally deform the sheet of plastic material to create weakened fold lines. According to a second aspect of the present invention there is provided a duct system comprising a first duct element in accordance with the first aspect connected to one or more other duct element(s) in accordance with the first aspect thus forming all or part of a Heating Ventilation and Air Conditioning (HVAC) system. Optionally, an annular connector element is provided for connecting together open ends of a pair of duct elements, the connector element comprising a pair of aligned annular channels for receiving transverse edges of said successive perimeter wall panels, said aligned annular channels being separated by an annular partition wall. A suitable connector is disclosed in granted UK patent Nos. GB2543909B and GB2585807B (both assigned to Unibrak Limited) which are incorporated herein by reference. It will be appreciated that the shape of the connector can be matched to the cross-sectional shape formed at the open ends of the duct elements to be connected. According to a third aspect of the present invention there is provided a method of manufacturing a duct element in accordance with claim 1, comprising the steps of: (i) cutting a sheet of flexible plastics material into a blank corresponding to the desired shape of a duct element in an unfolded state; (ii) forming a plurality of longitudinal fold lines along the blank to delimit successive perimeter wall panels of a duct element in an unfolded state; (iii) successively folding said sheet of flexible plastics material along said plurality of fold lines to transform said blank into a three-dimensional gas circulation conduit; (iv) forming an at least partial overlap between two of said wall panels; and (v) affixing said two wall panels together at the region of said at least partial overlap to maintain the three-dimensional shape of the gas circulation conduit. Optionally, the step of forming an at least partial overlap between two of said wall panels comprises folding a portion of one of said wall panels to form a tab overlapping all or part of the longitudinal extent of the other of said wall panels. Optionally, the step of forming a plurality of longitudinal fold lines, and / or folding a portion of one of said wall panels to form a tab, is preceded by weakening the sheet of flexible plastics material along the length of the intended folds by forming one or more cuts and / or scores and / or creases and / or perforations. Optionally, the method includes an additional step of forming one or more transverse fold lines to subdivide the corresponding perimeter wall panel(s) in the longitudinal direction. Optionally, method includes the step of collapsing said three-dimensional gas circulation conduit about said fold lines such that said successive perimeter wall panels are movable into a substantially parallel overlapping configuration. It will be appreciated that when the walls are collapsed, the hollow opening extending through the conduit is substantially eliminated and the three-dimensional shape of the gas circulation conduit moves towards a more two-dimensional configuration. Optionally, method includes the step of retaining the collapsed wall panels in said substantially parallel overlapping configuration against resilient deployment into said three-dimensional shape of the gas circulation conduit. It will be appreciated that the natural tendency of the duct element will be to adopt its intended three-dimensional shape, i.e. by virtue of the structure and resilient nature of the sheeted plastic material from which it is assembled. Factors including, but not limited to, the material choice, its thickness, and the presence or absence of cuts, scores, or perforations and the like, will of course affect the degree of resilience and can be controlled accordingly. Further features and advantages of the various aspects of the present invention will become apparent from the claims and the following description. Embodiments of the present invention will now be described by way of example only, with reference to the following diagrams, in which:- Fig. 1 is a perspective view of a duct element defining a linear gas-circulation conduit in accordance with a first embodiment of the present invention; Fig. 2 is a partial perspective view of the duct element of Fig. 1 showing the manner in which two adjacent wall panels are connected via a flanged tab; Fig. 3 shows a blank of flexible plastics sheet material from which the duct element of Fig. 1 is constructed; Fig. 4 is a first perspective view of an alternative T-shaped duct element in accordance with a second embodiment of the present invention; Fig. 5 is a second perspective view of the T-shaped duct of Fig. 3; Fig. 6 is a partial perspective view of the duct element of Fig. 4 showing the manner in which adjacent wall panels are connected via a flanged tab; Fig. 7 shows the T-shaped duct element of Fig. 4 in a partially collapsed configuration; Fig. 8 is a side view of a complex duct element defining an undulating gas-circulation conduit in accordance with a third embodiment of the present invention; Fig. 9 is a view from below of the complex duct element of Fig. 8; Fig. 10 shows a blank of flexible plastics sheet material which is foldable to form a major part of the duct element of Fig. 8; Fig. 11 shows a blank of flexible plastics sheet material which is foldable to form a minor part of the duct element of Fig. 8, and which is connectable to a major part of the duct element formed from the blank of Fig. 9; Fig. 12 is a partial perspective view of the duct element of Figs. 8 and 9 showing the manner in which adjacent wall panels are connected via a flanged tab; Fig. 13 shows the duct element of Figs. 8 and 9 in a partially collapsed configuration; and Fig. 14 shows the duct element of Figs. 8 and 9 at or near to a fully collapsed configuration. With reference to Figs. 1 and 2, these show a duct element 10 in accordance with a first example embodiment of the present invention. The duct element 10 may be formed from a blank 11 of corrugated flexible plastics sheet material, e.g. such as that sold under the brand name Correx®. A suitable blank 11 is shown in Fig. 3. The corrugations within the flexible plastics sheet material are sandwiched between two outer walls to provide a double-walled, air-filled structure exhibiting strength, rigidity and insulative properties. The sheet material is folded about a plurality of fold lines 12 extending longitudinally between two opposed open ends 14 of the duct element to define a unitary hollow gascirculation conduit. In some embodiments, one or all of the fold lines 12 may be replaced by one or more cut edges which have been affixed together to form a hingeable connection. The fold lines 12 delimit first, second, third and fourth major successive rectangular perimeter wall panels 16,18, 20, 22 of the duct element 10, the angle between adjacent wall panels preferably being maintained as right angles in order to maximise the internal gascirculation volume. In some embodiments the fold lines 12 extend in a direction which is parallel with the corrugations or flutes within the flexible plastics sheet material. In doing so, it is easier to form the fold lines 12 and to control their direction. However, in certain scenarios it may be preferable to alter the direction of the fold lines 12 relative to the orientation of the corrugations or flutes within the flexible plastics sheet material, e.g. to improve the strength characteristics and / or rigidity of the duct element. The first wall panel 16 comprises a longitudinally extending flanged tab 24 formed via a longitudinal fold line 12. As shown in Fig. 2, the uppermost wall of the double-walled structure of the tab 24 may be removed to expose the underlying corrugations, and to reduce the overall thickness of the tab 24 relative to the adjoining perimeter wall panel 16. A reduction in the thickness of the tab 24 facilitates its formation of a perpendicular hinge relative to the plane of wall panel 16. The lowermost wall of tab 24 is affixed, e.g. by means of an adhesive / sealant such as Mastic®, along a peripheral portion of an internally facing surface of wall panel 22. It will be appreciated that affixing of the tab 24 to wall panel 22 may be performed at the time of manufacture, or at the point of use; and may employ other suitable methods such as welding, taping, stitching and the like. The connection between tab 24 and wall panel 22 maintains wall panels 16, 22 in a substantially mutually perpendicular orientation. In doing so, a three-dimensional rectangular cross-sectional shape of the duct element 10 is established when viewed along a longitudinal axis extending between its two opposed open ends 14. In use, the resilient nature of the flexible plastics sheet material from which the duct element 10 is formed facilitates selective collapsibility of its perimeter wall panels 16,18, 20, 22 in the transverse direction about the longitudinal fold lines 12. For example, the application of a shear force in the transverse direction of the plane(s) of major wall panel(s) 18 and / or 22 causes the hollow conduit to reduce in height until wall panels 18, 22 move together, partially overlap one another, and ultimately contact one another. When fully collapsed, the internal volume of the hollow opening extending through the conduit of the duct element 10 is substantially eliminated for storage and transportation purposes. Fig. 7 shows a partially collapsed duct element 110 which is described in further detail below. In some embodiments, removal of the shear force causes the wall panels 18,22 to resiliently move apart (without the need for application of an external force) and leads to a progressive restoration of the overall rectangular cross-sectional shape of the duct element. Figs. 4-7 show a T-shaped duct element 110 in accordance with an alternative example embodiment of the present invention. In most respects, the structure of T-shaped duct element 110 is equivalent to that of duct element 10 described above in relation to the embodiment of Figs. 1 and 2 and so will not be repeated unnecessarily. However, a major extension panel 118a extends transversely away from a central portion of perimeter wall panel 118 and is delimited by longitudinal fold line 112. Similarly, a major extension panel 122a extends transversely away from a central portion of perimeter wall panel 122 and is delimited by longitudinal fold line 112 and a pair of transverse fold lines 113. A minor extension panel 122b and 122c is connected to major extension panel 122a about said pair oftransverse fold lines 113. Minor extension panels 122b and 122c each comprise a flanged tab 130 formed via an additional pair of said transverse fold lines 113. The flanged tabs 130 are each affixed, e.g. by means of an adhesive / sealant such as Mastic®, along a peripheral portion of an internally facing surface of major extension panel 118a. The flanged tabs 124 connecting parts of wall panel 116 to wall panel 122 are truncated in length. The resulting structure of the duct element 110 provides a T-shaped junction with three open ends 114 for connecting mutually perpendicularly extending lengths of gascirculation conduit within an HVAC system. As shown in Fig. 7, the resilient nature of the flexible plastics sheet material from which the duct element 110 is formed facilitates selective collapsibility of its perimeter wall panels 116,118,120,122 about the fold lines 112. For example, the application of a shear force in the transverse direction A and / or B of the plane(s) of major wall panel(s) 118 and / or 122 causes the hollow conduit to reduce in height until wall panels 118, 122 move together, partially overlap one another, and ultimately contact one another. When fully collapsed, the internal volume of the hollow opening extending longitudinally through the conduit of the duct element 110 is substantially eliminated. However, major extension panels 118a, 122a (and the latter's associated minor extension panels 122b and 122c) are restrained from collapsing relative to one another in the longitudinal direction. Despite this, the overall volume of the T-shaped duct element 110 can be greatly reduced for storage and transportation purposes. In some embodiments, removal of the shear force causes the wall panels 118, 122 to resiliently move apart (without the need for application of an external force) and leads to a progressive restoration of the overall rectangular cross-sectional shape of the T-shaped duct element 110. In the duct elements 10 and 110 described above in relation to Figs. 1-7, all perimeter wall panels 16,18, 20, 22,116,118,120,122 are rectangular in shape for constructing relatively simple, linearly extending gas-circulation conduits. However, more complex structures may be achieved by providing perimeter wall panels which adopt a non-rectangular polygonal shape. Such shapes may be achieved by forming a combination of fold lines and selective cuts in a rectangular blank of flexible plastic material to be subsequently assembled into a complexly shaped duct element. Such a complex duct element is described below in association with Figs. 8-13. Figs. 8 and 9 show a side view and a view from below of a complex duct element 210 extending longitudinally between its two opposed open ends 214 and defining an undulating gas-circulation conduit. A gap 250 is formed beneath an upper section 260 of the conduit and between a pair of inclined sections 270, 280 of the conduit which are each connected opposite ends of the upper section 260. In practice, another duct element (for example, a duct element 10 as shown in the embodiment of Figs. 1-2) may be routed through the gap 250 to allow two gas-circulation conduits at the same height to bypass or cross-over one another perpendicularly. The duct element 210 may be formed from two blanks 290, 295 of corrugated flexible plastics sheet material. A first blank 290 of sheet material (see Fig. 10) is folded about two longitudinally extending fold lines 212 to delimit first, second, and third major successive and contiguous perimeter wall panels 216, 218, 220 of the duct element 210. The first and third perimeter wall panels 216, 220 are polygons in the form of irregular dodecagons which are symmetrical about axis Y. The second perimeter wall panel 218 is rectangular in shape and includes two pairs of transverse fold lines 230 to subdivide it in the longitudinal direction to define: (i) longitudinally separated inclined wall panels 240 corresponding to the pair of inclined sections 270, 280 mentioned above; and (ii) longitudinally separated wall panels 245 forming part of each open end 214 of the duct element 210. In first blank 290, the wall panels 240, 245 are separated from the first and third perimeter wall panels 216, 220 by cut-away portions 225. A series of individual flanged tabs 224a and 224b are provided around the respective inner and outer longitudinally extending and inclined perimeter edges of the first and third perimeter wall panels 216, 220. As shown in Fig. 12, the uppermost wall of the double-walled structure of the tabs 224a and / or 224b may be removed to expose the underlying corrugations, and to reduce the overall thickness of the tabs relative to the adjoining perimeter wall panel(s) 240, 241, 245, 246. A reduction in the thickness of the tabs 224a, 224b facilitates formation of a perpendicular hinge relative to the plane of each adjoining wall panel. The sheet material of the first blank 290 is folded about the longitudinally extending fold lines 212 to initially define an upper part of a central portion of a unitary hollow gascirculation conduit. The inclined wall panels 240 are then folded about the transverse fold lines 230 until they are substantially aligned with the upper perimeter edges of inclined sections 270, 280 of the first and third perimeter wall panels 216, 220. The flanged tabs 224a provided around the inner perimeter edges of the first and third perimeter wall panels 216, 220 are then affixed to inner peripheral surfaces of the wall panels 240, 245. A fourth wall panel 222 is formed from a second rectangular blank 295 (see Fig. 11) of corrugated flexible plastics sheet material. The second blank 295 includes two pairs of transverse fold lines 230 to subdivide it in the longitudinal direction to define: (i) longitudinally separated inclined wall panels 241 corresponding to the pair of inclined sections 270, 280 mentioned above; and (ii) longitudinally separated wall panels 246 forming part of each open end 214 of the duct element 210. The sheet material of the second blank 295 is folded about the transverse fold lines 230 until it substantially matches the profile defined by the wall panels 218, 240, 245 of the assembled first blank 290. The lowermost wall oftab(s) 224a and / or 224b are affixed, e.g. by means of an adhesive / sealant such as Mastic®, along a peripheral portion(s) of an internally facing surface of wall panel(s) 222, 241, 246 as previously described in relation to the embodiments of Figs. 1-7. Once the first blank 290 is assembled and connected to the folded second blank 295, the angle between all adjacent wall panels (with the exception of those about the transverse fold lines 230 of the inclined wall panels 240) is preferably maintained as a right angle in order to maximise the internal gas-circulation volume of the duct element 210. In some embodiments, one or all of the fold lines 212 may optionally be replaced by one or more cut edges which have been affixed together (e.g. by adhesive, welding, stitching or the like) to form a hingeable / foldable connection. However, the inclined sections 270, 280 of the dodecagonal wall panels 216, 220 are necessarily (rather than optionally) created via the formation of the cut-away portions 225 as described above. In use, the resilient nature of the flexible plastics sheet material from which the duct element 210 is formed facilitates selective collapsibility of its assembled wall panels 216, 218, 220, 222 in the transverse direction about the longitudinal fold lines 212. For example, the application of a shear force in the transverse direction C and / or or D of the plane(s) of major wall panel(s) 218 and / or 222 causes the hollow conduit to reduce in height until wall panels 218, 222 move together, partially overlap one another, and ultimately contact one another. When fully collapsed, the internal volume of the hollow opening extending through the conduit of the duct element 210 is substantially eliminated for storage and transportation purposes. Figs. 13 and 14 show a progressive collapsing of duct element 210 as described in further detail below. As is best shown in Fig. 13, after the duct element 210 is collapsed about its longitudinal fold lines 212 in the transverse direction, it is subsequently collapsed perpendicularly about its transverse fold lines 230, 231 in the longitudinal direction indicated by arrows E. This concertina collapse causes: (i) both inclined wall panels 241 to pivot towards the adjoining fourth wall panel 222 about the corresponding transverse fold lines 230; (ii) both rectangular wall panels 245 to pivot towards their adjoining inclined wall panels 240 about the corresponding transverse fold lines 230; and (iii) the first and third perimeter wall panels 216, 220 to overlap about transverse fold lines 231. When fully collapsed (see Fig. 14), the undulating internal volume of the hollow opening extending through the conduit of the duct element 210 is substantially eliminated for storage and transportation purposes. In some embodiments, removal of the shear force causes the collapsed wall panels to resiliently move apart (without the need for application of an external force) and leads to a progressive restoration of the overall rectilinear cross-sectional shape of the undulating duct element 210. It is contemplated by the inventor that various substitutions, alterations, and modifications may be made to the invention without departing from the scope of the invention as defined by the accompanying claims. For example, some or all of the lines may be replaced by adjoined cut edges rather than folds between contiguous parts of the flexible sheet material. Therefore, the term "fold” may be interpreted to encompass adjoined cuts unless the context distinguishes the two or requires otherwise. The step of forming an at least partial overlap between two wall panels of a duct element need not require the formation of a flanged tab. Instead, an overlap may occur remote from the folded edges whereby two wall panels are joined together via heat sealing (with hot air) or otherwise affixed together. In some embodiments, the internal flutes of the flexible plastic material at an exposed edge are sealed shut by means of heat sealing, or via injection of an adhesive, sealant, or plastic material. It will be appreciated that this prevents air leakage and / or moisture ingress and improves the thermal efficiency of the duct element. The uppermost wall of the double-walled structure of a wall panel (or tab) may be removed to expose the underlying corrugations or flutes, and to reduce its overall thickness at the location of an overlapping joint between two wall panels (or between a wall panel and tab). Indeed, a joint may be formed by removing the uppermost wall of one wall panel (or tab) and the lowermost wall of another wall panel (or tab) to facilitate the formation of a lap joint along the length of their connection. A lap joint will avoid a localised line of increased thickness along the duct element. Traditionally, the thermal efficiency and acoustic absorbent properties of a ventilation duct element are improved by applying an insulation layer around it. For example, this may involve use of an aluminium foil-backed mineral wool layer such as that sold under the brand name Ductwrap. However, the inventor of the present invention proposes that the aluminium foil layer may be omitted without compromising the insulative performance of a duct element. Indeed, the primary purpose of the aluminium foil layer is to provide vapour / moisture resistance, and this can instead be achieved by encapsulating the mineral wool layer within a layer of plastic material such as polythene or polypropylene. Conveniently, the layer of plastic material may take the form of heat-sealable lay flat tubing. Advantageously, the layflat tubing may provided the dual advantage of eliminating costly aluminium whilst facilitating vacuum sealing of that duct element of the present invention in a collapsed or "flat-pack” configuration, thus reducing its volume for transportation and storage purposes.

Claims

1. A duct element comprising a sheet of flexible plastics material folded about a plurality of longitudinal fold lines to delimit successive perimeter wall panels of a unitary hollow gas-circulation conduit; a first wall panel of the conduit being connected to another wall panel to define a rectilinear annular perimeter extending between two opposed longitudinally separated open ends; and wherein said conduit is collapsible about said fold lines such that said successive perimeter wall panels are movable into a substantially parallel primary overlapping configuration.

2. A duct element according to claim 1, wherein the sheet of flexible plastic material further comprises one or more transverse fold lines extending between adjacent longitudinal fold lines to subdivide the corresponding perimeter wall panel(s) in the longitudinal direction.

3. A duct element according to claim 2, wherein said conduit is additionally collapsible about said one or more transverse fold lines such that said perimeter wall panels are movable into a substantially parallel primary overlapping configuration via said longitudinal folds; and then into a substantially parallel secondary overlapping configuration via said transverse folds.

4. A duct element according to any preceding claim, wherein each one of said successive perimeter wall panels is rectangular.

5. A duct element according to any of claims 1 to 3, wherein one or more of said successive perimeter wall panels is a non-rectangular quadrilateral.

6. A duct element according to any of claims 1 to 3, wherein, one or more of said successive perimeter wall panels is polygonal with more than four sides.

7. A duct element according to any preceding claim, wherein the sheet of flexible plastics material is corrugated or fluted.

8. A duct element according to claim 7, wherein the corrugations or flutes extend between two outer walls to provide a double-walled, air-filled structure.

9. A duct element according to any preceding claim, wherein said first wall panel of the conduit is connected to said another wall panel via an overlap formed between the respective panels.

10. A duct element according to claim 9, wherein the overlap is facilitated by a flange extension extending entirely or partly along a longitudinal edge of one or both of said panels.

11. A duct element according to claim 10, wherein the flange extension extends entirely or partly across one or both of said panels in a transverse direction.

12. A duct element according to claim 10 or 11, wherein the flange extension is defined by a folded tab portion.

13. A duct element according to any of claims 10 to 12, wherein the or each flange extension of one panel is affixed to the other panel to define said rectilinear annular perimeter of the hollow gas-circulation conduit.

14. A duct element according to any preceding claim, wherein the sheet of flexible plastics material is provided with weakened regions to facilitate the formation of said plurality of fold lines.

15. A duct element according to any of claims 10 to 13, wherein the sheet of flexible plastics material is provided with weakened regions to facilitate the formation of said flange (s).

16. A duct element according to claim 14 or 15, wherein the weakened regions are formed by cuts, score lines, or perforations formed in a surface of the sheet of flexible plastics material.

17. A duct element according to claim 14 or 15, wherein the weakened regions are formed by deformations or creases applied to the surface of the sheet of flexible plastics material.

18. A duct system comprising a first duct element in accordance with any of claims 1 to 17 connected to one or more other duct element(s) in accordance with any of claims 1 to 17 thus forming all or part of a Heating Ventilation and Air Conditioning (HVAC) system.

19. A duct system according to claim 18, wherein an annular connector element is provided for connecting together open ends of a pair of duct elements, the connector element comprising a pair of aligned annular channels for receiving transverse edges of said successive perimeter wall panels, said aligned annular channels being separated by an annular partition wall.

20. A method of manufacturing a duct element in accordance with any of claims 1 to 17, comprising the steps of:(i) cutting a sheet of flexible plastics material into a blank corresponding to the desired shape of a duct element in an unfolded state;(ii) forming a plurality of longitudinal fold lines along the blank to delimit successive perimeter wall panels of a duct element in an unfolded state;(iii) successively folding said sheet of flexible plastics material along said plurality of fold lines to transform said blank into a three-dimensional gas circulation conduit;(iv) forming an at least partial overlap between two of said wall panels; and(V) affixing said two wall panels together at the region of said at least partial overlap to maintain the three-dimensional shape of the gas circulation conduit.

21. A method of manufacturing a duct element according to claim 20, wherein the step of forming an at least partial overlap between two of said wall panels comprises folding a portion of one of said wall panels to form a tab overlapping all or part of the longitudinal extent of the other of said wall panels.

22. A method of manufacturing a duct element according to claim 20 or 21, wherein the step of forming a plurality of longitudinal fold lines, and / or folding a portion of one of said wall panels to form a tab, is preceded by weakening the sheet of flexible plastics material along the length of the intended folds by forming one or more cuts and / or scores and / or creases and / or perforations.

23. A method of manufacturing a duct element according to any of claims 20 to 22, wherein the method includes an additional step of forming one or more transverse fold lines extending between adjacent longitudinal fold lines to subdivide the corresponding perimeter wall panel(s) in the longitudinal direction.

24. A method of manufacturing a duct element according to any of claims 20 to 23, including the step of collapsing said three-dimensional gas circulation conduit about said fold lines such that said successive perimeter wall panels are movable into a substantially parallel overlapping configuration.

25. A method of manufacturing a duct element according to claim 24, wherein including the step of retaining the collapsed wall panels in said substantially parallel overlapping configuration against resilient deployment into said three-dimensional shape of the gas circulation conduit.

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