FIELD OF THE INVENTION

The present invention relates to a double-walled duct. BACKGROUND OF THE INVENTION

A double-walled duct or pipe typically comprises an inner conduit and an outer conduit disposed annularly around the inner conduit. The inner and outer conduits may be coaxial but need not be. An annular space is defined between the inner and outer conduits. Double-walled ducts are used for a variety of applications. For example, the annular space may be used to vent fluids escaping from the inner conduit in the event of a rupture. This may be particularly advantageous if the inner conduit is used to convey hazardous fluid. Alternatively, the inner and outer conduits may be used to convey dissimilar fluids, or fluids at dissimilar pressures. The annular space may provide a leak detection zone, incorporating one or more sensors for example to detect leakage of fluid from the inner active transfer conduit.

Conventionally, the inner and outer conduits are supported at each end of the duct. This tends to limit the uninterrupted duct length, requiring dedicated joining pieces to fluidly connect the inner and outer conduits. This increases cost, weight, complexity, maintenance etc. If the inner and outer conduits require continuous support along the length of the duct then a secondary structure is typically required which may restrict the flow through the annular space. Bending of double-walled ducts not having secondary support structures can be problematic due to distortion of the co-axial geometry. SUMMARY OF THE INVENTION

A first aspect of the invention provides a duct comprising a first conduit, a second conduit disposed annularly around the first conduit, and a support structure integrally formed with and connecting the first conduit to the second conduit and extending helically along the duct.

A further aspect of the invention provides an extruder comprising an inlet for receiving a hot flowable material, and an outlet having a former for discharging a cooled extruded product of the material in the shape of a duct, the former having a first portion for forming a first conduit, a second portion for forming a second conduit disposed annularly around the first conduit, and a third portion for forming a support structure integrally with and connecting the first conduit to the second conduit and extending helically along the duct. The support structure may extend radially between the first conduit and the second conduit.

The first conduit and the second conduit may be coaxial.

The support structure is preferably a helical wall. In particular, the support structure may be a single helical wall. In this way, over any given length of the duct, the complete circumference of the first and second conduits is supported and there are no longitudinal blockages or restrictions.

The first conduit may be connected to the second conduit only by the single helical wall substantially along the entire length of the duct.

The first conduit, the second conduit and the support structure preferably comprise the same material. In particular, the duct may be formed by extrusion.

The first conduit may be adapted to convey a first fluid within the first conduit. The second conduit may be adapted to convey a second fluid between the first conduit and the second conduit. Alternatively the second conduit may be adapted to act as a leak detection space. A leak detection space is a space adapted to contain a leaked fluid and detect the presence of the leaked fluid. The leak detection space may comprise one or more leak detection sensors for detecting the presence of a leaked fluid, for example fluid leaked from the first conduit.

The first conduit may be sealed with respect to the second conduit. The support structure may extend helically continuously from one end of the duct to the other end.

The duct may have a longitudinal axis, which includes a linear portion and/or a curved portion. The support structure may include one or more through holes. The support structure may occupy less than 20% of the cross sectional area between the first conduit and the second conduit, preferably less than 10%.

The duct may form part of a fuel system. The fuel system may be a vehicle fuel system, for example an aircraft fuel system. The first conduit and/or the second conduit may be used to convey fuel.

The duct may comprise or be formed of a plastics and/or composite and/or metal material.

In the extruder, the first portion may include a substantially cylindrical body. The second portion may include a substantially annular body arranged around the cylindrical body. The third portion may include a cut through the annular body, wherein the cut is twisted in the direction of material flow through the former.

The extruder may further comprise cooling means for cooling the extruded material at the outlet. The cooling means may cool the extruded material as it is discharged from the former. The cooling means may be part of the outlet and/or the former. The extruder may further comprise means for forming through holes in the support structure. In use, the means for forming through holes may be actuated by the progression of the flowable material through the extruder. The means for forming through holes may, for example, comprise a toothed wheel at the third extruder portion. The toothed wheel may be rotated in use by the progression of the flowable material through the extruder.

The hot flowable material through the extruder may be a hot plastics material or a molten metal material. Means for heating the material prior to entering the extruder may be provided. BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will now be described with reference to the accompanying drawings, in which:

Figure 1 illustrates a three dimensional view of a double-walled duct; Figure 2 illustrates cross sections through the duct at various points a-e in Figure 1;

Figure 3 illustrates a detailed view of the section between points a and b of the duct;

Figure 4 illustrates a cross section through an extruder for forming the double-walled duct; and

Figure 5 illustrates a view on the outlet side of the extruder. DETAILED DESCRIPTION OF EMBODIMENT(S)

Figure 1 shows a three dimensional view of a double-walled duct 1. The duct 1 comprises an inner conduit 2 and an outer conduit 3 disposed annularly around the inner conduit 2. The inner and outer conduits 2, 3 are coaxially arranged about common longitudinal axis X. The inner and outer conduits 2, 3 are each substantially circular in cross section. An annular space is defined between the inner and outer conduits 2, 3.

A support structure 4 connects and extends between the inner and outer conduits 2, 3. The support structure 4 extends in a helical manner along the duct about the axis X. The support structure 4 extends substantially radially outwardly between the inner conduit 2 and the outer conduit 3. The support structure 4 is formed as a wall having a wall thickness substantially the same as the wall thickness of the inner and outer conduits. The support structure 4 is therefore relatively thin and occupies an area of approximately only 5% of the cross sectional area of the duct.

Figure 2 illustrates cross sections taken through the duct 1 taken at spaced locations along the duct labelled a to e in Figure 1. The helical support structure 4 revolves through over 360 degrees along the length of the duct 1. The support structure 4 is arranged to support the inner and outer conduits 2, 3 over the complete circumference of the conduits. Due to the relatively thin walled construction of the support structure 4 there are substantially no longitudinal blockages or restrictions along the length of the duct.

Figure 3 illustrates a section of the duct between cross sections a and b in Figure 1 so as to best illustrate the helical walled support structure 4. In this embodiment the support structure 4 includes only a single helical wall but it will be appreciated that in alternative embodiments the support structure may comprise more than one helical wall so as to form, e.g. a double helix. Furthermore, the duct 1 is double-walled with an inner (first) conduit and an outer (second) conduit but it is contemplated that the number of ducts may be increased to three or more so as to form, e.g. a triple-walled duct. With three or more conduits comprising the duct a respective support structure is provided between each adjacent pair of conduits. The multiple support structures may be radially aligned or may be out of phase by approximately 180 degrees or indeed any other preferred angle. Returning to the duct 1 shown in Figures 1 to 3 the inner and outer conduits 2, 3 and the support structure 4 are integrally formed and comprise the same material. In particular, the duct 1 can be formed by extrusion. A suitable extruder is shown in Figures 4 and 5 and will be described in further detail below. The ability to extrude the duct 1 as a unitary component has several advantages in terms of reduced manufacturing cost and improved structural stability.

Since the support structure 4 occupies a relatively small cross sectional area of the duct, the duct can be bent to a wide variety of shapes. Therefore the longitudinal axis X of the duct 1 can have linear and non-linear portions, or may be non-linear along its entire length. Since the support structure extends helically continuously from one end of the duct to the other there is no requirement for additional inserts to be provided at specific bend locations along the duct prior to bending.

The duct 1 can be used to convey at least one fluid. For example, the interior space 5 within the inner conduit may be used to convey a first fluid whilst the annular space 6 between the inner and outer conduits may be used as a leak detection space so as to contain any leakage of the first fluid in the event of any rupture or leak from the inner conduit 2. The containment ensures that in the event of a leakage or rupture of the inner conduit 2 the fluid does not escape to the environment external to the duct 1, which may be particularly beneficial in the case of hazardous fluids.

Alternatively, the duct 1 may be used to convey two discrete fluid paths. A first fluid may be conveyed in the space 5 within the inner conduit 2 and a second fluid may be conveyed within the annular space 6 between the inner and outer conduits 2, 3. The first and second fluids may be dissimilar fluids, or may be at different pressures, or may be conveyed in different flow directions, or any combination of these. The construction of the support structure 4 provides minimal restriction to the flow of the second fluid through the annular space 6. Of course, if the duct comprises three or more conduits then at least three fluid paths may be provided.

The interior space 5 within the inner conduit 2 is sealed from the annular space 6 between the inner and outer conduits 2, 3, which in turn is sealed from the environment external to the outer conduit 3. This prevents mixing of the discrete fluid flows through the duct and also permits differential pressures between the fluid flows.

Whilst in the embodiment described with reference to Figures 1 to 3 the support structure 4 includes a solid wall it may in some instances be advantageous to provide through holes or perforations through the wall. With a solid wall the flow in the annular space 6 will tend to revolve about the axis X as it travels along the length of the duct. If the wall is perforated then the flow through the annular space 6 will have a lower tendency to rotate about the axis X. This may beneficially reduce flow losses through the annular space 6.

The duct 1 may be constructed of a variety of plastics or composite or metal materials. For example, polypropylene or copper could be used. If composites material is used then this may include a plastics material matrix containing short fibres or particles of electrically conductive material, a conductive composite material may be particularly beneficial if the duct is used in a flammable environment, such as when used as a fuel pipe, so that the duct 1 can be electrically coupled to a bonding network to provide lightning strike protection. It is envisaged that this may provide particular benefit in a vehicle fuel system, such as an aircraft fuel system. Turning now to Figures 4 and 5 an extruder for forming the double-walled duct 1 will now be described. The extruder 10 includes an inlet 11 for introducing a hot flowable material and an outlet 12 having a former for discharging a cooled extruded product of the material in the shape of the duct 1. The former has a first portion 13 for forming the inner conduit, a second portion 14 for forming the outer conduit disposed annularly around the inner conduit, and a third portion 15 for forming the support structure 4 integrally with and connecting the inner conduit 2 to the outer conduit 3 and extending helically along the duct 1. Finally, the outlet 12 shapes the external surface of the duct 1. The first portion 13 of the former includes a substantially cylindrical body. The second portion 14 of the former includes a substantially annular body arranged around the cylindrical body. The third portion 15 of the former includes a cut through the annular body, where the cut is twisted in the direction of material flow through the former.

Hot flowable material such as hot plastics material for a molten metal material or a hot flowable composite material is introduced into the inlet 11 and forced under pressure around the first, second and third portions 13, 14, 15 of the former. The extruded duct 1 exits the outlet 12 with a rotation about the longitudinal axis X of the duct 1. The continuously extruded duct 1 can be cut to desired stock lengths or to bespoke lengths in a conventional manner. Of course, it will be appreciated that the extruder 10 described with reference to Figures 4 and 5 is configured for producing an extrusion in the shape of the duct 1 shown in Figures 1 to 3. Variations to the shape of the duct 1 will necessitate modifications to the former shape which will be readily appreciated by those skilled in the art. For example, if a triple-walled duct is desired, or a double-walled duct with a double helix support structure, then additional features of the former will be required based upon the features described above, e.g. a further annular body and/or a further cut through the annular body.

It will also be appreciated that means for cooling the extruded material at the outlet of the extruder 10 will be required, as well as means for heating the material prior to entry into the extruder inlet 11. Since these heating and cooling means are conventional features of known extruders a detailed description of these heating and cooling means will not be reproduced here.

As discussed previously in some circumstances it may be desirable that the wall of the support structure 4 includes through holes or perforations. A rotatable toothed wheel or the like may be provided at the third extruder portion 15 for creating these perforations. The wheel is forced to rotate by the progression of the flowable material through the extruder with the teeth forming each perforation in the wall of the support structure 4. The duct 1 will exit the extruder 10 having a substantially linear longitudinal axis X. However, the duct 1 can be bent along its longitudinal axis at one or more locations. Bending of the duct may occur immediately upon exit from the extruder whilst the extruded material is still plastic, or alternatively the duct 1 can be bent to the desired shape during a subsequent forming operation.

The duct 1 has applications in a variety of fields of technology such as in the aircraft industry, the oil and gas industry, medical devices, or in any other industry where a duct for conveying one or more fluids in liquid or gaseous form is required.

Although the invention has been described above with reference to one or more preferred embodiments, it will be appreciated that various changes or modifications may be made without departing from the scope of the invention as defined in the appended claims.