GB2026648A

GB2026648A - Spacing Spiral for Coaxial Tube Systems

Info

Publication number: GB2026648A
Application number: GB7920147A
Authority: GB
Original assignee: KM Kabelmetal AG
Current assignee: KM Kabelmetal AG
Priority date: 1978-06-14
Filing date: 1979-06-08
Publication date: 1980-02-06
Also published as: JPS5843638B2; GB2026648B; FR2428779B1; DE2825937A1; DE2825937C2; SU1074416A3; JPS5510190A; FR2428779A1

Abstract

A spacing spiral for a coaxial tube system in which, in use, there is a temperature gradient between radially successive tubes of the coaxial system e.g. a coaxial tube system, constituted by an electric cable operated in a super-conductive state, or a pipeline for conveying a heated or cooled fluid medium, comprises individual cords (1, 2, 3 & 4) assembled in the form of a plait. Preferably the cords are of circular cross-section. The cords may for example be of metal and/or synthetic material. Furthermore, individual cords may be of composite construction, e.g. comprising synthetic and metal threads twisted together. A particularly useful specific construction employs four individual cords which follow an undulating course along the length of the spiral, two of the four cords (1 & 2) being disposed generally in one plane and the two other cords (3 & 4) being disposed generally in a plane at right angles to the first one. The plaited spiral enables heat transfer (in the radial direction of the coaxial tube system) to be kept down to a particularly low value without loss of flexibility. <IMAGE>

Description

SPECIFICATION Spacing Spiral for Coaxial Tube Systems This invention relates to a spacing spiral for a coaxial tube system in which, in use, there is a temperature gradient between radially successive tubes of the coaxial system. The coaxial tube system may, for example, be constituted by an electric cable operated in a superconductive state, or by a pipeline for conveying a heated or cooled fluid medium.

For the transmission of electrical energy, "cryogenically" cooled cables have become known in which a conductor is disposed within a tube filled with liquid helium. A heat barrier with respect to the outside is formed by further tubes, of larger and larger diameters, which surround the first tube coaxially, so as to form intermediate spaces of annular cross-section. In the space between the innermost tube and the one next to it, a vacuum is usually maintained. The space between the second and third tubes is customarily filled with liquid nitrogen in order to subdivide the temperature drop from the outside toward the inside. Usually the space between the third and fourth tubes, the latter being normally the last tube, which is at ambient temperature, is again evacuated.

Identical or similar tube systems can be used as low-temperature pipelines for conveying liquefied gases or other cryogenic media. The essential feature of the various structures under consideration is that heat transfer due to either conduction or radiation should be reduced to a minimum so that the operator can operate these systems as economically as possible.

For this purpose of minimising heat transfer, numerous suggestions have been made already for the use of thermally insulating spacers. Thus, among others, a spacing spiral for coaxial tube systems is known (German Patent 1 775 410) which consists of individual bands which are arranged loosely in layers on top of each other and which are themselves held together only at intervals. When installed, i.e., mounted, the individual bands rest against each other only at well spaced (statistically distributed) contact points, gaps being formed between them. This considerably reduces the total surface available for the transmission of heat.

Taking into account this design of a spacing spiral, which was itself advantageous with respect to the art previously existing, we have had as the basic object in the case of the present invention the provision of a spacing spiral which will reduce the heat transfer in coaxial tube systems even further, without reducing the flexibility of the system.

According to the invention, we provide a spacing spiral for a coaxial tube system in which, in use, there is a temperature gradient between radially successive tubes of the coaxial system, characterized in that the spiral comprises individual cords assembled, by being plaited or twisted together, in the form of a plait. Compared with the known spacing spirals, the present plaited construction offers considerable advantages. Thus, a first advantage is that the heat transfer between the individual cords making up the spacing spiral is confined to point-contact transfer, in contrast to the area-contact transfer in the spiral consisting of layered bands. Secondly, the diameter of the spiral according to the invention varies very greatly from one point along its length to another, so that the number of points of contact with the walls of the adjacent tubes is reduced greatly.A third advantage is that the spacing spiral, due to its plait-like construction, has great longitudinal elasticity, so that it exerts only small axial forces during cooling. In this connection it can be assumed that the number of points of contact with the walls of the adjacent tubes is reduced further, since during cooling the diameter of the spiral becomes smaller.

Compared with other spacing spirals, the present plaited spiral also contains less material, simplifying construction and making production more cost-effective. In a particularly advantageous embodiment of the invention the individual cords have a circular cross-section. This further reduces the contact points of the cords.

In the spacing spiral of the invention, it is often of advantage for the individual cords to be composed of synthetic material, for example, polyethylene or polytetrafluoroethylene. Also, however, it is possible, and sometimes of advantage, particularly if increased tensile loads are to be expected, to employ a metal for the individual cords. In the case of a high-temperature application, for example, a plainted structure formed from individual steel wires is particularly suitable for the purpose of the invention.

Combinations of metal cords and synthetic material cords can also be advantageous in the application of the present invention. The circumstances of any particular case will decide which embodiment is to be given preference.

The surfaces of the individual cords may be smooth, which simplifies production of the cords.

In order to reduce heat transfer, however, it may instead be of advantage in certain cases to roughen the surfaces of the individual cords, for example, by providing them with longitudinal or transverse corrugations.

Instead of using plain or unitary synthetic or metal cords for the purposes of the invention, it can be of advantage to employ cords of composite construction. This can be advantageous more particularly with metallic cords, for weight saving reasons. Both with nonmetallic and with metallic cords, however, it can be advantageous also as a means of effecting a further reduction in the heat-conducting capability of the spiral. In the cords of composite construction, a combination of metal and synthetic threads can be utilized. It will be understood that, in the first instance, cords are produced from these threads, and that the cords are then assembled, by being twisted or plaited together, to form the plaited spacing spiral.

The invention is explained in greater detail with reference to the accompanying diagrammatic drawing, whose single Figure is a fragmentary side view of a spacing spiral according to the invention.

The spacing spiral shown in the Figure, which can be used, for example, for the coaxial positioning of flexible corrugated tubes for liquid helium lines, comprises four individual synthetic cords 1, 2, 3 and 4. These individual cords are twisted together in the form of a plait, and advantageously they are twisted together in such a manner that the individual cords follow an undulating course along the length of the spiral, two cords (1 and 2) being disposed generally in one plane and the other two cords (3 and 4) being disposed generally in a plane at right angles to the first one.

Without regard to the position of any individual cord, for example, that of individual cord 2, the plaited spiral can be wound around the inner tube concerned; in each case (i.e. whatever the position of any individual cord) an optimum degree of heat insulation is produced. Provided that only curved surfaces are exhibited by the individual cords, as, for example, in the case of cords with a circular cross-section, there is only point contact at their crossing-over places, and this minimises heat transfer. Considered as a stranded element, the spacing spiral according to the invention has a sensitivity to tractive force such that it can be applied with the winding devices familiar in the art. Since the composite cross-section of the plaited spiral is approximately circuiar, a further advantage is that there is little danger of its being wound on incorrectly. Tilting or canting of the spiral does not impair its heat insulating properties.

In the embodiment shown in the Figure, there are four individual cords, but it is also possible, of course, to use a larger number of individual cords, or to increase the diameters of the individual cords if, for example, a larger total cross-section is required, so as to give a larger tube spacing. If desired, cross-sections which differ from the circular, e.g. elliptical, triangular or square crosssections, may be employed, but the individual cords can still be plaited or twisted together as indicated in the Figure.

Claims

1. Spacing spiral for a coaxial tube system in which, in use, there is a temperature gradient between radially successive tubes of the coaxial system, characterized in that the spiral comprises individual cords (1,2,3 84) assembled, by being plaited or twisted together, in the form of a plait.

2. Spacing spiral according to claim 1, characterized in that the individual cords have a circular cross-section.

3. Spacing spiral according to claim 1 or 2, characterized in that the individual cords are composed of synthetic material.

4. Spacing spiral according to claim 1 or 2, characterized in that the individual cords are of metal.

5. Spacing spiral according to any preceding claim, characterized in that the spiral comprises a combination of synthetic material cords and metal cords.

6. Spacing spiral according to any preceding claim, comprising four individual cords, characterized in that the individual cords follow an undulating course along the length of the spiral, two of the four cords (1 82) being disposed generally in one plane and the two other cords (3 Eft 4) being disposed generally in a plane at right angles to the first one.

7. Spacing spiral according to any preceding claim, characterized in that the surfaces of the individual cords are roughened.

8. Spacing spiral according to claim 7, characterized in that the surfaces of the individual cords have longitudinal or transverse corrugations.

9. Spacing spiral according to any preceding claim, characterized in that the individual cords themselves are of composite construction.

10. Spacing spiral according to claim 9, characterized in that the individual cords themselves comprise synthetic and metal threads twisted together.

11. Spacing spiral according to claim 1, substantially as described with reference to the accompanying drawing.

12. A superconductive electric cable in the form of a coaxial tube system, or heated or cooled fluid medium conveying coaxial tube system, incorporating at least one spacing spiral according to any preceding claim disposed between radially successive tubes of the coaxial system.