GB2390004A

GB2390004A - Flexible heating element

Info

Publication number: GB2390004A
Application number: GB0205477A
Authority: GB
Inventors: Martin Cook; David Cook
Original assignee: Individual
Current assignee: Individual
Priority date: 2002-03-08
Filing date: 2002-03-08
Publication date: 2003-12-24
Also published as: GB0205477D0

Abstract

A heating element (10) including first (12) and second (12') elongate, electrical bus conductors embedded in an insulating core (14), the conductors (12, 12') being mutually spaced and generally parallel and one or a plurality of resistance wires (16) made from a conductive material with a positive temperature coefficient, the resistance wire (16) extending between and being in electrical contact with the two conductors (12, 12'). The core may be silicone rubber and the wire an iron-nickel alloy which may be soldered to the bus conductors. In the plural arrangement the resistance wire are effectively connected in parallel and in the single arrangement the wire may be connected to each conductor at more than one point along the said wire.

Description

GB 2390004 A continuation (74) Agent and/or Address for Service: Forrester

Ketley & Co Chamberlain House, Paradise Place, BIRMINGHAM, B3 3HP, United Kingdom

(l- PATENTS ACT 1977

Al 0544GB-DJI /ACL Title: Heating Element Description of Invention

The present invention relates to a heating element, particularly, but not exclusively, to a self-regulating heating tape or cable for use in applications such a freeze protection for pipework and vessels, desnowing or de-icing of roofs and gutters, heating of roads, ramps, or walkways or underfloor heating.

Typically such a heating element is wound around pipes or vessels, or laid in loops adjacent to the surface to be heated. It is therefore essential that such a heating element is flexible.

Flexible self-regulating heating elements including two mutually spaced, generally parallel conductors or bus-bars embedded in an electrically conductive polymeric core are known. A polymeric insulating jacket is Wpically extruded over this core, and an optional over jacket may also be provided for additional mechanical or corrosion protection. For example, a braided tin plated copper over jacket is typically provided if additional mechanical protection is required, and if the element is to be used in a corrosive environment, a polymer overjacket is typically used.

The conductive polymeric core typically comprises conductive particles of carbon black, dispersed in a polymer matrix, for example a matrix of silicone rubber. The conductive particles allow a limited electrical current to pass between the two bus-bars, but as the electrical resistance of the polymeric core is significantly higher than that of the bus-bars, passage of current between the bus-bars causes ohmic heating of the element.

The materials used in forming the polymeric core are selected such that the polymeric core has a positive temperature coefficient (PTC) i.e. the resistance of the core increases with increasing temperature. Thus, as the element heats up the current flowing between the bus-bars is reduced until an

equilibrium state is reached at which energy loss through heat transfer to the surroundings equals that gained from the current flow. The element is thus self-regulating, and cannot overheat.

Existing heating elements do, however, have a relatively limited life, as the core tends to degrade, particularly where the element carries high currents, reducing the lifetime of the element to perhaps as low as 1000 hours.

Moreover, less expensive elements tend to be operable up to relatively low temperatures only, since carbon black tends to degrade at around 60- 70 C.

Creating an element which is capable of operating up to higher temperatures significantly increases the cost of the element.

Such elements which are capable of operating at relatively high temperatures also have the disadvantage that they have a relatively large in-rush current. In other words, at ambient temperatures, the resistance of the polymeric core is so low that there is a large initial in-rush of current when the power supply to the element is switched on. Thus, either the heating element requires a current carrying capacity vastly in excess of that required for steady state operation, or expensive control equipment is required to prevent the initial current in-rush from damaging the element.

According to the invention we provide a heating element including first and second elongate, electrical conductors embedded in a core, the conductors being mutually spaced and generally parallel, characterized in that the core is electrically insulating, and the element further includes a resistance wire made from a conductive material with a positive temperature coefficient, the resistance wire extending between and being in electrical contact with the two: conductors. By virtue of the provision of a resistance wire instead of a conductive polymer body to conduct electricity between the conductors and to provide ohmic heating, an inexpensive, self-regulating, heating element is produced I which does not exhibit a high initial current in-rush.

Preferably the resistance wire is metallic.

Thus, the heating element may be operated at high temperatures for long periods of time without the performance of the element deteriorating.

Preferably the resistance wire is wound around the core and thus around the conductors in a generally helical arrangement.

In this way, the resistance wire does not significantly limit the flexibility of the heating element.

The resistance wire may contact the conductors by means of a first and second aperture in the core, the first aperture extending from a surface of the core to the first conductor, and the second extending from a surface of the core to the second conductor, the resistance wire contacting portions of the conductors exposed by the first and second apertures.

Preferably the first aperture is displaced along the length of the conductors with respect to the second aperture.

The resistance wire may be soldered to the conductors.

The resistance wire may be made from a metal with a substantially linear characteristic resistance versus temperature graph.

The resistance wire may be made from an iron-nickel alloy.

The core may be made from silicone rubber and thus may be flexible.

The heating element may include a plurality of resistance wires connected in parallel between the two conductors, each resistance wire being wound around a different portion of the core.

In this case, preferably each resistance wire has at least one point of contact with one of the conductors which is adjacent to a point of contact of another resistance wire with one of the conductors.

Alternatively, the heating element may include a single resistance wire which is connected to each conductor at more than one point along the resistance wire. I

The invention will now be described with reference to the accompanying drawings of which, FIGURE 1 is an illustration of a cut away end of a heating element according to the invention, FIGURE 2 is an illustration of a side view of a heating element according to the invention with cut away portions.

Referring now to the figures, there is shown a heating element 10 including first 12 and second 12' elongate electrical conductors, embedded in an insulating flexible polymer core 14. The conductors 12, 12' in this example are both wires or bars with a generally circular cross section, are mutually spaced and generally parallel, and are made of a material with a high electrical conductivity such as copper. In this example, the core 14 has a generally ellipsoidal cross section, is made of silicone rubber, and has been formed by extruding the silicone rubber around the two conductors 12, 12'. The two conductors 12, 12' are thus electrically insulated from one another by the core 14. A plurality of resistance wires 16 are wrapped around the core 14 in a generally helical arrangement, one of which is shown in the figures. The resistance wires 16 extend between the two conductors 12, 12', and are electrically connected in parallel between the two conductors 12, 12'.

The resistance wires 16 are made from an electrically conductive material with a positive temperature coefficient, i.e. a material whose electrical resistance increases with increasing temperature. For example, an ironnickel alloy such as Nifethal 70 may be used. This material has a high temperature coefficient, low resistivity, low thermal expansion coefficient, and a substantially linear characteristic resistance versus temperature graph.

As the resistance wires 16 are wound around the core 14 in a helical arrangement, they do not significantly increase the stiffness of the core 14.

( l Electrical connection between the conductors 12, 12' and the resistance wires 16 is achieved by means of a plurality of apertures 20, 20' provided in the core 14 which extend from a surface of the core 14 to one of the conductors 12, 12'. A first end of each resistance wire 16 is soldered to a portion of the first conductor 12 exposed by a first aperture 20, and a second end of the resistance wire 16 in question is soldered to a portion of the second conductor 12' exposed by a second aperture 20'. The first end of the resistance wire 16 is displaced along longitudinal axes of the conductors 12, 12' with respect to the second end, typically by a distance of lm.

The first end of the next resistance wire 16 is connected to the first conductor 12 by means of another aperture which is either opposite or close to the aperture 20' via which the second end of previous resistance wire 16 is connected to the second conductor 12'. Thus, there is only a relatively small gap between the ends of adjacent resistance wires 16, and so none or only a small proportion of the length of the core 14 is not wound round with resistance wire 16.

It is not necessary to provide a plurality of resistance wires 16; a single, continuous resistance wire 16 may be used instead. In this case the resistance wire 16 must be connected to each conductor 12, 12' at more than one point along the wire 16. The resistance wire 16 may, for example, be connected to the first conductor 12, wound around the core 14, then connected to the second conductor 12', wound around the core 14 again, and then connected to the first conductor 12 and so on, such that a plurality of portions of the single resistance wire 16 are connected in parallel between the two conductors 12, 12'.

The core 14 and windings of resistance wire 16 are covered by an insulating oversheath 18, which in this case is also flexible and made for example of extruded silicone rubber. The oversheath 18 need not, of course be made from silicone rubber, and another flexible insulating material may be chosen according to the environmental conditions in which the heating element

10 is intended to operate. Markings indicating the origin and / or specifications

of the heating element may be printed on the oversheath 18.

If the heating element 10 is to be used in a particularly corrosive environment or if additional mechanical protection is required, a further oversheath may be provided around the insulating oversheath 18. For example a braided tin plated copper wire sheath may be used.

The heating element 10 is adapted to be used as follows.

Adjacent ends of the two conductors 12, 12' are connected to a power supply, and an electrical current flows along and between the conductors 12, 12' via the resistance wires 16. As current flows the resistance wires 16 heat! up, and as the temperature of the resistance wires 16 increases, their resistance increases. Thus, as the element 10 heats up, the current flowing between the conductors 12, 12' decreases, thus decreasing the ohmic heating effect. Thus element 10 is thus self- regulating and cannot overheat. Eventually an equilibrium state is achieved in which energy loss through heat transfer to the surroundings equals that gained from the current flow.

The temperature attained by the element may be selected, for example by selecting an appropriate voltage of the power supply, a material for the resistance wires 16 with an appropriate resistance versus temperature curve, or an appropriate gauge for the resistance wires 16.

A heating element 10 according to the invention may be used to heat articles to temperatures of up to around 200 C with a current range of to 30 amps without significant deterioration of the element 10, and such an element 10 does not exhibit significant in-rush current.

The materials from which the heating element 10 is made are preferably selected so that the element 10 is flexible, and therefore can be wound around a pipe or vessel, or laid in coils against an article to be heated such as a road or roof. I

( Extremely long lengths of heating element 10 according to the invention can be formed by extrusion, and a user may simply cut an appropriate length from a roll of heating element 10 according to how much is required for a particular application. Although this is likely to result in a resistance wire 16 at one end of the length of heating element 10 being cut into two, this does not have a significant effect on the performance of the heating element 10, as the current may still pass along all the other complete resistance wires 16 within the length of heating element 10.

The features disclosed in the foregoing description, or the following

claims, or the accompanying drawings, expressed in their specific forms or in terms of a means for performing the disclosed function, or a method or process for attaining the disclosed result, as appropriate, may, separately, or in any combination of such features, be utilised for realising the invention in diverse forms thereof.

Claims

CLAIMS I

1. A heating element including first and second elongate, electrical conductors embedded in a core, the conductors being mutually spaced and generally parallel, characterized in that the core is electrically insulating, and I the element furler includes a resistance wire made from a conductive material with a positive temperature coefficient, the resistance wire extending between and being in electrical contact with the two conductors.

2. A heating element according to claim 1 wherein the resistance wire is metallic.;

3. A heating element according to claim 1 or 2 wherein the resistance wire is wound around the core in a generally helical arrangement.

4. A heating element according to any preceding claim wherein the resistance wire contacts the conductors by means of a first and second aperture in the core, the first aperture extending from a surface of the core to the first conductor, and the second extending from a surface of the core to the second conductor, the resistance wire contacting portions of the conductors exposed by the first and second apertures.

5. A heating element according to claim 4 wherein the first aperture is displaced along the length of the conductors with respect to the second aperture.

6. A heating element according to any preceding claim wherein the resistance wire is soldered to the conductors.

!

7. A heating element according to any one of claims 2 to 6 wherein the resistance wire may be made from a metal with a substantially linear characteristic resistance versus temperature graph.

8. A heating element according to claim 7 wherein the resistance wire is made from an iron-nickel alloy.

9. A heating element according to any preceding claim wherein the core is made from silicone rubber.

10. A heating element according to any preceding claim wherein the heating element includes a plurality of resistance wires connected in parallel between the two conductors, each resistance wire being wound around a different portion of the core.

11. A heating element according to claim 10 wherein each resistance wire has at least one point of contact with one of the conductors which is adjacent to a point of contact of another resistance wire with one of the conductors.

12. A heating element according to any one of claims 1 to 9 wherein the heating element includes a single resistance wire which is connected to each conductor at more than one point along the resistance wire.

13. A heating element substantially as hereinbefore described with reference to and as shown in the accompanying drawings.

14. Any novel feature or novel combination of features described herein and/or in the accompanying drawings.