GB2225691A

GB2225691A - Parallel circuit heating cable

Info

Publication number: GB2225691A
Application number: GB8828140A
Authority: GB
Inventors: Kenneth Jack Palmer
Original assignee: ELECTRIC SURFACE HEATING Ltd
Current assignee: ELECTRIC SURFACE HEATING Ltd
Priority date: 1988-12-02
Filing date: 1988-12-02
Publication date: 1990-06-06
Also published as: GB8828140D0

Abstract

A parallel circuit heating cable comprises at least two insulated conductors (2, 3) for connection to an electrical supply and a resistive heating element (4). The heating element (4) is a foil strip (10) wound round the conductors (2, 3) such that the heating element (4) makes contact with each conductor (2, 3) at spaced intervals to form separate heating zones. The strip may be soldered to the conductors. The cable can be wrapped around pipes carrying fluids or liquids to prevent them from freezing or solidifying. <IMAGE>

Description

IMPROVEMENTS IN ELECTRICAL HEATING CABLES This invention relates to parallel circuit electrical heating cable of the kind comprising at least two insulated conductors for connection to an electrical supply, and a resistive heating element wound round the conductors, the arrangement being such that the heating element makes contact with each conductor at spaced intervals, to form separate heating zones.

Heating cables of the kind set forth are well-known, and commonly have flexible conductors, so that the cable can be wrapped round pipes carrying fluids or liquids, to prevent them solidifying and freezing. Because the heating zones are supplied independently, the cable can be cut to any required length. Normally the resistive element is a thin round wire, which is easy to handle in manufacture, but has disadvantages in use. This is because the area of contact between the wire and a conductor is relatively small, which leads to high operating temperatures at the contact points, in turn leading to high thermal stresses in the wire and its surrounding insulation, and loss of contact between the wire and the conductor.

According to the present invention, in a parallel circuit electrical heating cable of the kind set forth, the resistive heating element comprises a foil strip.

This has the advantage of providing a greater area of contact between the heating element and a conductor, thus reducing the operating temperature and the thermal stresses, and improving the contact between the heating element and the conductor.

Conveniently each conductor has, at spaced intervals, portions exposed from the insulation, with which several turns of the foil strip are in contact.

Preferably the foil strip is soldered to the exposed portions; a high temperature solder being used. This improves the contact further, and is especially useful where the conductors are flexible.

Preferably the foil strip is flat, and bare. The cable may be completed by an outer insulating sheath.

An embodiment of a parallel circuit electrical heating cable in accordance with the present invention is illustrated in the accompanying drawings, in which Figure 1 is a perspective view, part cut away, of a heating cable; and Figure 2 is a cross-section along the lines 2-2 of Figure 1.

The parallel circuit flexible heating cable 1 shown in the drawings comprises a pair of insulated conductors 2, 3 adapted for connection to an electrical supply, such as mains voltage (not shown), and a resistive heating element 4 wound round the conductors 2, 3 and contacting them at spaced intervals.

Each conductor 2, 3 comprises a multi-filament flexible wire 5, and they are both embedded in a silicone rubber sheath 6 which insulates them from each other. At spaced intervals along the outer edge 8 of each conductor 2, 3 the insulating sheath 6 is removed to expose a portion 9 of the conductor 2, 3, with which the element 4 makes contact.

The element 4 is in the form of a bare, flat foil strip 10, approximately 0.5mm wide and 0.04mmthick, which is wound helically round the sheath 6 as a core.

Several turns of the strip 10 contact each exposed portion 9, and high temperature solder 11 attaches these turns to the exposed portion 9. The cable is completed by an outer insulating sheath 12 of silicone rubber.

In use, a voltage applied across the conductors 2, 3 is also applied to the foil strip 10 through the exposed portions 9, to provide the heating effect. In fact, separate heating zones are formed, each supplied independently by the conductors 2, 3.

This enables the cable 1 to be cut to any required length. This type of flexible cable is particularly useful for wrapping round pipes to heat them, to prevent fluid or liquid carried by the pipes from solidifying or freezing.

In use, the heat generated by the foil heating strip 10 also heats the surrounding insulating sheaths 6, 12. In comparison with a round heating wire, the area of contact between the strip 10 and the exposed portion 9 is relatively large, so that the operating temperature at the contact point is relatively lower. The thermal stresses on the strip 10 and the insulating sheaths 6, 12 are therefore also lower, and the tendency for the contact between the strip 10 and portion 9 to be lost is counteracted. The use of the foil strip 10 also means that the separation between adjacent turns of the strip 10 is more reliable.

In a zcdification, any other suitable electrical insulating material may be used instead of silicone rubber. The dimensions of the foil strip may also be chosen to suit the requirements of the cable. The width will normally be in the range 0.25mm to 0.75mm, and the thickness 0.03mm to 0.lmm.

Claims

1. A parallel circuit electrical heating cable of the kind set forth in which the resistive heating element comprises a foil strip.

2. A heating cable according to claim 1, in which each conductor has, at spaced intervals, portions exposed from the insulation, with which several turns of the foil strip are in contact.

3. A heating cable according to claim 1 or claim 2, in which the foil strip is soldered to the exposed portions using a high temperature solder.

4. A heating cable according to any previous claim, in which the foil strip is substantially flat.

5. A heating cable according to any previous claims, in which the foil strip is bare only at the point of contact with the conductor.

6. A heating cable according to any previous claim, in which the cable is covered by an outer insulating sheath.

7. A heating cable according to any previous claim, in which the width of the foil strip is in the range 0.25mm to 0.75mm, and the thickness of the foil strip is in the range 0.03mm to 0.lmm.

8. A parallel circuit electrical heating cable substantially as described herein with reference to and illustrated in the accompanying drawings.