GB2098438A - Electrical heating tapes - Google Patents

Abstract

An electrical resistance heating tape comprising, say, two busbars 1 and 2 and a series of heating elements 4A etc. connected in parallel with each other across the busbars incorporates thermal switching means 7 controlling individual heating elements. The means 7 may, for example, consist of reed switches, thermistors or PTC materials. <IMAGE>

Description

SPECIFICATION Electrical heating tapes This invention relates to electrical heating tapes of the form comprising two or more busbars and a series of individual heating elements connected in parallel to the busbars.

Such tapes are well known in themselves and are usually used in conjunction with ancilliary control equipment including temperature sensing equipment which monitors the temperature at a particular location of the item being heated, for example a pipe or other vessel conveying or containing liquid, for switching the current supply off and on between predetermined upper and lower temperatures.

Existing heating tapes of the general form specified above consume full power when only a particular critical section of the complete installation requires heat to maintain a required minimum temperature, leading to excessive energy loss.

Also maintenance problems arise for example when servicing of valves involves local removal of heat insulation. The resultant heat loss may initiate actuation of the thermal control device, typically a thermo-couple, which is then manually overridden, with the result that the remainder of the system overheats and it is not uncommon for cancellation of the override mechanism to be overlooked when the installation has been serviced, resulting in the current being left on continuously, again with the result of overheating and avoidable energy loss.

The present invention aims at obviating or reducing these disadvantages and accordingly provides an electrical resistance heating tape comprising at least two busbars and a series of resistance heating elements connected across the busbars in parallel with each other, in which at least one individual element is controlled by thermally responsive switching means incorporated in the tape.

Conveniently the or each thermal switching means is connected between the element which it controls and a busbar, but other arrangements are possible and some are described below.

The thermal switching means employed may take various forms, such as thermal magnetic reed switches, electronic switching devices such as thermistors, or PTC materials, i.e. materials having a positive temperature coefficient such that their resistance increases with their temperature.

Various materials of this type are well known in the art.

Some specific forms of heating tape, all in accordance with the invention, will now be described, by way of example, with reference to the accompanying drawings, in which: Figures 1, 2 and 3 are scrap views of three forms of heating tape, partially broken away in the interests of clarity; Figure 4 is a similar scrap view of another tape, on a slightly larger scale; Figure 5 is a section on the line V-V of Figure 4; and Figures 1 A, 2A, 3A and 4A are circuit diagrams for the tapes of Figures 1, 2, 3 and 4 respectively.

Each of the illustrated tapes comprises a pair of spaced, parallel current supply busbars 1 and 2 held in an insulating sheath 3 of plastics or elastomer material conveniently extruded over the busbars. Individual resistance heating element wires 4A, 4B, are wound over the sheath 3 in spiral fashion, the sheath being locally cut away at spaced locations 6 to expose the busbars 1 and 2.

Thermal switches 7 are incorporated at intervals, conveniently in the manner best illustrated in Figures 4 and 5. Each switch 7 which may take various forms as described above, is housed in a recess 8 cut in the sheath between and spaced from the busbars and has each of its connecting tails 9 electrically connected, e.g. by soldering, either to an element or to a busbar. As can be seen from Figure 5, the switch is accommodated within the thickness of the sheath.

Over the whole assembly as so far detailed, there is provided an insulating outer cover 11 of plastics material which is conveniently applied by extrusion.

Optionally, but not illustrated, the assembly may be provided also with an outer mechanical protective cover, for example of braided metal.

Some possible electrical arrangements are illustrated in Figures 1 A to 4A, and correspond with the tapes shown in Figures 1 to 4, respectively.

In Figures 1 and 1A, thermal switches 7 are connected intermediate the ends of individual heating element wires 4A, 4B, 4D, each of which is connected at its ends to the respective busbars 1 and 2.

One wire 4C, is shown with a dotted line section to indicate that the element may either be continuous or may, like the others, incorporate a switch 7.

In Figures 2 and 2A, each switch 7 is connected on the one hand to the busbar and on the other hand to its element wire, whose other end is connected to the opposite busbar. The switches 7 are connected alternately to busbars 1 and 2 respectively.

The arrangement of Figures 3 and 3A differs from that of 2 and 2A only in that all the switches are connected directly to one busbar 1, and the other ends of the elements to the other busbar 2.

In Figures 4 and 4A, all the switches 7 are connected to the same busbar 1 and intermediate the ends of element wires 4A, 4B, etc. whose ends are all connected to the other busbar 2.

As indicated above, it is not essential for every resistance element to be controlled by a thermal switching means. Obviously those which are not will take power and heat up continuously when the busbars are energised. According to the requirements of any installation, the switches may be incorporated in every element, alternate elements or every third element etc. Nor is it essential for the switching means to be evenly distributed along the series of elements, a wide variety of arrangements being possible.

It will also be apparent to those skilled in the art that the invention may be applied to other forms of heating tapes, for example having three or more busbars for use with a multiple phase supply.

It will also be appreciated that the tapes can conveniently be manufactured in long lengths which may be cut up into individual lengths according to individual installation requirements, or as prepared modular lengths each complete with end terminations applied in the factory rather than on-site. In either case, the individual switches 7 will be preset to cut in and cut over a pre'determined temperature range, according to installation requirements and in use the main supply to the busbars can be controlled by a thermal overload to close down the supply complete if a maximum permitted temperature is sensed in a given critical area or areas.

In an extreme case, a tape may comprise a member of resistance heating elements, of which only one, to be positioned in a critical area is controlled by a thermally responsive switch means, the remaining element or elements remaining energised except when the power supply to the busbars is interrupted.

It will be appreciated that where a number of individual elements are controlled by respective thermally responsive means, different forms of such means may be provided for different elements.

The thermally responsive means may be of a construction to switch off and on at different temperatures to maintain the temperature of the respective sections of the tape with predetermined temperature ranges.

It would also be possible to employ N.T.C.

devices, i.e. devices having a negative temperature co-efficient which operate to reduce their internal resistance as they become hotter, or conversely to increase their resistance as their temperature drops.

These and many other variations and modifications will be possible within the scope of the invention.

Claims (5)

1. An electrical resistance heating tape comprising at least two busbars and a series of resistance heating elements connected across the busbars in parallel with each other, in which at least one individual element is controlled by a thermallly responsive switching means incorporated in the tape.

2. A heating tape according to claim 1, wherein the or each switching means is connected between the element which it controls and one of the busbars.

3. A heating tape according to claim 1, wherein individual elements are connected at opposite ends to the same busbar, and the switching means is connected between an intermediate point on the element and the opposite busbar.

4. A heating tape according to any preceding claim, wherein the busbars are held in spaced parallel relation by a sheath of insulating material, the thermal switching means being accommodated in recesses in the sheath, and the resistance heating elements being wound over the sheath, which is locally cut away to expose portions of the busbar or busbars to which the elements are connected, and the elements being covered by an outer continuous cover of insulating material.

5. An electrical resistance heating tape, substantially as herein described with reference to the accompanying drawings.