GB2121660A

GB2121660A - Electric heating element

Info

Publication number: GB2121660A
Application number: GB08315317A
Authority: GB
Inventors: Brian Carter
Original assignee: AIRELEC IND
Current assignee: AIRELEC IND
Priority date: 1982-06-04
Filing date: 1983-06-03
Publication date: 1983-12-21
Also published as: FR2528263A1; GB8315317D0; DE3320112A1

Abstract

An electric heating element comprises a helix of resistance-wire (1) supported along its length by spaced elongate support members (7, 8, 9) which space and support the helix at each turn. The support members may be flat strips with perforations through which the helix is wound. Alternatively the support members may be twisted strips of heat-resistant material. <IMAGE>

Description

SPECIFICATION Electric heating element The invention relates to an electric heating element, particularly for convector heaters. In order to heat air efficiency by convection it is important to ensure that the heating surface is distributed and offers little resistance to the flow of air. Also, of course, the resistance-wire must be electrically insulated from its surroundings.

A conventional form of heating element consists of a resistance wire supported concentrically within a metal sleeve by a jacket of electrically insulating particles, such as magnesium oxide powder. To ensure efficient heat transfer, fins are usually attached to the element. Such an arrangement is expensive to manufacture and it is difficult to provide a sufficiently unobstructed through-flow of air.

Another form of heating element consists of a helically wound resistance-wire supported to allow air to flow directly past it. However, such a helix of thin wire is difficult to support over its length in an insulated manner since it is liable to sagging and vibration. It is an advantage to use a long length of resistancewire with a consequently short pitch of the helix. However, there is then the risk of adjacent turns of the helix touching each other and short-circuiting. The nature of the support arrangement, places resitrictions on the rigidity of the helix and on the diameter of the resistance-wire and pitch of the helix.

An object of the present invention is to provide an improved support arrangement for a helical resistance-wire heating element.

According to the invention there is provided an electric heating element which comprises a helix of resistance-wire, and at least two elongate support members running the length of the helix substantially along respective generators thereof, the support members being electrically insulating and heat-resistant and being arranged to engage and retain the resistancewire at every turn of the helix.

There are two alternate preferred forms for the support members. In the first preferred embodiment of thetnvention the support members comprise flat insulating strips with spaced perforations and the helix of resistance-wire is threaded through the perforations. Again, there may be only two support members, but preferably there are three. The threading of the resistance-wire may be effected in a manner similar to the technique of spiral-binding note books.

In the second preferred form each support member comprises a pair of strings, and the string are twisted so as to hold the turns of the resistance-wire within the twists. Conveniently, the strings cross over once only between adjacent turns of the helix, but alternatively adjacent turns of the helix may be separated by more than one twist.

With two, but preferably three support members of this kind it is found that a rigid support structure can be achieved and the turns of the helix are spaced to allow efficient passage of air without there being a risk of adjacent turns of the helix touching. In particular it is found that if the pairs of strings are twisted in the same sense as each other, and then by twisting the whole assembly (helix and support strings) further in the same sense, the strings are tightened and the whole assembly becomes suprisingly rigid. This will result in the support strings skewing around the helical assembly and it is to be understood that such an arrangement lies within the scope of the present invention.

The invention will further be described with reference to the accompanying drawings, of which: Figure 1 is a perspective view of a heating element in accordance with one embodiment of the invention; Figure 2 is a drawing showing part of the detail of the Fig. 1 embodiment; Figure 3 is a perspective view of a heating element in accordance with another embodiment of the invention, and Figure 4 is a drawing showing part of the detail of the Fig.3 embodiment.

Referring to Figs. 1 and 2, a first embodiment of the invention comprises a helix 1 of resistance-wire with three spaced support members 7 to 9. In this embodiment each support member comprises a strip of insulating material, for example mica, which has spaced perforations 10 (Fig. 2). The helix 1 is wound through the perforations, thereby allowing the support members to support and space the turns of the helix.

Winding is effected by stacking the three strips one on top of another with the holes aligned, and then feeding the resistance-wire through the holes with a machine of the kind used for binding leaves of paper in the making of spiral-bound note-books. When the resistance-wire has been fully wound, the strips are separated 60 apart for mounting.

Referring to Figs. 3 and 4 there is again shown a heating element comprising a helix 1 of electrical resistance-wire. Three support members 2, 3 and 4 are spaced around the circumference of the helix. As can be seen from Fig. 4 each support element is constituted by a pair of heat-resistant and electrically insulating strings 5, 6. The strings twisted about the turns of the helix, there being a twist between each adjacent pair of turns.

It is to be noted that all the support members are twisted in the same sense as each other and in the same sense as the helix; in this case clockwise progressing from left to right. Finally, the whole assembly comprising helix 1 and support elements 2 to 4, is twisted in the same sense as the helix and strings are twisted, thus tightening the turns of the strings and rendering the assembly rigid. This results in the support members being skewed around the helix. In this way a substantial length of the helix can be supported without sagging. Clearly, for a very long helix, one or more intermediate supports may be used.

Claims

1. An electric heating element which comprises a helix of resistance-wire, and at least two elongate support members running the length of the helix substantially along respective generators thereof, the support members being electrically insulating and heat-resistant and being arranged to engage and retain the resistance-wire at every turn of the helix.

2. An electric heating element as claimed in claim 1 wherein the support members comprise flat insulating strips with spaced perforations and the helix of resistance-wire is threaded through the perforations.

3. An electric heating element as claimed in claim 2 wherein there are three support members.

4. A method of making an electric heating element as claimed in claim 2 or claim 3 which consists in stacking the strips with their perforations aligned, winding the helical resistance-wire through the perforations with a machine of the kind used for binding leaves of paper to make spiral-bound note-books, and separating the strips to space them aprt around the helix.

5. An electric heating element as claimed in claim 1 wherein each support member comprises a pair of strings, and the strings are twisted so as to hold the turns of the resistance-wire within the twists.

6. An electric heating element as claimed in claim 5 wherein the pairs of strings are twisted in the same sense as each other, and then by twisting the whole assembly (helix and support strings) further in the same sense, the strings are tightened.

7. An electric heating element substantially as hereinbefore described with reference to Figs. 1 and 2 of the accompanying drawings.

8. An electric heating element substantially as hereinbefore described with reference to Figs. 3 and 4 of the accompanying drawings.