GB2207590A

GB2207590A - Insulating coiled resistance heating wire

Info

Publication number: GB2207590A
Application number: GB08816468A
Authority: GB
Inventors: Derek Edward Morgan; James David Joseph Jackson
Original assignee: Micropore International Ltd
Current assignee: Micropore International Ltd
Priority date: 1987-07-25
Filing date: 1988-07-11
Publication date: 1989-02-01
Anticipated expiration: 2008-07-11
Also published as: JPS6454689A; GB8717668D0; DE3875855D1; EP0303354B1; US4987675A; GB2207590B; DE3875855T2; EP0303354A1; ATE82459T1; GB8816468D0

Abstract

A coiled heating element is manufactured by providing a coiled element (1) of bare resistance wire (2) in which the coil pitch is not greater than twice the cross-sectional dimension of the wire. The element (1) is stretched by an amount permitting recovery to its original configuration and an electrically insulating particulate material (3) is applied to the stretched element. The stretched element is then allowed to return towards its original configuration thus trapping particulate material between adjacent coils of the element and the element is heated so as to oxidise the surface of the wire.

Description

1 ll i 2207590 COILED HEATING ELEMENTS The present invention relates to

the manufacture of coiled heating elements and more particularly but not exclusively relates to the manufacture of coiled heating elements in which adjacent coils of the heating element are in contact with each other or are at least very close to each other. The invention also relates to electric heaters incorporating such heating elements.

Coiled heating elements (so-called close coiled heating elements) in which adjacent coils ofthe heating element are in contact with each other or are very close to each other are known. It is also known that electrical insulation between, adjacent coils is important because inadequate insulation can lead to short circuits and uneven heating of the element. When using such elements in heaters for glass ceramic top cookers any short circuited regions can be seen through the glass ceramic cooking surface as dark areas which may be several centimetres long. This makes such close coiled heating elements unsightly and inefficient and can reduce their working life.

It is usual to insulate adjacent coils of the heating element by oxidising the wire to produce a thin oxide scale on the surface thereof. However. oxidising the wire presents a number of problems because the wire requires to be heated to a temperature of the order of 1100 to 12000C in order to produce sufficient oxide scale in a reasonably short time. If the wire is oxidised with adjacent coils of the heating element in contact with each other welding can occur at the points of contact so that short circuited regions can still arise. if the heating element is stretched to separate adjacent coils, the heat treatment causes the wire to be annealed and to lose its previous resilience. This would require a further manufacturing stage to return the heating element substantially to its original configuration, assuming such an operation is even feasible.

It is an object of the present invention to provide an improved method of manufacturing a heating element in which the adjacent coils are at least very close to each other.

According to the present invention there is provided a method of manufacturing a coiled heating elementi which method comprises the steps of:

af providing a coiled element of bare resistance wire in which the coil pitch is not greater than twice the cross-sectional dimension of the wire; 1 stretching the element by an amount permitting recovery to its original configuration; applying to the element particulate material; an electrically insulating allowing the stretched element to return towards its original configuration thereby trapping particulate material between adjacent coils of the element; and heating the element so as to oxidise the surface of the wire.

The particulate material may be an oxide such as aluminium oxide or magnesium oxide or may be a refractory clay such as china clay.

The particle size may be less than the cross-sectional dimension of the wire and is preferably less than one tenth 15 of the cross-sectional dimension of the wire.

The particle size of the particulate material may be from 30 to 50 microns.

The particulate material may be applied to the element by dipping or spraying. The particulate material may be in the form of a dry powder or a slurry.

The element is preferably heated to a temperature from 1100 to 12000C. The heating may be effected by placing the element in a furnace or by passing an electric current through the element.

For a better understanding of the present invention and to show more clearly how it may be carried into effect reference will now be made, by way of example, to the accompanying drawings in which:

Figure 1 shows a part of a coiled heating element as produced by a coil winding machine; Figure 2 shows 'the coiled heating element stretched so as to separate adjacent coils; Figure 3 shows the heating element relaxed but with particulate material trapped between adjacent coils; Figure 4 is a partial cross-sectional view of a beating element according to the present invention stapled to a base; and T; Figure 5 is a partial plan view corresponding to the view shown in Figure 4.

When a coiled heating element 1 of bare resistance wire 2 is wound on a coil winding machine, adjacent coils of the element a re close to each other as shown in Figure 1 and are often in contact. It has previously been the practice to stretch such elements so that adjacent coils are well spaced and to secure the stretched elements to a base of thermally and electrically insulating material, for example by inserting staples between adjacent coils.

In accordance with the present invention the coiled heating element 1 is stretched as shown in Figure 2 so as to separate adjacent coils of the element. It is important not to stretch the element beyond the limits of its natural resilience so that the coils are able to return substantially to their original configuration.

An electrically insulating particulate material is then applied to the element. for example by dipping the element 1 into a dry powder or an aqueous slurry. However, other methods such as spraying the material, for example by electrostatic spraying. can be used. The particulate material can be any powder which is electrically insulating at elevated temperatures. Suitable materials include oxides such as aluminium oxide and magnesium oxide and refractory clays such as china clay. We have f ound that some bonding may occur between the wire and the particulate material. for example in the case of magnesium oxide powder and iron-chromium-aluminium resistance wire. This is advantageous in that it provides additional insulating characteristics.

The particle size of the particulate material must not be too large so as to avoid excessive stretching of the element. The particles should generally be smaller than the wire diameter and preferably should not be larger one tenth of the wire diameter.

than The particle size for commonly used wire diameters is typically from 30 - to 50 microns.

The degree of stretch required in the element before the application of the particulate material will depend on the particular dimensions of the wire. the coil and the particulate material. However, the degree of stretch can readily be established in each case by simple tests.

The stretched element is allowed to return substantially to its original configuration thereby trapping particulate material 3 between adjacent coils of the element. We have found that sufficient particulate material becomes trapped a ' k,' between adjacent coils to provide adequate electrical insulation between the coils to prevent short circuits. It is not necessary to ensure complete coverage of the wire with the particulate material because the element is heated so as to oxidise the surface of the wire and any exposed portions will therefore form an electrically insulating layer. The heating of the element to oxidise the surface of the wire is preferably carried out at a temperature of about 1100 to 12000C. Heating can be-effected for example by placing the element in a furnace or by passing an electric current through the element. This results in the heating element shown in Figure 3.

We have found that the method of the present invention reliably ensures the elimination of short circuits between adjacent coils of the heating element. The heating element can be incorporated into a heater for a glass ceramic top cooker by disposing the element on a base layer of thermal and electrical insulating material,, such as a microporous insulating material. The general arrangement may be as shown in GB-A-2 087 698. However. in the arrangement shown therein the element is secured by staples located between individual coils of the element. The substitution of a heating element made according to the present invention enables the use of staples which straddle the top of the heating element. as shown in Figs 4 and 5, without risk of j deformation such as could occur with more widely spaced coils. This is because the adjacent coils and insulating powder of the beating element made by the method of the pre-sent invention are self-supporting. It is therefore immaterial at what point along the heating element the staple is applied. and in particular there is no need to aim the staples between adjacent coils of the heating element as shown in GB-A-2 087 698. This positional flexibility in turn facilitates automation of the stapling operation. As an alternative to the use of staples# the beating element can be employed in conjunction with the technique of gluing coils for example to stakes as described in GB-A- 2 197 169.

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Claims

CLAIMS:

1. A method of manufacturing a coiled heating element. which method comprises the steps of:

providing a coiled element of bare resistance wire in which the coil pitch is not greater than twice the cross-sectional dimension of the wire; stretching the element by an amount permitting recovery to its original configuration; applying to the elen, ent particulate material; an electrically insulating allowing the stretched element to return towards its original configuration thereby trapping particulate material between adjacent coils of the element; and heating the element so as to oxidise the surface of the 15 wire.

2. A method according to claim 1, wherein the particulate material is an oxide such as aluminium oxide or magnesium oxide or a refractory clay such as china clay.

3. A method according to claim 1 or 2, wherein the particle size is less than the cross-sectional dimension of the wire.

4. A method according to claim 3, wherein the particle size is less than one tenth of the cross-sectional dimension of the wire.

5. A method according to any preceding claim, wherein the particle size of the particulate material is from 30 to microns.

6. A method according to any preceding claim, wherein the particulate material is applied to the element by dipping or spraying.

7. A method according to claim 6, wherein the particulate material is in the form of a dry powder or a slurry.

8. A method according to any preceding claimi wherein the element is heated to a temperature of 1100 to 12000C.

9 9. A method according to any preceding claim. wherein heating is effected by placing the element in a furnace or by passing an electric current through the element.

1 1

10. A method of manufacturing a coiled heating element according to claim 1 and substantially as hereinbefore described with reference to the accompanying drawings.

11. A coiled heating element made by the method 05 according to any one of the preceding claims.

12. An electric heater incorporating a coiled heating element as claimed in claim 11 disposed on a base layer of thermal and electrical insulating material.

Published 1988 at The Patent Office, State Rouse. W71 High Holborn, London WC1R 4TP. Further r.oDies may be obtained from T1-,e Pawnt Office, Sales Branch. St MarT Cray. Orping13n. Kent BR5 3RD. Printed by Multiplex techrAor.es h'. St Mary C:,ay. Kent Con 1 87