GB2275404A - Supporting radiant electrical heating element - Google Patents

Abstract

A radiant electric heater, particularly for a glass ceramic cooking appliance, comprises a dish (1) supporting a base layer (2) of microporous thermal and electrical insulation material. The layer (2) has at least one groove (9) formed therein into which is located edgewise an elongate electrically conductive strip (5) to serve as a heating element, the groove (9) being such that an interference fit exists between the strip (5) and walls of the groove (9). No mounting tabs staples or other additional securing means are required. The strip remains securely located during subsequent operation of the heater. <IMAGE>

Description

Radiant Electric Heater and Method This invention relates to radiant electric heaters particularly for use with glass-ceramic smooth top cookers, and a method of manufacture.

Radiant electric heaters are known in which an element of coiled bare electric resistance wire is supported on, and secured by staples to, a layer of microporous thermal and electrical insulating material compacted in a metal support dish. Such heaters are described, for example, in GB 1 580 909 and are incorporated in glass-ceramic smooth top cookers. The microporous insulation typically comprises a highly-dispersed metal oxide powder, such as silica aerogel or pyrogenic (fumed) silica, mixed with ceramic fibre reinforcement, titanium dioxide opacifier and a small quantity of alumina powder to resist shrinkage. Such insulation material is described in GB 1 580 909.

Radiant electric heaters have also been proposed in which, instead of an element of coiled resistance wire, an element comprising an elongate electrically conductive strip of a metal or metal alloy is provided, the element being supported on edge on an insulating base.

Arrangements of this kind are described, for example, in DE-OS 26 30 466, US 3 612 829 and US 4 161 648. In DE-OS-26 30 466, the conductive strip element is in the form of a spiral and is loose fitted in a pre-formed spiral groove in a rigid base of fire-resistant mortar.

In US 3 612 829, a convoluted conductive strip element in the form of a spiral is located in recesses pre-formed in the surface of a cast or moulded fibrous ceramic refractory material.

Staples are used to secure the strip element to the supporting base.

In US 4 161 648, a convoluted strip element of spiral form is provided with integral downwardly-extending mounting tabs which penetrate an electrically insulating sheet of high-temperature-withstanding board material and in the case of thin material may be bent over at the back of the material.

The board-like insulating sheet with the element thereon is then located on top of a layer of microporous thermal insulation material in a supporting dish.

In the present invention a radiant heater is provided in which an elongate electrically conductive strip heater element is secured directly to a base of microporous thermal and electrical insulation material without the need for mounting tabs or staples or any other additional securing means.

Accordingly, the present invention provides a radiant electric heater comprising a base of microporous thermal and electrical insulation material having at least one groove formed therein into which is located edgewise an elongate electrically conductive strip to serve as a heating element, said groove being such that an interference fit exists between said strip and walls of said groove.

The present invention also provides a method of manufacturing a radiant electric heater comprising: providing a base of microporous thermal and electrical insulation material; providing an elongate electrically conductive strip to serve as a heating element; forming at least one groove in said base of microporous insulation material; and locating said elongate electrically conductive strip edgewise into said groove, said groove being formed such that an interference fit is effected between said strip and walls of said groove when said strip is located.

Surprisingly, in view of the nature of the microporous insulation material, the strip remains securely located during subsequent operation of the heater.

The groove is suitably of a depth such that the strip protrudes to a desired extent from said base.

The said walls of said groove may be substantially parallel to each other or may, if required, be tapered such that said groove has a substantially V-shaped cross-section.

Preferably the said electrically conductive strip is of sinuous (also known as serpentine or convoluted) form along its length.

The said base of microporous insulation material is suitably provided as a compacted layer inside a supporting dish, suitably of metal.

The said base of microporous insulation material preferably has a surface of substantially planar form in which the said groove is provided.

The said strip of electrically conductive material suitably comprises a metal or a metal alloy, such as an iron-chromium-aluminium alloy.

Suitable microporous thermal and electrical insulation materials are well-known in the art, for example as described in GB 1 580 909, a typical composition being: Pyrogenic silica 49 to 97 % by weight Ceramic fibre reinforcement 0.5 to 20 % by weight Opacifier 2 to 50 % by weight Alumina 0.5 to 12 % by weight The invention is now described by way of example with reference to the accompanying drawings in which: Figure 1 represents a perspective view of a heating element comprising an electrically conductive strip, for use in a radiant electric heater according to the invention.

Figure 2 represents a plan view of a base of a radiant electric heater according to the invention, for receiving the heating element of Figure 1.

Figure 3 represents a plan view of a radiant electric heater according to the invention, comprising the components of Figures 1 and 2.

Figure 4 represents a cross-sectional view of the radiant electric heater of Figure 3.

A radiant heater is constructed comprising a metal dish 1 containing a base layer 2 of compacted microporous thermal and electrical insulation material, having a substantially planar surface and having a composition as described, for example in GB 1 580 909.

A heating element 4 is provided from an elongate strip 5 of a metal or metal alloy, such as an iron-chromium-aluminium alloy, having a thickness of, for example, 0.05 to 0.2 mm and a height h of, for example, 3 to 6 mm. The strip 5 itself is provided of sinuous form along its length (sometimes also known as serpentine or convoluted form) and is bent into a desired shape for the heating element as shown in Figure 1, using techniques well known in the art.

It should be noted, however, that the dimensions of thickness of the strip quoted above are for the strip before making into sinuous form. The surface of the base 2 of microporous insulation material is provided with grooves 9 in a pattem corresponding to the shape of the heating element 4. Such grooves 9 are suitably formed either by means of an appropriate moulding tool during compacting of the microporous insulation material into the dish 1 to form the base 2, or may be machined into the surface of the base material after compaction. The sides of the grooves can be made substantially parallel to one another, in which case the width of the grooves should be substantially the. same as the overall width of the sinuous strip 5.Alternatively, the wails of the grooves may be tapered so that the grooves have effectively a V-shaped cross-section, the width of the open top of the grooves then being made slightly greater than the overall thickness of the sinuous strip 5.

The element 4 is then located on the base 2 and the heating element strip 5 pushed edgewise into the grooves 9, such that the strip 5 forms an interference fit in the grooves 9 and is secured therein. The depth of the grooves 9 is selected such that, when inserted therein, the heating element strip 5 protrudes from the base 2, e.g. by at least 50 per cent of the height h of the strip 5.

Against the side of the dish 1 is located a peripheral wall 3 of thermal insulation material, such as a ceramic fibre material made from aluminosilicate fibres or alternatively microporous insulation material.

A terminal connector 6 is provided for electrically connecting the heating element 4 to an electrical supply for operation thereof.

A well-known form of thermal cut-out device 7 is provided, extending over the heating element 4, to switch off the heating element in the event of over-heating when the heater is installed and operating in a cooking appliance having a glass-ceramic cooking surface.

Claims (17)

1. A radiant electric heater comprising a base of microporous thermal and electrical insulation material having at least one groove formed therein into which is located edgewise an elongate electrically conductive strip to serve as a heating element, said groove being such that an interference fit exists between said strip and walls of said groove.

2. A radiant electric heater according to Claim 1, in which said groove is of a depth such that said strip protrudes to a desired extent from said base.

3. A radiant electric heater according to Claim 1 or 2, in which the said electrically conductive strip is of sinuous form along its length.

4. A radiant electric heater according to any preceding Claim, in which the said base of microporous insulation material is provided as a compacted layer inside a supporting dish.

5. A radiant electric heater according to any preceding Claim, in which the said base of microporous insulation material has a surface of substantially planar form in which the said groove is provided.

6. A radiant electric heater according to any preceding Claim, in which the said strip of electrically conductive material comprises a metal, or a metal alloy.

7. A radiant electric heater according to Claim 6, in which said metal alloy comprises an iron-chromium-aluminium alloy.

8. A radiant electric heater constructed and arranged substantially as hereinbefore described with reference to the accompanying drawings.

9. A method of manufacturing a radiant electric heater comprising: providing a base of microporous thermal and electrical insulation material; providing an elongate electrically conductive strip to serve as a heating element; forming at least one groove in said base of microporous insulation material; and locating said elongate electrically conductive strip edgewise into said groove, said groove being formed such that an interference fit is effected between said strip and walls of said groove when said strip is located.

10. A method according to Claim 9, in which said groove is formed of a depth such that said strip protrudes to a desired extent from said base.

11.A method according to Claim 9 or 10, in which the said electrically conductive strip is provided of sinuous form along its length.

12. A method according to any of Claims 9 to 11, in which the said base of microporous insulation material is provided as a compacted layer inside a supporting dish.

13. A method according to any of Claims 9 to 12 in which the said base of microporous insulation material is formed with a surface of substantially planar form in which the said groove is provided.

14. A method according to any of Claims 9 to 13, in which the said strip of electrically conductive material comprises a metal, or a metal alloy.

15. A method according to Claim 14, in which said metal alloy comprises an iron-chromium aluminium alloy.

16. A method of manufacturing a radiant electric heater substantially as hereinbefore described with reference to the accompanying drawings.

17. A radiant electric heater whenever produced by the method according to any of Claims 9 to 16.