GB2325604A - Electric heater support insulation - Google Patents

Abstract

An electric heater comprises a dish-like support (1) having a base (1A) and a surrounding side wall (1B). A base layer (2) of insulation material is provided in the support, at least one heating element (3) being supported on or adjacent to the base layer. A peripheral wall (7) of microporous thermal insulation material engages the interior of the surrounding side wall of the support and the base layer. A rebate (10) is provided in the peripheral wall such that an air gap (11) exists between the peripheral wall and the interior of the surrounding side wall of the support over a proportion of the height of the peripheral wall.

Description

Electric Heater This invention relates to an electric heater of the kind comprising a dish-like support, such as of metal, having a base and a surrounding side wall and a base layer of insulation material, such as microporous thermal insulation material provided in the support. At least one heating element, such as of wire, ribbon or lamp form, is supported on or adjacent to the base layer and a peripheral wall of insulation material engages the interior of the surrounding side wall of the support and the base layer.

Such a heater is well known to be used in cooking appliances and particularly with a top surface of the peripheral wall in contact with a sheet of material such as of glass-ceramic.

Various materials are known to be used for the peripheral wall of the heater. These include bound ceramic fibre, bound vermiculite and compacted microporous thermal insulation material.

A particular requirement for the peripheral wall is that it should function adequately as a thermal insulator to prevent the outside of the surrounding side wall of the support from reaching an excessively high temperature.

The most commonly used material over the years for the peripheral wall has been bound ceramic fibre material, comprising alumino-silicate fibres. This material has moderately good thermal insulation properties and on the basis thereof a well-defined standard wall thickness has been established which, for any specific size of dish-like support, results in a precisely specified heated area.

Recent trends have resulted in a move away from the use of ceramic fibre materials. An attractive alternative material for use for the peripheral wall has been found to be microporous thermal insulation material, particularly reinforced with glass-filaments instead of ceramic fibres. Such microporous insulation material for this application is described in EP-A0560479.

A particular advantage of microporous thermal insulation material is its exceptionally low thermal conductivity. It would therefore be possible to reduce the width of the peripheral wall compared with that of a wall made from ceramic fibre material while achieving the required maximum temperature for the outside of the side wall of the dish-like support. Although this would usefully save material of the peripheral wall, it would alter the heated area within the heater, which may be impermissible, or require new tooling for the dish-like support.

It is an object of the present invention to provide an electric heater having a peripheral wall of microporous thermal insulation material and in which the volume of material of the wall is reduced without changing the external dimensions of the wall.

The present invention provides an electric heater comprising: a dish-like support having a base and a surrounding side wall; a base layer of insulation material provided in the support; at least one heating element supported on or adjacent to the base layer; a peripheral wall of microporous thermal insulation material in engagement with the interior of the surrounding side wall of the support and with the base layer; and wherein a rebate is provided in the peripheral wall such that an air gap exists between the peripheral wall and the interior of the surrounding side wall of the support over a proportion of the height of the peripheral wall.

Preferably the peripheral wall engages the surrounding side wall of the support only at a top region of the peripheral wall.

The peripheral wall may have a top surface thereof arranged for engaging a sheet of material, such as glass-ceramic, for operation of the heater in a cooking appliance.

The top surface of the peripheral wall may be profiled such that it is higher at the centre than at the edges.

The base layer may comprise microporous thermal insulation material and may have a composition substantially the same as, or different from that of the peripheral wall.

The dish-like support may comprise a metal.

The invention is now described by way of example with reference to the accompanying drawings in which: Figure 1 is a plan view of an electric heater according to the invention; and Figure 2 is a cross-sectional view of the heater of Figure 1.

Referring to the drawings, an electric heater comprises a metal dish-like support 1 having a base 1A and surrounding side wall 1B and containing a base layer 2 of compacted microporous thermal and electrical insulation material of well known form and composition. Such base layer 2 may, for example, have a composition as described in GB-A-1580909 or alternatively as described in EP-A-0560479.

A heating element 3 is provided, supported on the base layer 2. The heating element 3 comprises a known form of corrugated ribbon, supported on edge and secured by partial embedment in the surface of the base layer 2. However the heating element 3 could be of any other form, such as wire, lamp, or other arrangements of ribbon. Multiple heating elements of the same type, or a combination of types, could alternatively be provided in the heater.

A terminal connector 4 is provided at the edge of the support 1 for electrically connecting the heating element 3 to an electrical supply for energisation.

A well known form of thermal cut-out device 5 extends across the heating element 3 and serves to switch off the heating element to prevent overheating when the heater is installed and operating, for example, in a cooking appliance behind a sheet 6 of a material such as glassceramic.

Against the surrounding side wall 1B of the metal dish-like support I is located a peripheral wall 7 of compacted microporous thermal insulation material, in the form of a ring. The peripheral wall 7 is preferably substantially free from ceramic fibre reinforcing material and preferably has a composition as described in EP-A-0560479 in which glass filaments such as of E-glass, are used as reinforcement.

The peripheral wall 7 has a top surface 8 which is arranged to contact the back of the sheet 6.

such as of glass-ceramic material. The top surface of the peripheral wall may be profiled, as shown by the dotted line 8A in Figure 2, such that it is higher at the centre than at the edges.

The peripheral wall 7 has a width W according to specific standard requirements for the heater and whereby a specified standard internal diameter of the heater is obtained.

Specifications of appliance manufacturers may define that the outer surface 9 of the surrounding side wall 1B ofthe dish-like support 1 must not exceed a predetermined maximum temperature, which may be of the order of 200"C. Microporous thermal insulation material has much better thermal insulation properties than the aluminosilicate ceramic fibre materials of the prior art. In theory therefore the peripheral wall of microporous thermal insulation material could be made of narrower width than the ceramic fibre wall of the prior art, while still maintaining the outer surface 9 of the surrounding wall I B of the dish-like support below the specified Ii t of temperature. However, although this would provide a useful reduction in material for the eripheral wall, the inside diameter of the heater would not comply with the specified standard.

This problem is overcome according to the invention by providing a rebate 10 in the peripheral wall 7 such that an air gap 11 exists between the peripheral wall 7 and the interior of the surrounding side wall 1B of the dish-like support. The air gap II exists over a proportion of the height of the peripheral wall 7 and such that the peripheral wall engages the interior of the surrounding side wall IB of the support only at a top region of the peripheral wall. A region 12 of reduced width of the bottom of the peripheral wall 7 engages the surface of the base layer 2 of insulation material.

The peripheral wall 7 is suitably retained in position in the heater by frictional engagement between the top region thereof and the inside surface of the surrounding side wall 1B of the support I.

By means of the invention, a useful reduction in material of the peripheral wall is achieved while maintaining the external dimensions of the wall. Sufficient material remains for support of the wall and to ensure that the temperature of the outside surface 9 of the surrounding wall 1B of the metal dish-like support does not exceed the maximum specified therefor. By way of example, the material of the peripheral wall can be reduced by one-sixth or more compared with that of a wall without the rebate 9.

Claims (9)

Claims

1. An electric heater comprising: a dish-like support having a base and a surrounding side wall; a base l*er of insulation material provided in the support; at least one heating element supported on or adjacent to the base layer; a peripheral wall of microporous thermal insulation material in engagement with the interior of the surrounding side wall of the support and with the base layer; and wherein a rebate is provided in the peripheral wall such that an air gap exists between the peripheral wall and the interior of the surrounding side wall of the support over a proportion of the height of the peripheral wall.

2. A heater according to claim 1, in which the peripheral wall engages the surrounding side wall of the support only at a top region of the peripheral wall.

3. A heater according to claim 1 or 2, in which the peripheral wall has a top surface thereof arranged for engaging a sheet of material, for operation of the heater in a cooking appliance.

4. A heater according to claim 3, in which the sheet of material comprises glass-ceramic.

5. A heater according to claim 3 or 4, in which the top surface of the peripheral wall is profiled such that it is higher at the centre than at the edges.

6. A heater according to any one of the preceding claims, in which the base layer comprises microporous thermal insulation material.

7. A heater according to claim 6, in which the base layer has a composition the same as or different from, that of the peripheral wall.

8. A heater according to any one of the preceding claims, in which the dish-like support comprises a metal.

9. An electric heater constructed and arranged substantially as hereinbefore described with reference to the accompanying drawings.