GB2324693A - Vapour barrier in a radiant electric heater - Google Patents

Abstract

An electric heater particularly of radiant form, has a bass (2, 2A) of microporous insulation material having supported thereon or adjacent thereto at least one heating element (4). A vapour barrier (6, 6A), particularly a barrier to water vapour, is provided between the at least one heating element and the base. The vapour barrier (6, 6A) is provided in sheet form, such as of mica, or in the form of one or more films, layers or coatings on the surface of the base. Alternatively the vapour barrier is provided by impregnating or enriching material of a surface of the base (2, 2A) with a suitable material.

Description

Electric Heater This invention relates to electric heaters, such as radiant or infra-red heaters in which one or more heating elements is or are supported on or adjacent to a base of thermal and/or electrical insulation material.

Such heaters are well known, particularly in cooking appliances, in which one or more heating elements such as of coiled wire form, or of ribbon form, or of lamp form, is or are supported on or adjacent to a base of insulation material which may be provided in a support dish such as of metal. It is particularly well known and advantageous to use, as a base, compacted particulate microporous thermal insulation material.

Heaters of this kind have generally been used beneath a sheet of glass ceramic material in a smooth top cooking appliance or behind such a sheet in oven or grill applications.

It is well known that cooking operations are commonly accompanied by issuance of steam and other vapours and spillages and that splattering of liquids and food particulates occur on and in cooking appliances. The sheet of glass-ceramic material normally prevents contact between the heater and such vapours, liquids and particulates. However such glass-ceramic material is expensive and also reduces the thermal performance of the heater.

There are applications for heaters, such as in ovens, grills, deep fat fryers, where it would be desirable to dispense with the use of a glass-ceramic sheet. However it is known that microporous insulation materials are seriously degraded when exposed to steam, water and other liquids or vapours and can also be damaged by contact with some particulate materials.

It is an object of the present invention to provide an electric heater in which this problem is eliminated or reduced.

The present invention provides an electric heater comprising a base of insulation material having supported thereon or adjacent thereto at least one heating element, a vapour barrier being provided between the at least one heating element and the base The base may comprise microporous thermal insulation material.

The vapour barrier may comprise one or more sheets, films, layers or coatings of one or more materials, on or adjacent to the base.

The one or more sheets may, for example, be selected from; mica, ceramic materials, woven glass or ceramic fibre fabric materials.

The one or more films, layers or coatings may, for example, comprise vermiculite sol, or glassceramic sol, or vapour deposited or flame-sprayed ceramic material.

The vapour barrier may alternatively comprise means impregnating or enriching material of a surface of the base adjacent to the at least one heating element.

Such impregnating or enriching means may, for example, comprise powdered mica, or powdered vermiculite, or a high density ceramic powder, such as alumina or zirconia.

When a vapour barrier comprising one or more sheets is provided, this may suitably be secured to the base by means provided to secure the at least one heating element to the base.

When staples are used to secure the at least one heating element, such staples may penetrate through the one or more sheets and into the base.

When the at least one heating element comprises a ribbon, such as of corrugated form, secured to the base by means of integral tabs extending therefrom and penetrating the base, the tabs may be arranged to penetrate through the one or more sheets and into the base.

In an alternative arrangement, the at least one heating element may be secured to the one or more sheets and the one or more sheets supported on or adjacent to the base and optionally secured to the base In this arrangement the at least one heating element may suitably comprise a ribbon, such as of corrugated form, provided with integral tabs extending therefrom, the tabs penetrating through the one or more sheets and being bent over or twisted after penetration.

The vapour barrier is preferably a barrier to water vapour.

By means of the vapour barrier, damage to the base of insulation material, particularly microporous insulation material, which would othenvise occur as a result of contact therewith by vapour, particularly water vapour, is minimised. Furthermore, contact with the insulation material by particulate materials, such as splattering food particles in cooking appliances, is prevented.

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: Figure 2 is a cross-sectional view of the heater of Figure 1 Figure 3A is a perspective view of a heating element for use in the heater of Figures 1 and 2; Figure 3B represents a sheet-form vapour barrier for use in the heater of Figures 1 and 2; Figure 4 is a cross-sectional view of an alternative construction of the heater of Figure 1: and Figure 5 is a cross-sectional view of a further embodiment of an electric heater according to the invention.

Referring to Figures 1, 2, 3A and 3B, a radiant electric heater according to the invention for use, for example, in a cooking appliance, has a metal supporting dish 1 containing a base layer 2 of compacted microporous thermal insulation material. Such microporous insulation material is well known to the skilled person a particular composition, by way of example only.

being Pyrotenic silica 49 - 97% by weight Ceramic fibre reinforcement 0.5-20% by weight Opacifier (e.g. TiO2) 2-50% by weight Alumina up to 12% by weight It is well known to the skilled person that microporous insulation materials are degraded by contact with water, or water vapour, such as steam.

A heating element 3 comprising a corrugated metal ribbon 4 provided with integral mounting tabs 5 as shown in Figure 3A is supported on the base layer 2 and secured in known manner by penetration of the mounting tabs 5 into the base 2 and frictional retention therein.

A vapour barrier 6 is provided between the heating element and the base 2. The barrier 6, as shown in Figure 3B, suitably comprises one or more thin sheets of mica of disc shape, although other sheet-form materials with adequate impermeability to vapours, particularly water vapour such as steam, could be provided. A tightly woven glass or ceramic fibre fabric material could be considered, as could thin ceramic materials.

The tabs 5 of the heating element ribbon 4 are pressed through the mica sheet 6 and into the base 2 of microporous insulation material. The mica sheet 6 consequently becomes secured in contact with the surface of the base 2 and protects the base 2 from attack of water vapour such as steam, and also from contact with any liquid or solid particulates when the heater is used in a cooking appliance, or other application, in the absence of a cover sheet of glassceramic, or other material, which has hitherto been provided overlying the entire heater and contacting the top surface of a peripheral wall 7 of thermal insulating material added in ring form to the heater.

Instead of the tabs 5 of the ribbon 4 of the heating element being inserted into the base 2 for securement, the tabs could be pushed through the mica sheet 6 and then bent over at the back of sheet 6 as shown in Figure 4. The mica sheet 6 with the heating element secured thereto is located in contact with the base 2 of microporous insulation material and secured in position.

for example, by means of the peripheral wall 7. Alternatively. the tabs 5 could be twisted after pressing through the mica sheet.

A well-known form of temperature limiter 8 may be provided for the heater, but this may not be essential for some applications.

The sheet 6 of vapour barrier material, such as mica, could also be provided secured to the base 2 of microporous insulation by means of an adhesive or by direct moulding with the base 2 during compacting of powdered microporous insulation material to form the base 2. With such an arrangement, the supporting dish 1 could be dispensed with. resulting, in effect, in a block or panel of compacted insulation material directly bonded to the sheet 6.

As an alternative to providing a vapour barrier 6 in sheet form, vapour barrier 6A could be provided as one or more films, layers or coatings on the surface of a base 2A, particularly of microporous insulation material as shown by the cross-sectional view of a heater in Figure 5.

Alternatively, the vapour barrier 6A in Figure 5 could be provided by impregnating or enriching material of a surface of the base 2 of microporous insulation material with a suitable barrier material.

Examples of film, layer or coating materials for the vapour barrier 6A are vermiculite sol, or glass-ceramic sol.

The vapour barrier 6A could also comprise a flame-sprayed or vapour deposited coating of a ceramic material.

Examples of impregnating or enriching materials for forming the vapour barrier 6A are powdered mica, or powdered vermiculite, or a high density ceramic powder, such as alumina or zirconia.

As shown in Figure 5, the base 2A is formed in a metal support dish 1, like that of the heater of Figure 1 and a heating element, as shown in Figure 3A, is provided having tabs 5 integral with a corrugated ribbon 4, inserted into the base 2A.

The heater of the invention is particularly suitable for cooking appliance applications such as ovens, including microwave ovens, grills, deep fat fryers, toasters.

Although the exemplified embodiments of the heater of the invention refer specifically to a heating element of corrugated ribbon form, the invention is by no means limited thereto and it will be evident to the skilled person that the vapour barrier could be likewise applied to heaters incorporating other forms of heating element or elements, such as coiled wire or coiled ribbon elements, lamp elements, or combinations of elements of different forms. Coiled wire or ribbon elements are well known to be secured to an insulation base by means of staples and such staples may pass through the vapour barrier 6, 6A and into the base.

Claims (16)

Claims

1. An electric heater comprising a base of insulation material having supported thereon or adjacent thereto at least one heating element, a vapour barrier being provided between the at least one heating element and the base.

2. A heater according to claim 1, in which the base comprises microporous insulation material.

3. A heater according to claim 1 or 2, in which the vapour barrier comprises one or more sheets, films, layers or coatings of one or more materials, on or adjacent to the base.

4. A heater according to claim 3, in which the one or more sheets is or are selected from: mica: ceramic materials; woven glass or ceramic fibre fabric materials.

5. A heater according to claim 3. in which the one or more films, layers or coatings is or are selected from: vermiculite sol: glass-ceramic sol; vapour deposited or flame-sprayed ceramic materials.

6. A heater according to claim 1 or 2, in which the vapour barrier comprises means impregnating or enriching material of a surface of the base adjacent to the at least one heating element.

7. A heater according to claim 6, in which the impregnating or enriching means comprises powdered mica, or powdered vermiculite, or a high density ceramic powder.

8. A heater according to claim 3 or 4 in which the vapour barrier comprises one or more sheets secured to the base by means provided to secure the at least one heating element to the base.

9. A heater according to claim 8, in which staples are used to secure the at least one heating element and penetrate through the one or more sheets and into the base.

10. A heater according to claim 8, in which the at least one heating element comprises a ribbon secured to the base by means of integral tabs extending therefrom and penetrating the base, the tabs penetrating through the one or more sheets and into the base.

11. A heater according to claim 3 or 4, in which the at least one heating element is secured to the one or more sheets and the one or more sheets is or are supported on or adjacent to the base.

12. A heater according to claim 11, in which the one or more sheets is or are secured to the base.

13. A heater according to claim 11 or 12, in which the at least one heating element comprises a ribbon provided with integral tabs extending therefrom, the tabs penetrating through the one or more sheets and being bent over or twisted after penetration.

14. A heater according to any one of the preceding claims in which the vapour barrier is a barrier to water vapour.

15. An electric heater constructed and arranged substantiallv as hereinbefore described witli reference to the accompanying drawings.

16. A cooking appliance provided with an electric heater according to any one of the preceding claims.