GB2357232A

GB2357232A - Further insulating dish/insulation interface in an electric heater

Info

Publication number: GB2357232A
Application number: GB9922915A
Authority: GB
Inventors: Peter Ravenscroft Wilkins
Original assignee: Ceramaspeed Ltd
Current assignee: Ceramaspeed Ltd
Priority date: 1999-09-29
Filing date: 1999-09-29
Publication date: 2001-06-13
Also published as: GB9922915D0; JP3076653U

Abstract

The metal dish (2) has at least an inside surface (3) provided at least in part with an electrically insulating coating (9), for example of vitreous enamel or alumina which isolates it from the microporous insulating material 5 thereby avoiding electrical leakage if insulation 5 adsorbs any liquid. The heating element 6 may be a corrugated ribbon or wire coil. Alternatively a heating lamp may be used.

Description

2357232 Ellectnic Heater This invention relates to an electric heater,

which may be of radiant form, having a metal support dish provided therein with a layer of thermal and electrical insulation material, at least one electrical heating element being supported relative to the layer of insulation material.

Radiant electric heaters of this kind are known, particularly in cooking appliances, such as ovens, hobs and grills. In a typical construction, a metal support dish, such as of steel, is provided with a layer of compacted microporous thermal insulation material inside it. At least one heating element is supported on or adjacent to the layer of insulation material, a particularly well-known form of heating element being a corrugated metal ribbon supported on edge and secured to or in the surface of the layer of insulation material.

Electrical safety legislation in some countries requires electric heaters to be subjected to a water immersion test, after which the insulation resistance is measured between the metal dish and the heating element or elements and must exceed a specified value.

Microporous insulation material is very water-absorbent and when this material is used as an insulating layer for supporting a heating element in a 2 heater, unacceptably low insulation resistance can result, measured between the dish and the heating element.

It is an object of the present invention to overcome or minimise this problem.

According to the present invention there is provided an electric heater comprising a metal support dish having therein a layer of thermal insulation material with at least one electrical heating element supported relative thereto and wherein the dish has at least an inside surface thereof provided at least in part with an electrically insulating coating.

The layer of thermal insulation material may comprise microporous thermal and electrical insulation material.

The electrically insulating coating may be arranged to cover at least that part of the inside surface of the dish where the layer of thermal insulation material is provided, and optionally substantially the entire inside surface of the dish.

The dish may also have an outside surface provided at least in part with the electrically insulating coating.

The electrically insulating coating may comprise a vitreous enamel, a glass ceramic, a ceramic, or a particulate metal oxide such as comprising alumina.

3 The invention is now described by way of example with reference to the accompanying drawings, in which:

Figure 1 is a plan view of a radiant electric heater according to the invention; and Figure 2 is a cross-sectional view of the heater of Figure 1.

Referring to the drawings, a radiant electric heater 1 comprises a metal support dish 2, such as of steel, having an inside surface 3 and an outside surface 4.

A layer 5 of compacted microporous thermal and electrical insulation material is provided inside the dish 2 and at least one electrical heating element 6 is secured to the layer 5.

A single heating element 6 is shown, comprising a known form of corrugated metal ribbon mounted edgewise on the surface of the layer 5 and secured either by partial embedding in the layer 5, or by other known means.

The heating element or elements 6 could instead comprise coiled wire or coiled ribbon or face-mounted ribbon or one or more lamps, or any other form of element known to the skilled person. More than one form of heating element could also be provided in the heater.

4 A terminal block 7 for the one or more heating elements 6 is mounted on the heater and allows the heater to be connected to a voltage supply for operation.

A peripheral wall 8 of thermal insulation material is provided in the heater. As shown, this wall is integral with the layer 5. However it could be provided as a separate member, as is well known in the art.

The inside surface 3 of the dish 2 is provided with an electrically insulating coating 9. Such coating 9 is provided over substantially the entire inside surface 3 of the dish, or at least over that area of the inside surface of the dish in contact with the layer 5 of insulation material.

The outside surface 4 of the dish 2 is optionally similarly provided with an electrically insulating coating 10, suitably of the same material as the coating 9.

The coatings 9 and 10 suitably comprise a known form of vitreous enamel, applied in known manner to the dish 2. Vitreous enamels and methods for their application to substrates such as steel are very well known to the skilled person.

Other well known materials and processes may be employed for providing the electrically insulating coatings 9 and 10. For example the coatings 9 and 10 could comprise a glass-ceramic material or a ceramic material. A coating material comprising a particulate metal oxide, such as alumina, could also be employed, being suitably applied by a process such as flame spraying or plasma coating.

If the resulting heater is subjected to a water immersion test, the microporous thermal and electrical insulation layer 5 absorbs water. In the absence of the electrically insulating coating 9 on the inside surface 3 of the metal dish, an unacceptably low value of electrical insulation resistance would be obtained, measured between the heating element or elements 6 and the metal dish 2.

However, with the provision of the electrically insulating coating 9, a high level of electrical insulation resistance is obtained. The provision of the electrically insulating coating 10 on the outside surface 4 of the metal dish 2 further enhances the electrical insulation of the dish.

The radiant heater 1 of the invention is particularly applicable in cooking appliances such as ovens, grills, toasters and hobs.

6

Claims

1 An electric heater comprising a metal support dish having therein a layer of thermal insulation material with at least one electrical heating element supported relative thereto and wherein the dish has at least an inside surface provided at least in part with an electrically insulating coating.

2. An electric heater according to claim 1, in which the layer of thermal insulation material comprises microporous thermal and electrical insulation material.

3. An electric heater according to claim 1 or 2, in which the electrically insulating coating is arranged to cover at least that part of the inside surface of the dish where the layer of thermal insulation material is provided.

4. An electric heater according to claim 3, in which the electrically insulating coating is arranged to cover substantially the entire inside surface of the dish.

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5. An electric heater according to any preceding claim, in which the dish has an outside surface provided at least in part with the electrically insulating coating.

6. An electric heater according to any preceding claim, in which the electrically insulating coating comprises a vitreous enamel, or a glassceramic, or a ceramic, or a particulate metal oxide.

7. An electric heater according to claim 6, in which the particulate metal oxide comprises alumina.

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