WO2002102115A1

WO2002102115A1 - Radiant electric heater

Info

Publication number: WO2002102115A1
Application number: PCT/GB2002/002678
Authority: WO
Inventors: Frederick James Davis
Original assignee: Ceramaspeed Limited
Priority date: 2001-06-08
Filing date: 2002-06-10
Publication date: 2002-12-19
Also published as: GB0113942D0

Abstract

A radiant electric heater comprises a support (1), a layer (2) of compacted finely divided metal oxide thermal and electrical insulation material supported by the support, and at least one electric heating element (5) supported relative to the insulation material. The insulation material comprises at least in part a compacted hydrophobic finely divided metal oxide distributed throughout the insulation.

Description

RADIANT ELECTRIC HEATER

This invention relates to a radiant electric heater incorporating compacted microporous thermal insulation material. The heater is particularly suited to environments which have high humidity or moisture levels.

It is known to use compacted powdery microporous thermal insulation as an electrical and thermal insulation material in radiant electric heaters such as ribbon or coil element heaters used in grills and cooking hobs. The use of microporous thermal insulation provides many benefits including an ability to form raised profiles and excellent thermal and electrical insulation properties.

The microporous thermal insulation typically comprises a dry particulate microporous material as defined hereinafter mixed with a high temperature ceramic or vitreous fibre reinforcement, titanium dioxide opacifier and, for high- temperature use, a small quantity of alumina powder to reduce high temperature shrinkage. Such material is described in GB-A-1 580 909.

The term "microporous" is used herein to identify porous or cellular materials in which the ultimate size of the cells or voids is less than the mean free path of an air molecule at NTP, i.e. of the order of 100 nm or smaller. A material which is microporous in this sense will exhibit very low transfer of heat by air conduction (that is collisions between air molecules) . Such microporous materials include aerogel, which is a gel in which the liquid phase has been replaced by a gaseous phase in such a way as to avoid the shrinkage which would occur if the gel were dried directly from a liquid. A substantially identical structure can be obtained by controlled precipitation from solution, the temperature and pH being controlled during precipitation to obtain an open lattice precipitate. Other equivalent open lattice structures include pyrogenic (fumed) and electrothermal types in which a substantial proportion of the particles have an ultimate particle size less than 100 nm. Any of these materials, based for example on silica, alumina or other metal oxides, may be used to prepare a composition which is microporous as defined above.

In applications where the radiant electric heater may be exposed for prolonged periods to very high levels of moisture (which may be air-borne) traditional microporous thermal insulation materials can become compromised as they can absorb water or other liquids after prolonged exposure to such moisture levels.

The absorption of water or other liquids leads to a reduction in the size of the cells or voids of the microporous thermal insulation and destroys the aforementioned porous structure. The decrease in pore size results in increased solid - solid conduction within the body of the microporous insulation reducing the electrical and thermal insulation properties of the material and consequently the benefits of the microporous thermal insulation material to the application.

For some applications, such as combination microwave ovens and grills, it is required that a radiant electric heater must be able to be energised successfully after it has been placed in a cavity with a container of water being boiled and producing steam continuously in excess of 500 hours.

Existing solutions to this problem include using a layer of high temperature glass to protect the microporous thermal insulation from moisture. However, the glass has a thermal mass associated with it which adversely affects the performance of such heaters and is also expensive. The use of a layer of glass also gives rise to other problems because a seal must be designed to ensure moisture does not penetrate around the glass and affect the microporous thermal insulation material.

It is therefore an object of the present invention to provide a means to overcome or minimise these problems. According to the present invention there is provided a radiant electric heater comprising: a dish-like metal support having a base and a surrounding wall; a layer of compacted finely divided metal oxide thermal and electrical insulation material in the support, the insulation material comprising at least in part a compacted hydrophobic finely divided metal oxide distributed throughout the insulation; and at least one electric heating element supported relative to the insulation material .

The thermal and electrical insulation material may comprise compacted microporous thermal and electrical insulation material .

The thermal insulation material may comprise 30 - 100 weight percent of finely divided metal oxide, 0 - 50 weight percent opacifier, 0 - 50 weight percent fibre material, and 0 - 15 weight percent inorganic binder as an intimate mixture.

The thermal insulation material may comprise substantially 58.5 weight percent of finely divided metal oxide, 30.0 weight percent opacifier, and 11.5 weight percent fibre material .

The finely divided metal oxide may comprise 5 - 100 weight percent, preferably 5 - 95 weight percent, and more preferably 15 - 35 weight percent of hydrophobic finely divided metal oxide. The thermal and electrical insulation material may be arranged to overlie the base of the dish-like metal support.

The radiant electric heater may have a peripheral wall of the thermal and electrical insulation material provided within the surrounding wall of the dish-like metal support.

The peripheral wall and the layer may comprise a single co- moulded component, or they may comprise two separate components.

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 drawing in which:

Figure 1 is a plan view of an embodiment of a radiant electric heater incorporating compacted microporous thermal insulation material; and

Figure 2 is a cross-sectional view of the radiant electric heater of Figure 1 taken along the line A - A in Figure 1.

The radiant electric heater shown in the drawings comprises a metal dish-like support 1 in which there is a base layer 2 of compacted microporous thermal and electrical insulation material. A wall 3 of thermal insulation material is provided around the periphery of the heater.

An electric heating element 5 is supported relative to the base layer 2 of thermal insulation material. The heating element 5 may comprise any of the well-known forms of elements, such as wire, ribbon, or foil, or combinations thereof.

A terminal block 6 is arranged at the edge of the dish-like support 1 and electrically connected to the heating element 5, whereby the heating element 5 can be connected to a voltage supply for energising.

A well known form of thermal cut-out device 4 is optionally provided, extending over the heating element 5, to switch off the heating element in the event of overheating.

The rod of the thermal cut-out device 4 passes through apertures in the peripheral wall 3 and is located in a space above the heating element 5 as shown in Figure 2.

The microporous thermal insulation material 2 provided in the radiant electric heater comprises 30 - 100 weight percent of finely divided metal oxide (such as silica) , 0 -

50 weight percent opacifier (such as titanium dioxide) , 0 - 50 weight percent fibre material, and 0 - 15 weight percent inorganic binder as an intimate mixture.

The microporous thermal insulation material 2 provided in the radiant electric heater may comprise substantially 58.5 weight percent of finely divided metal oxide, 30.0 weight percent opacifier, and 11.5 weight percent fibre material.

The microporous thermal insulation 2 comprises, distributed throughout the insulation, a percentage of finely divided metal oxide which does not exhibit any significant affinity to water. Such finely divided metal oxide is typically referred to as hydrophobic.

Such hydrophobic finely divided metal oxide is typically produced by treating dry, moving particles of non- hydrophobic finely divided metal oxide with an organic silane compound at elevated temperature. The non- hydrophobic finely divided metal oxide in powdered form is heated to a temperature typically of 200 degrees - 300 degrees Celsius while being agitated and is brought to a fluidized condition. The fluidized finely divided metal oxide is subsequently treated with an organic silane compound in droplet form to the fludized material to produce hydrophobic finely divided metal oxide. The hydrophobic finely divided metal oxide may be, for example, CABOT TS-720, TS-530 or TS-610, or DEGUSSA R202, R812 or R972.

The microporous thermal insulation material 2 contains sufficient hydrophobic finely divided metal oxide to render the overall thermal insulation material hydrophobic. To this end, the finely divided metal oxide of the insulation material comprises 5 - 100 weight percent of hydrophobic finely divided metal oxide, and preferably contains 15 - 35 weight percent of hydrophobic finely divided metal oxide.

Surprisingly, it has been found that such hydrophobic microporous thermal insulation material is able to withstand testing in high humidity atmospheres. That is the affinity of the hydrophobic microporous thermal insulation material to water is reduced to a level which allows a radiant electric heater incorporating the hydrophobic insulation material to be energised successfully after the radiant electric heater has been placed in a cavity with a container of water being boiled and producing steam continuously for a period in excess of 500 hours.

The present invention is not restricted to the use of the hydrophobic microporous thermal insulation material 2 solely as a base layer in the radiant electric heater. The hydrophobic microporous thermal insulation material may also be used as the peripheral wall 6 of the radiant electric heater, in which case the peripheral wall 6 may be made simultaneously with the base layer 2 or may be made separately therefrom.

Claims

1. A radiant electric heater comprising: a support (1) ; a layer (2) of compacted finely divided metal oxide thermal and electrical insulation material supported by the support; and at least one electric heating element (5) supported relative to the insulation material, characterised in that the insulation material comprises at least in part a compacted hydrophobic finely divided metal oxide distributed throughout the insulation.

2. A radiant electric heater as claimed in claim 1, characterised in that the thermal and electrical insulation material comprises compacted microporous thermal and electrical insulation material.

3. A radiant electric heater as claimed in claim 1 or 2, characterised in that the thermal insulation material comprises 30 - 100 weight percent of finely divided metal oxide, 0 - 50 weight percent opacifier, 0 - 50 weight percent fibre material, and 0 - 15 weight percent inorganic binder as an intimate mixture.

4. A radiant electric heater as claimed in claim 3, characterised in that the thermal insulation material comprises substantially 58.5 weight percent of finely divided metal oxide, 30.0 weight percent opacifier, and 11.5 weight percent fibre material.

5. A radiant electric heater as claimed in any preceding claim, characterised in that the finely divided metal oxide comprises 5 - 100 weight percent of hydrophobic finely divided metallic oxide.

6. A radiant electric heater as claimed in claim 5, characterised in that the finely divided metal oxide comprises 5 - 95 weight percent of hydrophobic finely- divided metallic oxide.

7. A radiant electric heater as claimed in claim 6, characterised in that the finely divided metal oxide comprises 15 - 35 weight percent of hydrophobic finely divided metal oxide .

8. A radiant electric heater as claimed in any preceding claim, characterised in that the support (1) comprises a metal dish-like support having a base and a surrounding wall .

9. A radiant electric heater as claimed in claim 8, characterised in that the thermal and electrical insulation material is arranged to overlie the base of the dish-like metal support (1) .

10. A radiant electric heater as claimed in any preceding claim, characterised in that the thermal and electrical insulation material is provided with a peripheral wall (3) .

11. A radiant electric heater as claimed in claim 10, characterised in that the peripheral wall (3) and the layer (1) comprise a single co-moulded component.

12. A radiant electric heater as claimed in claim 10, characterised in that the peripheral wall (3) and the layer (1) comprise two separate components.