GB2257002A - Ceramic electric hob - Google Patents

Abstract

A heating apparatus is for use in cooking appliances beneath an area of glass ceramic cook-top and comprises at least one high temperature electric heating element 3 and at least one low temperature heating element 4 disposed for substantially uniformly heating the said area. Preferably elements 3 and 4 are lamps. Insulating material disposed below the lamps absorbs radiation and re-radiates it at a lower frequency. <IMAGE>

Description

HEATING APPARATUS This invention relates to a heating apparatus incorporating at least one source of infrared radiation.

The heating apparatus according to the invention is particularly suitable for use in both domestic and commercial cooking appliances. In such cooking appliances, each source of infrared radiation has a peak radiation wavelength between 0.8 jim and 5 pm.

UK Patent No. 1273023 (Electricity Council), now lapsed, described and claimed an electric cooker comprising one or more tungsten filament lamps forming an infra-red radiation source or sources arranged below a surface plate of glass, ceramic or glass ceramic material transmitting infra-red radiation and having a reflector below the radiation source or sources for reflecting the infra-red radiation upwardly through the surface plate. The system proposed for use in a cooker made according to UK Patent No. 1273023 and operating beneath a glass ceramic cook-top material possessed a transmission characteristic such that some of the visible radiation from the lamps passes through the cook-top whereas certain shorter wavelength radiation, which may be injurious to a user is prevented from passing through the cook-top.

The cooker described in UK No. 1273023 employed a high purity aluminium reflector to redirect any radiation emitted away from the glass ceramic cook-top back again towards the glass ceramic cooking material.

One reflector proposed in UK No. 1273023 included a high purity aluminium electrolitically polished and brightly anodised reflective surface. This was intended to both improve the effectiveness of the heater and prevent overheating of other components associated with, or close to, the cooking appliance. Such a reflector is, however, unsatisfactory and does not afford adequate insulation to auxiliary components within an acceptable component depth for installation within a cooker or hob.

Several developments have been proposed and are disclosed in GB-2-132-060 (Thorn E.M.I. Ltd.) and European Patent, EP-0-117-346-B1. Such cookers produce to a user an illusion of heating and a visual indication of operation and are generally known by manufacture and user alike as Halogen Heaters (Registered Trade Mark).

European Patent No. 0117346 describes a heating assembly including heating means (7), a mass (2) of nonmetallic, thermally-insulative material located adjacent one side of said heating means (7), a plate (15) covering said heating means (7) adjacent the side thereof remote from said mass (2), and means for reflecting energy, initially emitted from said heating means (7) in a direction away from said plate (15), back towards said plate (15), that said heating means (7) includes at least one lamp (7) emissive of infra-red radiation, operative at a temperature within the range from (1800 to 3000)"K and comprising a tungsten filament (17) supported in a halogenated environment within a sealed, generally tubular envelope formed from an infrared transmissive material; in that said plate (15) has a transmitting characteristic selected to permit infrared radiation, substantially without change in its wavelength characteristics as emitted from said at least one lamp (7), to emerge from said assembly through said plate (15); in that said assembly includes optical filter means located relative to said at least one lamp (7) to inhibit emission from said assembly of undesirable radiation emitted from said at least one lamp (7), whilst permitting emission from said assembly of sufficient visibile radiation to provide a visual indication of the amount of infra-red radiation being emitted from said at least one lamp (7); in that said reflective means, includes a reflective constituent coated upon, or incorporated in said thermally-insulative material and is effective to reflect infra-red radiation emitted from said at least one lamp (7) to enhance the amount of infra-red radiation which emerges from said assembly through said plate (15); and in that said assembly also includes temperature-responsive means (11) responsive to the temperature of said plate (15) and switching means (12) co-operable with saidtemperature responsive means (11) to control power supplied to said at least one lamp (7), in dependence on the temperature of said plate (15).

It is an object of the present invention to provide a alternative heating apparatus to those previously proposed and, at the same time, produce an apparatus which is aethetically pleasing to a user, cost effective, readily controllable and compatible with the dimensional constraints of currently required cooking appliances.

According to the present invention there is provided a heating unit intended to operate beneath an area of a glass ceramic cook-top material to be heated, wherein the heating unit comprises at least one high temperature heating element and at least one low temperature heating element disposed for substantially uniformly heating the said area, and wherein each heating element possesses electrical characteristics which are so chosen that their respective power ratios achieve a substantially uniform heat flux distribution within said heating area.Conveniently the heating elements are disposed between the cook-top and an insulating material possessing characteristics which, in use, absorbs and thereafter re-radiates at least some short wavelength radiation to alter that primary spectrum emitted by each heating element to create a secondary spectrum and thereby achieve the substantially uniform heat flux distribution within said heating area. Preferably, the re-radiating insulation material is a substantially nonreflective material.

In contrast with the heating assembly of European Patent No. 0117346, the heater unit of the present invention uses tungsten filament heater lamps which are operated in a non-halogenated and oxygen free atmosphere.

Furthermore, the insulation material used in a heater of the present invention does not include , nor is it coated with a reflective medium nor is such a reflective coating deposited upon that side of the heating element remote from the cook-top. Specifically, the insulation material used in a heater unit according to the present invention absorbs and, thereafter, reradiates as opposed to relfects at least some shortwave radiation thereby, as indicated above, altering the primary spectrum emitted by each heating element to create a secondary spectrum. The insulation material used in the heater of the present invention is, therefore, unmodified and although the maximum theoretical flux density of re-radiated infrared heat is less than that obtained using a material including d reflector, we have found that more than adequate heating capacity is available for cooking purposes using the non-modified material.

We have found that a so-called known insulation material sold under the Trade Mark W.D.S. and manufactured by Wanker GmbH is satisfactory. In contrast a version of this material was previously modified for use in conjunction with Halogen heaters to increase its reflectivity properties. To this end iron and titanium oxides contained in the W.D.S. material are replaced by Zirconium dioxide which functions as a reflective component. In addition to creating a reflective insulation material necessary for the adequate fulfillment of a heater assembly made in accordance with European Patent No. 0117396B1, the colour of the so-modified material changed significantly from brown to white.

A series of comparative experiments to test the characteristics of the un-modified and modified insulation material indicate that the un-modified material functions as an absorbing and re-radiating medium whereas the modified material functions as substantially reflective medium.

Electrical control of the heating elements may be configured using a multi-pole switch according to know techniques for selecting different power levels required during cooking from simmering through to boiling.

Control may also for example follow similar conventional principles as disclosed in UK 1273023 and EP 0117346.

In addition to the advantages of the present invention when compared with the heater units previously proposed, a temperature limiter to switch heating elements which has been specifically modified so as not to respond to short wave infrared radiation which is transmitted through the cook top is not required. The limitex may be used to limit the power supplied by the low temperature heating elements only.

We have found that suitable cook-top glass ceramic materials are those sold under the trade marks CERAN and CERAN KOLUR. These materials possess characteristics such that some short wave-length infrared radiation is not significantly attenuated whereas potentially harmful shorter visible components are attenuated. In the heater unit according to the present invention the short wave-length infrared transmission coefficient is not required to be as high as for those systems previously proposed and as discloed in UK 1273023 and EP 0117346.

One form of heating unit is shown diagrammatically in the accompanying Figure. The heating unit is supported in a conventional bowl (1) and lined with insulation material (2). High (3) and low (4) heating elements are disposed as shown with a thermal limiter (5) supported diagonally across the element. It will be seen that the embodiment of heater illustrated includes four individual heating elements and the precise operation and control thereof may be carried out as described in our pending EPA 0331369 which is herein incorporated by reference.

Claims (11)

1. A heating unit intended to operate beneath an area of a glass ceramic cook-top material to be heated, wherein the heating unit comprises at least one high temperature heating element and at least one low temperature heating element disposed for substantially uniformly heating the said area and wherein each heating element possesses electrical characteristics which are so chosen that their respective power ratios achieve a substantially uniform heat flux distribution within said heating area.

2. A heating unit according to claim 1 wherein the heating elements are disposed between the cook-top and an insulating material possessing characteristics which, in use, absorbs and thereafter re-radiates at least some short wavelength radiation to alter the primary spectrum radiation emitted by each heating element to create a secondary radiation spectrum and thereby achieve the substantially uniform heat flux distribution within said heating area.

3. Heating unit according to claim 2 wherein the reradiating insulation material is a non-reflective material.

4. A heating unit according to claim 2 or claim 3 wherein the insulation material possesses a A value within the range .01-.05 W/m.k.

5. A heating unit according to claim 4, wherein the insulation material has a A value of 0.02 W/m.k.

6. A heating unit according to any preceding claim wherein each heating element is a sealed tungsten filament lamp.

7. A heating unit according to claim 6 wherein each said sealed tungsten filament lamp contains a nonhalogenated and oxygen free atmosphere selected from the group consisting of nitrogen, carbon dioxide and the noble gases.

8. A unit according to any preceding claim including two high temperature heating elements and at least one low temperature heating element wherein the power ratios of the respective high and low temperature heating elements are such that at maximum total power 50% power is produced by the high temperature heating elements and 50% by the at least one low temperature heating element.

9. A unit according to claim 8 with a disposition of said elements as shown un the accompanying Figure.

10. A unit according to any preceding claim in which the heating unit includes a temperature limiting device which is not shielded from incident short wavelength radiation.

11. A unit according to claim 10 wherein the temperature limiting device is operable to limit power supplied by the low temperature heating element(s).