GB2144956A - Heating apparatus - Google Patents

Abstract

Heating apparatus consists of a shallow circular tray (1) having a layer (2) of thermally insulative material disposed therewithin and supporting a number of infra-red- emitting, tunsten-halogen lamps (7) on flanges (3, 4), beneath a glass ceramic cooktop. A moulding (8) of ceramic fibre material is press-fitted around the ends of the lamps (7) and a thermal limiter (11) is provided to limit the operating temperature of the cooktop. The tray is constructed so as to constrain infra-red energy from the lamps to preferentially heat a defined area of the cooktop so as to form a hot plate in said area. <IMAGE>

Description

SPECIFICATION Heating apparatus This invention relates to heating apparatus and in particular, though not exclusively to such apparatus including one or more sources of infra red radiation of a wavelength within the band 0.8-5 m, having a peak at approximately 1.2 m.

Heating apparatus incorporating sources of infra-red radiation is disclosed in U.K. Patent No. 1273023, to The Electricity Council, wherein one or more sources, each comprising a tungsten filament lamp, are arranged below a glass ceramic cooking hob. A metallic reflector is disposed below the sources so as to reflect radiation, emitted in a downward direction from the sources, upwardly onto and through the underside of the glass ceramic hob. The metallic reflector is preferably made of high purity Aluminium, which is polished and anodised, and shaped so as to reflect radiation onto the underside of the hob in that area which would be covered by the base of a utensil standing thereon.

However, it has been found that such an arrangement, incorporating a metallic reflector, raises a number of problems, namely that, by placing the reflector close to the infra-red radiation sources to obtain the optimum effect thereof and to produce a relatively shallow arrangement, the reflector may be caused to melt or, at the least, to be greatly distorted and discoloured by the considerable heat emitted from the sources, unless it is not provided with heat insulation, in which case a substantial amount of heat can be lost. This problem may only be alleviated by placing the reflector at a substantial distance from the sources and by not using any heat insulation, thereby reducing the effect of the reflector to an unacceptable level.

It is an object of the present invention to alleviate the above-identified problems by providing a more efficient heating apparatus than that disclosed heretofore, having a relatively rapid response time, which is at least comparable with that of gas-fuelled heating apparatus, whilst retaining the inherent advantage of cleanliness.

According to the present invention, there is provided heating apparatus including a layer of glass ceramic material and means for preferentially heating an area of said layer to form a hot plate upon which food or drink may be heated, said means including a source of infra-red radiation and a structure, formed of or including non-metallic, thermally insulative material, constructed to constrain radiation, emitted by said source, to the vicinity of said area, the layer, the source and the structure being disposed in closely proximate relationship to one another with the source interposed between said layer and at least a major part of said material.

The invention will now be further described by way of example only with reference to the accompanying drawings, wherein: Figure 1 shows a plan view of an embodiment of the present invention, Figure 2 shows a sectional view on X-X in the direction indicated, of the embodiment shown in Fig. 1, Figure 3 shows a sectional view on Z-Z, in the direction indicated, Figure 4 shows a spectral transmission curve for a preferred type of glass ceramic utilised in the present invention, Figure 5 shows various switching arrangements for power input control of the embodi ment shown, and, Figure 6 shows a schematic sectional view of part of the embodiment shown in Fig. 1.

Referring to Fig. 1, a generally circular shallow tray 1, preferably made of metal, has disposed therewithin, on the base thereof, a layer 2 of insulative material, which may be fabricated from a microporous material, for example that known as Microtherm. The tray 1 has two extending flanges, 3 and 4, arranged on opposite sides of the rim of the tray 1, each flange having upturned end portions, 5 and 6, respectively.

A number of sources of infra-red radiation, preferably four, one being shown at 7, are disposed above the layer 2 of insulative material and are supported at each end by the flanges, 3 and 4.

A moulding 8 of ceramic fibre material is disposed above the tray 1 and press-fitted around the ends of each source 7 to provide a suitable packing therefor.

Each source 7 of infra-red radiation comprises a quartz, halogenated tubular lamp including a tungsten filament (not shown in Fig.

1), one suitable example of which is described and claimed in copending Application No.

8308103, in the name of THORN EMI plc.

Each lamp has moulded ceramic end caps, one shown at 9, enclosing a pinch-seal (not shown) with an amp tag connector connected to an end of the filament sealed therein, each end cap 9 being provided with a location tab 10, so that the tubes can easily be inserted in gaps provided in the upturned portions 5 and 6, on the flanges 3 and 4.

The tray 1 and flanges 3 and 4 are preferably made of metallic material, and sufficient clearance is allowed in each gap provided for the end caps 9 to permit expansion of the tray and flanges without breaking the lamps, whilst providing sufficient support for the lamps during attachment of electrical wiring to the amp tag connectors. It also permits conduction of heat away from the lamp pinchseals via the flange to maintain satisfactory operating temperatures. Heat is also conducted away from the lamp ends by way of the electrical wiring attached thereto.

If further cooling of the pinch seals is required, heat sinking and conventional cooling techniques disclosed in any of copending Application Nos. 8314451, 8316304, 8316306 and 8318457 may be employed, or any other suitable technique known to those skilled in the art.

The ceramic fibre moulding 8 is also sufficiently flexible to ailow a certain amount of movement, caused by expansion and contraction of the tray and/or flanges whilst providing positive location for the lamps.

A number, preferably four, of the heating apparatuses shown in Fig. 1 are preferably disposed below a layer of glass ceramic, which is in this example fabricated from Corning Black Cooktop 9632, to provide a slimline cooking hob, which may be of depth comparable with that of a standard worktop.

A thermal limiter 11, which is intended to limit the operating temperature of the glass ceramic layer, comprises a bimetallic rod arranged so as to operate a microswitch 1 2 and the limiter is provided between the lamps 7 and the layer 2 of insulative material and is adjusted so that expansion of the rod, due to heat emitted by the lamps, causes one end of the rod to operate the microswitch 1 2 when the temperature has reached a threshold value, thereby disconnecting the power to the lamps. During adjustment of the limiter, the effect of incident infra-red radiation thereon, which can cause variations in readings, should be taken into account.

Figs. 2 and 3, in which like parts are labelled with like reference numerals with respect to Fig. 1, show sectional views of the apparatus shown in Fig. 1, indicating the shape of the features thereof, particularly of the tray 1 and the end caps 9, as well as showing the overall shallowness of the apparatus.

The properties of the glass ceramic material provide optimum transmission of infra-red radiation emitted from the infra-red lamps by matching the frequency of infra-red transmission through the glass ceramic with frequency of emission of the lamps.

The transmission characteristics of the glass ceramic material are such that wavelengths below 0.6 m are substantially absorbed. However, some visible radiation above this wavelength is transmitted, as red light, thus providing a visible indication of power level.

The heating arrangement, as described hereinbefore, is further advantageous, in that it provides an advantageously high nominal energy loading per surface area of the cooking hob. A typical nominal energy loading per surface area is approximately 6W/cm2, whereas in this embodiment, the matching between the energy emission characteristic of the lamps and the energy transmission characteristics of the cooktop is such that an increased energy loading of up to as much as 8W/cm2 may be achieved.

Fig. 4 shows a spectral transmission curve for the preferred ceramic, approximately 4mm in thickness, and it can be seen at line A on the horizontal axis indicating wavelength that, at the peak value, ie. approximately 1.2 m, within the wavelength band of the infra-red radiation emitted from the sources utilised in the present invention, this material has a transmission factor of nearly 80%.

Operation of the apparatus is controlled by a multi-pole, preferably seven-pole, switching arrangement, used in conjunction with the preferred configuration of four 500W filament lamps, to provide a range of powers of approximately 2KW to 147W, by switching the filaments into various series and/or parallel combinations.

Fig. 5 shows six switching combinations of the four 500W filament lamps, one shown at 7 in Fig. 1, thus providing six discrete control settings on a user-rotatable control knob (not shown) which correspond to six power outputs as shown to produce an optimised characteristic heat output curve. Fig. 5 also indicates the percentage of each power output relative to the total output i.e. 2000W. It can be seen that a diode 1 3 is used in two of the six combinations to ensure that each control setting, especially the lower settings, provide an aesthetically-pieasing balanced effect of the visible radiation emitted from the filaments as seen through the layer of glass ceramic, as well as enabling lower powers, which are suitable for simmering purposes, to be provided by the combinations.

The diodes employed in each of the switching arrangements used respectively for the heating apparatuses incorporated within the cooking hob may be randomly poled to ensure that the loading on the mains is distributed evenly instead of being concentrated on one particular sequence of half-cycles of the mains waveform.

Moreover, implementation of the switching arrangement ensures that any malfunction of one of the infra-red lamps still allows operation of the hob at reduced power levels.

A phase control device, incorporating diacs, triacs, etc, or any alternative conventional control, may be implemented at powers below approximately 200W, so as to comply with international standards.

However, as an alternative to phase control, mark space control may be employed at higher power settings, in conjunction with one or more continuously energised lamps, so as to mask the disturbing flickering effect produced by the so controlled lamp or lamps. It may be further advantageous to employ, for example, two continuously-energised lamps, together with two burst-fire controlled lamps, as the two burst-fire controlled lamps may thus be operated at a considerably higher frequency than if four burst-fire controlled lamps were utilised.

The thermal limiter, shown at 11 on Figs. 1 and 2, is used to ensure that the maximum operating temperature, ie. approximately 700"C, of the undersurface of the glass ceramic is not exceeded. Ther thermal limiter 11 needs to be adjusted to avoid nuisance tripping of the microswitch 12, thereby disconnecting the power supply to the lamps.

The incorporation of a thermal limiter into the apparatus is further advantageous, in that it allows the use of utensils of any material in conjunction therewith. However utensils having certain characteristics will perform differently with the present invention, than with other cooking hobs. As heating is substantially increased by infra red transmission to the utensil base, distorted infra-red absorbing utensils will operate more efficiently with the present invention, than with other electrical cooking hobs, where good contact is required between the utensil base and the heated area, to allow conduction of heat. Conversely utensils having highly reflective bases, which are not flat, will operate less efficiently with the present invention, as the infra red radiation will be reflected back to the hob surface.This will cause the operating temperature of the apparatus to increase and the thermal limiter to operate. In such circumstances the thermal limiter will switch the lamps on and off to maintain a satisfactory glass ceramic temperature, thereby providing a visual indication that the utensil being used is causing inefficient operation.

The insulative layer 2 is preferably approximately 12mm thick, and it may have grooves provided in the surface thereof to accommodate a portion, preferably about one half, of the diameter of each of the lamps.

The use of quartz, halogenated lamps as the source of infra-red radiation is advantageous in that the lamp construction provides longevity of the filament, whilst providing high efficiency, the temperature of the filament reaching approximately 2400K, as well as providing a rapid response time for the cooking hob control.

As shown in Fig. 6, wherein a schematic view of a cross section of a lamp 14, in association with the glass ceramic layer 1 5 is illustrated, the lamp 1 4 has an integral oxide or other suitable reflector in the form of a coating 1 6 on the lower part thereof. A filament 1 7 of the lamp 1 4 is positioned at the focal point of the coating 16, so that downwardly-emitted radiation from the filament 1 7 is reflected either back towards the filament, or towards the glass ceramic layer 15.

As an alternative to, or in combination with, the reflective coating on each of the lamps, the surface of the insulative material maybe provided with a reflective coating, such as a metallic oxide, or the surface layer of the insulative material may be enriched therewith, so that a reflective layer is disposed between the lamps and a major part of the body of the insulative material, thereby ensuring that the insulative material is substantially opaque to infra-red radiation.

The layer 2 of microporous insulative material, used in conjunction with the reflective coating on the lamps and/or the surface of the layer, is advantageous over conventional infra-red cooking hobs, as emission from the lamp matches transmission by the glass ceramic layer, consequently reflected radiation passes through the glass ceramic layer also.

Furthermore, the insulative material or reflective coating thereon has better reflectivity at higher frequencies, minimising that portion of radiation which is absorbed by the layer and re-emitted at frequencies which do not pass through the glass ceramic layer.

The envelope of the lamp may have an alternatively shaped cross-section to the preferred circular cross-section, such as the coated half of the envelope being parabolic in cross-section, the filament 10 being positioned at the focal point of the parabola.

Alternative materials, such as glass ceramic, may be used instead of quartz for the envelope of the lamp, so that an optical filter may be incorporated within the tube.

The tube may also include a second quartz envelope having optical filter properties.

As well as, or instead of, incorporating an optical filter within the envelope, a separate optical filter may be used.

Alternatively a clear glass ceramic, such as Corning 9618, may be used in conjunction with a lamp envelope incorporating an optical filter to block out undesirable visible light. The filter may be provided in the form of a coating on the glass ceramic itself or alternatively, a wafer of filter material could be interposed between the lamp and the glass ceramic, or on the quartz envelope of the tube.

As an alternative. a conventional, mechanical cam-operated, bimetal switch may be used to set the amount of radiation required, thereby providing the advantages of low cost and reliability. Similarly, devices such diacs, triacs and phase controllers can be used.

A feed back temperature control device, such as that disclosed in Patent No.2071969, may also be used, such as a device based on 'fibre optics'.

The apparatus may be used with or without the layer of glass ceramic, as any other supporting means may be utilised to provide support for a utensil and to protect the lamps.

instead of placing utensils to be heated on the hob, the hob itself may be used as a cooking utensil.

To ensure that the infra-red radiation, or heat provided thereby, is transmitted to the food to be cooked, glass ceramic cooking utensils, which transmit infra-red radiation directly to the food, or utensils having an infra red absorbent base, may be utilised.

The area of the hob surface illuminated by the lamp is not, of course, limited by the present invention to a substantially circular shape, but may be varied by using different shapes and/or sizes of the tray, such as a square or rectangular shape, as well as other suitable shapes and/or configurations of the lamps, such as circular, semi-circular, horseshoe shape, concentric rings with aligned end portions, or lamps which can be tapped at various points along their lengths.

Flying leads may be used, as an alternative to amp tag connectors, at each end of the lamps.

The thermal limiter 11 may be disposed in any suitable position relative to the lamps, either above, below or at the same level as, and parallel to, the lamps. As a further alternative, it may be mounted in a vertical position relative to the lamps. The thermal limiter may be shielded from incident infra-red radiation so that it responds primarily to the temperature of the glass ceramic layer 2. The shield may take the form of a suitable infrared reflective coating, such as a metallic oxide coating, or the limiter may be enclosed in a tube of ceramic fibre, or other suitable material. The limiter may, alternatively, be disposed within the insulative layer, in such a way as to provide shielding from incident infra-red radiation.

Alternative means for sensing and limiting the temperature of the glass ceramic layer, such as an electric control system, may be employed in the present invention, incorporating a temperature sensor which may be disposed in any suitable position within the heating apparatus. Such sensors may of course be shielded from incident infra-red radiation in a similar manner to the bimetallic thermal limiter.

Alternatively, a thermostat, disposed outside the tray, may be employed. The thermostat can be adjusted to sense a temperature equal to the required glass ceramic temperature, either directly from the tray or via a thermal window open to the temperature within the tray.

Furthermore, the infra-red lamps may be disposed in any vertical or horizontal position relative to each other below the glass ceramic layer, so as to obtain an even distribution of infra-red radiation over the cooking area of the layer, whilst still maintaining a relatively high level of infra-red transmission therethrough.

Instead of utilising the material, Microtherm, any other suitable thermally insulative material may be used, for example microporous materials manufactured by Ego-Fischer, Wacker or Johns-Manville, or mineral wool, glass fibre, calcium silicate, ceramic fibre, or alumina fibre, although in some cases a substantial thickness of the insulative material may be required to ensure efficient operation.

A suitably strong material may also be fabricated so as to be self-supporting, thereby eliminating the need for a tray to support the material and lamps.

Alternatively, if a tray is utilised, it may be formed from a plastics material instead of a metal.

The preferred embodiment of the present invention operates at a colour temperature of approximately 2400K, but, however, operation is possible at other colour temperatures within the range of approximately 1800K-3000K.

Heating apparatus in accordance with the present invention may be suitably orientated so that it may be employed in alternative applications, such as microwave ovens, grills, barbecues, toasters, electric fires and rotisseries.

The present invention therefore provides a substantially improved heating apparatus, using infra-red radiation, of relatively slim construction, having a surprisingly rapid thermal response time and low boiling time due to high efficiency and power density, comparing favourably with that of conventional gas-fuelled cooking apparatus, as well as providing a smooth hob surface, which can easily be cleaned and which can be used in conjunction with a cooking utensil made of any material.

Claims (10)

1. Heating apparatus including a layer of glass ceramic material and means for preferentially heating an area of said layer to form a hot plate upon which food or drink may be heated, said means including a source of infra-red radiation and a structure, formed of or including non-metallic, thermally insulative material, constructed to constrain radiation, emitted by said source, to the vicinity of said area, the layer, the source and the structure being disposed in closely proximate relationship to one another with the source interposed between said layer and at least a major part of said material.

2. Heating apparatus as claimed in Claim 1 wherein said non-metallic, thermally insulative material comprises a microporous material.

3. Heating apparatus as claimed in Claim 1 or 2 wherein said source of infra-red radiation consists bf one or more tungsten-halogen lamps, each including a tungsten filament supported within a generally tubular quartz envelope.

4. Heating apparatus as claimed in Claim 3 wherein each of said lamps includes an infra-red-reflective coating formed on a lower portion of the surface of said envelope.

5. Heating apparatus as claimed in any one of Claims 1 to 4 wherein said structure comprises a shallow tray member, within which said non-metallic thermally insulative material is accommodated.

6. Heating apparatus as claimed in any preceding claim and including a thermal limiting device for ensuring that a maximum operating temperature of said layer of glass ceramic is not exceeded.

7. Heating apparatus as claimed in any preceding claim and including support means for supporting respective end regions of said sources of infra-red radiation.

8. Heating apparatus as claimed in any preceding claim and including a temperature control arrangement consisting of switching means for interconnecting filaments of said sources in various series and/or parallel combinations, so as to provide a range of power outputs from said sources.

9. Heating apparatus substantially as herein described with reference to the accompanying drawings.

10. A cooking hob incorporating at least one heating unit, as claimed in any preceding claim, said infra-red-transmissive layer forming a cook-top for supporting cooking utensils and the, or each of, said heating units defining a hotplate area of said cook-top.

10. A cooking hob incorporating at least one heating apparatus as claimed in any preceding claim.

1. A heating unit including a piurality of halogenated, tungsten filament lamps supported between a layer of material transmissive of infra-red radiation generated by said lamps and a mass of microporous material, the respective surfaces of said microporous material and said layer adjacent said lamps being disposed substantially parallel and in closely proximate relation to each other, and electrical coupling means connecting said lamps to a source of electrical energy and including switching means for selecting the intensity of said generated radiation, said mass of microporous material being conditioned to reflect infra-red radiation generated by said lamps towards said infra-red-transmissive layer.

2. A unit as claimed in claim 1 wherein said surface of said mass of microporous material adjacent said lamps is coated with an infra-red-reflective material.

3. A unit as claimed in claim 1 wherein said mass of microporous material is surfaceenriched with an infra-red-reflective material.

4. A unit as claimed in claim 2 or 3 wherein said infra-red-reflective material is a metallic oxide.

5. A unit as claimed in any preceding claim wherein said lamps are each provided with an infra-red-reflective coating to reflect radiation generated thereby towards said infra red-trahsmissive layer.

6. A unit as claimed in any preceding claim wherein said mass of microporous material is accommodated within a shallow tray member.

7. A unit as claimed in any preceding claim and including a thermal limiting device arranged to regard primarily to the temperature of said infra-red-transmissive layer to ensure that said layer does not exceed a predetermined maximum operating temperature.

8. A unit as claimed in any preceding claim wherein said switching means includes a multi-pole switching arrangement for switching the filaments of said lamps into series and/or parallel combinations to achieve a range of radiation intensities output from said lamps.

9. A heating unit as claimed in any preceding claim, substantially as herein described with reference to the accompanying drawings.