IE55415B1 - Heating apparatus - Google Patents

Description

This invention relates to a thermal limiting device and, in particular, though not exclusively, the invention relates to a thermal limiting device suitable for controlling power supplied to a heating apparatus incorporating one or more sources enissive of radiation In the wavelength range Ο.^μη to with the peak emissivity occurring at approximately- l.^ptm.

One such heating apparatus is described in GB-A-1273023, tp the Electricity Council, wherein one or more sources of infra-red radiation, each consisting of a tungsten filament lamp, are arranged below a glass ceramic cook top. A metal reflector is disposed below the sources so as to reflect radiation, emitted in a downward direction from the sources, upwardly towards the glass ceramic cooktop.

GB-A-2071969 describes s known thermal Halting device which comprises a tenperature responsive resistor, operatively connected to an annular, electrically resistive heating element, and a metallic reflector for focussing heat from the glass ceramic cooktop onto the resistor.

GB-A-2069300 describes a thermal limiting device which includes a rod made of a thermally expansive material which is arranged, when subjected to a maximum temperature/ to activate a snap-action switch connected operatively to sources of infra-red radiation· It has been found that these known thermal limiting devices may not be primarily responsive of the actual temperature of the glass ceramic cooktop· It is an object of the present invention to provide a thermal limiting device which at least alleviates the afore-mentioned problem· Accordingly there ia provided a thermal limiting device for monitoring the operating tenperature of a glass ceramic top of a cooking hob which includes at least one source of infra-red radiation and for controlling operation of said at least one source to prevent the tenperature of the glass ceramic top exceeding a predetermined maximum temperature, said thermal limiting device including a rod member made of a thermally - expansive material, the rod member being arranged, when subjected to said maximum temperature, to activate switching means arranged to de-energise said at least one source the thermal limiting device being characterised by provision of an infra-red reflective coating on the rod member, the coating being effective to reflect infra-red*radiation, produced directly -by said at least one source, away fron said rod nenber thereby enabling said rod nenber to respond primarily to the operating tenperature of said glass ceramic top· The Invention will now be further described by way of example only with reference to the accompanying drawings, tfhereln: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 Figure 1, Figure 3 shows a sectional view on ϊ-X, in the direction indicated.

Figure 4 shows s 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 embodiment shown, and, Figure 6 shows a schematic sectional view of part of the embodiment shown in Figure 1.

Referring to Figure 1, a generally circular shallow tray 1, preferably made of metal, has disposed therewithin, on the base ' thereof, a layer 2 of thermally insulative material,, which may be fabricated frcm 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 l, each flange having upturned end portions, 5 and 6, repectlvely, 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 nnd 4.

A moulding 8 of ceramic fibre material Is disposed above the tray 1 and press-fitted around the ends of esoh source 7 to provide a suitable packing therefor, Each source 7 of Infra-red radiation comprises a quarts, - halogenated tubular lamp including a tungsten filament (not shown in Figure 1), one suitable example cf which is described and claimed In Patent Specification Ho. , ia the name of THORN EMX pic.

Each lamp has moulded ceramic end caps, one shown at 9, enclosing a pinch-seal (not shown) with an amp tag connector connected to aa 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 cape 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. Italso permits conduction of heat away from the lamp pinch-seal a via the flange to maintain satisfactory operating temperatures. Heat is also conducted away from the lamp ends hy way of the electrical wiring attached thereto'.

If further cooling of the pinch seals is required, heat sinking end conventional cooling techniques disclosed in any of European Applications EF-A-0 123 344, EP-A-O 131 372 and EP-A-0 132 041 may be employed, or any other suitable technique known to those skilled in the art.

The ceramic fibre moulding 6 Is also sufficiently flexible to allow 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 Figure 1 are preferably disposed below a layer of glass ceramic, which is la this example fabricated from Corning Black Cooktop 9612, 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 mlcroswlteh 12 and V * 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 12 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.

Figures 2 and 3, in which like parts are labelled with like reference numerals with respect to Figure i, show sectional views of the apparatus shown in Figure 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 optima transmission of Infra-red radiation emitted free 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 glees oeramlc materiel are suoh that vaveloigths below 0.6 b are substantially absorbed. However, some visible radiation above thia wavelength la transmitted, as red light, thus providing e visible Indication of power level.

The heating arrangement, aa 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 ia approximately 6W/cm, whereas In thia embodiment, the matching between the energy emission characteristic of the lamps and tbe energy . transmission characteristics of the cooktop is suoh that an increased energy loading of up to as much'as 8H/cm2 may be achieved.

Figure 4 shows a spectral transmission curve for the preferred ceramic, approximately 4mm In thickness, and It can be seen at line A on tba horizontal axis indicating wavelength that, at the peak value, le. 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 conjuction with the preferred configuration of four 500W filament lamps, to provide a range of powers of approximately 2KW to 147V, by switching the filaments Into various series and/or parallel combinations.

Figure 5 shows six switching combinations of the four 500H filament lamps, one shewn at 7 in Figure 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. Figure 5 also Indicates the' percentage of each power output relative to the total output I.e. 2000W. It can be seen that a diode 13 Is used In two of the six combi nations to ensure that each control setting, especially tbe lower settings, provide an aesthetloally-pleaslng 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 sinering 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 oooklng 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 tbe. mains waveform.

Xt has been found that, in some circumstances, harmonic disturbances may tend to be Imposed on the mains supply in the switching combination, providing control setting Ho. 3· To mitigate this problem, It may be preferable to.replace diode 13 with two oppositely-directed diodes, respectively, in the two parallel arrangements forming this combination, thereby suppressing the second and fourth mains harmonies.

Moreover, Implementation of tbe 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 oontrol device, incorporating dlacs, trises, etc, or any alternative conventional oontrol, may be Implemented at powers below approximately 20W, 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 laaps. It may be further advantageous to eaploy, for exanple, two continuously-energised lamps, together vlth two burst-fire controlled leaps, as the two burst-fire controlled lanps nay thus be operated at a considerably higher frequency 5 than if four burst-fire controlled leaps were utilised.

The theraal Uniter, shown at 11 on figures 1 and 2, is used to ensure that the naxinwa operating tenperature, le. approxlaately 700°C, of the wdersurface of the glass cereaic is not exceeded. The thermal Halter 11 needs to be adjusted to 10 avoid nuisance tripping of the aloroswitch 12, thereby disconnecting the power supply bo the lamps.

The incorporation of a theraal llniter into the apparatus is further advantageous, in that it allows the use of utensils of any aaterial in oonjmction therewith. However utensils 15 having certain characteristics will perf ora differently with the present invention, than with other oooking hobs. As heating is substantially increased by infra red transmission to the utensil base, distorted infra-red absorbing utensils will operate nore efficiently with tha present invention, than with other 20 electrical oooking hobs, where good contact is required betveen the utensil base and the heated area, to all cm conduction of heat. Conversely utensils having highly reflective bases, which are not flat, will operate leaa efficiently with the present invention, as the infra red radiation will be reflected back to 25 the hob surface. This will cause the operating temperature of the apparatus to inorease and the theraal llniter to operate.

In such circumstances the theraal llniter will switch the laape on and off to aaintain a satisfactory glass cereaic tenperature, thereby providing a visual Indication that the utensil being 30 used is causing inefficient operation.

The insulative layer 2 ls preferably approximately 12ma thick, and it asy have grooves provided in the surface thereof to accoonodate 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.

Aa shown in Figure 6, wherein a schematic view of a cross section of a lamp 14, in association with the glass ceramic layer 15 is illustrated, tha lamp 14 has an integral oxide or other suitable reflector in the form of a coating 16 on the lower part thereof. A filament 17 of the lamp 14 is' positioned at the focal point of the coating 16, so that downwardly-emitted radiation froa the filament 17 is reflected either back1 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 aa a metallic oxide, or the surface layer cf 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 oonJunction with the reflective ooating on the lamps and/or the surface of the layer, is advantageous over conventional infra-red cooking bobs, as amission from the lamp matches •transmission by the glass ceramic layer, consequently reflected radiation pisses 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 oeramlc -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 aa, or instead of. Incorporating an optical filter vithift 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 coat and reliability. Similarly, devices such discs, triacs and phase controllers can be used· 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-rad 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 bees, may be utilised.

The area of the hob surface illuminated by the lamp le 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. it. .aa wall aa other suitable shapes and/or configurations'of the leaps, such as circular, seal-circular, horse-shoe shape, concentric rings with aligned end portions, or lamps which can be tapped at various points along their lengths.

Plying leads may be used, as an alternative to sap tag connectors, at each end of the leaps.

The theraal -Halter 11 aay be disposed in any suitable position relative to the lamps, either above, below or at the sane level as, and parallel to, the leaps. As a further alternative, it aay be mounted in a vertical position relative to the lamps. The theraal Halter is shielded froa incident Infra-rad radiation ao that It responds primarily to the tenperature of the glass ceramic layer 2 and the shield takes the form of a suitable infra-rad reflective coating, such as a metallic oxide coating.

Furthermore, the infra-red leaps aay be disposed in any vertical or borlsontal position relative to each other below the IS glass ceramic layer, ao as to obtain an even distribution of Infra-red radiation over the ooo Id ng 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, Vaclcer or Johns-Manville, or mineral wool, glass fibre, calcium silicate, ceramic fibre, or alialna fibre, although in some cases a substantial thickness of the insulative material may he required to ensure efficient operation. A suitably strong material may also be fabricated ao as to be self-supporting, thereby eliminating the need for a tray to support the material and lamps.

Alternatively, lf 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 oolour temperature of approximately 240«, but, hcwever, operation is possible at other oolour temperatures within the range of approximately 1800K * 300«.

Boating apparatus in aocordanoe with the present Invention may be suitably orientated ao that it may be employed in alternative applications, suob as microwave ovens, grills, barbecues, toasters, eleotrle fires and rotisseries.

In the preferred embodiment of the cooking hob, four heating apparatuses, in accordance with the present Invention, are provided below the layer of glass ceramic. However, any ntmber of such beating apparatuses may be employed and, in particular, a single heating apparatus may ba used in a cooking hob of substantially smaller size than that of the preferred hob.

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 3 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 oooklng utensil made of any material. .

Claims (8)

1. a thermal limiting device for monitoring the operating temperature of a glass ceraaic top of a cooking hob which includes at least one source of infra-red radiation and for controlling operation of said at least one source to prevent the tenperature of the glass ceramic top exceeding a , predetermined maximum temperature, said thermal limiting device including a rod member made of a thermally expansive material, the rod member being arranged, when subjected to said maximum temperature, to activate switching means arranged to de-energise said at least one source , the thermal limiting device also including an infra-red reflective coating on the rod member, the coating being effective to reflect infra-red radiation, produced directly by said at least one source, away froa said rod member thereby enabling said rod member to respond primarily to the operating temperature of said glass ceramic top·

2. A thermal limiting device according to Claim 1 wherein said infra-red reflective coating consists of a metallic oxide·

3. Beating apparatus for mounting beneath a glass ceramic top of a cooking hob, said apparatus including at least one source of infra-red radiation supported above a layer of thermally-ineulative material, and a thermal limiting device as claimed in any preceding claim.

4. · Beating apparatus ss claimed in Claim 3 wherein each of said sources of infra-red radiation consists of a quarts, halogenated, tubular lamp including a tungsten filament .

5. A cooking hob including a glass ceramic top and at least one heating apparatus, ss claimed in Claim 3 or Claim 4, mounted beneath said top..

6. A thermal limiting device according to Claim 1, substantially as hereinbefore described with reference to the accompanying drawings.

7. Heating apparatus according to Claim 3, 5 substantially as hereinbefore described with reference to the accompanying drawings.

8. A cooking hob according to Claim 5, substantially as hereinbefore described with reference to the accompanying drawings.