HEATING APPARATUS

Abstract: A heating unit (Figures 4, 5) suitable for mounting in a cooking hob consists of a circular tray having a base layer (2) of a thermally-insulative material, an inner, thermally insulative wall (8) which defines an inner, hot-plate region and an outer, peripheral wall (3) defining an outer hot-plate region, between the inner and outer walls. The walls support four infra-red emissive lamps (6) each having a first length (5) of linear filament associated with the inner, hot plate region and a second length (16) of linear filament associated with the outer hot plate region. The first length can be selectively energised independently, of and concurrently with, the second length.

Description: This invention relates to improvements in heating apparatus according to the preamble of Claim 1 and in particular though not exclusively, to heating apparatus of the type described in the later published U.K. Patent Application GB-A-2132060 and in the later published corresponding EP-A2-0117346, EP-A1-0132888 and EP-A2-0149267 (forming state of the art only in the meaning of Art. 54(3) EPC) incorporating one or more sources of infra-red radiation. The heating apparatus described in U.K. Patent Application GB-A-2132060 consists of, in one example, a shallow tray member containing a layer of thermally-insulative material, above which four infra-red-emitting, tungsten-halogen lamps are supported. The tray member, usually along with a number of similar members may be mounted beneath a layer of glass ceramic to form a cooking hob. FR-A-2,448,693 and corresponding GB-A-2 044 057, from which is known a heating unit according to the preamble of Claim 1, describe a dual-ring heating unit including a first wire-wound, electrically resistive heating element located in a central heating zone bounded by an inner cylindrical wall made of a thermally-insulative material, and a second wire-wound electrically resistive heating element located in an outer, annular heating zone bounded by the inner, cylindrical wall and an outer cylindrical wall. EP-A1-0037638 and GB-A-2092869 also describe dual-ring heating elements comprising inner and outer heating zones, each zone being provided with a respective, electrically resistive heating element. US-A-3,375,346 describes an infra-red surface heating unit comprising a support plate made of an infra-red transmissive material and an infra-red emissive lamp underlying the support plate. The lamp has a bulbous envelope having a domed top surface immediately adjacent the support plate. GB-A-1,273,023 describes an electrical surface cooker which comprises a support plate made of a glass ceramic material and a source of infra-red radiation in the form of one or more tubular lamps emissive of infra-red radiation. DE-A1-2,809,131 describes a heating unit which comprises an infra-red transmissive support plate and a wire-wound resistive heating element which is mounted below the support plate and supported by an open-ended quartz tube. A thermal limiter is preferably provided in the heating apparatus to ensure that the maximum operating temperature of the glass ceramic is not exceeded. A number of discrete temperature settings are also provided by switching the lamps into series and/or parallel configurations, wherein one or more diodes are usually required to achieve one or more of the lower settings. However, some problems may be found to occur under particular circumstances of operation of the apparatus, such as operation of the limiter causing it to trip and disconnect the power supply to the lamps, which may be a source of annoyance for a user of the apparatus, or it may not be desirable to use the diodes in the switching arrangement, as they tend to interfere with the harmonics of the main supply. It is therefore an object of the present invention to provide heating apparatus which alleviates at least the above-mentioned potential problems, as well as being cost-effective. According to the present invention, there is provided a heating unit suitable for mounting in a cooking hob, said unit comprising, a base layer of a thermally insulative material, an inner, thermally-insulative wall defining an inner hot-plate region, an outer, peripheral thermally-insulative wall defining an outer hot-plate region, between said inner and said outer walls, the heating unit being characterised by the provision of at least one lamp comprising a tubular envelope and means emissive of infra-red radiation, supported within said envelope, the lamp having a first end and a second end and being so supported that said envelope extends across said inner and outer hot-plate regions and said first and second ends are located outside said outer, peripheral-thermally insulative wall, said means for emitting infra-red radiation comprising a first length of linear filament associated with said inner hot-plate region and a second length of linear filament associated with said outer hot-plate region, and by connection means enabling said first length of linear filament to be selectively energised independently of, and concurrently with, said second length of linear filament. The invention will be described by way of example only with reference to the accompanying drawings, wherein:- Figure 1 shows a schematic plan view of one embodiment of the invention, Figure 2 shows an alternative embodiment to that shown in Figure 1, Figures 3a and 3b show switching arrangements to provide discrete power outputs in accordance with the invention, Figures 4 and 5 show two further embodiments of the invention, and Figure 6 shows a schematic sectional view through I-I in Figure 1 of the embodiment shown, mounted in a cooking hob. Referring to Figure 1, there is shown a heating unit consisting of a circular tray 1 having a layer 2 of thermally-insulative material, such as microporous material, disposed therein and a peripheral thermally-insulative wall 3, within which four infra-red lamps, one shown at 4, are supported. Each lamp 4 consists of tungsten filament 5 supported within a tubular quartz envelope 6, and each end of each lamp 4 is formed with a pinch seal (not shown), having a connection between the respective end of the filament 5 and an electrical connector sealed therein, each pinch seal being housed within a ceramic end cap, such as at 7. The tray 1 also includes an inner thermally-insulative wall 8, which is concentric with the peripheral wall 3 and of such a diameter as to enclose the entire length of all of the filaments 5 within the lamps 4. Between the peripheral wall 3 and the inner wall 8, two conventional, electrical, wire-wound heating elements 9 and 10 are disposed. As shown in Figure 6, the heating unit is preferably mounted beneath, and urged up towards, a glass ceramic plate 19, by a suitable mounting arrangement 20, to form a hotplate of a cooking hob. The mounting arrangement 20 is fixed to a base 21 of a housing for the cooking hob. The four lamp filaments 5 and the two elements 9 and 10 may be energised independently and/or concurrently, so that only the central hotplate region within the inner wall or alternatively the whole region may be heated. Furthermore, any combination of one or more lamps and one or both elements may be energised to provide various discrete power outputs, and thus temperature settings, whilst maintaining an aesthetically pleasing balanced effect of the lamps, as seen through the glass ceramic plate. Preferably, a thermal limiter (not shown) either mechanical or electronic, is also employed within the heating apparatus to limit the operating temperature of the apparatus to prevent damage to the glass ceramic by overheating. It may be preferable for the limiter to control the heating elements 9 and 10 only, so that tripping of the limiter would not be so conspicuous, as only the elements, and not the lamps, would be de-energised if overheating occurred. It may be preferable to arrange that switching of the lamp filaments into various series and/or parallel configurations provides relatively high power outputs and that energisation of one or both of the heating elements 9 and 10 provides lower power outputs. Figure 2, wherein like parts are labelled with like reference numerals with respect to Figure 1, shows an alternative embodiment to that shown in Figure 1, wherein only three lamps, such as at 4, are provided in combination with only one heating element 9. Such an arrangement, as shown in Figure 2, may be advantageous in that one lamp has been replaced by a conventional heating element, thereby providing a cost saving in manufacture of the apparatus. Figures 3a and 3b show the configurations of four lamp filaments 11 to 14 required to generate the third lowest and the lowest power outputs of the apparatus. The remaining configurations for the power outputs provided are shown in the above-mentioned U.K. Patent Application No.GB-A-2 132 060, wherein the configurations giving the same power outputs as those shown in Figures 3a and 3b also include one or more diodes to generate the required power output. However, the use of these diodes may be disadvantageous, as they tend to cause harmonics disturbances in the mains power supply. The present configurations shown in Figures 3a and 3b overcome this disadvantage by including an additional element 15 in series with the configuration formed by lamp filaments 11 to 14, which, when switched into the two configurations generates the same respective power outputs as when diodes are used. The element 15 may be either a fifth lamp filament or a conventional heating element as shown in Figures 1 and 2. A further use of the additional element 15 may be as a pre-heating device to produce faster warm-up periods of the apparatus. The use of the element 15 would provide a high power output fur an initial warm-up period, the length of which could be controlled by a timer and/or a thermal sensor device. The element 15 could also be used at various times during energisation of the lamp filaments 11 to 14, but preferably not continuously. Subsequent use of the element 15 after the initial warm-up period could be controlled manually. The initial warm-up period could be controlled by any suitable type of timing device, so that the element 15 is de-energised at the end of a predetermined time, such as 6 mins. for example. Alternatively or additionally, the initial warm-up period could be controlled by any suitable thermal device, including a pre-set thermostat disposed at any suitable location within the apparatus to cause de-energisation of element 15 when a certain operating temperature is attained. Possible advantages of using a pre-heating element 15 may be that power outputs exceeding the limit that the glass ceramic is capable of withstanding may be utilised, because it is only energised until the required operating temperature is attained, and additionally at lower outputs, a thermal limiter to protect the glass ceramic may not be required, as the glass ceramic should not exceed its limit at these lower outputs. This therefore may provide an additional cost saving. However, if a limiter to protect the glass ceramic is employed, it may be necessary to cause the timer and/or thermal device controlling the pre-heating element 15 to by-pass the limiter, at least while the element 15 is initially energised, to prevent nuisance tripping of the limiter. Figures 4 and 5, wherein like parts are also labelled with like reference numerals with respect to Figures 1 and 2, show two alternative embodiments which include an additional filament accommodated within each lamp, instead of a conventional heating element. Each lamp, as at 4 shown in Figure 4 accommodates a filament 5, the length of which is commensurate with the area of the inner hotplate region within the wall 8, and a second filament 16, the length uf which is commensurate with the area of the whole hotplate region within the outer wall 3. The two filaments may be energised independently and/or concurrently as required. The embodiment shown in Figure 5 is provided with a non-central inner hotplate region within the inner wall 8, which is integral around part of its circumference with the outer wall 3. A lamp filament 18 is accommodated within each lamp 4, having a length commensurate with the area of the whole hotplate region. However, each filament 18 is tapped at a point 19 along its length, so that, if required only the filament length coinciding with the area of the inner hotplate region may be energised. Filament 18 thus effectively consists of a filament 22 commensurate the inner hotplate region and an extended portion 23 thereof commensurate with the whole hotplate region. The embodiments shown in Figures 4 and 5 may be advantageous in providing an increased number of switching combinations of the filaments, especially if they are maintained at a 100% output, as well as being cost-effective, because two filaments are accommodated within only one lamp. In the preferred embodiment of the heating apparatus described in U.K. Patent Application No.GB-A-2 132 060, each lamp is provided with an infra-red-reflective coating applied to the lower half of the quartz envelope, to reflect radiation emitted downwardly from the filaments back in an upwards direction towards the glass ceramic plate. It may however be preferable to provide a suitable infra-red reflector between the lamps and the layer 2 of thermally-insulative material, which may be cost-effective and easier to manufacture. It can of course be envisaged that some of the different embodiments shown in the Figures could be used in combination with each other, as well as each embodiment being used in isolation.

Claims: A heating unit suitable for mounting in a cooking hob, said unit comprising, a base layer (2) of a thermally-insulative material, an inner, thermally-insulative wall (8) defining an inner hot-plate region, an outer, peripheral thermally-insulative wall (3) defining an outer hot-plate region, between said inner and said outer walls, the heating unit being characterised by the provision of at least one lamp (4) comprising a tubular envelope (6) and means (5; 18) emissive of infra-red radiation, supported within said envelope (6), the lamp (4) having a first end (7) and a second end (7) and being so supported that said envelope (6) extends across said inner and outer hot-plate regions and said first and second ends (7) are located outside said outer, peripheral thermally-insulative wall (3), said means (5; 18) for emitting infra-red radiation comprising a first length (5; 22) of linear filament associated with said inner hot-plate region and a second length (16; 23) of linear filament associated with said outer hot-plate region, and by connection means enabling said first length (5; 22) of linear filament (18) to be selectively energised independently of, and concurrently with, said second length (23) of linear filament (18; 16). A heating unit according to Claim 1 wherein said second length of linear filament comprises an extension (23) of said first length (22) of linear filament (18) and said connection means includes a tapping connection (19) effective to by-pass said extension (23) of said first length (22) of linear filament (18) (Fig. 5). A heating unit according to Claim 1 wherein said first and second lengths of filament comprise separate filaments (5; 16) arranged substantially parallel to each other in said envelope (6) (Fig. 4). A heating unit according to any one of Claims 1 to 3 wherein said inner and outer hot-plate regions are concentric. A cooking hob comprising an open-topped housing a glass ceramic plate, at least one heating unit according to any one of Claims 1 to 4 and mounting means (20, 21) for mounting said at least one heating unit adjacent to the underside of said plate (19) (Fig. 6).

Inventors: BROOKS, STEVE MARTIN, MAY, DAVID ROBERT
Applicants: THORN EMI PATENTS LIMITED
Application year: 1985
Main classification: H05B3
IPC codes: H05B3