GB2187070A - Heating apparatus - Google Patents

Abstract

An oven cavity 1 contains a turntable 2, upon which food is placed to be cooked by microwave energy directed into the cavity 1. To obtain browning of the food cooked by the microwave energy, two infra-red-emissive heating units 4,5 are located outside the cavity 1, so that infra-red radiation emitted therefrom is transmitted directly into the cavity 1. Each heating unit 4,5 includes an open, meandering wire element 8 formed from molybdenum disilicide, which emits near-infra-red radiation in a wavelength band of 0.8-5 mu m. <IMAGE>

Description

SPECIFICATION Microwave ovens This invention relates to microwave ovens.

The advantages of cooking by microwave energy, particularly the rapidity of the cooking process, are well-known, but, because the microwave energy does not brown the food, it is now also common practice to include additionally a heating means for effecting the browning of food cooked by the microwave energy.

Conventional browning means in microwave ovens generally consist of a metal-sheathed, wire-wound, resistance heating element mounted outside of the oven cavity and heat from the element is directed into the oven cavity for browning the food by air convection, with the use of a fan.

However, this known arrangement tends to have a relatively slow response, particularly as compared with the rapid response of the microwave energy, and tends to undermine the advantages of the microwave oven. Furthermore, the dimensions of the oven are considerably increased by the inclusion oftheairconvection unit.

An alternative arrangement, which alleviates the problems associated with the above-described known arrangement, is described in our copending UK Patent Publication No.21 52790A, wherein a number oftungsten-halogen lamps emissive of near-infra-red radiation are utilised to effect browning by transmission of the emitted infra-red radiation directly into the oven cavity. The thermal response of the infra-red lamps has been found to be at least comparable with that of the microwave energy and, due to the direct transmission of the infra-red radiation, an air convection unit is not necessary and therefore the oven can be made more compact.

However, the infra-red lamps are considerably more expensive than conventional metal-sheathed heating elements and also require careful protection from damage caused by exposure to the microwave energy.

It is therefore an object of the present invention to provide a microwave oven including browning means, which does not suffer from the disadvantages of either of the known browning means described hereinbefore.

According to the present invention there is provided a microwave oven including a source of microwave energy for cooKing food placed in the oven cavity and a heating element emissive of infra-red radiation for browning the food, said element being formed from molybdenum disilicide.

Preferably, the molybdenum disilicide element is mounted in a heating unit outside of the oven cavity with an opening into the oven cavity, through which the infra-red radiation emitted from the element is directly transmitted.

The use of a molybdenum disilicide element is advantageous, in that, whilst the thermal response ofthe element is almost as rapid as the infra-red lamps, it is considerably less expensive than the lamps and does not itself require protection from exposuretothe microwave energy.

Furthermore, like the infra-red lamps, the molybdenum disilicide element also emits near-infra-red radiation, which has been found to be far more efficient at browning than the infra-red radiation of longer wavelength emitted by conventional heating elements. However, whereas the lamps require a filter for filtering out the relatively high levels of visible radiation generated by the lamps, which may be harmful to a userofthe oven, the levels of visible radiation generated by the molybdenum disilicide element are sufficiently low to remove the necessity for such a filter, thereby increasing the amount of radiation for browning.

The molybdenum disilicide element is also advantageous, like the infra-red lamps, over the conventional, wire-wound, metal sheathed element in that it provides directtransmission of the emitted infra-red radiation into the oven cavity, thereby enabling a more compact oven to be made duetothe redundancy of an air convection unit.

The invention will now be further described by way of example only with reference to the accompanying drawings,wherein: Figure 1 shows a schematic sectional view of a microwave oven, in accordance with the invention, and Figure2 shows an enlarged plan view of a particular feature of the oven shown in Figure 1.

Figure 1 shows an oven cavity 1 containing a turntable 2, upon which food is placed to be cooked by microwave energy emitted from one or more sources (not shown) and directed intotheoven cavity 1 by a conventional waveguide 3, stirrer and/or suitable other means.

To obtain a browning effect of the food cooked by the microwave energy, two infra-red-emissive heating units 4,5 are provided outside of the cavity 1 at predetermined locations on the top and/or side walls of the oven cavity, so that infra-red radiation emitted therefrom is transmitted directly into the oven cavity 1, via respective openings in the cavity walls.

Each heating unit 4,5 comprises a metallic casing 6 accommodating a mass of thermally-insulative material 7. An open heating element 8, formed from molybdenum disilicide, is mounted in a recess 9 of the insulative material 7. The molybdenum disilicide element 8 is emissive of near-infra-red radiation in the wavelength band of 0.8-5 > m.

The metallic casing 6 is open on one side and mounted adjacentan opening 10 inthecavitywall, sothatinfra-red radiation emitted by the molybdenum disilicide element 8 is transmitted directly through the opening 10 into the oven cavity 1 to brown food supported on the turntable 2.

The opening 10 is closed by a plate 11 of an infra-red transmissive material, preferably a glass ceramic material, such as Schott (Robax), or quartz, which prevents the molybdenum disilicide element 8 and the interior of the heating unit 4,5 from being soiled by food particles or other cooking products dissipated from the food being cooked in the cavity 1.

Thethermally-insulative material 7 preferably consistsofaceramicfibreora microporous material, which may be opacified with a suitable infra-red reflective material, such as a metal oxide, so that it acts as a diffuse reflector ofthe infra-red radiation.

The metallic casing 6 is preferably provided with side flanges 12, 13,whichenabletheunittobe attached and earthed to the top or side wali of the oven cavity 1.

Figure 2 shows a schematic plan view of one of the heating units 4, wherein like parts are labelled with like reference numerals, with respectto Figure 1.

It can be seen that the molybdenum disilicide element8 consists of an open meandering wire having an end terminal block 14, within which the ends of the wire are connected to electrical power leads 15,16, provided externallyofthe unit 4.

It is preferable to provide the power leads 15, 16 with a microwave choke 17, such as a conventional quarter wave choke, which inhibits leakage ofthe microwave energy to the exterior of the cavity 1.

As an alternative to a reflective microporous material, the mass ofthermally-insulative material 7 may be provided with a surface coating or surface enrichment of a suitable reflective material, such as titanium dioxide or aluminium oxide, or any other suitable reflective means may be utilised.

To achieve optimum browning effects offood placed within the cavity 1,when using a turntable such as 2, it has been found advantageous to position the centre ofthe upper heating unit 4 ata location approximately 1/7 of width 18 ofthe cavity 1 from cavity sidewall 19 and to position the centre of the side heating unit Sat a location approximately 1/4 of height 20 of the oven cavity 1 from cavity base 21.

However, it may be preferable to provide a user-operable mechanism for adjusting the height of the side unit 5from the base 21, in dependence on the height of a food container placed on the turntable 2. Alternatively, the heating unit 5 or some component thereof, such as a reflector, could be provided with a pivotal arrangement so thatthe angle of emission of infra-red radiation from the element 8 could be varied.

A user-operable control arrangement for controlling operation of the microwave oven may include a number of settings, such as only energisation of microwave energy, energisation of microwave energy and the upper heating unit 4, or energisation of microwave energy together with alternate energisation of upper unit4 and side unit 5.

Alternatively, energisation of infra-red radiation only may be used for grilling purposes.

The control arrangement may provide pulsing of the microwave energy and infra-red radiation art a user-adjustablevariablefrequency and time proportions.

Furthermore, operation ofthe relative emission of microwave energy and infra-red radiation may be controlled by mark-space or phase-controlled techniques, orthe power supplied to respective microwave and infra-red sources may be varied by a user of the oven overtime.

Suitable control devices may be electronic or consist of electromechanical timers and switches arranged to provide required combinations of microwave energy and infra-red radiation, in accordance with a pre-setcooking programme.

It will be appreciated that a turntable is not vital to this invention and that if a turntable is not used the lamps could be re-positioned for optimum results.

Claims (7)

1. Amicrowaveoven including asourceof microwave energy for cooking food placed in the oven cavity and a heating element emissive of infra-red radiation for browning the food, said element being formed from molybdenum disilicide.

2. An oven as claimed in claim 1 wherein said element is mounted in a heating unit outside ofthe oven cavity, said cavity being provided with an opening to communicate said element with the interior of said cavity, so that infra-red radiation emitted from said element is transmitted directly into said cavity.

3. An oven as claimed in claim 1 or 2 wherein said element consists of a meandering open wire of molybdenum disilicide.

4. An oven is claimed in claim 2 or 3 wherein said heating unit consists of a metallic casing accommodating a massofthermally-insulative material, to which said element is mounted.

5. An oven as claimed in any one of claims 2,3 or 4wherein said opening is closed by a plate of infra-red transmissive material.

6. An oven as claimed in any one of claims 2 to 5 wherein power leads from said heating unit are provided with a microwave choke arrangement.

7. A microwave oven substantially as herein described with reference to the accompanying drawings.