GB2383126A

GB2383126A - An oven with a magnetron and a thick-film heating means

Info

Publication number: GB2383126A
Application number: GB0129561A
Authority: GB
Inventors: Gavin John Coleman
Original assignee: Ceramaspeed Ltd
Current assignee: Ceramaspeed Ltd
Priority date: 2001-12-11
Filing date: 2001-12-11
Publication date: 2003-06-18
Anticipated expiration: 2021-12-11
Also published as: GB2383126B; GB0129561D0

Abstract

The oven comprises a main heating means and an auxiliary heating means, the auxiliary heating means comprising a thick-film heating element 29 disposed in the base 3 of the oven cavity 17. Preferably the main heating means is a magnetron 23 which produces microwaves or a resistance heating element with a fan (71, 73 fig. 7), and a grill type heating element (77, fig. 7) directing heat downwardly on to a food item disposed on a supporting means. The supporting means is preferably a turntable 19, and is used to support the food item above the base. The thick-film heating element is preferably applied to a substrate such as a coated metal or a ceramic material. The construction of the thick-film heating element is such that it includes a track 33 which is meandered so that the track is spaced closer together as it approaches the periphery of the turntable (figure 1). Preferably the auxiliary heating means is shaped as a rectangle which is offset from the central vertical axis of the turntable, or it could be circular shaped with the central support shaft of the turntable passing though it.

Description

OVEN WITH AUXILIARY HEATING MEANS This invention relates to an oven with auxiliary heating means.

Ovens, such as microwave ovens and fan-assisted ovens, generally have one or more main heating means at the top or in an upper region of an oven cavity. Nevertheless, it is known to incorporate an auxiliary heating means in the region of the base of the oven cavity. Such an auxiliary heating means is generally employed because current rapid cook ovens, such as microwave ovens and top-grill ovens, cook the top and upper region of a food item placed in the oven cavity more effectively than the bottom or lower region thereof.

It has been proposed to arrange an auxiliary heating means beneath the lower wall of the oven cavity of a microwave oven, but such an arrangement has the disadvantage that it is necessary first to heat the base of the oven cavity and then to rely on convection and conduction from the base to the food item being cooked. In cases where the oven is provided with a turntable, the turntable forms a further barrier to the transfer of heat to the food item being cooked.

It has also been proposed to provide an auxiliary heating means within the oven cavity of a microwave oven. Such an

arrangement is known, for example, from US-A-3 172 987 and US-A-4 326 113. According to US-A-3 172 987 a sheathed resistance heating element is mounted slightly above the base of an oven cavity beneath a turntable. A similar arrangement is shown in US-A-4 326 113 except that the sheathed heating element substantially protrudes beyond the periphery of the turntable. The arrangement of a sheathed heating element within the oven cavity has the disadvantage that the heating element is relatively exposed and readily becomes soiled, while being relatively difficult to clean.

It has further been proposed in US-A-4 132 216 to provide a hot-air oven for heating food-loaded cartridges which are arranged in an annular array on a turntable. Air is heated at the top of the oven cavity and blown by a propeller into the hollow core formed by the annular array, such that part of the air passes through holes in the cartridges and past food containers therein, while the remainder of the air passes through a flow passage beneath the cartridges and above the turntable. The arrangement of a heating element and a propeller within the oven cavity gives rise to the disadvantage that the internal components are difficult to clean, while the propellor is potentially dangerous and requires to be shielded from the user.

It is therefore an object of the present invention to provide an oven with auxiliary heating which overcomes or ameliorates the above disadvantages.

According to the present invention there is provided an oven comprising an oven cavity, a main heating means for heating a food item placed in the oven cavity and auxiliary heating means arranged in a base of the cavity for heating a lower region of the food item, the auxiliary heating means comprising a thick film heating element.

The main heating means may comprise a magnetron or a resistance heating element and fan means for propelling air past the heating element and into the oven cavity. The main heating means may further comprise a grill element for directing radiation downwardly towards the food item.

The auxiliary heating means may comprise a thick film element applied to a substrate, such as of ceramic or coated steel.

The auxiliary heating means may be mounted in a recess formed in the base of the oven cavity.

The thick film heating element may be provided with thermal insulation to reduce loss of heat from the oven cavity.

The thermal insulation may be provided in a tray.

The oven may include means for supporting the food item above the base of the oven cavity.

The supporting means may be provided with a downwardlyextending peripheral lip.

The supporting means may comprise a turntable.

The track of the thick film element may be arranged such that the heat emitted by the auxiliary heater increases with distance from the axis of rotation of the turntable.

The auxiliary heating means may be substantially rectangular and offset laterally relative to an axis of rotation of the turntable. Alternatively, the auxiliary heating means may be substantially circular, a support shaft of the turntable passing through the auxiliary heating means.

For a better understanding of the present invention and to show more clearly how it may be carried into effect reference will now be made, by way of example, to the accompanying drawings in which: Figure 1 is a plan view of an embodiment of a microwave oven incorporating an auxiliary heating means according to the present invention, the top of the oven being omitted for clarity; Figure 2 is a front elevational view of the microwave oven shown in Figure 1, with the door removed for clarity;

Figure 3 is a cross-sectional view showing on a larger scale one embodiment of an auxiliary heating means arranged in the base of the microwave oven of Figures 1 and 2; Figures 4 and 5 are similar to Figures 1 and 2 and show an alternative configuration of auxiliary heating means arranged in the base of a microwave oven; Figure 6 is a plan view of an embodiment of a fan-assisted oven incorporating an auxiliary heating means according to the present invention, the top of the oven being omitted for clarity; and Figure 7 is a front elevational view of the fan-assisted oven of Figure 6, with the door removed for clarity.

The microwave oven shown in Figures 1 and 2 comprises a body having a top 1, a base 3, a back 5, side walls 7 and 9 and a door 11 provided with a handle 13 for opening and closing the door. A control panel 15 is provided for controlling operation of the oven. Provided within an oven cavity 17 is a turntable 19 which is rotatable in known manner when the oven is in operation. The outer periphery of the turntable has a lip which extends downwardly towards the base of the oven so as to retain heat in the space between the base and the underside of the turntable. A plurality of feet 21 are provided beneath the oven body to support the oven on a suitable surface (not shown).

A magnetron, shown diagrammatically at 23, is provided in a chamber behind the control panel 15 and functions as a main heating means to supply microwaves in known manner into an upper region of the oven cavity 17 as indicated by arrows 25. The microwaves are very effective in heating the upper part of a food item 27, but are not as effective in heating the lower part of the food item.

The upper part of food item 27 may additionally be heated with a grill heater (not shown) provided in the top 1 of the oven body and directed downwardly onto the upper part of the food item.

However, while the magnetron and a grill if provided effectively heat the upper part of the food item, the lower part of the food item is not effectively heated and this can lead either to the upper part of the food item being over-cooked or the lower part of the food item being undercooked. Either case is visually undesirable, while undercooking of the lower part of the food item can be hazardous to health.

According to the present invention there is provided in the base of the microwave oven an auxiliary heater 29. The auxiliary heater 29 is not required to provide heat to the entire oven cavity, only to the lower region of food item 27. Thus the power output of the auxiliary heater can be relatively low, for example 200 watts.

One embodiment of the auxiliary heater 29 shown in Figures 1 and 2 is illustrated in more detail in Figure 3. The auxiliary heater of Figures 1 to 3 underlies part only of the turntable 19 and is offset laterally relative to the axis of rotation of the turntable such that the food item rotates in use into and out of the area heated by the auxiliary heater. Such a relatively small heater is economical to manufacture and install in the microwave oven.

The auxiliary heater 29 comprises a thick film heating element applied to, for example the underside of, a suitable substrate 31, such as of ceramic or steel which is coated with a layer of glass or other suitable electrically insulating material. The auxiliary heater is mounted in a shallow recess formed in the base of the oven cavity such that the floor of the cavity is substantially smooth. The thick film heating element demonstrates significant resistance to abrasion and provides a surface which is easy to clean. As can be seen from Figure 1, the track 33 of the thick film heating element becomes progressively closer as the distance from the axis of the turntable increases in order to emit more heat and to provide substantially even heating of the food item irrespective of the distance from the axis of the turntable.

Loss of heat from the base of the oven cavity can be minimised in a number of ways. For example, thermal

insulation material may be arranged between the base of the oven cavity and the outer casing of the microwave oven. Alternatively, the auxiliary heater may be mounted in a tray or the like (not shown) which provides thermal insulation material between the substrate and the base of the tray.

Figures 4 and 5 show a microwave oven provided with an alternative configuration of auxiliary heater 29. As can be seen, the auxiliary heater of Figures 4 and 5 underlies substantially the entire area of the turntable 19 with a shaft for rotating the turntable passing through an aperture provided in the heater. Thus, the relatively large auxiliary heater is more effective in heating the lower region of the food item.

The fan-assisted oven shown in Figures 6 and 7 comprises a body having a top 51, a base 53, a back 55, side walls 57 and 59 and a door 61 provided with a handle 63 for opening and closing the door. A control panel 65 is provided for controlling operation of the oven. Provided within an oven cavity 67 is a turntable 19 which is rotatable in known manner when the oven is in operation.

A main resistance heating element 71 is provided behind the back 55 of the oven cavity and a fan 73 propels air past the heating element and distributes the hot air around the oven cavity for relatively even heating of the top and

sides of a food item 75 to be heated. The hot air from the main heating element 71 is very effective at heating the upper part of the food item 75, but is not as effective at heating the lower part of the food item.

The upper part of food item 75 may additionally be heated with a grill heater 77 provided in the region of the top 51 of the oven body and directed downwardly onto the upper part of the food item.

However, while the hot air from main heating element 71 and grill 77 if energised effectively heat the upper part of the food item 75, the lower part of the food item is not effectively heated and this can lead either to the upper part of the food item being over-cooked or the lower part of the food item being under-cooked. Either case is visually undesirable, while under-cooking of the lower part of the food item can be hazardous to health.

According to the present invention there is provided in the base of the fan-assisted oven an auxiliary heater 29. The auxiliary heater 29 is not required to provide heat to the entire oven cavity, only to the lower region of food item 75. Thus the power output of the auxiliary heater can be relatively low, for example 200 watts.

The auxiliary heater 29 shown in Figures 6 and 7 is similar to that illustrated in more detail in Figure 3 and

described hereinabove except that the heater is circular as shown in Figures 4 and 5. If desired, the auxiliary heater of Figures 6 and 7 could be the same as that shown in Figures 1 to 3 and underly part only of the turntable 19.

It should be noted the turntable 19 is not always required, particularly in relation to a fan-assisted oven. If the turntable is omitted an alternative means (not shown) is provided to support the food item 27,75 a short distance above the base of the oven cavity.

Claims

CLAIMS 1. An oven comprising an oven cavity, a main heating means for heating a food item placed in the oven cavity and auxiliary heating means arranged in a base of the cavity for heating a lower region of the food item, the auxiliary heating means comprising a thick film heating element.
2. An oven as claimed in claim 1, wherein the main heating means comprise a magnetron.
3. An oven as claimed in claim 1 or 2, wherein the main heating means comprises a resistance heating element and fan means for propelling air past the heating element and into the oven cavity.
4. An oven as claimed in claim 2 or 3, wherein the main heating means furthers comprise a grill element for directing radiation downwardly towards the food item.
5. An oven as claimed in any preceding claim, wherein the auxiliary heating means comprises a thick film element applied to a substrate.
6. An oven as claimed in claim 5, wherein the substrate is of ceramic material.

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7. An oven as claimed in claim 5, wherein the substrate is of coated steel.
8. An oven as claimed in any preceding claim, wherein the auxiliary heating means is mounted in a recess formed in the base of the oven cavity.
9. An oven as claimed in any preceding claim, wherein the thick film heating element is provided with thermal insulation to reduce loss of heat from the oven cavity.
10. An oven as claimed in claim 9, wherein the thermal insulation is provided in a tray.
11. An oven as claimed in any preceding claim and including means for supporting the food item above the base of the oven cavity.
12. An oven as claimed in claim 11, wherein the supporting means is provided with a downwardly-extending peripheral lip.
13. An oven as claimed in claim 11 or 12, wherein the supporting means comprises a turntable.
14. An oven as claimed in claim 13, wherein the track of the thick film element is arranged such that the heat

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emitted by the auxiliary heating means increases with distance from the axis of rotation of the turntable.
15. An oven as claimed in claim 13 or 14, wherein the auxiliary heating means is substantially rectangular and offset laterally relative to an axis of rotation of the turntable.
16. An oven as claimed in claim 13 or 14, wherein the auxiliary heating means is substantially circular, a support shaft of the turntable passing through the auxiliary heating means.
17. An oven substantially as hereinbefore described with reference to, and as shown in, the accompanying drawings.