GB2044057A

GB2044057A - Smooth top cookers

Info

Publication number: GB2044057A
Application number: GB8003748A
Authority: GB
Original assignee: Micropore International Ltd
Current assignee: Micropore International Ltd
Priority date: 1979-02-07
Filing date: 1980-02-05
Publication date: 1980-10-08
Also published as: DE3004187C2; AU533560B2; SE8000898L; DE3004187A1; GB2044057B; AU5526480A; IT1150077B; ZA80693B; NZ192821A; IT8019767A0; ATA65080A; CA1144969A; AT398874B; FR2448693A1; BE881566A; CH649621A5; FR2448693B1; US4327280A

Description

1 GB 2 044 057 A 1.

SPECIFICATION Smooth Top Cookers

This invention relates particularly to the socalled "smooth top" cookers of the type in which a smooth top normally of glass ceramic overlays one or more generally circular electric heater elements supported on a layer of thermal and electrical insulating material such that the element is spaced from the top. In use, a utensil placed on the top over an element is heated by the transmission of heat from an element to and through the top by air convection and infra red radiation. Such elements are referred to herein as 11 radiant heaters". The insulating material substantially prevents heat being transmitted away from the element except towards the top and as the preferred materials for the top are essentially thermally non-conductive, only areas of the top which are "exposed" to the element will be heated. In order to prevent heat being transmitted to parts of the top not covered by a utensil placed thereon, a peripheral wall of insulating material is also normally provided around the coil.

While the arrangement described above has been very successful, its best use efficiency is limited to use with utensils having bases of area substantially equal to that of the heater or 11 exposed" area of the top. Where the areas are not equal, either the heating of the utensil is non uniform, or local areas of the top are exposed which is wasteful of heat.

The present invention is directed primarily at the problem of adapting radiant heaters to the different sizes and shapes of cooking utensils which are in popular use. In one broad aspect, the invention contemplates non-circular heaters which are suited to utensils having non-circular bases such as oval casseroles which are currently popular. In another aspect, heaters of the invention comprise two or more heater colis of which at least one may be energised independently of the other. For example, the combination of an independent circular coil and an adjacent part-circular coil enables the heater to be used for circular utensils, over the circular coil, and for oval utensils over both coils. Similarly, a central primary coil may be surrounded by a secondary annular coil. Heaters according to this aspect of the invention are thus adapted to define a single continuous large heating area, when both coils are energised, which provides substantially uniform transmission of heat to the entire 11 exposed area" of the top for larger utensils, or a smaller area for smaller utensils.

Where two or more heater elements are used, it is also preferable to separate the elements by one or more dividing walls to confine the transmitted heat to separate distinct heating zones on the top. In this way, undesirable spread of the heat transmitted from the element is 125 minimized, bearing in mind that the top is normally substantially thermally non-conductive.

The use of one or more dividing walls is especially advantageous in substantially circular heaters. For example, a central circular element can define a heating area for small utensils and an annular element surrounding the central element can be used to extend the area for larger utensils.

The provision of a dividing wall confines the smaller area to inhibit wastage. Of course, additional annular elements may be employed also. The material of the dividing wall is preferably a-ceramic fibre insulation material (made from alumino silicate fibres) but other materials such as calcium silicate or Microtherm can be used.

In all heaters of the invention, a thermal cut out is preferably employed to avoid overheating. Where two or more elements are used, it may be located only over the larger or largest element but operable to switch off all elements simultaneously.

In a cooking appliance with a glass ceramic surface the cut out device serves to prevent the glass ceramic temperature from exceeding its safe operating temperature during all forms of normal cooking use and during abnormal use. While it is possible for a radiant heater to be designed so that a cut out is not needed by ascribing to the heater a rather low watts density, this leads to a slow cooking, performance, but in some instances this may be acceptable as a means of avoiding the cost of the cut out device. It is generally preferred however, to use a thermal cut out to allow provision of better cooking performance. (Avoidance of extremely high temperatures-greater than 6000C at the exposed glass surface-can also avoid some problems of staining of the glass ceramic).

The electrical and thermal insulating material used in heaters of the invention is preferably that known as Microtherm and available from Micropore International Limited. This material can be formed with shallow grooves in which the element or elements sit. We have found that this arrangement assists in holding the element in position, preventing it from losing its shape, while having no undesirable effect on the efficiency of the heater. In some cases we have found the heating effect to be increased.

The or each heating element in heaters of the invention is preferably an open coil which is stapled or cemented, (for example by a ceramic fibre cement) onto the base. The shape of the coil should be as stable as possible for maximum life of the heater. The shape of a coil can be stabilised by forming it into the desired shape and heating it to its annealing temperature. Upon cooling, the desired shape will be retained. Such method is described in our copending Application No: to which reference is directed.

Two embodiments of the invention will now be described by way of example and with reference to the accompanying drawing wherein like numerals refer to like parts throughout and in which:

Figure 1 is a plan view of a heater according to the invention; 2 GB 2 044 057 A 2 Figure 2 is a section taken on line 11-11 of Figure 11; Figure 3 is a plan view of a heater according to a second embodiment of the invention; and Figure 4 is a section taken on line N-R of 70 Figure 3.

The heater illustrated in the drawings comprises a metal dish 2 containing a base 4 of electrical and thermal insulating material. Against the side 6 of the dish is located a peripheral wall 8 of thermal insulation. Set in grooves formed in the base 4 are two electric heater coils 10 and 12 which are separated from each other by a dividing wall 14. Extending over the larger coil 10 is a thermal cut out 16 which is operable to switch off both coils in the event of overheating.

Each coil is controllable independently through terminal connectors 18 and 20 enabling a circular pan or utensil to be heated solely by the coil 10 and a larger possibly oval casserole or similar utensil be heated on both. Of course, a smaller pan might be heated on coil 12 alone. Typically, the coil 10 is a 1400 watt unit while the coil 12 is an 800 watt unit.

Each coil is unprotected and secured in the base 4 by means of staples (not shown). Each coil is preferably made from an iron chromium aluminium resistance heating wire and shaped by the method described in our copending Application referred to above.

The thermal cut out is of the differential expansion type and comprises a quartz tube containing a length of Inconel wire, differential expansion as a consequence of overheating operating a mechanical switch 22 to disconnect both coils 10 and 12 from the power source.

Although a thermal cut out could be located adjacent each coil, we have found that one over the larger coil is quite satisfactory.

It will be appreciated that the principle of using two separated and independently operable 105 heating coils in a radiant heater of the kind described herein can be extended to all shapes of heater. For example, a central coil surrounded by an annular coil as referred to above provides a heater having two different circular heating zones definable but the same principle may be applied to square or rectangular heaters. On a smooth top cooker however, where the top is substantially thermally non-conductive it is advantageous to provide a dividing wall of thermal insulating material such as 14 in Figure 1, to define distinct and separate heating zones. Without a dividing wall, heat radiating from each coil would extend beyond the surface of the top immediately above it with consequent wastage of heat.

The lateral thickness of the dividing wall 14 should be thick enough to prevent substantial transmission of heat from the zone heated by coil 10 from that heated by coil 12, but thin enough so as not to create a "cold spoV or "line" between the zones when both coils are energised. We have found a thickness of 10-15 mm to be satisfactory in a heater of the kind illustrated herein. The height of the wall 14, in the optimum arrangement will be the same as that of the peripheral wall 8 so that in use, both the peripheral wall 8 and the dividing wall 14 engage the underside of the top in the cooker In practice though, this is difficult to achieve and the dividing wall 14 is designed 1 or 2 mm shorter to ensure that at least the peripheral wall 8 engages the top to inhibit the lateral dissipation of heat from the total heating area.

An embodiment of the invention comprising two concentric coils is illustrated in Figures 3 and 4. In this heater, the primary coil 10 is located in the centre with the secondary coil 12 encircling it.

The dividing wall 14 is thus circular and divides the heating area defined by the peripheral wall 8 into a central and an annular zone. As with the embodiment of Figures 1 and 2, each coil 10, 12 is operable independently through terminal connectors 18 and 20.

The circular embodiment of Figures 3 and 4 requires a different form of thermal cut out from that of Figures 1 and 2. For the same reasons, the cut out need only be located over the primary coil but to be reliably effective, it must be thermally isolated from the secondary coil 12. To achieve this the thermal cut out 16 is enclosed by a block 26 of thermal insulation where it extends over the secondary coil 12 between the peripheral wall 8 and the dividing wall 14. The thermal cut out 16 terminates in the dividing wall on the other side of the primary coil 10.

The heater illustrated in the drawings has a step junction 24 between the underneath and side of the dish 2 to facilitate mounting of the heater in a cooking appliance. The horizontal flange may be provided with screw holes for securing the heater.

Claims

1. A radiant heater for use in a smooth top cooker, the heater comprising at least two heater elements supported on a base of electrical and thermal insulating material at least one of which elements is energisable independently; and a peripheral wall of thermal insulating material surrounding the elements.

2. A radiant heater according to Claim 1 including a dividing wall of thermal insulating material between adjacent elements to define separate and distinct heating zones.

3. A radiant heater according to Claim 1 or Claim 2 wherein each element is energisable independently.

4. A radiant heater according to any preceding Claim which is substantially circular and includes a central element and one or more annular elements surrounding the central element.

5. A radiant heater according to any of Claims 1 to 3 which is non-circular, the elements being arranged adjacent one another. 125

6. A radiant heater according to Claim 5 wherein one element is circular and an adjacent element is part circular.

7. A radiant heater according to any preceding 3 GB 2 044 057 A 3 Claim wherein each element is an unprotected coil.

8. A radiant heater according to Claim 7 wherein the coils are held in grooves formed in the base.

9. A radiant heater according to any preceding Claim including a thermal cut out device extending over at least one of the elements.

10. An element according to any preceding Claim wherein the base is supported in a metal 25 dish and having a peripheral wall of thermal insulating material of height greater than that of the dish.

11. A radiant heater in a smooth top cooker, the heater comprising at least one heater element 30 supported on a base of electrical and thermal insulating material, which heater is non-circular.

12. A heater according to Claim 11 wherein the heater is oval.

13. A heater according to Claim 11 or Claim 12 including at least two heater elements, one of which is circular, the circular element being energisable independently of the other element or elements.

14. A heater according to Claim 13 wherein a dividing wall of thermal insulating material is provided between the elements.

15. A heater according to Claim 13 or Claim 14 including. a thermal cut out device extending over the circular element.

Printed for Her Majesty's Stationery Office by the Courier Press, Leamington Spa, 1980. Published by the Patent Office, 2 5 Southampton Buildings, London, WC2A 1 AY, from which copies may be obtained.