GB2069300A - Radiant heating elements for smooth top cookers - Google Patents

Abstract

An electric heater unit for "smooth top" cookers employs two or more heater elements in the same unit. It is known to use a thermal cut-out device extending over the heater unit where a single element is used, but design problems develop when two or more elements are employed in the same unit, of which at least one is independently energisable. According to the invention an elongate thermal cut-out device (16) extends across said one (central) element but is thermally isolated from the other (peripheral) element or elements. Thus, the device (16) operates only in response to the one element. Such thermal isolation can be accomplished by enclosing a portion (30) of the device (16) which extends across the other element or elements in a block (26) of thermal insulation material, or in a sheath of thermally conductive material which carries any heat from the other element or elements toaheat sink. In a third alternative, the effective length of the device terminates at the boundary (14) of the one element. <IMAGE>

Description

SPECIFICATION Radiant heating elements for smooth top cookers This invention relates to electric radiant heater units of the kind used in "smooth top" cookers. More particularly, it relates to such heaters which employ two or more heater elements in the same unit.

A "smooth top" cooker is one 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, conduction and infra red radiation. Such elements are referred to as "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 whch 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.

It is usual, and in some jurisdictions mandatory in radiant heaters to include a thermal cut-out device to protect both the element and the top from overheating. While it is possible to design a heater with a low watts density to obviate the need for a cut-out device, this leads to a slow cooking performance.

Thus, a thermal cut-out device is desirable from both the point of view of safety and that of performance.

Further, excessive temperatures can result in damage to or discoloration of the top in a smooth top cooker. For example, a glass ceramic top can discolour if the temperature at the exposed surface exceeds 600"C (700"C at the surface nearest the heater element or elements).

In radiant heater units which employ two or more adjacent heater elements of which one is of larger thermal capacity than any of the others, we have found that a thermal cut-out can satisfactorily protect the unit from overheating if its response is limited to the heat generated by that larger element.

however, a problem exists if the or another element also has an influence. Typical cut-out devices are of elongate form, designed to extend across the heater unit, and the present invention provides an electric radiant heater unit having at least two adjacent heater elements of which at least one is energisable independently from the other or others, wherein a thermal cut-out device extends across said one element but is thermally isolated from said other element or elements such that it is operative in response only to heat generated by said one independently energisable element.

Thermal isolation of the cut-out device can be achieved in a number of ways. Most simply perhaps, that portion of it which would otherwise be affected by the other element or elements is enclosed by a thermal insulation material, typically in the form of a block which can be shaped to fit neatly into the heater unit. Another way is to sheath the portion of the device in a thermally conductive material which transmits the heatwhich would otherwise influence the device away to a heat sink or to a point external of the unit. Yet another way is to limit the effective length of the cut-out device to that part of it which extends across the one element.This aim can be fulfilled by terminating the responsive part of the device at the boundaries of the one element and connecting the cut-out switch by for example a microswitch, at that boundary, or by connecting that part of the device across the other element or elements in a manner which precludes the other element or elements from influencing the response given by the device.

The invention is particularly suited to heater units in which one heater element surrounds another, for example in a circular heater unit having two concentric elements. However it can also have use where two elements are located adjacent one another in the same unit where positioning of the cut-out switch with respect to the unit is predetermined and cannot conveniently be moved to a location contiguous with one element only.

The heater elements in units of the present invention are preferably bare coiled wires supported in a microporousthermal insulation material. Such a coiled wire may be straightened in the vicinity of the element from which the cut-out device is to be isolated to reduce the amount of heat radiated therefrom which might influence the device. Where the device is enclosed in a block of insulation material, this facilitates the formation of the block and enables a greater thickness of material to be interposed between the cut-out device and the respective element. In some embodiments, the element is discontinuous along a peripheral zone in which it is effective and in these cases, the cut-out device may traverse the discontinuity.

The cut-out device is normally of the differential expansion type, a suitable device comprising a quartz tube containing a length of Inconel wire, differential expansion of the tube and wire operating a switch which de-energises the entire unit. Such a device is available from Therm-o-disc Mansfield, Ohio, United States of America under the designation "12.T.B Limiter".

The invention will now be described by way of example and with reference to the accompanying drawing wherein: Figure lisa plan view of a heater according to the invention; Figure 2 is a section taken on the line ll-ll of Figure 1; Figure 3 is a detail section, to a larger scale, taken on the line Ill-Ill of Figure 1; and Figure 4 is a view similar to that of Figure 3, showing an alternative means by which the cut-out device may be thermally isolated.

The heater unit illustrated in Figures 1 and 2 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 large 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 iarger 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.

The thermal cut-out is of the differential expansion type and comprises a quartz tube 28 containing a length of Inconel wire (not shown in Figures 1 and 2), a differential expansion as a consequence of overheating operating a mechanical switch 22 to disconnect both coils 10 and 12 from the power source. 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 a portion 30 of 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.

It will be appreciated that the principle of using two separated and independently operable heating coils in a radiant heater of the kind described herein can be extended to all shapes of heater. The circular unit illustrated herein 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 nonconductive 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. The dividing wall 14 is circular and divides the heating area defined by the peripheral wail 8 into a central and an annular zone.Without a dividing wall, heating radiating from each coil would extend beyond the surface of the top immediately above it with consequent wastage of heat.

As shown in Figures 1 to 3, the block 26 of insulation material is shaped to rest on the secon dary coil 12 and receive the quartz tube 28 of cut-out device 16. Its height is such as to reach substantially the same level as the peripheral wall 8 and dividing wall 14 such that all may engage the under surface of the smooth top when the unit is installed in a cooker.

The block 26 may be formed with channels 34 (as shown in Figure 2) in its under surface, to allow passage of the coil 12 therethrough or alternatively the coil 12 may be straightened, as shown in Figure 3 to pass directly under the bulk of the block 26. This has the advantage of reducing the heat generated by the coil 12 in the vicinity of the device 16 and minimizing energy wastage. In another alternative, the device may be located to extend over the gap 36 between the points of maximum curvature of the coil 12, thereby foregoing anynecessity of the coil 12 bypassing the cut-out device 16.

If desired, the block 26 might totally enve,lope the quartz tube 28 but we have found that this is not absolutely necessary to achieve satisfactory results.

The material of the block 26 may be a ceramic film or a microporous insulation materia, a preferred exam ple of the latter being that marketed by Micropore International Limited under the Trade Mark MICROTHERM.

Figure 4 illustrates an alternative means by which the portion 30 of the device 16 may be thermally isolated. In Figure 4 the quartz tube 28 is enclosed in a tube 32 of thermally conductive material, preferably a metal such as copper. The tube 32 can extend through the peripheral wall 8 to connect with the dish 2, transmitting heat thereto which will be dissipated around the body of the unit. The shape of the sheath 34 is not critical; it is its capacity to carry heat away from the secondary coil zone that is important. Once again, and for the reasons given above, the coil 12 may be straightened to pass below the tube 32, or the device 16 located overthe gap is to minimize the influence of the coil 12 and energy wastage.

Another manner (not illustrated) by which the cut-out device may be thermally isolated from the secondary coil 12 is to terminate the tube 28 at the dividing wall 14 at both ends. At one end, a microswitch may be coupled to the tube and wire which is separately connected to the cut-out switch 22.

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 (19)

1. An electric radiant heater unit having at least two adjacent heater elements of which at least one is energisable independently from the other, wherein a thermal cut-out device extends across said one element but is thermally isolated from said other element such that it is operative in response only to heat generated by said one independently energisable element.

2. A radiant heater unit according to Claim 1 wherein the cut-out device includes a portion which extends across the other element, which portion is - enclosed by a thermal insulation material.

3. A radiant heater unit according to Claim 2 wherein said other element is in the form of an unprotected coil, a length of which is straightened to pass under the thermal insulation material and said portion of the cut-out device.

4. A radiant heater unit according to Claim 2 or Claim 3 wherein said portion is enclosed in a block of microporousthermal insulation material.

5. A radiant heater unit according to Claim 2 or Claim 3 wherein said portion is enclosed in a block of ceramic thermal insulation material.

6. A radiant heater unit according to Claim 1 wherein the cut-out device includes a portion which extends across the other element, which portion is enclosed in a thermally conductive material, connected to a heat sink.

7. A radiant heater unit according to Claim 6 wherein the heater unit includes a metal dish in which the heater elements are supported, and wherein the dish constitutes the heat sink.

8. A radiant heater unit according to Claim 6 or Claim 7 wherein said portion is enclosed in a metallic sheath.

9. A radiant heater unit according to Claim 8 wherein the sheath is of copper.

10. A radiant heater unit according to any preceding Claim wherein said other element defines a peripheral heating zone in the unit, but is discontinuous along a portion of said zone, and wherein the cut-out device traverses said portion of the zone.

11. A radiant heater unit according to Claim 1 wherein the effective length of the cut-out device terminates at the boundary of said one element.

12. A radiant heater unit according to any of Claims 1 to 10 including a primary substantially central heater element defining a central zone and a secondary element surrounding the primary element defining a peripheral zone, the cut-out device extending fully across the central zone and once across the peripheral zone, the portion extending across the peripheral zone being thermally isolated from the secondary element.

13. A radiant heater unit according to Claim 12 wherein each of the primary secondary elements is circular.

14. A radiant heater unit according to any preceding Claim including a dividing wall of thermal insulation material between adjacent heater elements.

15. A radiant heater unit according to any preceding Claim including a peripheral wall of thermal insulation material surrounding all the heater elements in the unit, the cut-out device extending through the wall to a switch assembly mounted externally of the unit.

16. A radiant heater unit according to Claim 14 and Claim 15 wherein the dividing wall and the peripheral wall are of substantially the same height and wherein the cut-out device extends through both walls.

17. A radiant heater unit according to any preceding Claim wherein the cut-out device of the differential expansion type.

18. A radiant heater unit according to Claim 14 wherein the cut-out device comprises a quartz tube containing a length of Inconel wire coupled to a switch.

19. An electric radiant heater unit substantially as described herein with reference to and as illustrated by the accompanying drawings.