WO1998019499A1

WO1998019499A1 - Heating element with a plurality of parallel tracks mounted on a substrate

Info

Publication number: WO1998019499A1
Application number: PCT/GB1997/002966
Authority: WO
Inventors: Stylianos Panaghe
Original assignee: Delta Theta Limited
Priority date: 1996-10-31
Filing date: 1997-10-28
Publication date: 1998-05-07
Also published as: GB9622695D0; AU4787397A

Abstract

There is disclosed a heating element comprising a plurality of parallel-connected electric thermal element tracks on a substrate. The element is thermally-controlled by a temperature-sensor or PTC-means and is failure-tolerant because of its parallel structure. Could be used in a stove, grill or toaster.

Description

HEATING ELEMENT WITH A PLURALITY OF PARALLEL TRACKS MOUNTED ON A SUBSTRATE

This invention relates to electric thermal elements such for example as heating elements for cookers and radiant elements for grills and toasters.

Conventionally, such heating elements comprise a wire, usually encased in a ceramic insulator or in the case of radiant heaters, mica or other insulator that can withstand high temperatures, the wire having an ohmic resistance so as to determine a current when an appropriate, usually mains, voltage is applied which heats the wire usually to a glowing temperature. The wire is usually coiled into a spiral in circular "rings" on electric stoves or arranged in some other appropriate layout in toaster elements and grills.

One requirement, to which the precise design of the coil or other layout is addressed, is to provide an even temperature over the cooking ring or a grill, certainly to avoid "hot spots", or a graded temperature in the case of a vertically-orientated toaster panel.

Particularly, however, in the case of electric cooker rings or hot plates, the temperature distribution is affected by the characteristics of the utensil used on it. If the base of the utensil is uneven, those areas of it in more intimate contact with the plate will conduct more heat away from the plate over the area in contact, while hot spots will build up under those areas less in contact or not in contact at all, leading to uneven pan base temperatures (food sticking and burning on to the pan base) and inefficient use of power, as well as shortened element life, even catastrophic failure if a hot spot melts the wire and the element goes open circuit. The present invention provides new thermal elements that can better meet the requirements of levelness of temperature or maintaining a predetermined temperature distribution and efficiency of conversion between electrical and thermal energy.

The invention comprises a thermal element comprising a plurality of parallel-connected electric thermal element tracks on a substrate.

The tracks may be printed on to the substrate or they may be fashioned by removal of parts of a layer (which may be a solid layer, so that the tracks may be created by such removal, or which may be a printed track, for fine adjustment thereof) as by etching, abrasion or laser or spark ablation or erosion.

At least some tracks may be thermally controlled. Control may be effected by a temperature sensor and an electric control arrangement or by the tracks having a high temperature coefficient of resistance, this latter measure giving rise to an inexpensive means of automatic temperature regulation inasmuch as any tendency for any particular element track to rise in temperature is accompanied by an increased resistance in that track which cuts down the power dissipated in that track which then leads to cooling.

The thermal element could also be a cooling element operating, perhaps, on the Peltier effect, automatic temperature regulation being effected using a high negative temperature coefficient of resistance.

The parallel connected tracks may be served by bus bars of low temperature coefficient of resistance.

The element may have a network structure tolerant of partial open-circuit failure of the track layout and may for example comprise a radial arrangement of thermally effective tracks on a circular bus bar arrangement, or a grid arrangement of parallel straight bus bars and thermally effective tracks interconnecting the same.

The element may be comprised in a heater plate for a cooking stove or a radiant element for a grill or a toaster, but may also be adapted for laboratory heaters and Peltier effect refrigerators.

Embodiments of thermal elements according to the invention will now be described with reference to the accompanying drawings, in which:-

Figure 1 is an element track layout for a cooking stove hot plate;

Figure 2 is an element track layout for a toaster heater element.

The drawings illustrate thermal elements comprising a plurality of parallel- connected electric thermal element tracks 11 on a substrate 12.

The tracks 11 are either printed on the substrate 12 or fashioned by removal of parts of a solid layer (i.e. one covering the entire surface of the substrate 12 or of a part thereof) as by etching, abrasion or laser or spark ablation or erosion. These latter techniques can also be used to "fine tune" a printed layout by removing excess material, perhaps in accordance with an infra-red image of the element on power, from the tracks.

The substrate 12 can be stainless steel for example that known as 430 S 17, which is ferritic with about 17% Cr content. A glass ceramic insulating material can be printed on to the plate after oxidation at about 900 °C in a furnace. Repeated applications of the ceramic each about 15 microns thick can build up sufficient to withstand the required 1250 V ac - about four layers are needed. The thermal element tracks can be printed on to the ceramic layer and fired at about the same temperature after which an additional glass ceramic layer is printed on top, except for connector pads.

The materials used for the tracks may be selected from:

nickel - TCR about 5500 ppm/degC or 55%/100 deg C

platinum - TCR about 3500 ppm/degC or 35%/100 deg C

mixture of elements with a TCR of about 50 ppm/degC

In both embodiments some of the tracks 11, identified as 1 la, are thermally controlled. While, clearly, the controlled tracks could, either individually or collectively, be subject to an electronic control arrangement with a temperature sensor measuring the temperature and the control arrangement using an error signal to control the voltages applied to the tracks, the invention also provides for an inexpensive but robust automatic control arrangement by making the tracks l ie of a material of high temperature coefficient of resistance. At any given applied voltage, the tracks 11a will tend to maintain a predeterminable temperature - if the temperature falls, perhaps because of improved local heat transfer to a pan base, say, on a cooking stove, the resistance will drop and move current flow as a result, bringing the track 11a back up to temperature. Conversely, if the temperature rises, the resistance will also rise reducing the current flow and bringing the temperature down again.

The tracks 1 la are served by bus bars 1 lb which are of material with a low temperature coefficient of resistance - indeed desirably these tracks have a low ohmic resistance, either through being of a material having a low resistance such for example a gold or silver resinate, or through being of relatively large cross-section, or both. In the embodiments of Figures 1 and 2, the tracks 11a, l ib form a network structure tolerant of partial open-circuit failure of the track layout. The bus bars 1 lb in Figure 1 are essentially circular, the thermally effective tracks 11a being in a radial arrangement like spokes in a wheel. The areas 1 lc are termination pads for attachment to the supply.

In Figure 2 the bus bars 1 lb form a grid of parallel straight sections - all horizontal tracks are bus bar tracks together with the edge sections 1 Id and the central section l ie carrying the termination pads l ie, while all other vertical tracks are thermally effective tracks 1 la. The track density is higher at the bottom of the element than at the top, this being appropriate for an electric toaster. Such an element used for a grill would have an even density or perhaps a greater density around the edges than in the middle.

As mentioned, the thermally effective tracks 1 la could have a high negative coefficient of resistance when the Peltier effect is being exploited to produce a cooling effect.

Whilst the main advantages will be perceived, in the context of electric domestic appliances such as cooking stoves, electric clothes irons, electric blankets possibly and toasters, as self-regulation (in the case of high temperature coefficient of resistance materials) and the resulting protection against failure due to local melting at a hot spot, together with the capability to operate even when partially open-circuit, the improved efficiency due to better heat distribution than can normally be attained using a single resistance wire and the more even temperature distribution, in some circumstances the improved efficiency, coupled with the low cost of printed circuit rather than wire configuration can be paramount. It becomes possible, for example, to contemplate converting mechanical power into useful heat, for example for cooking, using inexpensive but highly efficient hot plates fed from a hand-cranked generator, possibly with spring energy storage - like the "clockwork" radio finding utility in Third World areas where batteries are too expensive or hard to come by. Of course, the mechanical power requirements are considerable merely to boil a kettle of water or simmer a cooking pot But the total energy requirement might be no more (though more highly concentrated) than required for the gathering of scarce combustible material that would be better conserved.

Claims

1. A thermal element comprising a plurality of parallel-connected electric thermal element tracks on a substrate.

2. A thermal element according to claim 1, in which the tracks are printed in to the substrate.

3. A thermal element according to claim 1 or claim 2, in which the tracks are fashioned by removal of parts of a layer as by etching, abrasion or laser or spark ablation or erosion.

4. A thermal element according to any one of claims 1 to 3, in which at least some tracks are thermally controlled.

5. A thermal element according to claim 4, in which control of said tracks is effected by a temperature sensor and an electronic control arrangement.

6. A thermal element according to claim 4, in which control of said tracks is effected by the tracks having a high temperature coefficient of resistance.

7. A thermal element according to any one of claims 1 to 6, in which the parallel-connected tracks are served by bus bars of low temperature coefficient of resistance.

8. A thermal element according to any one of claims 1 to 7, having a network structure tolerant of partial open-circuit failure of the track layout.

9. A thermal element according to any one of claims 1 to 8, comprising a radial arrangement of thermally effective tracks on a circular bus bar arrangement.

10. A thermal element according to any one of claims 1 to 8, comprising a grid arrangement of parallel straight bus bars and thermally effective tracks interconnecing the bus bars.

11. A thermal element according to any one of claims 1 to 10, comprised in a heater plate for a cooking stove.

12. A thermal element according to any one of claims 1 to 10, comprised in a radiant element for a grill or toaster.