GB2307385A - Radiant heater having resistance heating element with dynamic TCR - Google Patents

Description

Radiant Electric Heater This invention relates to a radiant electric heater for a glass-ceramic top cooking appliance and particularly to such a heater providing fast heat up to radiance and with high operating temperature and good uniformity of heat distribution over an associated region of glass-ceramic cooking surface.

The present invention provides a radiant electric heater for a glass-ceramic top cooking appliance, which heater comprises: a dish-like support comprising or containing a base of thermal insulating material and a peripheral wall of thermal insulating material; at least one electrical resistance heating element supported in the dish-like support having a negative temperature coefficient of electrical resistance over a first temperature range from 01,C to at least 700"C and a positive temperature coefficient of electrical resistance over a second temperature range higher than the first temperature range.

The first temperature range may, for example, be from 0 C to 10000C. or from O"C to 9000C or from OC to 800 C.

The at least one electrical resistance heating element maq compnse a refractory semiconducting material, particularly silicon carbide and more particularly alpha silicon carbide. It may Include one or more dopants or additions to provide required electrical resistance and/or temperature coefficient of resistance charactenstics.

The at least one electrical resistance heating element may be provided in the form of at least one elongate rod or wire or strip.

At least one elongate strip of rectangular cross-section may advantageously be provided, supported edgewise in the dish-like support and having a height significantly greater than its thickness.

Such edgewise support may be provided by securing the at least one elongate strip at opposite end regions thereof in the dish-like support and preferably such that a region of the strip intermediate the end regions is spaced from the base of thermal insulating material. The end regions of the strip may be held in mounting means, suitably comprising ceramic components, provided in peripheral regions of the dish-like support.

The at least one elongate strip may be substantially straight or may be of bent or curved form.

When arranged to traverse the dish-like support, the at least one elongate strip may, if desired, be formed with a region intermediate the end regions thereof having a height and/or thickness dimension which is different from, preferably greater than. corresponding height and/or thickness dimension in regions on either side thereof whereby a required temperature distribution along the strip Is obtained during operation of the heater.

The at least one elongate stnp may taper m the height and/or thickness dimension from the intermediate region towards the regions on either side thereof.

The at least one electrical resistance heating element may have end terminal regions thereof of lower electrical resistance than the remainder of the element. Such end terminal regions provide for connection thereto of connecting leads and during energisation of the heater attain a lower temperature than the remainder of the element.

When the at least one electrical resistance heating element compnses silicon carbide, the end terminal regions may suitably comprise beta silicon carbide, or alpha silicon carbide enriched with silicon (such as by having pores therein filled with silicon), the remainder of the element comprising alpha silicon carbide. Each of the end terminal regions may also or alternatively, be provided with a sleeve or layer of a metal or alloy having good electrical conductivity.

If desired, at least one additional heating element of different type or material from that of the at least one electrical resistance heating element may be provided in the dish-like support. Such at least one additional heating element may, for example, comprise at least one infra-red lamp, or at least one coiled wire resistance element, or at least one corrugated ribbon resistance element.

Combinations of additional heating elements may also be provided.

The or each infra-red lamp suitably comprises a halogen lamp.

The or each coiled wire resistance element or corrugated ribbon resistance element may suitably comprise iron-chromlum-alummium alloy.

The at least one additional heating element may be arranged to be energised independently of. or temporarily or permanently in senes or in parallel with, the at least one electrical resistance heating element.

The dish-like support may comprise a metal dish containing the base of thermal insulating material and the peripheral wall the base of thermal insulating material suitably comprising microporous thermal insulating matenal.

The heater of the invention exhibits a fast light-up time to radiance which may be less than 1.5 seconds. Although the electrical resistance of the at least one electrical resistance heating element is relatively high at the point of initial energisation, it then falls rapidly, accompanied by a rapid rise in temperature until it reaches a minimum value at the point where the temperature coefficient of resistance changes from negative to positive sign. The subsequent increase in electrical resistance prevents thermal runaway from occurring.

High operating temperatures of, for example, 1 5000C available with the at least one electrical resistance heating element result in excellent heater performance and good heat distribution is obtained particularly with an element of rectangular cross-section mounted edgewise.

The invention is now described by way of example with reference to the accompanying drawings. m which: Figure 1 Is a perspective view of one embodiment of radiant electric heater according to the invention; Figure 2 is a side view of a modified shape of heating element for use in the heater of the invention; Figure 3 Is a top view of another modified shape of heating element for use in the heater of the invention.

Figure 4 Is a top view of a further modified shape of heating element for use in the heater of the invention; Figure 5 is a perspective view of a further embodiment of heater according to the invention; Figure 6A is a perspective view of another embodiment of heater according to the invention; Figure 6B is a cross-sectional view of the heater of Figure 6A shorn underneath a glass ceramic cook top ; and Figure 7 Is a perspective view of a still further embodiment of heater according to the invention.

Referring to Figure 1, a radiant electric heater for a glass-ceramic top cooking appliance is constructed as follows. A circular metal dish 1 contains a base layer 2 of thermal and electrical insulating material, such as microporous thermal and electrical insulating material. A peripheral wall 3, of well-known form. is arranged to contact the underside of a glass-ceramic cook top (not shown) when the heater is installed in a cooking appliance for operation. The peripheral wall comprises a thermal and electrical insulating material such as, for example, bound ceramic fibres. bound glass fibres, or vermiculite.

An elongate electrical resistance heating element 4, in the form of a strip having a rectangular cross-section, Is mounted edgewise inside the dish and diagonally traverses the dish, whilst being spaced from the base layer 2.

The element 4 is inserted into the heater through openings provided in the side walls of the dish 1 and through supports 5, compnsing ceramic components in the form of inserts provided diametrically opposed in the peripheral regions of the base layer 2. Such supports 5 may suitably be installed during formation of the base layer 2 in the dish 1.

The electrical resistance heating element 4 has a negative temperature coefficient of electrical resistance over a first temperature range from 0 C to at least 700 C and a positive temperature coefficient of electrical resistance over a second temperature range higher than the first temperature range. The first temperature range may, for example be from 0 C to 1000 C or from O"e to 900"C, or from Oc to 800"C.

The element 4 comprises a refractory semiconducting material and suitably comprises a central exposed region of alpha silicon carbide which may be in a pure form or contain a proportion of beta silicon carbide and/or small amounts of impurities or dopant material, and which may have end regions of lower electrical resistance than the central region. A possible strip material for the element 4 Is recrystalllsed alpha silicon carbide, supplied bv Kanthal AB, having the registered trade name Hot Rod. This material has a 30% porosity and the opposite end regions are filled or enriched with silicon to provide reduced electrical resistance at these end regions. As an alternative, the stnp-form element 4 could comprise a central region of dense recrystallised or reaction bonded alpha silicon carbide having end regions of lower-resistivity beta silicon carbide reaction bonded thereto.

Electrical leads (not shown) are arranged to be connected to the ends 6. 7 of the heating element 4, for example by means of friction-grip connectors or by using small threaded fasteners passing through holes at the ends of the element and fitted with suitable nuts To facilitate connection, the ends 6, 7 of the element 4 may be provided with sleeves or metallic coatings of good electrical conductivity, such as of aluminium or an alloy such as Kovar.

Multiple elements 4 may be provided in the heater if desired, such as in parallel spaced relationship.

In the case of a 165 mm diameter heater, the edgewise mounted strip-form heating element 4 could. by way of example, have a strip height of about 6 mm, a strip thickness of about 1.2 mm and a central exposed region of about 140 mm in length. When energised from a 240 volts supply. the element 4 dissipates about 1300 watts.

A thermal limiter 8, of well known form is provided, with its sensor rod extending across the heater. The limiter 8 serves to interrupt the voltage supply to the heating element 4 at a predetermined temperature to prevent thermal damage to the glass-ceramic cook top during operation of the heater in a cooking appliance.

When the resulting heater is switched on in a cooking appliance, the electrical resistance of the heating element 4 falls quite rapidly as the element heats up and reaches a minimum value at about 800-900"C. As the temperature of the element continues to rise, the electrical resistance increases, preventing thermal runaway and the electric current reduces. A final operating temperature of about l500-15500C is obtained.

From vlsual aspects, the heater is appealing to the user slnce it has a light-up time, on switching on, of about 1 second, or possibly less, and is attractively bright. with good operating performance, at its element running temperature of 1500 - l5500C.

Because the end regions of the heating element 4 have lower electrical resistance than the central exposed region, the end regions operate at a lower temperature than the central region which is desirable from the aspects of mounting and terminating of the element, as well as facilitating cooling of the element on switching off the heater.

The heat distribution from the heater across the glass ceramic cook top may be optimised according to requirements by providing a heating element 4 which is of non-uniform thickness and/or height, for example as shown in Figures 2 and 3. In Figure 2, a heating element 4 is shown which has a height dimension in the middle region which is greater than that at regions on either side thereof, the middle region tapering in height to the regions on either side thereof. The middle region therefore operates at a lower temperature than the regions immediately on either side thereof which raises the temperature of a peripheral region of a zone of a glass-ceramic cook top heated by the heater.

Alternatively. as shown In Figure 3, instead of increasing the height dimension of the element 4 in the middle region, a similar effect could be achieved by increasing the thickness of the element 4 in the middle region compared with the thickness of regions on either side thereof.

A combination of the elements 4 of Figures 2 and 3 could also be considered in which the element 4 has a middle region which has greater thickness and height dimensions than those of the regions on either side thereof.

Curved or bent forms of the electrical resistance heating element may be advantageous for some applications to further optimise heat distribution. Such a curved element 40 is illustrated in Figure 4 and finds particular application in an oval-shaped heater as shown in Figure 5. In the heater of Figure 5 a circular heater arrangement 9 and a part-circular heater arrangement 10 are provided in an oval metal support dish 100. The construction of the heater arrangement 9 is substantially the same as that of the heater of Figure 1 and the same reference numerals are used as in Figure 1 to denote common components. Similar constructional details apply to the part-circular heater arrangement 10 except that in order to take account of the part-circular shape and provide optimum heat distribution, a curved heating element 40, as illustrated in Figure 4, is utilised and also modified as shown in Figure 2.

This curved heating element 40 is otherwise constructed as described with reference to Figure 1.

By way of specific example with reference to Figure 5, the circular heater arrangement 9 has a diameter of 165 mm and incorporates a strip-form heating element 4 comprising silicon carbide as described with reference to Figure 1. The strip form element 4 has a height of about 6 mm and a thickness of about 1.0 mm, the exposed length being about 140 mm. When connected to a voltage supply of 240 volts the power dissipated In the element 4 is about 1100 watts.

The part circular heater arrangement 10 in Figure 5 has a curved strip form heating element 40 as shown in Figure 4, but (as shown in Figure 2) also with a central region of greater height dimension, at 6 mm, than that of the regions on either side thereof, at 5 mm. The exposed length of the element 40 is about 150 mm, the thickness being about 1.0 mm. When connected to a voltage supply of 240 volts, the power dissipated in the element 40 is about 900 watts, the shape and contour of the element 40 resulting in excellent heat distribution over the part-circular heated area.

The heater arrangements 9 and 10 can be operated independently or in association with one another.

For some applications it Is advantageous to provide a heater according to the invention in which, in addition to the electrical resistance heating element 4, 40, one or more other forms of heating element are incorporated. One such heater is shown in Figures 6A and 6B. In Figures 6A and 6B, a heater Is constructed substantially as shown in and described with reference to Figure 1, except that a further heating element in the form of a circular halogen lamp 11 is provided, supported in the peripheral region of the heater. The lamp I 1 is arranged to pass over the strip form heating element 4. The lamp 11 may be operated independently of, or in series or in parallel with, the heating element 4. It is particularly advantageous to provide the lamp 11 connected in series with the strip-form element 4. In addition to the lamp 11 providing increased heating power in the peripheral region of the heater, the strip-form element 4 also operates as a ballast resistance element to damp inrush current though the lamp when the heater is switched on from cold, such inrush current being a consequence of the lamp Il having a filament comprising a material such as tungsten which has a substantial positive temperature coefficient of resistance. In a particular specific example of the heater of Figure 6A, the heater is 200 mm in diameter. and required to provide a power of 1800 watts at 240 volts. The circular lamp 11 has an inside diameter of about 136 mm and Is rated at 1350 watts at 160 volts. The lamp is connected in series with the strip form element 4 comprising silicon carbide as descnbed with reference to Figure 1, which is about 2 mm thick and about 8 mm high with an exposed central region of about 100 mm in length and lower resistance end regions each of about 60 mm in length. The material composition and supporting of the element 4 Is as described with reference to Figure 1.

On switching on from cold, the electrical resistance of the series combination of the lamp 11 and heating element 4 is about 32 ohms, the current being about 7.4 amps with a supply voltage of 240 volts. Inrush current which would otherwlse occur in the lamp is consequently damped. At this stage the element 4 is dissipating about 1643 watts and the lamp 11 only about 131 watts. Under these conditions the element 4 is heating up faster than the lamp 11. As soon as the element 4 begins to heat up, its resistance starts falling and the current increases from its initial level of about 7.4 amps up to a maximum of about 14 amps. At this stage and with the continuing increase in electrical resistance of the lamp 11 the temperature of the lamp rapidly increases and verbs quickly overtakes the temperature of the element 4. After a lew seconds. the electrical resistance of the series combination of the element 4 and lamp 11 reaches a value of about 30 ohms, with a current of about 8 amps and a total power dissipation of about 1920 watts.

As the glass ceramic cook top heats up, the element 4 and the lamp 11 continue to nse in temperature. Although the resistance of the tungsten filament in the lamp 11 does not significantlx change further. the resistance of the element 4 now increases from about 6 ohms to about 8 ohms as a result of its positive temperature coefficient of resistance charactenstic at such a temperature.

Thus In final steads state operation. the senes combination has a resistance of about 32 ollms and the power disslpation of the heater Is 1800 watts.

During operation of the heater. the amount of heat generated in the infra-red lamp is maintained substantialls greater than 50% of the heat generated in the heater as a whole.

Such a heater Is intended to meet the stnngent switching (PST) specifications of official regulators authorities For this purpose the element 4, used in series with one or more lamps 11, Is arranged to operate with its minimum electrical resistance at about 900"C.

Figure 7 shows a further arrangement in which a heater is constructed substantially as shown in and described with reference to Figure 1, except that a further heating element 12 of coiled wire form is provided, supported on the base layer 2 in the peripheral region of the heater. The coiled wire element 12 may be operated independently of, or m series or in parallel with the heating element 4. Such further heating element 12 is advantageous in providing additional heating power, particularly in large heaters. Furthermore it maq' be advantageous to connect the further heating element 12 in series with the heating element 4 in order to damp any inrush current in the element 4 at temperatures where the temperature coefficient of resistance of the element 4 is of negative sign.

By way of specific example, the stnp-form element 4 compnsing silicon carbide, as described with reference to Figure 1, has a height of about 10 mm and a thickness of about 1.2 mm. In a heater of 200 mm diameter. the exposed central region of the element 4 has a length of about 150 mm. The coiled wire element 12, comprising, for example, iron-chromium-aluminium alloy, is connected in senes with the element and arranged such that, with a supply voltage of 240 volts, the total heater power Is 1800 watts, 1350 watts being dissipated in the element 4 and 450 watts being dissipated in the coiled wire element 12.

Instead of the coiled wire heating element 12, a corrugated ribbon heating element, comprising, for example, iron-chromlum-alumlnlum alloy, could be provided, supported edgewise on the base layer 2.

Instead of the element 12 being confined to the peripheral region of the heater, it could be provided In the heater on either side of the stnp-form element 4. One or more additional heating elements, such as the coiled wire element in Figure 7 or the lamp 11 of Figure 6A, could also be provided in the heater of Figure 5, if required.

Claims (1)

1. Claims 1. A radiant electric heater for a glass-ceramic top cooking appliance, which heater comprises: a dish-like support comprising or containing a base of thermal insulating material and a peripheral wall of thermal insulating material; at least one electrical resistance heating element supported in the dish-like support having a negative temperature coefficient of electrical resistance over a first temperature range from 0"C to at least 700"C and a positive temperature coefficient of electrical resistance over a second temperature range higher than the first temperature range.

   2. A radiant electric heater according to claim 1, in which the first temperature range is from 0 C to 10000C.

   3. A radiant electric heater according to claim 2, in which the first temperature range is from 0 C to 9000C.

   4. A radiant electric heater according to claim 3, in which the first temperature range is from 0 C to 8000C. t. A radiant electric heater according to any preceding claim, in which the at least one electrical resistance heating element compnses a refractory semiconducting material.

   6. A radiant electric heater according to claim 5, in which the refractory semiconducting matenal comprises silicon carbide.

   7. A radiant electric heater according to claim 6 in which the silicon carbide comprises alpha silicon carbide at least in part.

   8. A radiant electric heater according to claim 6 or 7, in which the silicon carbide includes one or more dopants or additions to provide required electrical resistance and/or temperature coefficient of resistance characteristics.

   9. A radiant electric heater according to any preceding claim, in which the at least one electrical resistance heating element is provided in the form of at least one elongate rod or wire or stnp.

   10. A radiant electric heater according to claim 9, in which at least one elongate strip of rectangular cross-section Is provided, supported edgewise in the dish-like support and having a height significantly greater than its thickness.

   11. A radiant electric heater according to claim 10, in which the edgewise support Is provided by securing the at least one elongate strip at opposite ends regions thereof in the dish-like support.

   12. A radiant electric heater according to claim 11, in which a region of the strip intermediate the end regions is spaced from the base of thermal insulating material.

   13. A radiant electric heater according to claim 11, or 12, in which the end regions of the stnp are held in mounting means provided in peripheral regions of the dish-like support.

   14. A radiant electric heater according to claim 13, in which the mounting means compnse ceramic components.

   15. A radiant electric heater according to any of claims 10 to 14, in which the at least one elongate strip is substantially straight or of bent or curved form.

   16. A radiant electric heater according to any of claims 10 to 15, in which the at least one elongate strip traverses the dish-like support and is formed with a region intermediate the end regions thereof having a height and/or thickness dimension which is different from a corresponding height and/or thickness dimension in regions on either side thereof, whereby a required temperature distribution along the strip is obtained during operation of the heater.

   17. A radiant electric heater according to claim 16, In which the height and/or thickness dimension in the intermediate region is greater than the corresponding height and/or thickness dimension in the regions on either side thereof.

   18. A radiant electric heater according to claim 17, in which the at least one elongate strip tapers in height and/or thickness from the intermediate region towards the regions on either side thereof.

   19. A radiant electric heater according to any preceding claim, in which the at least one electrical resistance heating element has end terminal regions thereof of lower electrical resistance than the remainder of the element.

   20. A radiant electric heater according to claim 19, in which the at least one electrical resistance heating element compnses silicon carbide, the end terminal regions comprising beta silicon carbide or alpha silicon carbide ennched with silicon. and the remainder of the element compnsing alpha silicon carbide.

   21. A radiant electric heater according to claim 19 or 20, in which each of the end terminal regions is provided with a sleeve or layer of a metal or alloy having good electrical conductivity.

   22. A radiant electric heater according to any preceding claim, in which at least one additional heating element of different type or material from that of the at least one electrical resistance heating element is provided in the dish-like support.

   23. A radiant electric heater according to claim 22, in which the at least one additional heating element comprises at least one. infra-red lamp, or at least one coiled wire resistance element, or at least one corrugated nbbon resistance element.

   24. A radiant electric heater according to claim 22 or 23, in which a combination of additional heating elements of different types or materials is provided with the at least one electrical resistance heating element.

   25. A radiant electric heater according to claim 23 or 24, in which the or each infra-red lamp compnses a halogen lamp.

   26. A radiant electric heater according to claim 23 or 24, in which the at least one coiled wire resistance element or the at least one corrugated ribbon resistance element compnses iron chromium-aluminium alloy.

   27. A radiant electric heater according to any of claims 22 to 26, in which the at least one additional heating element is arranged to be energised independently of, or temporarily or permanently in series or in parallel with, the at least one electrical resistance heating element.

   28. A radiant electric heater according to any preceding claim, in which the dish-like support compnses a metal dish containing the base of thermal insulating material and the peripheral wall.

   29. A radiant electric heater according to claim 28, in which the base of thermal insulating material comprises microporous thermal insulating material.

   30. A radiant electric heater for a glass-ceramic top cooking appliance substantially as hereinbefore described with reference to the accompanying drawings.