GB2293952A

GB2293952A - A heating unit for a ceramic hob

Info

Publication number: GB2293952A
Application number: GB9420078A
Authority: GB
Inventors: Maurice Hugh Carter
Original assignee: Redring Electric Ltd
Current assignee: Redring Electric Ltd
Priority date: 1994-10-05
Filing date: 1994-10-05
Publication date: 1996-04-10
Also published as: DE19536830A1; GB9420078D0; ES2113298A1; ES2113298B1

Description

1 A HEATING UNIT FOR A HOB -2293952 The present invention relates to a

heating unit for mounting under a plate of a ceramic hob.

Heating units for use with ceramic hobs typically comprise a canister, formed out of appropriately worked sheet metal, having an electrically insulating layer on which a radiant heating element is supported. The canister is then mounted under a translucent glasslceramic plate through which energy from the heating element is radiated to a cooking utensil.

Conventionally cerarnic hobs have had one or more heating elements each formed from a resistive wire wound in a coil. The element is normally laid out across the insulating layer of the canister in an aesthetically pleasing arrangement. the heating element in use being visible through the glass/ceramic plate, the element being retained in place by simply being stapled to the insulating layer.

The gauge of wire used for the heating element anddiameter of the wound coil are dictated by the requirement that the coil substantially retain its initial form throughout the working life of the unit, it being impractical to sheath the heating element in a silica tube due to the limited space in which the. heating element has to be located. This in turn dictates the operating temperature, for if for a given operating temperature the ratio of the diameter of wire to the diameter of the coil is too small then the wire distorts on heating. Conversely if the gauge of the wire is sufficient relative to the diameter of the coil such that the coil does not distort then either the diameter of the wire has to be so great that the resistive 2 P/60195/RED value per metre is impracticably low, or the diameter of the coil has to be relatively small which limits the quantity of heat that can be radiated for that given operating temperature.

In an attempt to overcome the above problems certain units supplement or replace the conventional wound resistive wire by one or more halogen "lamps", comprising a tungsten wire sealed in a silica envelope filled with a halogen gas. The adoption of halogen lamps provides a more rapid response time, the fine tungsten wire heating up and cooling down more quickly than the conventional wound resistance wire. Halogen lamps also operate at far higher temperatures than conventional resistance wires, and enable a higher radiant heat intensity to be achieved for a given area.

Although halogen lamps have a quicker response time, about 1.5 seconds compared to about 11 seconds for conventional wound resistive elements, they also have a number of drawbacks. Firstly, they are more expensive. Also because the element is tungsten, which has a relatively. high thermal coefficient of resistance compared with the negligible thermal coefficient of resistance associated with convention wound resistive wire elements, and because of the high operating temperatures, the halogen lamps have a very low "cold" resistance which results in a large inrush current when switched on. This problem is further compounded by the output of the lamps normally being controlled by cyclic switching. This results in a requirement for more complex switching apparatus than is necessary with convention wound resistance wire elements.

According to the present invention there is provided a heating unit of the aforementioned type wherein the heating element consists of a plurality of wire strands twisted together to 3 P/60195/RED form a multi-stranded wire, the multi-stranded wire being wound in the form of a coil.

By employing the present invention it has been found_ that the response time and heat output of a heating unit can be increased, compared to a unit comprising conventional wound resistive wire elements, such that the performance is comparable to that obtained using the relatively expensive halogen lamp heating elements.

Although multi-stranded wire or wire rope is normally more pliable than an equivalent single strand wire it has been found that employing the present invention provides a more stable coil which pernks a larger ratio of the diameter of coil to cross section of wire. This in turn permits a larger diameter coil to be used for a given resistive value Which enables more power to be dissipated for a given area of canister in which the heating element is placed. Conversely a shorter coil of a conventional diameter may be employed. The current through this shorterelernent will be greater than.for a conventional element and it will operate at a higher temperature, now possible due to the greater stability of the coil structure. This again makes it possible to radiate a given power from a smaller element.

The total cross section of a multi-stranded element is significantly less than that of a "solid" single strand element of a comparable overall diameter. This results in a proportional higher resistance per unit length of the multi-stranded element. If it is desired to produce a multi-strand heating element with the same power output as a conventional single strand element then a shorter length of element is required to draw the same current and operate at approximately the same temperature, the multi-stranded element having a greater surface per unit length area from which heat can be radiated. Below is a table which compares the 4 P/60195/RED dimensions and values of a conventional single strand wire element and a multi-strand element in accordance with the present invention. Both elements have a resistance of 39.4 ohms which at 24OV provides a 1.4 kW output.

Strand Overall Wire Total wire Cross section diameter Diameter 0m---1 length surface area Single Strand Element 0.635 0.635 4.39 8.97 0.317 Seven Strand Element 0.18 0.535 8.1 4.86 0.178 Preferably the wire strands are twisted together in the opposite sense to the coil for this has been found to provide a more stable coil. It has also been found to be particularly advantageous if the ratio of the diameter of the coil to the mean diameter of the multi-stranded wire is in the range of 35-45: 1, this being a particularly stable configuration.

A wire rope comprising seven strands of wire is particularly stable.

Preferably the resistance of one strand of wire is used to control the supply of electrical energy to the heating element. If one wire having a significant thermal coefficient of resistance is included in the wire rope then the resistance of this single strand of wire can be used to control the current in the heating element. This is possible because although the wire strands are wound closely together there is little electrical conductivity between them due to oxidisation of the outer surfaces.

If desired, at least one strand of wire can be connected to an electrical source independently P/60195/RED from at least one other wire strand, and this provides a means of providing different outputs from the core simply by energising an appropriate number of strands. Advantageously at least some strands of wire are of different diameters to the others, thereby providing different resistive loadings and corresponding heat settings.

A particularly pleasing aesthetic effect is achieved if a ceramic hob comprises a heating unit in accordance with the invention located under a glass/ceran-fic plate having a substantially uniform repetitive pattern over a region of one surface. The bigger diameter of coil made possible by employing the present invention causes a particularly pleasing visual pattern to be produced which resembles a holographic effect.

According to a second aspect of the invention there is provided a method of forming a heating element for a hob, the method comprising twisting a plurality of wire strands together to form a multi-stranded wire and winding the wire to form a coil. Preferably the method comprises annealing the wire prior to winding the wire into a coil for this increases the resistance of the coil to distortion during cyclic heating. Preferably the method additionally comprises annealing the multi-stranded wire after it has been wound into a coil for the same reason.

One embodiment of the present invention will now be described by way of example only with reference to the drawings, of which:

Figure 1 illustrates an enlarged section of heating element in accordance with the present invention; 6 P/60195/RED Figure 2 is a side elevation of a heating unit in accordance with the present invention; and Figure 3 is a plan view of the heating unit of Figure 2.

Referring to Figure 1, there is illustrated a section of wire rope heating element 1 comprising seven strands of wire twisted together. The strands of wire are 0.18 mm diameter "Kanthal AF" which is an alloy comprising iron, chromium and cobalt. The wire is wound such that it has a lay distance of 5.08 mm (the lay distance is the distance in which the wire rope undergoes one complete revolution). This provides a wire rope of 10 mean diameter "D" equal to 0.535 mm. When wound, the wire rope is then annealed before being wound on a mandrel to form a coil as shown. The diameter of the coil "D" of the particular element illustrated is 4. 75 mm, but another preferred diameter is 12 mm. When the coil is wound it is then annealed again before being removed from the mandrel. The coil is wound in opposite sense to the direction in which the wire rope is twisted. The 15 resistance of the rope is 8.1 ohms/metre.

In Figures 2 and 3 there is illustrated a heating unit 2 consisting of a metal canister 3 in which an insulating layer 4 and insulating annular wall 5 are positioned such as to insulate heating element 6 from the canister 3. The heating element 6 comprises the wire rope of Figure 1 wound in a coil as shown in Figure 2. Staples 7 retain the heating element 6 in fixed relationship relative to the insulating layer 4. The heating element is connected at connection block 8 to multicore cable 9 with each of the strands of the wire rope 6 being attached to individual switching means so that current can be supplied separately to each wire of the heating element in order to control the heat output. The total length of wire 7 rope is 4.86 metres providing a maximum power output 140OW at 240V.

P/60195/RED The heating unit 2 is retained in position under a glass-ceramic hob plate 11 by means Of springs 10 acting between lugs 11 of the canister and a support structure not shown. The glass-ceramic plate 11 has a repetitive pattern on its lower surface 12 such that when the heating unit is in operation the relatively large structure of the coil produces a holographic type effect when viewed through the translucent glass-cerarnic plate 11.

In an alternative embodiment the strand can be of a material such as tungsten having a high thermal coefficient of resistance, and the current supplied to the heating element can be controlled in dependence on this resistive value.

8 P/60195/RED

Claims

A heating unit adapted for mounting under a plate of a ceramic hob, the unit comprising a canister having an electrically insulating layer on which a radiant heating element is supported, characterised in that the heating element consists of a plurality of wire strands twisted together to form a multi-stranded wire, the wire being wound in the form of a coil.
2. A heating unit as claimed in claim 1 wherein the wire strands are twisted together in the opposite sense to the coil.
3. A heating -unit as claimed in claim 1 or 2 wherein the ratio of the diameter of the coil to the mean diameter of the multi-stranded wire is in the range of 35-45 to 1.
4. A heating unit as claimed in any preceding claim wherein the multistranded wire comprises a seven stranded wire rope.
5. A heating unit as claimed in claim 4 wherein the diameter of each wire strand is between 0. 16 and 0.20 mm.
6. A heating unit as claimed in any preceding claim wherein the resistance of one strand of the multi-stranded wire is used to control the supply of electrical energy to the heating element.

9 P/60195/RED
7. A heating unit as claimed in any preceding claim wherein said one strand has a negative resistance.
8. A heating unit as claimed in any preceding claim wherein at least one strand of wire can be connected to an electrical source independently from at least one other wire strand.
9. A heating unit as claimed in claim 9 wherein the output from the heating element is controlled by connection of different ones of the wire strands to an electrical supply.
10. A heating unit as claimed in claim 8 or 9 wherein at least some strands of wire are of different diameters.
A ceramic hob comprising a heating unit as claimed in any preceding claim located under a glass/ceramic plate having a substantial uniform repetitive pattern over a region of one surface.
12. A method of forming a heating element for a hob, the method comprising twisting a plurality of wire strands together to form a multi- stranded wire and winding the wire rope to form a coil.
13. A method as claimed in claim 12 comprising winding the coil in opposite sense to the wire strands.
14. A method as claimed in claim 12 or 13, comprising annealing the multistranded P/60195/RED wire prior to winding the wire into a coil.
15. A method as claimed in claim 12, 13 or 14 comprising annealing the multi-stranded wire after it has been wound into a coil.
16. A method of producing a heating element for a hob substantially as hereinbefore described with reference to the accompanying drawings.
17. A hob substantially as hereinbefore described with reference to, and as illustrated in, the accompanying drawings.