GB2230410A

GB2230410A - Radiant heating device

Info

Publication number: GB2230410A
Application number: GB9008231A
Authority: GB
Inventors: Joseph Hecht
Original assignee: AKO Werke GmbH and Co KG
Current assignee: AKO Werke GmbH and Co KG
Priority date: 1989-04-11
Filing date: 1990-04-11
Publication date: 1990-10-17
Also published as: GB9008231D0; DE3911761A1

Description

11 - Radiant heating_device This invention relates to a radiant heating

device, in particular for a cooker hob, having a heating element composed of heating wire, which is laid on an insulating support and which forms a current-carrying cross- sectional area and a heat-radiating surface.

A radiant heating device of this kind is- described in DEOS 36 23 130, for example. In known radiant heating,devices, the heating wire is formed as a heating coil, and the heating coil is laid spirally on the insulating support below a glass ceramic hob. The heating wire from which the heating coil is wound is, according to the prior art., a wire of circular cross-sectional area.

An electrical radiant heating element is described in DE AS 1094383, that is forrhed from a tubular heating body. The tubular heating body comprises a tubular metal sleeve in which are arranged two heating wires that extend in the form of a coil in the usual manner. The non-round tubular metal sleeve is twisted to a screwed line. The heating wires thus do not touch one another. They can be switched on independently of one another.

In DE 3509985A1 there is described an electrical resistance heating element with a non-circular cross-section. This has a ribbon-shaped or cross-shaped profile that is twisted about its longitudinal axis.

If the service life of the heating element is to be extended, according to the prior art,_ the heating coil is wound from a heating wire of relatively large - circular - cross-sectional area. This increases the surface correspondingly, so that the specific temperature load of the surface is reduced. This has the effect, ijrter alia of making the heating coil more rigid, which opposes the spiral laying thereof on the insulating support. Furthermore, the heating wire is less quick to glow and glows less intensively. This is undesirable, since the lowing heating coil should be visible to the user through the glass ceramic hob.

1 t 2 In order to make the heating coil glow quickly and intensively, it could be manufactured from a heating wire of comparatively small crostsectional area. However, this would he associated with a shortening of its service life.

The object of the invention is to propose a radiant heating device of the kind described at the beginning, wherein the surface of the heating wire is formed according to the desired heat radiation conditions.

According to an embodiment of the invention, the abovementioned object is achieved in a radiant heating device of the kind described at the beginning by dividing the currentcarrying cross-sectional area between at least two single wires, which are twisted together. This heating element consisting of twisted single wires of circular cross-section has a larger heat-radiating surface than a heating element which consists of only one heating wire of equal currentcarrying cross-section. Thus the service life of the heating element consisting of a plurality of single wires twisted together is increased, because the heat radiation is distributed over a larger area. Furthermore, the heating element can be more easily laid in a spiral on the insulating support, as it is less rigid than a heating element composed of only one heating wire of equal currentcarrying cross-sectional area.

The single wires twisted together necessarily touch one another in certain places due to their twisting, s.o that in these places a desirable, rapid glowing occurs, which is visible to the user through the glass ceramic hob. This can be ascribed to the fact that the single wires heat each other up in the places mentioned.

k A further advantage is that if, during manufacture, heating elements of different current-carrying cross-sectional areas are to be manufactured, the same single wire cross-section 1 j i i 1 i i i 1 1 11 3 1 or the same single wire cross-sections can be used. if larger current - carrying cross-sectional areas are desired, correspondingly more singTe wires are twisted together than if smaller current -carrying cross- sectional areas are to be provided.

The invention also solves the following problem: the heating wire conventionally consists of an iron-chromium-aluminium alloy, which is ferromagnetic and thus when coiled generates a considerable magnetic field. Consequently, during heating, oscillations occur in the heating wire, which give rise to a humming noise, which lasts aPproximately 12 s to 20 s. This humming disappears if the temperature-de-Dendent Curie point is exceeded. When the Curie point is exceeded, the heating wire becomes diamagnetic, so that, the magnetic field weakens. The larger the current -carrying crosssectional area chosen in a heating element composed of only one heating wire according to the prior art in order to extend the service life, the louder the humming noise during heating. With the invention, this humming is considerably reduced and barely audible, since in the '--wisted single wires, the Curie point is rapidly exceeded, and furthermore, due to -the comparatively thin single wires, which are oDerated electrically in parallel, the magnetic field is less strong.

In one embodiment of the invention, the single wires twisted together have equal, circular diameters. The twisted single wires may, however, also have a current-carrying crosssectional area which differs from the circular shape, so that the surface even of each single wire is larger than the surface of a single wire of circular cross-sectional area. The current -carrying cross-sectional area of the twisted single wires may be stellar or oval-',tn shape, for example.

4 The heating element can be laid uncoiled, spirally on the insulating support. It may have a narrow gap. Not forming the heating wires as heating coils permits a reduction in the structural height of the radiant heating device.

The heating wires may be pre-oxidised by a prior treatment before being laid on the insulating support.. Pre-oxidation forms an electrical insulation of the surfaces.

Further advantageous embodiments of the invention will appear from the sub-claims and from the following description of examples. The drawing shows:

Figure 1, a radiant heating device in section along the line I-I according to figure 2, Figure 2, a plan view of the radiant heating device according to Figure 1, Figure 3, a part of a heating wire for the radiant heating device, Figure - 4, a heating wire similar to that of Figure 3 in a coiled embodiment, consisting of three single wires and Figure 5, a heating wire consisting of two single wires.

A,radiant heating device 1 is mounted under a glass ceramic hob 2. The radiant heating device 1 has an insulating support 3 with an annular edge 4. The insulating support 3 rests in a metal dish 5 on an Asulating layer 6.

A heating wire 7, which in the embodiment according to Figures 1 and 2 is not coiled, is laid spirally on the 11 1 i i insulating support 3. One terminal 8 of the heating wire 7 is guided out through the annular edge 4. The other terminal 9 of the heating wire extends from the centre of the spiral, out between the insulating support 3 and the insulating layer 6.

The heating wire 7 in the embodiments according to Figures 3, 7 and 8 consists of a plurality of single wires 10, which are twisted together. The heating wire 7 in the embodiment according to Figure 3 consists of approximately twenty single wires, whereas in the embodiment according to Figures 7 and 8, it is manufactured from three or two single wires 10. The heating element formed of the single wires 10 acccording to Figures 3, 7 and 8 is very flexible and can therefore be easily laid uncoiled in the narrow (cf.Figure 2). Its current- carrying cross-sectional area is formed by the sum of cross-sectional areas of the single wires 10. The heat-radiating surface of the heating wire 7 is larger than if the heating element only consisted of one wire having the same cross- sectional area as the sum of the single wires 10.

Enlargement of the surface is effected in the outer regions 11 of the single wires 10 in such a manner that the radiation per unit area in these regions is less than in a wire of circular diameter of equal crosssectional area. In the regions 12 where the single wires 10 twisted in the manner of a braid touch one another, heat accumulates, which in that place gives rise to more rapid and more intensive glowing than in the areas 11.

The single wires 10 may have an., equal diameter, e.g. 0.25 mm or 0.3 mm. However, it is also possible to twist together single wires 10 with different diameters, the one single wire 10 having a diameter of e.g. 0. 25 mm and the other -s.ingle wire 10 having a diameter of e.g..

6 4 0.3 mm. By having single wires 10 of different diameters, the glow pattern and the rapid appearance of the glow pattern can be influenced, in that the single wire 10 of smaller diameter glow more rapidly than the other single wire 10.

i 1 1 1 1 In the embodiments according to Figures 4 and 5, the twisted single wires 10 are wound to form a heating coil 13. This heating coil 13 may be laid spirally or in another configuration on the insulating support 3.

The heating wire 7 according to Figures 3, 4 and 5 may also be manufactured, by twisting, from single wires 10 with circular crosssection and single wires of non-circular cross-section. Thereby, the heatradiation conditionsf in particular the glow pattern, can be further improved.

i 1 i 1 1 i i i i 1 i 1 i i 1 1 i j A.

i 1 7 j, r

Claims

1 Radiant heating device, in particular for a hob, having a heating element composed of heating which is laid on an insulating support dish and forms- a current- carrying cross-sectional area and a radiating surface, wherein the current-carrying sectional area is divided between at least two wires which are twisted together.

cooker wire, which heatcrosssingle

2. A radiant heating device according to Claim 1, wherein the single wires have the same, circular diameter.

3. A radiant heating device according to Claim 1, wherein the single wires have different, circular diameters

4. A radiant heating device according to Claim 1, wherein the single wires have a cross-sectional area which deviates from the circular form.

5. A radiant heating device wherein the cross-sectional area shape.

according to Claim 4, is stellar or oval in

6. A radiant heating device according to Claim 1, wherein one or more single wires have a circular diameter and'one or more single wires have a cross-sectional area which deviates from the circular form.

7. A radiant heating device according to any one of the preceding claims, wherein the heating element is laid uncoiled and spirally on the insulating support.

AI k 8

8. A radiant heating device according to any one of the preceding claims, wherein the wires are pre-oxidised.

i I i J.1 Publishea 1990 at The PatentOffice. StAte Emue. 66 71 54h Rolborn.14ndon WC1R 4TP. Further oopits maybe obtained from ThePatantOtrice. E&As ZrawJi, St Marj Cray. OrpU4wr4 Kent BR5 3RD. Printed by Multiplex ques ltd. St Mary CrAy, Kent, Con. 1187 1