Electric heating – Heating devices – Combined with container – enclosure – or support for material...
Reexamination Certificate
1999-06-11
2001-03-27
Paik, Sang (Department: 3742)
Electric heating
Heating devices
Combined with container, enclosure, or support for material...
C219S542000
Reexamination Certificate
active
06207935
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention concerns a radiant heating body for a cooking hob, in particular a glass ceramic cooking hob, having a heating conductor comprising a metal foil and an insulating bottom under the heating conductor, wherein the heating conductor extends in a geometrical pattern between the cooking hob and the insulating bottom.
2. Discussion of the Prior Art
A radiant heating body of that kind is described in DE 42 29 375 A1 and EP 0 585 831 A2. The heating conductor comprises a flexible flat strip which is laid in a spiral configuration on the insulating bottom in such a way that its large surface is perpendicular to the insulating bottom and to the cooking hob. Only its narrow upper edge is towards the cooking hob. By virtue of that arrangement the large surfaces radiate heat to each other.
For the purposes of fixing the heating conductor strip in position it has at its lower edge holding plate portions or tongues which are fitted into the insulating bottom itself. Assembly of the heating conductor is a complicated and expensive operation as the flat strip must be laid in spiral-shaped turns and the holding tongues have to be pressed into the insulating bottom.
At the high temperatures, greater than 600° C., involved in radiant operation, corresponding expansion phenomena at the heating conductor occur, which may not result in the heating conductor coming loose from the insulating bottom.
EP 0 175 662 B1 describes a heating conductor whose flat geometrical pattern is separated by stamping out of a foil laminate including a metal foil. That heating body is only suitable for temperatures below 200° C., a maximum of 400° C., and cannot therefore be used as a radiant heating body.
EP 0 175 662 B1 provides for stamping out of the foil laminate two heating bodies in metal form, which are in mutually internested relationship, which reduces the amount of stamping wastage.
EP 0 229 928 A2 discloses a heating element for cooking units, which has a heating conductor path using flat conductor technology, preferably thick-film paste technology, which is applied to a carrier element which is stable in respect of shape. That publication does not describe a radiant heating body in which glowing operation of the heating conductor path is an important consideration.
U.S. Pat. No. 3,567,906 describes a flat heating wire which is wound in a sinusoidal configuration and which is laid in parallel strips beneath the cooking surface. The flat heating wire is not to be visible through the cooking surface. This therefore does not involve a radiant heating body in which it is important for the heating conductor to glow visibly for the user.
DE 41 22 106 A1 discloses a radiant heating body whose heating turns are held down on the insulating bottom by means of a shaped member.
SUMMARY OF THE INVENTION
The object of the present invention is to propose a radiant heating body of the kind set forth in the opening part of this specification, whose heating conductor can be easily fitted over the insulating bottom, wherein temperature expansion phenomena of the heating conductor are to be reliably accommodated and advantageous radiation properties are to be achieved, in particular in regard to rapidly attaining a condition of visible glowing, and in regard to the level of efficiency involved.
In accordance with the invention, in a radiant heating body of the kind set forth in the opening part of this specification, that object is attained in that the geometrical heating conductor pattern is separated out of a metal foil, and that the heating conductor pattern forms heating limbs whose thickness is substantially smaller than their width and whose large surface is directed upwardly and which are held at least at an outer annular rim of the insulating body.
As the heating conductor is separated out of a metal foil, for example stamped out, cut out or etched out, it is fairly stable in the plane of its flat geometrical pattern, that plane being parallel between the plane of the cooking hob or surface and the plane of the insulating bottom. The geometrical pattern is for example star-shaped, in a meander form or in the form of a spiral. It may also adopt the configuration of a company logo, for example a character or symbol, a characteristic sequence of letters, or letters and/or digits.
The heating conductor can be integrally separated out of a metal foil if there is available a metal foil which is of a width corresponding to the diameter of the heating conductor. It may however also be composed of a plurality of portions which form a respective part of the geometrical pattern. In that case the heating conductor portions are preferably connected together in those enlarged-width zones which in operation remain cold in comparison with the heating limbs.
The heating conductor when prefabricated in that way can be easily fitted over the insulating bottom, in which respect laying procedures such as for example winding it in a spiral are eliminated.
The heating limbs of the heating conductor have their large surface facing towards the cooking surface or hob. That therefore provides that the heating conductor occupies a large part of the surface area of the cooking hob, thereby affording advantageous radiation properties. The spacings between the narrow edges of the heating conductor only have to be of such a size that the necessary electrical insulation between the heating limbs is guaranteed. With that arrangement the available surface area of the cooking hob can be substantially covered by heating conductors.
Comparative tests have shown that the glow-up or initial glow time is substantially shorter than in the case of known radiant heating bodies. As the radiating surface area is large in relation to the cross-section, that affords good heat dissipation which makes it possible for the heating limbs to be loaded with a comparatively high level of current density. That also provides for a short time to start cooking, with a high level of efficiency.
A further advantage is that the structural height of the radiant heating body can be small, or, with the same structural height, the thickness of the insulating bottom and thus its thermal insulating effect can be increased.
Preferably the actual heating limbs float freely over the insulating bottom, especially in the cold condition and preferably also in the hot condition. With the thermally induced expansion phenomena which inevitably occur as a result of the high radiant temperatures which are over 600° C. and which can reach up to 1200° C., the heating limbs can sag in the direction of the insulating bottom without the heating conductor coming loose in that situation. The air space between the heating limbs and the insulating bottom has a heat-insulating action. In order to reduce the degree of sag the stiffness of the heating limbs which are flexible in themselves can be increased by the heating limbs being profiled in a U-shape or a V-shape, in regard to their cross-section. The profiling may extend over the entire length or a part of the length.
The heating conductor is preferably fixed by means of holding tongues which are fashioned on its geometrical pattern and which are seated for example on an annular rim of the insulating bottom. The annular rim can be fitted as a separate part on to the insulating bottom or can be formed in one piece thereon.
Preferably the outer annular rim is formed by an annular outer zone of the insulating bottom, which zone projects beyond the insulating bottom, and an annular portion which is fitted on to the outer zone and which comprises thermally and electrically insulating material, with the holding tongues engaging between the outer zone and the annular portion. By virtue of that arrangement, in the assembly operation the heating conductor can be easily fitted on to the annular outer zone, in which case it is at the required spacing from the insulating bottom. The heating conductor is fixed in position by fitting the annular portion in place.
The frictional and/or positively lockin
Dittmar Gunter
Hecht Josef
Wildner Gunther
Diehl AKO Stiftung & Co. KG
Paik Sang
Scully Scott Murphy & Presser
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