EP0031514B1 - Electric heating radiator, and process and apparatus for its manufacture - Google Patents

Abstract

A method for the manufacture of a radiant electrical heater, intended more particularly for heating glass ceramic hotplates, comprises so inserting or pressing helical heating resistors in helical slots in a support, such that the support or the heater coils themselves are deformed and consequently fixed to the support. The coils penetrate the wall or bottom areas of the slot or are otherwise secured by parts of the support by positive engagement. An apparatus for performing the method has a tool with ribs which deforms the support or heater coils for positive engagement. A radiant heater is obtained, whose support has partly overlapping deformations in the slot area of the heater coils.

Description

The invention relates to a process for the production of electric radiant heaters, in particular for glass ceramic hotplates, with a carrier body made of an electrically and heat-insulating material with grooves, in which helical heating resistors are partially embedded, parts of the groove side wall engaging between the heating resistor turns and wherein the effective groove depth is at most approximately as large as the dimensions of the heating resistor windings in the direction of the groove depth, as well as a device for this and the radiant heater thus produced.

With radiant heaters of this type, it is always a problem to fix the heating coils in the grooves in such a way that they are properly fixed during transport and use of the radiant heaters. It is important that the spirals are not only secured against falling out, but also do not tend to move (creep) in the longitudinal direction of the spirals or grooves. The thermal movements during heating and cooling support the tendency to creep, as a result of which individual sections of the coils close together while others pull apart. This results in a stronger heating in the densified areas, which leads to an early failure of the heating resistor.

So far, such heating coils have been fixed in the grooves by heat-resistant cement or putty. If you wanted to shape the grooves in such a way that protrusions or undercuts were created during the manufacture of the carrier body, it would be very difficult to insert the heating coil.

A radiant heater of the type mentioned is known from BE-A-464 026. The insertion takes place there so that the heating resistors protrude over the groove with more than half of their circumference. An indentation in the groove side walls can therefore only fix the heating coils in the longitudinal direction, while they are not secured against falling out upwards.

DE-A-2 729 930 and 2 820 114 describe fixing heating coils by pressing heating coil areas into rib-like projections, but the remaining part of the heating coils lies largely freely on the surface of the insulating support. This definition, which is tried and tested per se, requires high-quality insulation materials and certain manufacturing processes for the correct determination.

Furthermore, from DE-A-2 203 642 and US-A-3 221 396 methods for fixing tubular heating elements are known which are thermally conductively connected to this by partial deformation of a metal plate. In the case of tubular heaters, there are other requirements that cannot be compared with the definition of free heating resistance coils. The heat from the tubular heating element is to be given off to the metal material surrounding it, while in the case of radiant heating elements a heat transfer to the surrounding insulating material is to be avoided. The tubular heating elements are also fixed there by enclosing the smooth tubular heating element jacket.

The object of the invention is therefore to propose a radiant heater and method and device for its manufacture, with which a secure determination of the heating resistors with little work and without additional aids such as putty, cement or the like. is possible.

This object is achieved according to the invention in that the helical heating resistors are introduced into the grooves to such an extent that the side regions of the heating resistor windings with the largest dimensions in the direction of the groove width lie within the groove, and that at least parts of the groove side wall and lateral sections of the windings of the Heating resistors are moved relatively laterally towards one another until parts of the groove side wall overlap the side regions of the heating resistor turns in the direction of the slot opening and engage between the heating resistor turns.

These measures therefore fix the heating coils in the area of the groove by penetrating the material of the carrier body itself. The determination itself takes place after the introduction of the heating coils and does not require any significant additional time. The tools used for pressing in or pressing in can largely integrate these work steps into the process of inserting the helix.

It is advantageously possible, when the heating coils are inserted into suitable grooves, to deform the heating coils, for example from a circular into an oval cross section, by pressing from above, the narrow sides of the oval being formed being pressed into the groove side walls. It is also possible, after inserting the heating coils into suitable grooves, the webs provided between the grooves, i. H. in the area of the groove side walls a little above the coils, so that a section overlapping the heating coils is created. Here too, the heating coils can be pressed into the groove side wall.

In these embodiments, the pressing can be carried out both in the fully cured state of the carrier body, which consists of an insulating ceramic or fibrous material. However, it is also possible to press-in or press-in before the carrier body has hardened, for example by firing or drying. In this case, the heating coils can be pressed in with very little resistance, so that the method can also be used for the thinnest heating coils. However, it is also possible to insert the heating coils into suitable grooves in the not yet cured, still moist support body and then to carry out the curing or firing process. By the shrinkage of the wearer's material body then there is also a reduction in the groove dimensions, so that the spiral is pressed into its walls.

In principle, it is possible in all embodiments to make the determination by penetrating the groove walls over the entire length of the heating coils. However, it is also possible, by providing corresponding projections and recesses in the groove walls, to make the determination only in sections at certain longitudinal intervals, for example if the stability of the heating coils is large enough to get by without gaps in the interstices due to the large wire thickness.

• Fig. 1 einen schematischen Schnitt durch einen Strahlheizkörper und einen Teil einer Glaskeramik-Kochstelle,
• Fig. 2 und 3, 4 und 5 sowie 6 und 7 jeweils zwei Herstellungsschritte von Ausführungsformen im Detailschnitt.

Further features and advantages emerge from the dependent claims or the description in connection with the drawings. Embodiments of the invention are shown in the drawing and are explained in more detail below. Show it:

• 1 is a schematic section through a radiant heater and part of a glass ceramic hob,
• 2 and 3, 4 and 5 and 6 and 7 each two manufacturing steps of embodiments in detail section.

Fig. 1 shows a radiant heater 11 which is arranged below a glass ceramic plate 12 which forms the cooking surface of a cooking appliance. The radiant heater 11 is supported by springs 14 on a support structure 13, which may consist, for example, of a sheet metal recess. It is used to heat cooking vessels placed on the glass ceramic plate 12 by means of radiation, which can partially penetrate the glass ceramic plate and heat it on the other hand.

The radiant heater 11 consists of a normally circular flat carrier shell 15 made of sheet metal, in which a carrier body 16 made of an electrically and thermally insulating material is introduced. It can be made from a ceramic or from an insulating material consisting of inorganic fibers. The carrier body 16 also has the shape of a flat circular shell with an essentially flat bottom 18 and a raised edge 17 which is pressed against the underside of the glass ceramic plate 12 under the action of the springs 14.

In the exemplary embodiment, the carrier body 16 has spirally arranged grooves 19 in which electrical heating resistors in the form of heating coils 20 are located. Their arrangement and fastening in the grooves can be seen more clearly from the following drawing figures. The heating resistors are connected to a domestic electrical network via a switch or controller.

Fig. 2 shows in detail the arrangement of the heating coils 20 in the grooves. In this embodiment, a heating coil with a circular coil cross section is introduced into a groove whose width corresponds to the outside diameter of the coil, while the depth is less than the coil diameter.

2 shows the carrier body 16c after the heating coils 20 have been inserted, but before their final fixing. The heating coils are inserted into a groove 19c, the width of which corresponds to the outside diameter of the heating coils 20, which have a circular cross-section when inserted, and the depth of which lies between half and full coil diameters. Thereafter, the heating coils are acted on with a tool 24, which has the shape of a plate with downwardly projecting, in the exemplary embodiment triangularly shaped ribs 25 which extend in the longitudinal direction centrally over the grooves 19c.

As can be seen from FIG. 3, the heating coils are given a shape which corresponds to a lying, rounded B, the outer coil section 26 of the deformed heating coils 20c lying on the open groove side having an indentation. As a result of this deformation, the overall shape of the coil is widened, so that it digs into the side walls 21 of the groove 19c and forms indentations 23c there, which ensure the heating coils are fixed both in the longitudinal direction of the groove and upwards. Instead of the triangular ribs 25 in cross-section, ribs of any basic shape can be used, so that the most favorable helical shapes in terms of the definition and the radiation properties can be produced, for example also a flat oval. The inflection in section 26, however, has the advantage that a larger section of the heating coils lies on the upper side directly facing the glass ceramic plate.

A carrier body 16d is also shown in FIG. 4 before its completion. The grooves 19d correspond to those according to FIG. 4, however the carrier body 16d in FIG. 1 is formed from the ceramic or fiber-like mass, but has not yet been dried or fired, as a result of which it gets its final hardness. In Fig. 4 the Heizwendein 20 are suitable, but without indentations in the grooves 19d.

5 shows the same carrier body 16d after drying or firing. When burning, a shrinkage occurs in the material of the carrier body, i. H. the carrier body shrinks. Among other things, 4 also reduces the dimensions of grooves 19d according to FIG. 4 to those of groove 19d 'according to FIG. 5. The heating coils, which are not influenced in terms of their mass by the hardening, therefore press into the side walls 21d of the grooves and form indentations 23d there, which have the same effect as in FIG. 3. Here, slight deformations of the heating coils can occur as a result of the lateral indentation. which, however, can be prevented by an appropriate support when pressing in.

6 also shows a production stage of a carrier body 16e, in which the grooves 19e are substantially wider than they are deep, so that heating coils 20e with a largely suitable therein flat oval spiral cross section could be inserted. After the heating coils have been inserted, these webs 29 are acted on using a tool 27 which has triangular projections 28 on its underside, but which are arranged centrally above the webs 29, so that they press apart in the manner of a rivet head 30 (FIG. 7 ) and thus determine the heating coils 20e which remain essentially unchanged in shape. In addition to the fixing in the longitudinal direction, which occurs when the side walls 21 move inwards, the rivet head-like deformations 30 provide particularly good protection against the heating coils 20e falling out. The deformation between FIGS. 6 and 7 can follow in the already hardened or also wet state, the shrinkage (method according to FIGS. 4 and 5) helping to determine the latter in the latter case.

The fixing methods according to the individual described embodiments are particularly advantageous in their combination, especially when it comes to fixing very thin wires, which are also particularly critical with regard to creep. In the case of relatively thick wires, on the other hand, the fixing can be carried out with fewer fuses; in this case, for example, the partitions between the grooves can also be partially omitted or only partially present. This improves the heat transfer conditions. In the embodiment according to FIGS. 6 and 7 in particular, it is possible for thicker wires to provide the deformations 30 only in sections. The depth of the indentations of the heating coils in the walls of the groove consisting of the side and bottom walls depend on the winding diameter of the wire load and the specific load on the heated surface and the wire thickness. In the embodiments, the heating coils are also protected against falling over. This could occur if the wire forming the heating coils loses so much strength at high temperatures that the individual turns of the coils collide laterally, unless they are supported.

When selecting the individual embodiments for the definition, the type of shaped body will also have to be taken into account. For example, in the case of the types of fixing made by indentations in the fully cured state, a relatively soft molded body made of fibrous material is preferred, while in the embodiment according to FIGS. 4 and 5 a ceramic material that is hard after firing is primarily suitable.

The heating coils can also be introduced into the carrier body, which has been pre-dried or pre-hardened but not yet fully hardened.

Claims (10)

1. Method for the manufacture of a radiant electrical heater, particularly for glass ceramic hotplates (12) with a support (16,16c-e) made from an electrically and thermally insulating material with slots (19, 19c-e) in which are partially embedded helical heating resistors (20, 20e), parts of the slot side wall (23c, d, 30) engaging between the heating resistor turns, the effective slot depth being at a maximum of the same magnitude as the dimensions of the heating resistor turns in the direction of the slot depth, characterized in that the helical heating resistors (20, 20e) are introduced into the slots (19, 19c-ee) to such an extent that the lateral regions of the heating resistor turns with the largest dimensions are located within the slot in the direction of the slot width and that at least parts of the slot side wall (21, 21d) and lateral portions of the turns of the heating resistors (20, 20e) are moved relatively laterally towards one another to such an extent that parts of the slot side wall overlap the lateral regions of the heating resistor turns in the direction of the slot opening and engage between the heating resistor turns.

2. Method according to claim 1, in which the support (16, 16c-16e) comprises a fibrous or ceramic material hardenable during the manufacturing process.

3. Method according to claims 1 or 2, characterized in that the insertion of the heater coils (20) is carried out prior to the final hardening of the material of support (16d) and deformation takes place through the shrinkage of support (16d).

4. Method according to claims 1 or 2, characterized in that the deformation of the heater coils (20c) takes place through modifying the coil cross-section after its insertion in slots (19c).

5. Method according to claims 1, 2 or 4, characterized in that the heater coils (20e) are inserted with a circular coil cross-section and deformed by pressing from the outside of the slot (19e).

6. Method according to one of the claims 1 to 5, characterized in that the heater coils (20) are pressed into parts of the slot wall (slot side walls 21), which are preferably spaced from one another in the longitudinal direction of the slot.

7. Method according to one of the claims 4 to 6, characterized in that at least after deformation, the coil cross-section is oval, the smaller axis of the oval being in the radiation direction.

8. Method according to claims 1 or 2, characterized in that the deformation of the support (16e) is brought about by compression of at least part of the web (29) of support (16e) located between two adjacent slots (19e).

9. Apparatus for the manufacture of radiant heaters using the method according to one of the claims 1, 2 or 4 to 8, characterized in that a tool (24, 27) with a plate-like working surface and projections or ribs (25, 28) is provided, which can be pressed against the radiant heater (11), the projections or ribs (25, 28) deforming parts of the support (16e) and/or the heater coils (20c).

10. Radiant heater manufactured according to the method of one of the claims 1 to 8.

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