EP0056150B1 - Electric heater - Google Patents

Abstract

An electric hotplate, having an insulator bearing at least one electrical heating resistor, comprising: a metal plate heatable by the at least one electrical heating resistor, having substantially flat upper and lower surfaces and a downwardly directed rim near its outer circumference, the rim and lower surface defining a space therebetween, and the upper surface forming a cooking surface for receiving cooking vessels placed thereon; the insulator having a plurality of insulating layers, the at least one heating resistor being disposed on the upper surface of the uppermost layer; a basket-shaped metal lattice surrounding the insulator about its outer surface and holding the plurality of insulating layers together, the insulator and metal lattice forming an independently positionable heater/insulator assembly; and, fasteners at the rim for engaging the metal lattice and holding the heater/insulator assembly in position at least partly in the space.

Description

The invention relates to an electric heater according to the preamble of claim 1. DE-OS 2820 139 describes such a heater, which consists of an insulating body carrying the electrical heating resistors, the outside of which is reinforced by a metal grid, for example a wire mesh or an expanded metal grid, is surrounded.

From US-A-2 261 496 an electric heater has become known, in which two insulating layers arranged at a distance from the heating elements lie in a metal bowl, but are not held together by the same. There is a risk that the insulating layers will move out of position during transport or operation and lead to malfunctions. It is therefore not advisable to attach the heating elements to the upper insulating layer.

The present invention aims to further improve the properties of the radiator mentioned in the introduction, in particular with regard to the good handling and strength as well as the insulating properties of the insulating body.

This object is achieved according to the invention by the features of claim 1.

The multi-layer, insulating body held together makes it possible to design the insulating materials in such a way that they are optimally used in accordance with their particular purpose. For example, an upper, relatively thin insulating layer can consist of a mechanically stronger insulating material that supports the heating resistors, while the layer underneath is selected primarily with regard to good insulating properties. Everything is held together by the metal grille, so that a manageable body is created, which can advantageously be used both as a unit that can be used directly in this form for heating the underside of a glass ceramic plate, or in conjunction with a plate made of metal with side flanges as a single hot plate can be used. In most cases, it is not necessary to provide a further lower cover, for example a sheet metal cover, on the underside of the hotplate, because the metal grid not only ensures sufficient strength, but can also be grounded. The invention can be used in connection with exposed heating coils as radiant heating or in connection with tubular heaters in which the heating resistors are insulated in a mostly triangular metal jacket. In any case, the top insulating layer carries the heating coils or the tubular heating elements, preferably on protruding ribs. In the case of contact radiators (tubular radiators), sufficient pressure is ensured against the metal or glass ceramic plate by placing springs on the underside of the insulating body, i.e. the lower part of the metal grille.

By means of ribbing between the individual insulating layers, air spaces which are preferably enclosed by peripheral ribs can be formed, which further contribute to improving the insulating properties without increasing the thermal inertia in the slightest. The ribs should, if possible, be designed in such a way that they ensure the distance under all circumstances, for example by a combination of circular and radial ribs.

It is also advantageously possible to include temperature protection switches or other temperature-sensitive switching elements in the insulating body. Both the entire switch body can be arranged within the insulating body. the top insulating layer can provide a shield for the not so temperature-resistant switch body. In the case of a temperature switch with a rod-shaped temperature sensor, the switch head can be arranged outside the insulating body and only the rod-shaped sensor can protrude transversely through the insulating body, preferably lying in a recess in one of the lower insulating layers, while the upper insulating layer has openings in the area of the temperature sensor to ensure a good and, if possible, instantaneous temperature coupling between the temperature sensor and the heating resistors. For this purpose, additional coupling elements can be provided, such as a channel-like reflector part, which lies below the temperature sensor and couples it in terms of radiation in the manner of a parabolic mirror, or a heat-conducting bridge between a tubular heater and the temperature sensor, which projects through one of the openings.

• Fig. 1 einen vertikalen Schnitt durch einen elektrischen Heizkörper,
• Fig. 2 eine Unteransicht des Heizkörpers nach Fig. 1,
• Fig. 3 eine seitliche Ansicht der unteren Randkante des Heizkörpers nach Fig. 1,
• Fig. 4 bis Fig. 6 vertikale Schnitte durch weitere Ausführungbeispiele eines elektrischen Heizkörpers,
• Fig. 7 eine abgebrochene Draufsicht auf den Heizkörper nach Fig. 6, mit Schnittangabe VI-VI für Fig. 6,
• Fig. 8 bis Fig. 10 unterschiedliche Ausführungen der Anordnung eines Temperaturfühlers, geschnitten nach der Linie VII-VII in Fig. 7.
• Fig. 11 einen vertikalen Schnitt durch einen weiteren elektrischen Heizkörper,
• Fig. 12 einen Detailschnitt aus Fig. 11 und
• Fig. 13 eine Teildraufsicht auf ein Streckmetallgitter.

Further advantages and features of the invention emerge from the subclaims and the description in connection with the drawings, the features shown and described being advantageous individually or in combination with one another, in particular also in combination with other embodiment variants. Embodiments of the invention are shown in the drawing and are explained in more detail below. Show it:

• 1 is a vertical section through an electric radiator,
• 2 is a bottom view of the radiator of FIG. 1,
• 3 is a side view of the lower edge of the radiator of FIG. 1,
• 4 to 6 vertical sections through further exemplary embodiments of an electric radiator,
• 7 shows a broken top view of the radiator according to FIG. 6, with section VI-VI for FIG. 6,
• 8 to 10 different versions of the arrangement of a temperature sensor, cut along the line VII-VII in Fig. 7th
• 11 is a vertical section through a further electric radiator,
• Fig. 12 shows a detail section from Fig. 11 and
• Fig. 13 is a partial plan view of an expanded metal grid.

1 to 3 show an electric radiator 11, which is used to heat a plate 12d, which is an essentially flat and relatively thin-walled plate made of steel or cast material.

It has an upper flat cooking surface 32 and a lower, also flat underside 51, against which tubular heating elements 31 are pressed for contact heat transfer. The tubular heaters consist in a known manner of inner, usually helical heating resistors 33, which are located in an electrically insulating investment material and are surrounded by a thin-walled jacket 34 made of stainless steel tube, which is pressed in a triangular shape, so that an upper contact surface results the bottom 51 rests. Because of the good heat transfer property from the tubular heater to the cooking surface 32, which results from the short distance of the heating resistor to the plate 12d with good heat transfer of the highly compressed investment material in the tubular heater and good heat cross-conduction in the plate 12d, the tubular heater can be made with relatively large distances that are larger than the width dimensions of the tubular heater itself, be arranged and still generate a sufficient power density.

A downwardly directed, essentially cylindrical outer edge 13 is formed on the outer circumference of the plate 12d and has an oblique groove in its upper area adjacent to the cooking surface 32 for receiving an outer hold-up ring 19 made of stainless sheet metal material. The plate thus has the shape of an inverted flat box, which receives the heater 52, which is designed as a coherent unit, in its interior. It has an insulating body 28d, which in the example shown consists of two insulating layers 29, 30, which are held together by a metal grid 49d.

The upper insulating layer 30 is in the form of a relatively thin disk made of a mechanically stronger, temperature-resistant heat insulating material, which is pressed, for example, from mineral fibers, as are known under the trade name “Fiberfrax”. It has radially extending ribs 53 on the top, on which the tubular heating elements 31 rest. On the underside, the insulating layer 30 also has a ribbing 54, which, however, is arranged in the form of concentric circles in the example shown. Radially arranged ribs 55 of the lower insulating layer 29 in turn work together with the ribbing 54, so that an air gap 56 is created between the insulating layers 29, 30, which is largely closed to the outside by the outer annular rib 54 on an annular rib 57 encircling the outer circumference lower insulating layer 29 rests.

The insulating layers 29, 30 are held together by the metal grid 49d, which is a relatively fine-mesh steel wire mesh with mesh sizes on the order of 2 mm. However, other types of wire mesh or expanded metal mesh 49 'can also be used as the metal mesh, which are produced from a metal sheet by slitting and stretching transversely to the slot direction and accordingly have a somewhat flat hexagonal honeycomb structure.

The metal grid 49d is deformed into the shape of a flat circular shell and takes in the two insulating layers. The upper edge 58 is bent inwards over the edge of the upper insulating layer, so that the two insulating layers are held firmly together. The radiator 52 is produced in such a way that a shell is first formed from the metal grid, which shell does not yet have cylindrical, but rather somewhat conical, side walls. Then the material for the lower insulating layer 29, which consists of a mechanically not so strong material, but which has very good insulating properties with good temperature resistance, for example an inorganic molding compound made of short fibers or powdery material, this mixture, if necessary with a binder can be offset, is pressed into the metal grid shell, the ribs 55 also being produced. Then the second layer, to which the tubular heating elements 31 are possibly already attached, is placed on top and the metal grid is deformed into its final shape with cylindrical side walls and an inwardly bent edge. The bottom 58 of the metal grid 49d is profiled, so that radial or star-shaped recesses 59 are preferably formed (FIG. 3), which not only provide stiffening of the underside, but also, in cooperation with a pressure spring 25d, a security against rotation of the radiator 28d prevented relative to the plate 12d.

2, the pressure spring 25d has the shape of a three-pointed star made of spring plate, the arms of which are bent in such a way that their central region press against the metal grid 49d, while the arm ends lie on projections 60 of tab-like fastening elements 44d which are welded on the outside at the edge 13 of the plate 12d three and protrude essentially vertically downwards. At their lower end they have barbs 45 which reach through a lower cover 43 of a hob or a corresponding support bracket and thereby hold the entire radiator 11 in place. This attachment is preferably carried out resiliently by an underlying leaf spring 48.

The insulating body 28d is thus pressed upwards by the pressure springs 25d, so that the relatively thin flexible, spirally wound tubular heating elements are pressed resiliently onto the underside 51 of the plate 12d. The metal grid 49d forms the lower surface of the insulating body and thus also of the entire radiator 11. There is therefore no need for a separate cover plate. Electrical safety is also ensured because the metal grid can be grounded.

In a recess of the insulating body, a temperature protection switch 37 is used, which is held by the fact that its lower Shoulder 61 is supported on the edge of a recess in the metal grid 49d. As a result, its connection ends with the insulation surrounding them are freely accessible from the outside. The switch body of the temperature protection switch 37 is located in a recess in the lower insulating layer 29, during which the temperature sensor, for example a bimetal element, protrudes through a narrower recess in the upper insulating layer 30 and rests on the underside of a tubular heating element 31 in order to sense its temperature as quickly as possible .

The upper insulating layer 30 thus protects the more temperature-sensitive switch part of the temperature protection switch against excessive heating.

• In der Metallgitter-Schale 49d des Isolierkörpers 28e befindet sich eine untere Isolierschicht 29, die der vorherbeschriebenen entspricht. Die obere Isolierschicht 30e hat jedoch an ihrer Oberseite eine flache Ausnehmung, so dass der Rand 58 des Metallgitters auf einen nach oben vorstehenden Rand der Isolierschicht 30e greift. An der Oberseite der Isolierschicht 30e sind radiale Rippen 53e ausgeformt, in die wendelförmige Heizwiderstände 33e bei der Verformung der Isolierschicht 30e teilweise eingepresst sind. Die Einpressung erfolgt so, dass im Bereich der Rippen der Drahtquerschnitt der Heizwendeln von dem Isoliermaterial umgeben ist, jedoch nicht der gesamte Wendelumfang von der Isoliermasse abgedeckt ist. Im Bereich zwischen den radialen Rippen verlaufen die Heizwiderstände weitgehend uneingebettet auf der Oberfläche der Isolierschicht 30e. Eine derartige Festlegung der Heizwiderstände ist aus der deutschen Patentschrift 2 729 929 bekannt geworden. Sie schafft einen besonders wirksamen Strahlheizkörper, bei dem an der plattenförmigen oberen Isolierschicht 30e die Heizwiderstände sicher und leicht handhabbar festgelegt sind, ohne dass partielle Überhitzungen an den Festlegungsstellen oder eine Lösung der Heizwiderstände zu befürchten sind.

4 shows a variant which corresponds to that of FIGS. 1 to 3 except for the following differences:

• In the metal grid shell 49d of the insulating body 28e there is a lower insulating layer 29 which corresponds to that described above. However, the upper insulating layer 30e has a flat recess on its upper side, so that the edge 58 of the metal grid engages with an upwardly projecting edge of the insulating layer 30e. Radial ribs 53e are formed on the upper side of the insulating layer 30e, into which helical heating resistors 33e are partially pressed when the insulating layer 30e is deformed. The pressing is carried out in such a way that the wire cross section of the heating coils is surrounded by the insulating material in the area of the ribs, but not the entire coil circumference is covered by the insulating compound. In the area between the radial ribs, the heating resistors run largely without being embedded on the surface of the insulating layer 30e. Such a definition of the heating resistors has become known from German Patent No. 2,729,929. It creates a particularly effective radiant heater in which the heating resistors are securely and easily handled on the plate-shaped upper insulating layer 30e without partial overheating at the fixing points or a solution of the heating resistors being feared.

The insulating body 28e thus obtained is attached to the underside of a plate 12d which, like the one according to FIGS. 1 to 3, represents a hotplate to be installed in a hob. The insulating body is installed in such a way that the heating resistors 33e are at a distance from the underside 51 of the plate 12d, but this is due to the good fixing of the heating resistors 33e to the upper insulating layer 30e, which has no tendency to bulge, and because of the possibility of the plate 12d and to ground the metal grid 49d can be very low, so that not only the overall height becomes low, but also the radiation heat transfer to the plate 12d is very good.

The insulating body 28e is fixed to the plate in that projections 60e in the form of lateral, upright tabs of the fastening elements 44e, which are otherwise the same as in FIG. 1, engage in depressions 62 on the outer circumference of the metal grid shell 49d (see detail in FIG dash-dotted circle in Fig. 4).

It is also possible to provide a projecting shoulder 63 in the area of the outer circumference of the metal mesh shell, which shoulder rests on the lower edge of the edge 13 (right part in FIG. 4).

A temperature protection switch 37e lies flat in a recess in the lower insulating layer 29 and is completely covered by the upper insulating layer 30e in order not to be directly exposed to the high temperatures of the heating resistors 33e.

FIG. 4 also shows the hotplate connection 64, which consists of a conventional connection block 65, into which the internal connection lines 66 lead and can be connected there to the outer connection lines. The connecting block 65 is fastened to a connecting plate 67 which projects beyond the edge of the hotplate and is attached to the bottom section 58 of the metal grid 49d by spot welding or other fastening means. It should be noted here that the attachment of parts to the metal grille is particularly simple, since this can be easily accomplished with wood or self-tapping screws.

The embodiment according to FIG. 5 differs from that according to FIG. 4 in that instead of the lower insulating layer 29, a plurality of thin plate-shaped insulating layers 29f are provided, which are each provided with mismatched profiles on their top and bottom sides, so that they are between them Create insulating air gaps 56. In this case, the insulating layers could consist of a mechanically stronger, possibly also ceramic material.

Fig. 6 shows an electric heater 11g, which is pressed below a glass ceramic plate 12g, the top of which forms the cooking surface 32g, by means of spring elements, not shown. It lies with an upwardly projecting peripheral edge 67 of the upper insulating layer 30g on the underside of the plate 12g.

The upper insulating layer 30g has on its top in a shallow recess radially extending grooves which, as in FIGS. 4 and 5, define helical heating resistors 33e. The circular disk-shaped central part of the insulating layer 30g is crossed somewhat eccentrically by the rod-shaped temperature sensor 68 of a temperature switch 69 which runs in a groove 70 of the lower insulating layer 29g which can be seen in FIG. 8. Openings 71 in the sensor area are provided in the upper, thinner insulating layer 30g between the ribs 53e, which ensure sufficient heat coupling between the heating and the temperature sensor. The switch head of the temperature switch 69 is arranged outside the radiator.

• Aus dem im strichpunktierten Kreis herausgezeichneten Detail ist zu ersehen, dass der obere, nach innen gebogene Rand, der bei den vorigen Ausführungsformen durch eine Einwärtsbiegung des Metallgitters selbst gebildet war, hier durch einen kreisförmigen Blechring 72 mit L-förmigem Querschnitt gebildet wird, an dessen nach unten weisenden, am Aussenumfang der oberen Isolierschicht 30g anliegenden Schenkel der aufwärtsgerichtete Rand des Metallgitters durch Punktschweissung o.dgl. angebracht ist. Der horizontale L-Schenkel liegt auf einer Schulter 73, die zwischen dem Aussenumfang des Isolierkörpers und dem Rand 67 gebildet ist. Durch diesen Ring wird der freie Rand des Metallgitters zusammengefasst und ohne hervorstehende Einzeldrähte abgeschlossen. Bei einem aus Streckmetall bestehenden Metallgitter könnte dieser Rand aus dem dort nicht unterbrochenen Streckmetallmaterial selbst bestehen. Es ist auch möglich, den freien Rand des Metallgitters in das Material des oberen Isolierringes hineinzudrücken, so dass dadurch die freien Enden des Randes verdeckt werden. Auf jeden Fall hält auch hier das Metallgitter die beiden Isolierschichten 30g, 29g zusammen.

The metal grid 49g corresponds to the previous figures with the following exception:

• From the detail drawn in the dash-dotted circle it can be seen that the upper, inwardly curved edge, which in the previous embodiments was formed by an inward bend of the metal grid itself, here is formed by a circular sheet metal ring 72 with an L-shaped cross section, on the downward-pointing leg lying against the outer circumference of the upper insulating layer 30g, the upward edge of the metal grid by spot welding or the like. is appropriate. The horizontal L-leg lies on a shoulder 73 which is formed between the outer circumference of the insulating body and the edge 67. With this ring, the free edge of the metal grille is combined and completed without protruding individual wires. In the case of a metal grid made of expanded metal, this edge could itself consist of the expanded metal material which is not interrupted there. It is also possible to press the free edge of the metal grid into the material of the upper insulating ring, so that the free ends of the edge are thereby covered. In any case, the metal grid also holds the two insulating layers 30g, 29g together here.

9 shows other embodiments of the arrangement of the temperature sensor 68. The embodiment according to FIG. 9 corresponds to that according to FIG. 8, except for the fact that the recess 70h in the lower insulating layer 29h is larger.

FIG. 10 shows a sensor arrangement in which the recess 68i corresponds in size to that according to FIG. 9. Under the temperature sensor there is inserted a reflector part 74, which consists of a sheet of metal running approximately below the temperature sensor 68 and having an approximately semicircular cross section, which reflects the radiation coming in through the openings 71i back onto the temperature sensor 68.

The heating element 11k is also provided for heating a glass ceramic plate 12g, but by means of contact heating elements designed as tubular heating elements 31, which are pressed against the underside of the plate 12g by ribs of the upper insulating layer 30k. An outer upright edge 67k of the upper insulating layer 30k largely closes off the space 27 formed between the plate and the insulation, but does not lie firmly against the plate. The pressure is also applied here via spring elements, not shown, from a support structure of the cooker or the built-in hob.

The temperature sensor 68 of the temperature switch 69 runs in a recess 70k (FIG. 12) of the lower insulating layer 29k. The recess 70k is connected to the space 27 via openings 71k in the upper insulating layer 30k. It is shown in FIG. 12 that a heat-conducting coupling part 76 extends through the openings 70k. It consists of a sheet metal clamp which extends around the temperature sensor 68 and rests on the underside of the tubular heater 31 with two flange-like bends. This ensures a secure coupling between the temperature sensor and the tubular heater.

The invention creates an electric radiator that has numerous advantages. It is very light, so that it can follow a glass ceramic plate even when subjected to impact stresses and does not form an anvil on which the glass ceramic plate could be destroyed. Its low thermal inertia leads to good efficiency and safe and energy-saving parboiling. Efficiency is also increased by the good insulation, which at the same time results in a low floor temperature for the radiator, which can therefore also be used in lower built-in hobs without a cover plate. Nevertheless, the mechanical strength is excellent. It can be seen that in all of the designs the center of the circular cooking surfaces remains free, so that a central sensor, for example a sensor socket of a temperature controller which is located on the underside of the plate or also breaks through it, can be provided there without further ado. Even with radiant heaters, a small distance to the plate (minimum contact distance of 3 mm) is possible, so that the heat coupling to the plate is good. The properties of the insulating materials are ideally exploited due to the multilayer structure. The thin and relatively strong upper carrier plate provides for the mounting and storage of the heating resistors (coils or tubular heating elements) and is preferably pressed from a firmer long-fiber inorganic fiber, while the lower insulating layer only needs to have enough mechanical strength to cover its entire surface the total contact pressure, if there is one, must take up. The metal grid takes care of all other wearing properties. In any case, a manageable unit is created, which contains the heating elements and any temperature limiters and can be easily installed and replaced. It is possible to provide the outer surfaces of the insulating body with a coating 77 on its surfaces surrounded by the metal grid and possibly also on its upper side, for example by spraying with a heat-resistant lacquer or an organic binder layer, in order to also use insulating materials with low abrasion resistance can without fear of dust. This coating is preferably carried out only after the insulating body, which is pressed in the moist state, has dried out.

The result is a radiator with a floor temperature that is approx. 80 to 100 ° lower than that of conventional radiators. This not only improves efficiency, but also protects the environment against high temperatures, so that less effort is required even when routing cables and arranging controllers and switches.

In Fig. 4-7 a radiator is shown, the heating coil of which is arranged in the form of a catchy spiral. Two heating coils can advantageously be essentially par as a two-start spiral allel to each other, which can also be switched on separately to improve controllability.

Claims (12)

1. Electric heater (11) for heating a plate (12d, 12g) with an insulator (28d, 28e), to which are fitted electric heating elements (31, 33), the insulator (28d, 28e) being connected to a basket-shaped metal lattice structure (49d, 49g), which surrounds it in the vicinity of its outer faces, characterized in that the insulator (28d, 28e) comprises several insulating layers (29, 29f, 29g, 29k, 30, 30e, 30g, 30k) which are held together by the metal lattice structure (49d, 49e), in that the upper edge of the metal lattice structure (49d, 49e) terminates at a distance from plate (12d, 12g) and is bent round towards the insulator in the vicinity of the upper edge and in the vicinity of said edge the ends of the wire or strip-like elements forming the metal lattice are combined to give a continuous edge boundary and/or are enclosed in the insulator.

2. Heater according to claim 1, characterized in that the edge boundary is a sheet metal ring (72) with an angular cross-section, which engages over a shoulder (53) of the upper insulating layer (39g).

3. Heater according to claims 1 or 2, characterized in that the metal lattice (49) is provided in its surface forming the underside of the insulator (28d) with preferably radially directed depressions (59).

4. Heater according to one of the preceding claims, characterized in that the outer faces of the insulator (28d, 28e) are at least partly provided with a temperature-resistant coating (77).

5. Heater according to one of the preceding claims, characterized in that the upper insulating layer (30) has ribs (53), which carry heating resistors (33) enclosed in tubular heaters (31).

6. Heater according to one of the claims 1 to 4, characterized in that the upper insulating layer (30e) has ribs (53e), in which are embedded by part of the coil circumference thereof, coil-like heating resistors (33e), this preferably taking place during the compression process of the insulating layer, optionally two preferably separately switchable heater coils being arranged spirally parallel to one another.

7. Heater according to one of the preceding claims, characterized in that the insulating layers (29, 30, 29f) preferably made from compressed granular or fibrous material are provided on there upper and/or lower faces with ribs for forming air gaps (56) and that preferably the upper, thinner insulating layer (30) comprises mechanically stronger material than the underlying insulation (29).

8. Heater according to one of the preceding claims, characterized in that the plate (12d) heated by it is on the top and bottom surface a substantially planar circular metal plate with a substantially circular cylindrical, downwardly directed rim (13) on the outer circumference and that between the plate underside (51), rim (13) and insulator (28d) is formed an area (27) receiving the heating means (31, 33e).

9. Heater according to claim 8, characterized in that fixing elements (44), preferably in the form of clips with barb-like bends (45) are fitted to rim (13), cooperate with detachable accessories (43) in the form of hobs, support members, etc. and optionally secure the insulator (28d, e) on plate (12d) and on the fixing elements (44d) is preferably supported at least one leaf spring-like pressure member (25d) which presses the insulator (28d) upwards and optionally on the fixing elements (44e) are constructed projections (60e), which engage in depressions (62) shaped in the metal lattice (49d).

10. Heater according to one of the claims 9 to 12, characterized in that a step (63) supported on the rim (13) of plate (12d) is provided on the circumference of insulator (28e).

11. Heater according to one of the preceding claims, characterized in that a temperature protection switch (37, 37e) is provided in insulator (28d, 28e) and the switch part thereof is located in a recess of a lower insulating layer (29) and is preferably at least partly thermally shielded by the upper insulating layer (30, 30e), the temperature sensor of the temperature protection switch (37) in particular projecting through the upper insulating layer (30) and engaging on the underside of a tubular heater (31).

12. Heater according to one of the preceding claims, characterized in that the insulator is horizontally traversed by a rod-like temperature sensor (68) of a temperature switch (69), which runs in a recess of a lower insulating layer (29g), that the upper insulating layer (30g, 30k) in the vicinity of temperature sensor (68) has passages (71, 71k) leading to the heating resistors (33) or tubular heaters (31), that preferably recess (70i) contains a channel-like reflector part (74) for the temperature sensor (68) and which is positioned below the latter and that optionally between tubular heater (31) and temperature sensor (68) is provided a thermally conducting coupling part (76), which contacts the casing (34) of the tubular heater and the temperature sensor (68).

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