Heating element with sections of varying heating power and cooking appliance

Abstract

Heating element (10, 10', 10") for a heating system for cooking appliances, comprising a support (12), a heating layer (14) applied to the support (12) and forming a heating surface with a normal area (20), and at least one functional section (18) of the heating surface, wherein the functional section (18) locally modifies the specific heating power of the heating surface, wherein the heating layer (14) at least partially contains a carbon nanotube dispersion and water glass.

Description

[0001] The invention relates to a heating element for a heating system for cooking appliances and to a cooking appliance with such a heating element.

[0002] Heating elements for cooking appliances can consist of a heating layer made of an electrically conductive material. When a voltage is applied to the heating layer, a current flows through it, and due to the electrical resistance of the heating layer material, the current-carrying portion heats up. The electrical energy converted into heat in a given section of the heating layer is, over a wide range, proportional to the current flowing through the heating layer.

[0003] The heating element is usually made of a homogeneous material and therefore has a homogeneous heating output. Nevertheless, uneven heating of the cooking container and / or the food inside it has been observed, making precise temperature control very difficult.

[0004] Furthermore, the heating elements' heating layers are usually not without defects, such as electrical contacts, mounting recesses, or bends. These defects prevent homogeneous heating of the heating layer, as areas of elevated temperature, known as hotspots, develop around them. Such inhomogeneous temperature distributions further complicate temperature control.

[0005] From DE 10 2004 044 352 A1 a heating conductor for a heating device of an electric heating appliance is known, which generates heat by flowing current, wherein the material of the heating conductor has carbon nanotubes.

[0006] Furthermore, a household appliance for preparing food is known from DE 10 2009 003 135 A1, with a receiving chamber in which the food can be placed for preparation and in which a preparation device is arranged which has several separately heatable heating zones, wherein a heating zone heating device is designed as a printing by means of a printing process on the underside of a plate of the preparation device.

[0007] Furthermore, EP 0 069 298 A1 discloses a hotplate with a flat hotplate body having a substantially flat top and bottom surface in the heated area, an electric heating element located on the underside of the hotplate body, and a base plate and / or a raised edge. The hotplate body is formed from a flat substrate of highly thermally conductive, electrically insulating material, on the underside of which a layer of resistive material is applied.

[0008] The present invention is based on the objective of providing a heating element that heats the food container or the food to be heated evenly.

[0009] This problem is solved by a heating element for a heating system for a cooking appliance, comprising a carrier, a heating layer applied to the carrier and forming a heating surface with a normal area, and at least one functional section of the heating surface, wherein the functional section locally modifies the heating power of the heating surface. The heating layer contains at least a portion of a carbon nanotube dispersion and water glass. This provides a heating layer with a low heat capacity and enables the heating layer to withstand temperatures up to 500°C. The invention is based on the understanding that a heating element with a homogeneous heating power distributed over its surface produces an uneven heat distribution, since different areas of the heating element experience different heat losses. For example, the heat loss at the edge of the heating element is greater than in the center.To counteract this, functional sections are provided that locally modify the specific heating output of the heating surface, for example, by providing higher heating output in the edge areas. This allows the heat losses from the different areas of the heating surface to be compensated for. This ensures a homogeneous heat distribution and thus uniform heating of the cooking container or the food being cooked.

[0010] For example, an area of ​​the heating layer with a layer thickness that differs from the layer thickness of the heating layer in the normal area forms the functional section, so that the functional section can be formed in a simple way.

[0011] In one design variant, the functional section is an area of ​​the heating layer on which another heating layer is located, allowing the heating power of the functional section to be adjusted.

[0012] For example, the heating layer and the subsequent heating layer have different compositions, especially different specific resistances, so that the difference in heating power can be precisely adjusted.

[0013] Preferably, the additional heating layer contains carbon nanotubes, in particular a carbon nanotube dispersion. This allows the advantageous properties of a heating layer made of carbon nanotubes to also be used for the additional heating layer.

[0014] In one embodiment of the invention, a temperature sensor is also provided in the heating element, which has a measuring surface containing carbon nanotubes, in particular a carbon nanotube dispersion, and whose temperature is determined by resistance measurement. This enables a very precise and direct temperature measurement of the heating layer.

[0015] Preferably, the heating layer containing the carbon nanotubes, in particular the carbon nanotube dispersion, also contains water glass, thereby ensuring temperature resistance of the heating layer up to 500°C. Particularly preferably, the heating layer containing the carbon nanotubes, in particular the carbon nanotube dispersion, is silicone-free.

[0016] For example, the heating element is curved or forms a free shape, so that the heating element can be used with any cooking container.

[0017] In one embodiment of the invention, a dielectric is provided between the support and the heating layer, so that the support and the heating layer are insulated from each other.

[0018] In another embodiment, the heating element features current distributors that are distributed across the heating layer in such a way that the current density generated by electrical contacts is almost homogeneous within the heating layer. This avoids undesirable areas of high heating power, so-called hotspots, which tend to form around defects.

[0019] The power distributors can be located on the functional section and / or the normal area, so that there is an electrical contact between the heating layer and the power distributor.

[0020] In another embodiment, the current distributors are arranged between the two heating layers, so that both heating layers are equally in contact with the current distributor.

[0021] In another variant, the power distributors are arranged section by section in place of one of the two heating layers, thus enabling a simpler manufacturing of the layer system.

[0022] Further features and advantages of the invention will become apparent from the following description and from the accompanying drawings, to which reference is made. The drawings show: - Fig. 1a a section through the heating element according to the invention, - the Fig. Figures 1b to 1d show a section through further embodiments of the heating element according to the invention, - Fig. 2 the heating element after Fig. 1a in top view, - Fig. 3 the heating element after Fig. 1b in top view, and - Fig. 4 a section of a further embodiment of the heating element according to the invention.

[0023] In Fig. Figure 1a shows a partial section of a heating element 10. The heating element 10 has a carrier 12 on which a dielectric 13 and a heating layer 14 are applied. In certain areas, the heating layer 14 is additionally provided with a further heating layer 16, with this area forming a functional section 18. The heating power in the functional section 18 differs from the heating power of a normal area 20 without functional section 18.

[0024] The heating layers 14, 16, individually or together, form a heating surface and contain carbon nanotubes, in particular a carbon nanotube dispersion, and may contain water glass. They are preferably free of silicone to achieve a temperature resistance of at least 500°C.

[0025] As a rule, the heating layers 14, 16 have different compositions with respect to the carbon nanotubes, which lead to different specific resistances.

[0026] The functional section 18 can also be formed by a region of the heating layer 14 with a different layer thickness. In this case, the additional heating layer 16 would be part of the heating layer 14 and made of the same material.

[0027] The functional section 18 is therefore a section of the heating surface that has a different layer thickness than the normal area 20, a different composition than the normal area 20 and / or an additional heating layer 16.

[0028] In Fig. Figure 1b shows a second embodiment of the heating element 10' in section. It essentially corresponds to the embodiment according to Figure 1b. Fig. 1a, wherein a current distributor 22 is provided on the further heating layer 16. The current distributor 22 can also be arranged in the normal area 20 on the heating layer 14 and contains metal, in particular silver, or a highly conductive carbon nanotube dispersion.

[0029] Fig. 1c shows a similar pattern to the Fig. 1b a third embodiment with a current distributor 22. However, the current distributor 22 is partially provided in the functional section 18 in place of the heating layer 14. The current distributor 22 is therefore located between the further heating layer 16 and the dielectric 13 and interrupts the heating layer 14.

[0030] Fig. Figure 1d shows a fourth embodiment of the heating element 10', also with a current distributor 22. In contrast to the embodiment according to Fig. In 1c, however, the power distributor 22 is arranged between the heating layer 14 and the further heating layer 16, so that the heating layer 14 is not interrupted by the power distributor 22. The layer thicknesses of the heating layer 14 and the further heating layer 16 are reduced in the area of ​​the power distributor 22 to create space for the power distributor 22.

[0031] In Fig. Figure 2 shows a top view of the heating element 10. In this embodiment, two functional sections 18 are formed at the lateral edges of the heating element 10. The inner normal area 20 is formed without functional sections 18. Contacts 24 are arranged on the upper and lower sides of the heating element 10 (as shown in the figure) and extend along the entire width of the heating element 10.

[0032] Furthermore, the heating element 10 has openings 26, for example, holes for mounting the heating element 10 in a cooking appliance, which represent defects with regard to the uniform heat generation in the heating element 10. Defects can arise not only from openings but also from other fastening elements or the like. Bends can also act as defects.

[0033] To operate the heating element 10, a voltage is applied between the contacts 24, so that a current is generated through the heating layer 14 and, if necessary, the further heating layer 16.

[0034] This current causes the heating layers 14 and 16 to heat up. The heating power of the functional sections 18 and the normal area 20 depends on the layer thickness, the materials used for the heating layers 14 and 16, and the current flowing through them.

[0035] The edges of the heating element 10, for example due to thermal radiation, have a greater heat loss than the inner areas of the heating element 10. Therefore, the heating power of the outer functional sections 18 is chosen to be greater than the heating power of the inner normal area 20 in order to compensate for the greater heat losses at the edges. The heating powers are matched to each other in such a way that the heat distribution over the entire heating element 10 is essentially homogeneous.

[0036] Furthermore, a temperature sensor can be implemented by attaching two conductors. A section of the heating layer 14 or 16 serves as the measuring surface 28, which is contacted by means of two conductors 30 connected to evaluation electronics (not shown). The evaluation electronics measure the resistance of the heating layer 14, 16 between the contact points of the conductors 30. From this, the temperature of the heating layer 14, 16 can be deduced, since the resistance of the carbon nanotubes is temperature-dependent. Thus, it is possible to directly determine the temperature of the heating layer 14, 16.

[0037] The heating element 10', which is in Fig. The element shown in 3 essentially corresponds to the heating element 10 according to Fig. 2. However, as in the embodiment according to Fig. 1b, two current distributors 22 are arranged in the area between the two contacts 24. The current distributors 22 are located on opposite sides of an imaginary line between the two openings 26 and extend parallel to this line. The current distributors 22 have a length that is greater than the distance between the two openings 26, so that both openings 26 lie completely within the area between the current distributors 22.

[0038] The current distributors 22 reduce the current density in the areas around the openings 26 by distributing the current across their surface. This prevents areas of increased current density from forming around the openings 26, which would otherwise lead to hotspots.

[0039] In Fig. Figure 4 schematically shows another embodiment for the arrangement of the functional sections 18 in a top view of a section of the heating element 10". In this embodiment, a plurality of functional sections 18 are provided, so that the functional sections 18 and the normal areas 20 alternate.

[0040] In this embodiment, a change in the specific heating power over a larger range is achieved by different densities of the functional sections 18 in the normal ranges 20.

[0041] In this way it is possible to gradually change the heating power, so that an even better uniformity of the heat distribution of the 10" heating element can be achieved.

[0042] Naturally, the design according to Fig. 4 can be combined with the aforementioned embodiments.

[0043] Furthermore, the shape of the heating element is not limited to a flat plate. Rather, the heating element can be curved or form a freeform shape. This allows heating elements to be manufactured for any type of cooking container, which can be fitted flush against the outside of the container.

Claims

[1] Heating element (10, 10', 10") for a heating system for cooking appliances, comprising a support (12), a heating layer (14) applied to the support (12) and forming a heating surface with a normal area (20), and at least one functional section (18) of the heating surface, wherein the functional section (18) locally modifies the specific heating power of the heating surface, wherein the heating layer (14) at least partially contains a carbon nanotube dispersion and water glass. [2] Heating element according to claim 1, characterized by , that a region of the heating layer (14) with a layer thickness that differs from the layer thickness of the heating layer of the normal region (20) forms the functional section (18). [3] Heating element according to claim 1 or 2, characterized by , that the functional section (18) is an area of ​​the heating layer (14) on which another heating layer (16) is located. [4] Heating element according to claim 3, characterized by, that the heating layer (14) and the further heating layer (16) have different compositions, in particular different specific resistances. [5] Heating element according to claim 3 or 4, characterized by , that the further heating layer (16) contains carbon nanotubes, in particular a carbon nanotube dispersion. [6] Heating element according to any one of claims 3 to 5, characterized by , that the further heating layer (16) which contains the carbon nanotubes, in particular the carbon nanotube dispersion, has water glass. [7] Heating element according to any one of the preceding claims, characterized by , that a temperature sensor is provided on the heating element (10) which has a measuring surface (28) containing carbon nanotubes, in particular carbon nanotube dispersion, and whose temperature is determined by resistance measurement. [8] Heating element according to any one of the preceding claims, characterized by, that the heating element (10) is curved or forms a freeform shape. [9] Heating element according to any one of the preceding claims, characterized by , that a dielectric (13) is provided between the support (12) and the heating layer (14). [10] Heating element according to any one of the preceding claims, characterized by , that the heating element (10', 10") has current distributors (22) which are distributed over the heating surface in such a way that a current density generated by electrical contacts (24) in the heating surface is almost homogeneous. [11] Heating element according to claim 10, characterized by , that the power distributors (22) are arranged on the normal area (20) and / or the functional sections (18). [12] Heating element according to claim 10, insofar as it refers back to claim 4, characterized by , that the power distributors (22) are arranged between the two heating layers (14, 16). [13] Heating element according to claim 10, insofar as it refers back to claim 4, characterized by, that the power distributors (22) are arranged section by section in place of one of the two heating layers (14, 16). [14] Cooking appliance with a heating element (10, 10', 10") according to one of the preceding claims, characterized by , that the heating element (10, 10', 10") can be attached to the outside of a food container to be heated.

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