WO2000019774A1

WO2000019774A1 - Electrical cooking system which transfers contact heat

Info

Publication number: WO2000019774A1
Application number: PCT/EP1999/007258
Authority: WO
Inventors: Gerhard Schmidmayer; Lars Schubert
Original assignee: BSH Bosch und Siemens Hausgeräte GmbH
Priority date: 1998-09-30
Filing date: 1999-09-30
Publication date: 2000-04-06
Also published as: EP1116416B1; ATE240028T1; EP1116416A1; ES2198959T3; DE19845102A1; DE59905488D1; TR200100806T2

Abstract

A known contact heat transfer electrical cooking system has a metallic cooking plate body whose top face is covered by a protective layer, for heating a cooking vessel which can be placed on said protective layer. The cooking plate body also has at least one heating element mounted on its bottom face and a control unit, which is connected to the heating element in order to control the heating capacity of the cooking system. According to the invention, the protective layer is configured as a sol-gel layer in order to produce a cooking system with good application properties which is also simple to manufacture.

Description

Contact heat-transferring electrical cooking system

10

The present invention relates to a contact heat-transferring electrical cooking system with a hotplate body according to the preamble of patent claim 1.

15 Such a cooking system is known from the publication DE 41 09 569 A1, the heating plate having a cover layer made of a material which is a good conductor of heat, electrically non-conductive or only poorly conductive, preferably glass ceramic or ceramic which has only a few mm wall thickness, in particular a thickness in the range from 6 to 10 mm. The metal plate serving as a heating plate is preferably made of steel, preferably has a thickness

20 is in the range from 4 to 8 mm, and is covered on its underside with an enamel layer, the wall thickness of which moves in the um area. The enamel layer is printed with a heating conductor arrangement.

1. Object of the present invention is to provide a cooking system according to the preamble of

Provide 25 claim 1, which has good performance properties with simple manufacture.

According to the invention this is achieved in a cooking system with the features of claim 1. On the one hand, it is a protection against tarnishing of hotplate bodies

30 made of stainless steel or scratching or other soiling prevented. On the other hand, extremely thin protective layers can be easily implemented using sol-gel technology. The sol-gel layer can be applied to the hotplate body, for example, in a simple immersion process. In the sol-gel technique, in particular, the low baking temperatures are compared to the enamelling technique

35 from about 450 to 500 ° C particularly favorable. The applied sol-gel layers are also suitable for the temperatures typical in such cooking systems. The Layer thicknesses are only a few μm. Owing to the sol-gel technique, the layers applied, despite their low thickness, have great stability and great adhesion both in the case of a multilayer technique on one another and on the substrate material itself, in particular metal.

An electrical insulation layer is advantageously also applied to the underside of the hotplate body using the sol-gel technique. In this way, the cover layer and the insulation layer can be applied in one technology and possibly even in the same production step. If the insulation properties of the sol-gel layer are not sufficient, an additional enamel insulation layer can be applied to the sol-gel insulation layer in accordance with a preferred embodiment. This additional enamel could then be a glass enamel, which, in contrast to ceramic enamels, could already be baked at around 550 ° C. If necessary, the additional enamel can be stoved together with heating elements, sensors and electronic circuit elements applied to the additional enamel using thick-film technology.

As a result of the low layer thickness of the sol-gel layer, only low voltages occur in the contact heating element. Furthermore, very good heat conduction from the heating elements to the pot is ensured, and the protective layer and / or the insulating layer are less susceptible to cracking or there is a low probability of flaking off. This thin protective layer provides adequate protection against corrosion and oxidation as well as a hard surface protection for the metal.

A particularly good heat transfer from the hotplate body or to the pot and an excellent heat distribution in this regard can be achieved in that the hotplate body has an insulating protective layer in the μm range, to which the heating element is applied directly by means of thick-film technology. If necessary, the heating element can also be implemented using more complex coating processes, for example thin-film technology.

Due to the thin protective layers, only slight stresses occur in the cooking system when the contact radiator is heated up, despite the different thermal expansion coefficients of the sol-gel protective layer and the metal plate.

According to a preferred embodiment, the hotplate body is cold at about 20 ° C. deepened in the shape of a plumb line. This curvature away from the underside of the pot bottom can be realized in particular by crowning a metal plate serving as a hotplate body. This ensures that pots with cup bases that curve downward in the form of a dome can be placed on the cooking system in a stable manner and at the same time a large-area thermal contact between the hotplate body and the pot base is possible.

Extensive series of tests have shown that the depth of the calotte or of the bowl-shaped hotplate body is at most about OJ mm. This ensures, on the one hand, that almost all cooking vessels available on the market can be placed on the cooking system without any problems, and, on the other hand, that the curvature of the hotplate body can also be controlled in the direction of upwardly curved cooking vessel bases.

The material of the hotplate body is advantageously stainless steel or aluminum. Compared to, for example, silicon nitride, metal has in particular the better heat conduction properties and also cost advantages. On the one hand for reasons of stability and on the other hand for reasons of cost, the thickness of the metal plate advantageously ranges between approximately 2 and 5 mm. Applying the protective layer using a sol-gel technique to a stainless steel plate is much easier in terms of manufacturing technology compared to the enamelling technique.

According to a preferred embodiment, an expansion plate is held essentially in the central region of the hotplate body, the coefficient of thermal expansion of which differs from that of the hotplate body. When the hotplate body is heated, the different length expansion coefficients cause the hotplate body to bulge in the direction of the pot bottom or upwards. As an alternative to this, the hotplate body has a flat recess on its underside in the central area. In terms of production technology, this is easier than holding the insert in the hotplate body. As a result of the central recess, for example in the form of a spherical cap, on the underside of the hotplate body in its central region, the heating process leads to tangential and radial tensile stresses. These cause the hotplate body to bend upwards or towards the underside of the cooking vessel base. In order to be able to control the extent of the curvature of the hotplate body, a sensor system is preferably provided to detect the large-area contact between the bottom of the cooking vessel and the top of the hotplate body. This can be implemented, for example, by a capacitive sensor arrangement. The changing capacitance between the two plates with changing distance between the top of the hotplate body and the underside of the pan base is used in a manner known per se as a measurement signal. Another alternative is that the control unit evaluates the rate of change in the temperature of the hotplate body over time during the heating process. This takes advantage of the fact that the temperature rise of the hotplate body is significantly reduced when the heating power supplied is known, if there is sufficient thermal power contact between the pan base and the hotplate body. Furthermore, it would also be possible for the cooking vessel manufacturers to announce the flatness or the degree of curvature of the base of the pan, and for the operator to be able to predefine the cooking system via an input unit at the start of the respective cooking process. The control unit then calculates the corresponding heating outputs or heating output profiles of the first and second radiators from the specified curvature and the desired or set heating output.

In order not to unnecessarily deteriorate the thermal contact between the bottom of the pot and the top of the hotplate body, the control unit keeps the temperature difference between the central area and the peripheral area of the hotplate body and thus its curvature essentially constant from the detection of sufficient thermal contact between the two. Furthermore, the control unit guarantees, by means of a correspondingly adapted heating output of the two heating elements, that the heating output specified by an operator via the operating unit is reached.

According to a preferred embodiment, the control unit first controls the second heating element arranged in the peripheral area of the hotplate body for a certain time. As a result, the peripheral area is heated relative to the central area of the hotplate body. Tangential and radial tensile stresses are created in the peripheral area of the hotplate body. As a result of these tensile stresses, the circumference of the heating element increases and excellent flatness of the surface of the hotplate body is achieved. Three exemplary embodiments of the cooking system according to the invention and the corresponding method for operating the cooking systems are described below using schematic representations.

Show it:

1 is a greatly simplified side view, partly in section, of the cooking system with the pot placed thereon according to the first exemplary embodiment,

2 is a diagram greatly simplified with the time course of the heating power of the cooking system,

3 the hotplate body of the cooking system according to the second exemplary embodiment,

Fig. 4 shows the hotplate body of the cooking system according to the third embodiment, and

Fig. 5 highly schematic three phases of the heating process in a hotplate body according to the second or third embodiment.

1, a hob has a glass ceramic plate 1, in or below which a cooking system 3 is held. A circular stainless steel hotplate body 5 is placed in a circular opening of the glass ceramic plate 1 from above. On the top of the hotplate body 5, a pot 6 known per se is placed with an underside of the pot bottom (shown in broken lines). As a result, according to FIG. 1, an undesirable air gap is formed at room temperature between the top of the hotplate body 5 and the underside of the pot base 6, which adversely affects the heat transfer from the hotplate body 5 to the pot base 6. The hotplate body 5 is designed as a 4 mm thick disc, the upper side of which is provided with an approximately 5 μm thick transparent protective layer 13 applied using the sol-gel technique.

A colloidal system on a micrometer scale (gel) is generated from a solution (sol) by controlled condensation methods and applied to the substrate. brings. This gel is compacted by drying as a result of solvent removal and then cured in a suitable manner or baked at a temperature of about 450 to 500 ° C. During this process, the sol-gel layer is particularly firmly bonded to the substrate via chemical compounds. The resulting sol-gel layer forms a tarnish and oxidation protection for the stainless steel. The protective layer 13 further protects the stainless steel from being scratched. Alternatively, it is also possible to color the protective layer and / or to make it opaque. On the top of the hotplate body 5 facing the pot 6, the hotplate body 5 has a circumferentially extending shoulder 7 with which the hotplate body 5 lies on the edge region of the opening of the glass ceramic plate 1. A sufficient gap 9 is formed on the circumferential side between the side wall of the hotplate body 5 and the wall of the opening of the glass ceramic plate 1 in order to allow a radial expansion of the hotplate body 5 when it is heated in the heating process in accordance with the operating procedure. For fastening and sealing the arrangement, at least the gap 9 is partially filled with silicone adhesive 11. Furthermore, the hotplate body 5 can be held in the opening of the glass ceramic plate 1 by holding devices (not shown).

1, a dome-shaped recess 15 is formed on the underside of the stainless steel plate 5 in the central area. This extends over about half the diameter of the hotplate body 5 and reaches its maximum depth in the center or center of the circular disk 5. On the underside of the hotplate body 5 is an electrical insulation layer 16 using sol-gel technology in the same thickness as that of Protective layer 13 applied. In order to improve the insulation properties of the insulation layer 16, further sol-gel layers or a glass enamel layer can be applied to it (not shown). A large area of a first heating element 17 is printed on the insulation layer 16 in the region of the recess 15, that is to say in the central region of the hotplate body 5, in particular using thick-film technology with a suitable paste. The first heating element 17 can, for example, run in a spiral and have a plurality of sub-heating circuits connected in series and / or in parallel (not shown). Furthermore, a first temperature sensor 19, likewise applied in thick-film technology, is provided in the region of the recess 15. This is suitably arranged in order to be able to detect the temperature in the area of the recess 15 of the hotplate body 5. Corresponding to the first heating element 17 and the first temperature sensor 19, in the annular peripheral area of the hotplate body 5 there are a second heating element 21 outside the recess 15 over a large area and a second temperature sensor 23 is printed. The heating elements 17, 21 and sensors 19, 23 can in turn be covered with a protective layer (not shown). Furthermore, a thermal insulation layer is provided below the heating elements 17, 21 in order to reduce the energy losses of the cooking system 3 below the glass ceramic plate 1 (not shown).

The cooking system 3 has an electronic control unit 25, which is connected to the first and second heating elements 17, 21 and the first and second temperature sensors 19, 23 via connecting lines 27. Furthermore, the control unit 25 is connected via control lines 29 to circuit breakers (not shown in more detail) which serve to control the heating power of the heating elements 17, 21. In order to make the power control particularly sensitive, this can be done by a vibration or Pulse packet control or a suitable phase control can be implemented. The appropriate switching or control of mains half-waves ensures that the prescribed flicker rates are observed. An input unit 31 is also connected to the control unit 25. The operator can use this to specify, for example, the desired heating output and, if appropriate, also the nature, in particular the curvature, of the base of the pot.

The functioning of the cooking system according to the first exemplary embodiment shown in FIG. 1 can be, for example, the following: The operator specifies a desired output and at the same time the degree of curvature of the pan base used, which is known to him, in the input unit 31. Via parameters stored in a table of the control unit 25, the latter controls the chronological sequence and the values of the heating outputs of the two heating elements 17 and 21 to the known degree of curvature, as will be explained in more detail below.

On the other hand, the heating process can also take place fully automatically according to FIG. 2 if the curvature of the pot base 6 is unknown. First, at time t1, the control unit 25 switches a limited heating output to the second heating element 21, which is arranged in the peripheral region of the hotplate body 5. This causes tangential and radial tensile stress in the peripheral area, which results in an increase in the circumference or an extension of the hotplate body 5. In this first step, a complete flatness of the top of the hotplate body 5 can be achieved. This first phase can take a few seconds, for example 15 to 30 seconds. If a pot 6 is placed on the hotplate body 5 with a completely flat underside of the pot bottom, the actual heating process can be started immediately afterwards. If, however, a pot with the pot base curved upward is placed on the hotplate body 5, the heating power of the first heating element 17 is increased at time t2 according to FIG. 2. As a result of the central, spherical cap-shaped recess 15 in the underside of the hotplate body 5, the plate deformation continues in the direction of the bottom of the pot 6 due to the mechanical stresses caused by the heating of the central area, ie the stainless steel plate 5 bulges upwards. At time t3, the control unit 25 recognizes that the thermal contact between the bottom of the pot 6 and the top of the hotplate body 5 is sufficiently large, ie that the air gap originally present between them is reduced to a minimum. This contact detection is based on the fact that from the time of sufficient thermal contact between the bottom of the pot and the top of the hotplate body 5, the temperature rise per unit of time decreases significantly in the central and peripheral areas. This is caused by the fact that as a result of the good heat-conducting contact between the pot 6 and the hotplate body 5, significantly more heat is removed from the overall system. Typical values for the time interval from time t2 to time t3 can be 30 to 60 seconds.

At the time of contact (t3) between the pot base 6 and the hotplate body 5, there is a defined temperature difference between the central area and the peripheral area of the hotplate body 5. Due to the given geometric configuration of the hotplate body 5, a certain degree of curvature of the hotplate body 5 is assigned to each such temperature gradient. So that the extent of the curvature of the hotplate body 5 is retained when the contact is established, the individual heating powers of the two heating elements 17 and 21 are now increased in a coordinated manner to adjust the heating power desired by the operator. The aim is to keep the temperature difference between the central and peripheral areas measured at the time of contact detection approximately constant. The desired heating output is then set at time t4 and, at the same time, the required degree of curvature of the hotplate body 5 is ensured for producing a heat-conducting contact with the underside of the pot base 6. If it is determined when the desired heating output is reached that the distance between the top of the plate body 5 and the underside of the pot bottom 6 has undesirably increased, the said temperature gradient is reset by the control unit 25.

In the second and third exemplary embodiments according to FIGS. 3 and 4, only the hotplate body 5 is in each case slightly modified from that of the first exemplary embodiment. In order to be able to heat cooking vessel bases 6 that are curved downward with the cooking system 3 with the desired efficiency, the hotplate body 5 is each provided with a crown of approximately OJ mm. As a result, the hotplate body 5 is designed as a dome that is curved downward in the central region away from the bottom of the pot. As an alternative to the second exemplary embodiment, according to the third exemplary embodiment in FIG. 4, a round plate-shaped insert part 43 can also be inserted into a correspondingly shaped circular recess in the underside of the hotplate body 5. This has a larger coefficient of thermal expansion than the hotplate body 5. The function of the hotplate body 5 according to the third exemplary embodiment corresponds to that of the first and second exemplary embodiment, the mechanical stresses in the hotplate body 5 being caused in particular by the different material properties or coefficients.

5 shows three essential phases (a, b, c) of the targeted deformation of the hotplate body 5, which is controlled by the control unit 25, according to the second or third exemplary embodiment. In the unheated state (phase a), the hotplate body 5 has a dome-shaped contour that curves downward. To change this, heat is supplied to the peripheral region of the hotplate body 5 via the second heating element 21. In phase b, as a result of the mechanical stresses that arise, as explained above, the hotplate body 5 is completely flat. In phase c, the first heating element 17 in the region of the recess 15 is first subjected to heating power in order to curvature the hotplate body 5 to reach the bottom of the pot 6. Due to the temperature gradient present in the hotplate body 5, the hotplate body 5 bulges into the upwardly curved pot base 6 until sufficient thermal contact is made between the pot base 6 and the hotplate body 5.

Claims

claims

1.Contact heat-transferring electrical cooking system for heating cooking vessels with a metallic hotplate body, which is covered on its top with a protective layer, with at least one heating element held on its underside, and with a control unit, which is connected to the heating element for controlling the heating power of the cooking system , characterized in that the protective layer (13) is implemented as a sol-gel layer.

2. Cooking system according to claim 1, characterized in that the thickness of the protective layer (13) in sol-gel technology is at most about 5 to 10 microns.

3. Cooking system according to claim 1 or 2, characterized in that an electrical insulation layer (16) on the underside of the hotplate body (5) is realized as a sol-gel layer.

4. Cooking system according to one of the preceding claims, characterized in that the protective layer (13) and the insulation layer (16) have substantially the same thickness.

5. Cooking system according to one of the preceding claims, characterized in that the material of the hotplate body (5) is metal, in particular

Is stainless steel.

6. Cooking system according to claim 5, characterized in that the metal plate (5) is about 2 to 5 mm thick.

Cooking system according to one of the preceding claims, characterized in that an additional enamel insulation layer is applied to the insulation layer (16).