WO1999021401A1

WO1999021401A1 - Highly efficient heating element

Info

Publication number: WO1999021401A1
Application number: PCT/CH1998/000450
Authority: WO
Inventors: Dusko Maravic
Original assignee: Dusko Maravic
Priority date: 1997-10-21
Filing date: 1998-10-21
Publication date: 1999-04-29
Also published as: AU9427298A

Abstract

A highly efficient heating element has a flat heat source and a surface that radiates heat outwards and serves in particular as a hot plate. A layer of heat-insulating material (1) is provided with an electrically insulating layer (2) upon which is applied a resistance layer (3) that serves as a heat source.

Description

Highly efficient heating element

The invention relates to a heating element according to the preamble of claim 1.

The rapid development in the area of rational use of energy requires ever greater efforts to create new, better solutions. In the field of cooking and roasting, i.e. in kitchen stoves, improvements are increasingly being made. Old-fashioned cheap solutions such as mass hot plates are increasingly being replaced by glass ceramic cooking systems, although the latter are several times more expensive. The fact that the purchase price of a stove is becoming increasingly insignificant is due to the development of electricity prices. There is also an important size, namely the speed. The latter requirement has significantly favored the introduction of the induction cooker. If one takes the customer's two desired properties seriously, then the induction cooker is given a great future if it weren't for the purchase price. At the moment, an induction cooker is almost ten times more expensive than a mass hob and at least three times more than an ordinary glass ceramic hob. The future will show whether the enormous energetic advantages of the induction cooker justify its high price or whether a reduction in its purchase price is inevitable. One problem with the induction cooker is unavoidable, namely the existence of a magnetic field, the effects of which on human health have not yet been sufficiently investigated. A still completely untouched topic is the water chemistry and the health effects of the food prepared with the induction cooker. These two properties of induction cooking represent a huge obstacle on the way to market entry through induction technology.

In the area * of electric cooking and baking, two market-dominating systems are currently present, namely the bil- mass hot plates and the more expensive glass ceramic hobs. Both systems represent a market share of more than 95% in Europe. The development in the past five years clearly shows that there has been a noticeable shift in market shares in favor of glass ceramics. In 1996, six out of ten cookers sold were made of glass ceramics. From an energetic point of view, this is negative, since glass ceramic hobs make poor use of energy. The advance of the induction center somewhat corrects this undesirable development.

Apart from gray cast iron and steel, no metallic materials are known for the production of hot plates. The warming plates are only made of aluminum for some household heating devices, for example coffee machines. These plates are only warmed up to 100 ° C. The use of such plates as a heat source for cooking and roasting, i.e. in the temperature range well above 100 ° C, is still unknown.

From DE-A-3728466 it is known that metal, preferably chromium steel, for example 1.4742 and 1.4016, can be used as the plate material. There is no indication of the dimensional stability of the panels in the event of thermal overloads.

From DE-A-29702813 Ul it is known to use metal, preferably nickel steel with 36-42% nickel content - trade name INVAR - as the plate material. This material is suggested because of its small size. It is an unsuitable material in terms of food compatibility. There is no evidence that you should treat your surface accordingly.

The use of high-tech ceramics for hot plates has recently shown that induction cooking and roasting is still not the most perfect solution. This material is characterized by unprecedented performance data such as energy consumption, heating time and working temperature. The invention has for its object to provide a hotplate that is superior to the known hotplates in terms of performance and manufacturing costs.

According to the invention, this object is achieved by a heating plate which is characterized by the features specified in the characterizing part of patent claim 1.

Preferred exemplary embodiments are described below with reference to the accompanying drawings. Show it:

r -y. a sectional view of a hot plate according to the

Invention;

Figure 2 is a plan view of the heating plate.

Fig. 3 shows an alternative embodiment of the plate;

Fig. 4 is a heating element in the form of a tube and

Fig. 5 shows an alternative embodiment of a rohrför

Heating element.

Fig. 1 shows the functional structure of the new heating plate. A metal plate 1 made of aluminum or an aluminum alloy, with a diameter between 100 and 300 mm and a thickness between 2 and 10 mm, is covered at least on one side with an electrical insulation layer 2. An electrically conductive layer, so-called heating layer 3, is then applied to this layer. It is not important how the heating layer 3 is produced. It can preferably be achieved by applying a heating compound (resistance pastes) using screen printing. It can also be produced by a deposition process, for example PVD or CVD. Another possibility is the pressing of a foil heating meander from known heating materials. For all these solutions it applies that the heat transfer from the source (layer, foil) to the heat sink (metal plate) takes place by heat conduction, i.e. a form of heat transfer that is associated with the lowest energy losses. The material of the insulation layer 2 must have good electrically insulating properties. In addition, their adhesion to plate 1 must be good. Finally, layer 2 must not stand out from plate 1, for example due to loss of adhesion or excessive expansion in relation to plate 1. Very good adhesion is achieved if an insulation layer is formed by galvanic aluminum oxidation. The resulting Al ₂ 0 ₃ layer has an extremely good adhesion. The insulation layer 2 can also be produced by other thermal processes, for example plasma spraying, etc. The layer thickness must be chosen so that it has sufficient adhesion to the plate 1 in order to guarantee long-term stability of this connection. As a rule, that is

Elongation of a thin layer is much greater than that of the bulk material. Nevertheless, there are limits to the layer thickness, for example because the adhesion of the layers with a distinct life of their own can weaken over time and can even be completely lost.

The insulation layer 2 must have a sufficient dielectric strength. At operating voltages of 230 or 400 V and at operating temperature of plate 1 of approx. 230 ° C, the layer must have a dielectric strength of 1250 V for 1 minute.

In addition to good electro-insulating properties, the layer 2 must also be a good heat conductor so that the heat generated in the heating layer 3 can reach the plate 1 through the insulation layer 2. The thermal conductivity coefficient of layer 2 should preferably be greater than 3 W / mK. This means that with an assumed layer thickness of 5 = μm, a heating surface load of up to 10 W / cm ² can be handled without any problems. A thermal insulation 4 is advantageous for reducing the heat losses. The operating temperature of the hotplate is limited to a maximum of 230 to 250 ° C. Higher temperatures are not necessary for cooking and roasting because, on the one hand, the heat is better used with good cookware and, on the other hand, the hotplate has an excellent response behavior. This can be demonstrated by a numerical example: With a medium hot plate with a diameter of 180 mm, which is heated with 2000 W, a maximum plate temperature of less than 120 ° C arises when water is boiled. As a result, a high plate temperature is not required when cooking. Only for frying are plate temperatures at a higher level required. The reaction behavior can be seen from a further figure: the temperature gradient in the plate is at almost 10 ° K / s when the plate is idling, i.e. without the presence of the pot. A negative temperature change of the plate of only 1 ° K is compensated for or replenished within one second. Such a reaction speed is not known from conventional cooking heating.

The properties described above are due to the excellent thermal properties of the heating plate. They refer to the aluminum sheet material. Aluminum has a very high heat penetration coefficient b (greater than 20,000 Wsl / 2 / Km2) and can therefore sense the slightest changes in temperature. This property is welcome on the one hand, on the other hand temperature sensors with a very short response time are required. An ordinary Pt 100, for example, cannot be used for this because it is too slow. That is why the hotplate is operated with fully electronic, power-controlled temperature control. Thin resistance layers made of nickel, titanium or other suitable materials are used as temperature sensors on the hotplate. The use of fast reacting thermocouples and special thermocouples is also conceivable. Because the temperature difference between the bottom of the hotplate and the inside of the bottom of the pot is only a few ° K, these sensors can also easily determine the current temperature of the pot contents.

An important requirement that a hotplate has to fulfill is to ensure its constant evenness. In order to be able to meet this requirement, the geometric design must be adapted to the corresponding mechanical loads. The aim is to choose a minimum plate thickness in order to keep the temperature differences in the axial direction (plate height) low. If these become too large, the plate may deform (bulge). This problem can be remedied by suitable, surface-optimized heating surface loading. When using aluminum or another good heat-conducting material, such as copper and its alloys, the temperature difference across the plate thickness is less than 1.5 ° K with a connected load of 2000 W and the geometric dimensions of the plate described above. The resulting thermo-induced bending stress, which causes the so-called bimetal effect, is so low that no significant curvature of the plate can occur.

There are also known constructive solutions for cookware that allow flat pot bases to be built. Such solutions have already been successfully tested and will be available on the market in the foreseeable future. In contrast to today's metallic floor constructions, they do not have a floor indentation and thus ensure very great contact with the heating plate, ie they transfer the heat optimally. Level potty plates that are not firmly connected to the pot body can preferably be made of high-tech ceramic, aluminum or aluminum alloys with good heat conductivity, non-ferrous metals or their alloys with good heat conductivity. Such pots do not cause a high working temperature of the heating plate and can lead to optimal results in connection with the present invention. In addition, the cost of such cooking systems is significantly lower than is the case with previously known cooking systems. Since the expansion coefficient of the heating plate no longer has to be given any significant significance, any metallic or non-metallic material can be used for the heating plate, provided that it is a good conductor of heat. Whether it also has other good physical properties at the same time is secondary, since it is possible at any time to achieve the desired properties, such as hardness, color, etc. with additional treatments. Gray cast iron and other iron alloys, provided they are good heat conductors, can also be used.

It is easy to see that the proposed hot plate is provided as a hot plate, preferably in connection with a new cookware. It has been proven that working temperatures, i.e. the hotplate temperatures can be kept very low with optimally coordinated cooking systems, although it initially appears that especially when roasting high temperatures of well over 250 ° C, usually over 350 ° C are required. However, it is not true that these temperatures are required. There is a tendency to heat a frying pan to such an extent that, thanks to the high temperature level, it has sufficient heat capacity so that it does not get too cold when the food, for example meat or fish, is inserted. One should not forget that when a large piece of meat is placed in a pan, the pan or the oil temperature below the meat cools from 300 ° C to around 70 ° C within a few seconds. As a rule, however, it takes more than 30 seconds for the

Plate heating (for example the radiant heating of a glass ceramic field) reacts and starts heating again. However, this reaction time is far too long because a lot of meat juice is lost to the meat. Here you can see how important the reaction time of a cooking system is. It is not for nothing that cooks love the gas stove. The heating plate proposed here, combined with the suitable cookware, is similar to the reaction behavior of a gas stove.

3 shows a version of the hotplate in which there is a functional separation between the plate 9 which emits the heat to the cookware and the plate 1 provided with the resistance layer 3. This version is useful where the dielectric strength of the insulation layer 2 has to be very high and this is not given by the actual layer or by layer material and thickness. A layer of an electrically insulating compensation medium 7 is arranged between the two plates 1 and 9. Thermally conductive pastes (WLP) with insulating properties can preferably be used as the compensation medium. They are usually provided with good thermal conductive ceramic fillings, which one

Insulation resistance of more than 1014 Ω. From an energetic point of view, this solution is somewhat inferior to that shown in FIG. 1, since the heat-emitting plate 9 has an additional mass. This difference can be kept small, however, since the thickness of the plate 1 can now be reduced significantly. As a rule, a thickness of up to 2 mm is sufficient.

This version also allows you to choose from a number of additional materials. For example, materials with very specific properties can preferably be used without having to be good heat conductors. Such a material can be, for example, gray cast iron, stainless steel, hardened non-ferrous metal alloys, electrically conductive high-tech ceramics, such as liquid phase sintered silicon carbide.

Everything previously disclosed related to a simple geometry, namely a plate shape and the use of this as a hotplate. Basically, however, all properties of this heating element can also be applied to another Geometry can be applied. One form that is indispensable in practice is the tube. The same applies to the remaining plate shapes, which have not yet been mentioned here. The geometrical design of the resistance layer carrier 1 certainly plays a decisive role in application-specific cases, but not in fundamental considerations. In this respect, the differences, such as, for example, application of the resistance layer 3 to the inside or outside surface of the tube, are not discussed further here. What has been said above about the function and property of the insulation layer 2 also applies to the tubular geometry.

Fig.4 represents a heating tube, which is analogous to

Version shown in Fig. 1 is available. A detailed description can therefore be dispensed with.

A further tubular solution, which is analogous to the version in FIG. 3, is shown in FIG. 5.

Claims

claims

1. High-performance heating element with a flat heat source and one that emits the heat to the outside

Surface, in particular for use as a hotplate, characterized in that a layer of heat-conducting material (1) is provided with an electrical insulation layer (2) to which a resistance layer (3) is applied as a heat source.

2. High-performance heating element according to claim 1, characterized in that the heat-conducting material consists of aluminum, aluminum alloys, non-ferrous metal, intermetallic phases such as aluminum titanium, molybdenum disilicide, iron disilicide, iron, iron alloys, steel and steel alloys.