EP1422972A1 - Shielded PCT heating element - Google Patents

Abstract

A heating element (2) for electric food heating or cooking equipment comprises a tubular metal envelope (6) inside which is lodged a resistance wire (4) surrounded by insulation (5). The two principal elements constituting the resistance wire are nickel and iron and the wire has a temperature coefficient greater than 1,500 ppm/ degreesC, preferably greater than 3,000 ppm/degreesC. An Independent claim is also included for electric food heating or cooking equipment incorporating a hot plate with this heating element.

Description

The present invention relates to the field of type heating elements shielded where a resistive wire is housed, in a spiral, in a metal tube being surrounded by an insulator such as magnesia. The present invention is in particular relating to such elements having electrical characteristics special.

It is known, in devices of the water heater type, the use of elements resistive heaters whose resistance value has a coefficient significant thermal, i.e. with a significant increase in the resistance value when the temperature rises. This characteristic is better known under the name of CTP effect (for temperature coefficient positive).

This characteristic is expressed by the formula: ρ = ρ o [1 + α ( T - 25)] where ρ _o is the resistivity of the wire at 25 ° C, ρ the resistivity of the wire at temperature T expressed in ° C, and α the temperature coefficient.

This property causes a decrease in the power of these elements, since the power P is given by the formula: P = V ² / R, where V is the supply voltage and R the resistance of the heating element directly linked to its resistivity value.

These heating elements are however used in "all or nothing", that is to say in as long as thermal safety avoids any malfunction. The variation of resistance is around 25% between 20 ° C and 800 ° C approximately, which allows generate power drops of 25%, sufficient for normative tests.

In addition, the heating wires commonly used in elements heaters for domestic cooking appliances, including maximum temperature of the heating plates is around 300 ° C, have a variation of the order of 10%, for Ni-Cr or Ni-Cr-AI type wires.

The PTC effect therefore has little effect on the operation of the device. he seems interesting to try to get more out of this effect, protection and / or regulation of such devices.

We also know, from document US 2,767,288, a heating element the heating wire of which has a temperature coefficient at least equal to 0.003, the heating element having improved transfers thermal at the level of the heating element, by a double tube, the tube interior with high thermal conductivity, such as copper, tube exterior being corrosion resistant.

If, of course, such a heating element allows an automatic limitation of the power when the temperature rises, its use is limited to a range of high temperature and for remote heating of the products, or contact at certain points with the object to be heated.

Such an embodiment of the heating element remains however expensive, by the materials used for the two tubes. In addition, such a device has the disadvantage of poor contact between the heating element and the element to heat.

The present invention aims to remedy the above-mentioned drawbacks. She is notably achieved using a heating element for an electrical appliance for heating food or cooking food, comprising an envelope metallic tubular inside which is housed a resistive wire surrounded of insulation, the two main elements constituting said wire being nickel and iron, said wire having a temperature coefficient α greater than 1500 ppm / ° C and preferably greater than 3000 ppm / ° C, characterized in that the wire is wound in a spiral with an outside diameter greater than 0.7 times the inside diameter of the tubular casing.

One of the objects of the present invention therefore aims at the production of elements heaters with a very significant PTC effect, with a value of hot resistivity (for example 300 ° C) which can reach several times the initial value at room temperature. By such an effect, by feeding electrically such heating elements, as they are heating, their resistance will increase and therefore their power decrease, until stabilization at a certain temperature which depends, in first approximation, the importance of the CTP effect as well as the conditions heat transfer.

By the use of wires having such values of the temperature coefficient α, it can be envisaged to obtain a stabilization temperature for the element substantially identical to that obtained using a device specific regulation including for example a temperature probe associated with means for stopping the electrical supply of the element heating.

However, one of the consequences of using wires having a high temperature coefficient α is their low initial resistivity (at ambient temperature). However, a lower resistivity value obliges either to increase the length of the heating wire, or to decrease the section of the wire in order to find the adequate value of the resistance R ₀ of the wire at ambient temperature which conditions the value R at a given temperature , via the temperature coefficient α. Indeed, the formula linking resistance to resistivity is given by: R = ρ l s s where R is the resistance, ρ the resistivity of the wire, / the length of the wire and s its section. It is therefore possible, to increase the resistance value, either to increase the length of the wire, or to decrease its section.

One of the constraints linked to the increase in the length of the wire concerns the increase in the tubular envelope, which increases the cost price and can generate an increase in the size of the heating plates and therefore the device, generating excessive additional costs.

So try, as much as possible, to house a larger length of wire in a given volume of tubular casing.

One way to solve this problem is to wind the yarn into a spiral with an outside diameter greater than 0.7 times the inside diameter of the tubular casing. It is usually common to wind the wire spiraling inside the tubular casing, but the outside diameter of the winding does not exceed 60% of the inside diameter of the tubular casing. he however, keep a minimum distance of 0.8 mm to 1 mm between wire and tubular casing.

A relative increase in this diameter compared to the inside diameter of the tubular casing therefore makes it possible to increase the total length of the wire for a same size of the tubular envelope.

Furthermore, the thickness of the coating of the insulation is effectively reduced, which increases the heat transfer between the resistive wire and the envelope tubular.

Other techniques can be used to accommodate a longer length of wire in a tube and thus limit the increase in the size of the heating element: tighter winding, concentric, coaxial turns, double spiral, ....

According to a particular characteristic of the invention, the proportion of nickel in the constitution of the wire is more than 40%. This value provides wires with high temperature coefficients α.

Another object of the present invention is to provide an apparatus electric food heating or food cooking, comprising at at least one heating plate for said food, said plate being connected with a heating element comprising a metallic tubular casing with the interior of which is housed a resistive wire surrounded by insulation, characterized in that the two main elements constituting said wire are nickel and iron, and in that the wire has a temperature coefficient α greater than 1500 ppm / ° C and preferably greater than 3000 ppm / ° C.

The temperature range being relatively small, of the order of 300 ° C., since the appliances concerned are food cooking appliances, we will look for the resistive wire has a high temperature coefficient, while ensuring that the implementation of a heating element comprising such a wire does not lead to a considerable increase in the cost of such an element, and either compatible with the practical realization of said household appliance.

The production of a heating plate is however conditioned by the heat exchanges between the heating element and said plate. this is all the more important as the equilibrium temperature is dependent on the charge of the heating plate, i.e. of the product being cooked. The plaque heater must therefore be in intimate contact with the heating element so that the CTP effect plays its full role.

Advantageously, the heating element conforms to one of the previously stated characteristics.

Advantageously, the resistance of the wire is adjusted so that the heating generated by the power supply to the heating element causes a increase in wire resistance to an equilibrium value corresponding to a temperature of the heating plate which is the operating temperature of said heating plate for heating of food or cooking food in cooking appliances of the type croque-monsieur, waffle irons, meat grills, ... This temperature is usually set, in common appliances, by a thermostat comprising a temperature probe associated with means for stopping the heating element supply.

The present invention therefore more particularly aims at eliminating the thermostat for regulating the heating elements fitted to certain electric cooking appliances, ensuring the regulation of the elements heaters without specific device.

The CTP effect must be significant because, as regards the regulation of cooking food, the temperature difference is much less than in the case of water heater malfunctions.

By this characteristic, when the heating element is supplied, it heats up the heating plates, which leads to an increase in its resistance. he therefore heats up less and less as its temperature gets up. A thermal equilibrium is therefore obtained fairly quickly. In carefully determining the resistance of the wire, the temperature can be adjusted thermal balance of the wire, and therefore of the heating plate. In other words, a such an appliance no longer requires thermal regulation of the heating plate, the latter self-regulating by the CTP effect of the wire constituting the core of the heating element.

However, a compromise must be found between the value of the CTP effect and the value of the initial resistivity of the wire because, as already mentioned previously, the greater the CTP effect, the more the resistivity decreases.

According to another presentation of the characteristics of an electrical appliance according to the present invention the power of the heating element to the temperature required plate for heating or cooking food is included between 0.4 and 0.7 times the power of the heating element at temperature ambient, under the same supply voltage of said heating element.

According to the present invention, the variation in power is only due to the thermal variation of the resistance of the wire resulting from the value of the coefficient of temperature α.

Advantageously, the electrical appliance for heating food or cooking of foods according to the present invention comprises means promoting heat exchange between the heating element and the heating plate.

Indeed, the goal being the realization of a household appliance, the wire, even if is at the heart of the problem, is not the only parameter on which it be careful so that, overall, we obtain this self-regulating effect the device.

Indeed, when an appliance comprising heating plates, of the type croque-monsieur or waffle iron, is powered, power is important at start of use of the device until the plates stabilize when empty. Then, when the food is arranged, the plate drops in temperature. The whole operation of the device then lies in this drop in temperature and the power recovery which must follow, over a narrow range temperature (about 50 ° C).

This rise or recovery of power is necessary for cooking to take place. performs correctly. This increase or recovery of power is a important parameter which is a function of the nuance of the wire but also, for summarize, heat exchanges between the wire and the heating plate since this resumption of power can only take place if information of the fall in temperature of the plates reaches the heating wire.

One of the means to promote heat exchange consists in providing, in the heating plate, a groove for housing the heating element, which allows a more intimate connection between the heating element and the heating plate.

Advantageously, the groove surrounds the heating element over at least half a perimeter of the tubular casing of said heating element.

According to an alternative embodiment of the element housing in the groove of the heating plate, the heating element undergoes a compression step in the groove in order to increase the contact surface between said element and said groove.

It is possible to improve the heat exchanges between the heating element and the heating plate by modifying the emissivity characteristics of the element the parts of the heating element in contact with the heating plate heater having a surface emissivity higher than the parts which are not not in contact with the heating plate. Radiation on the back heating element is reduced.

A complementary method to increase the heat transfer of the heating element towards the heating plate is to cover the parts which do not are not in contact with the heating plate of a diffusion plate in one material good thermal conductor, such as aluminum or copper. Preferably, this diffusion plate is also in contact with the heating plate by extending over a significant area, for example of the order of 30% of the surface of the heating plate.

Another way to promote the transfer of energy from the heating element to the heating plate is to offset the resistive wire, inside the envelope tubular, towards the heating plate.

Thus, by improving the heat exchanges between the heating element and the heating plate, said heating element detects any variation in temperature of the plates, automatically causing a change in its power.

• une meilleure réactivité, par la diminution de la charge du fil à haute température,
• un meilleur vieillissement des éléments en diminuant le nombre de coupures par rapport à une régulation "classique" qui sollicite par ailleurs les zones de soudures,
• la suppression éventuelle du fusible.

This principle of self-regulation generates other advantages:

• better reactivity, by reducing the charge of the wire at high temperature,
• better aging of the elements by reducing the number of cuts compared to a "classic" regulation which also solicits the weld zones,
• possible removal of the fuse.

• la figure 1 présente une loi de comportement de la puissance et de la température d'une plaque de chauffe selon la présente invention, équipant un appareil électrique de type croque-monsieur ou gaufrier,
• les figures 2 à 6 présentent des agencements particuliers entre un élément chauffant et une plaque de chauffe.

Other advantages resulting from the tests will appear on reading the description which follows, in relation to a nonlimiting example of embodiment of the present invention, with reference to the appended figures, among which:

• FIG. 1 presents a law of behavior of the power and the temperature of a heating plate according to the present invention, equipping an electrical appliance of the croque-monsieur or waffle maker type,
• Figures 2 to 6 show specific arrangements between a heating element and a heating plate.

The example illustrating the present invention is a cooking appliance of the type croque-monsieur, or waffle iron, comprising heating elements based on wires with a strong CTP effect.

As indicated in the introduction to this application, obtaining a significant CTP effect is essentially linked to the choice of the material constituting the resistive wire, and in particular its temperature coefficient. Among the many available references of yarns, the selection according to the invention focused on yarns having a temperature coefficient α of between 0.0015 and 0.0050, which corresponds to a relative increase in resistivity from 1500 to 5000 ppm / ° C. In other words, a temperature increase of 300 ° C leads to an increase in resistivity, and therefore in resistance of the heating wire, by a factor between 1.4 and 2.4, which induces, at this temperature, a power drop of the same ratio.

A value lower than 0.0015 would not have given a CTP effect sufficiently large, and a value greater than 0.005 results in heating element and / or cooking appliance feasibility issues.

Indeed, such a variation of the temperature coefficient leads to values low resistivity, around 0.2 Ω.mm instead of 1 Ω.mm for wires traditional. The two parameters on which it is possible to play for find the nominal value are the length of the wire (to be increased) and / or the section (to decrease).

However, it should be borne in mind that an increase in the length of the wire results in an increase in the exchange surface, which can lead to "get out" of the typical wire load curve.

In addition, wire diameters of 0.18 mm or even 0.14 mm were thus used (usually 0.25 to 0.30 mm).

Two typical heating curves are illustrated in Figure 1. A first curve, dotted, shows the variation of power of the heating elements since the device was started. The second, in solid lines, shows the variation of the temperature of the heating plates.

Thus, as soon as the heating elements are supplied, a power important, denoted Pf (for cold power) is generated, power necessary for the temperature rise of the plates. These last heating up, the PTC effect causes a decrease in power up to thermal balance of the plates. The power thus generated is called Pc (for hot power). A first fact to take into consideration is therefore this difference in power (Pf-Pc) / Pf, denoted ΔP (in%), since the expected temperature of the plates in operation is determined by the equilibrium power Pc, the latter depending on Pf by the coefficient of temperature α.

We thus obtain here valuable information on the value of the coefficient of temperature. It should be noted, however, that although the actual "reaction" of the wire component of the heating element can be predicted by calculation and simulation, experience is necessary here to obtain this information, since the equilibrium reached also depends on the heat exchanges on which it is possible to intervene. The supply voltage of the heating elements can also be changed to adjust the equilibrium temperature of plates when the equilibrium power is a little too strong.

Tests have therefore been carried out by varying the cold ohmic value of the different heating elements, to determine what cold value is necessary to obtain a given power in stabilization regime towards 300 ° C.

It is important to note that such tests are made difficult by the very realization of the heating element whose size should not increase considerably, the heating element must include a wire resistive of temperature coefficient α as previously defined and present a resistance R making it possible to obtain a determined power at a given temperature, with the constraints as above mentioned.

Such tests are illustrated by the following table showing, from different cold powers, the evolution of said powers as a function of the temperature, the heating elements being made of a steel tube inside which is housed a wire whose temperature coefficient is 3600 ppm / ° C. The different nominal powers are obtained in modifying the length of the wire, essentially by playing on the winding pitch of said wire in the tube.

The table also indicates the variation in power between 160 ° C and 210 ° C, these two temperatures being estimated as, on the one hand, the temperature of the plate when it receives food to be cooked or heated (160 ° C), and on the other hand the temperature of said plate during cooking or heating (210 ° C). Power at 25 ° C Power at 160 ° C Power at 210 ° C Power at 300 ° C ΔP 25 ° C / 300 ° C ΔP 160 ° C / 210 ° C 1318 W 708 W 670 W 625 W 52.6% 5.4% 1128 W 605 W 570 W 524 W 53.5% 5.8% 977 W 540 W 506 W 461 W 52.8% 6.3% 893 W 470 W 440 W 400 W 55.2% 6.4% 796 W 430 W 402 W 367 W 53.9% 6.5% 754 W 401 W 373 W 335 W 55.6% 7.0%

Under the same conditions, using aluminum tubes, the following results are obtained: Power at 25 ° C Power at 160 ° C Power at 210 ° C Power at 300 ° C ΔP 25 ° C / 300 ° C ΔP 160 ° C / 210 ° C 1060 W 700 W 630 W 542 W 48.9% 10.0% 890 W 580 W 520 W 445 W 50.0% 10.3% 802 W 500 W 448 W 383 W 52.2% 10.4% 700 W 450 W 400 W 335 W 52.1% 11.1% 646 W 400 W 356 W 298 W 53.9% 11.0% 552 W 355 W 315 W 265 W 52.0% 11.3%

The results show, for steel tubes as for aluminum tubes, a relatively stable value of ΔP whatever the starting power, with slightly higher values for steel than for aluminum. Through against, aluminum presents a greater variation of power between 160 ° C and 210 ° C, linked to better heat transfer in aluminum than in steel.

Referring again to Figure 1, at time t _A , food is arranged on the plate, which causes a significant drop in temperature of the plate. This information is relayed by thermal transfer to the heating wire which reacts by reducing its resistance, which causes a rise or recovery of power, denoted ΔPc.

This resumption of power conditions the quality of cooking, a resumption of too low power resulting in little or no roasting of the product and / or a longer cooking time.

• de la nuance du fil,
• de la qualité de l'isolant pulvérulent, tel la magnésie et des deux interfaces fil/isolant et isolant/tube métallique,
• des échanges thermiques entre le tube métallique et la plaque de chauffe.

Thus, the value of the power recovery ΔPc is a function:

• the nuance of the thread,
• the quality of the powdery insulation, such as magnesia, and of the two wire / insulator and insulator / metal tube interfaces,
• heat exchanges between the metal tube and the heating plate.

We can thus estimate that the wire used is for 60 to 70% of the self-regulating effect. and quality of roasting, thermal transfers playing for 30 to 40%.

According to a practical example of implementation of the invention, the apparatus illustrating the present invention is an apparatus for making croque-monsieur or waffles, depending on the shape of the heating plates used. Her starting power is between 500 and 600 W, while its power, when the plates are hot enough, is only 250 to 300 W.

As already mentioned previously, it is necessary to use a wire with significant CTP effect. However, it is also important to have good thermal conduction between the heating plates and the heating element. In in other words, there is a need to increase and improve trade compared to products with a regulator. For these last indeed, a temperature probe is often linked directly to the cooking plate. Tests on such products with the envisaged yarns show that heat exchange can be improved in order to increase the sensitivity of the wire to the temperature variation of the heating plates.

The wire being housed in a tube filled with insulation, itself in connection with a diffusion plate, connected to the heating plate receiving the product to be cooked, various parameters influencing the heat transfer between the resistive wire and the food being cooked can be modified, in conjunction with the use of different resistive wires with different coefficient values temperature.

Of course, preliminary tests have taken place, for each grade of yarn, and as previously specified, in order to determine the initial resistance of the heating element to stabilize the cooking temperature adequate.

Other tests have thus been carried out in an attempt to improve the transfer thermal so that the heating element is sensitive to the change in load of the heating plate, and can react quickly. Some trials have been made using heating elements made of aluminum tubes or copper rather than steel or stainless steel. Other tests have concerned improving heat exchange between the heating element and the plate heating, either by the presence, in the cooking plate, of grooves heating element housing, either by the quality of the insulation surrounding the wire in the tube, either by adapting the surface properties of the tube, these different improvements that can be combined for a more significant effect.

The table presents various tests carried out. In the contact with plate column, the indication "N" corresponds to a contact as it is usually made, while the indication "A" corresponds to an improvement in the contact between the heating element and the heating plate, by producing a housing groove for said heating element which has an important role in heat transfer. Coefficient of wire temperature (ppm / ° C) Tube type Contact with plate ΔP (%) ΔPc (%) 1350 steel NOT 30 % 8% 1350 steel AT 30 % 11% 3600 steel NOT 55% 19% 3600 steel AT 55% 29% 3600 aluminum AT 52% 40% 4500 steel NOT 66% 25% 4500 steel AT 59% 41% 4500 aluminum AT 59% 47%

The tests carried out show that from a wire whose temperature coefficient is 1350 ppm / ° C, the power variation, like the power recovery have significant values, respectively 30% and 11% in the best of times. The ensuing effect on the principle of self-regulation and cooking food can thus be considered.

By choosing higher values of the temperature coefficient, we allows to choose a higher cold power, which reduces the setting heats hotplates. Furthermore, the recovery of power is more high, which improves the cooking quality of food.

• un dépassement limité de la température dite de "régulation", notamment une diminution, voire suppression du phénomène "d'overshoot" lié au premier pic de température lors de la régulation,
• une valeur de régulation qui peut donc être montée de 10 à 30°C,
• une diminution du différentiel de température lors de la régulation (écart entre la température minimale et maximale autour de la valeur de régulation)
• pas d'augmentation de puissance en cas de survoltage,

Furthermore, during the tests carried out, and unexpectedly, the following additional advantages appeared compared to a "classic" regulation:

• a limited overshoot of the so-called "regulation" temperature, in particular a reduction or even elimination of the "overshoot" phenomenon linked to the first temperature peak during regulation,
• a regulation value which can therefore be increased from 10 to 30 ° C,
• a decrease in the temperature differential during regulation (difference between the minimum and maximum temperature around the regulation value)
• no increase in power in the event of overvoltage,

Improvement of heat exchanges and reduction of thermal inertia between the heating element and the plate can be obtained by the quality of the tube of the heating element, made for example of a material having a very good thermal conduction, such as aluminum, in conjunction with a bond intimate between the heating element and the heating plate.

As commonly used, with reference to Figure 2, the heating element 2 comprises a resistive wire 4 centered in a tubular envelope 6 surrounded insulator 5. This insulator is preferably a mineral insulator, for example a oxide such as magnesia, alumina or zirconia. Boron nitride can also be used.

The heating element 2 is linked to a heating plate 8 by a cord of solder 10. The exchange surface between the heating element and the plate heater is relatively weak.

Figures 3 to 6 show different configurations improving the transfer between the heating element and the heating plate.

Thus Figure 3, a groove 12 for receiving the heating element is provided in the heating plate 80, said groove being delimited by flanks 14. The groove can be flush with the surface, as shown in Figure 3, or be located more inside the heating plate, as shown in Figure 4, thus reducing the distance d between the heating element and the active surface 81 by the heating plate 82. A winding along a larger diameter of the wire 4 is also presented in FIG. 3, this operation making it possible to house a longer length of wire in the same tubular casing.

In FIG. 4, the heating element 20 is shaped to the shape of the groove, by example by pressing, which further increases the surface of contact between the heating element and the heating plate.

Together with the shape of the wire in the shape of the groove, the resistive wire 40 is presented eccentrically in the envelope of the heating element in the direction of the heating plate 82. This configuration, independent of the conformation wire to the shape of the groove, localizes the heating mainly at level of the heating plate, thus limiting the radiation opposite to the heating plate.

For the same purpose, special surface treatment may be provided. the heating element so that it has a high emissivity on the surface in contact with the heating plate and low emissivity elsewhere.

It can also be provided, as shown in Figure 5, an overmolding of the heating element 2 by a diffusion plate 84, the heating element 2 being disposed on the heating plate 86, the latter having, or not, a positioning groove.

In Figure 6 is presented an advantageous version of practical realization improved heat transfer between the heating element and the plate heating.

The heating sub-assembly 30 includes a heating plate 36, a heating element 37 and a diffusion plate 38. The heating element 37 has a resistive wire with strong PTC effect according to one of the characteristics previously mentioned.

The heating plate 36 comprises at least one cavity comprising at least an imprint taking the shape of the food to be cooked. All of the footprints of a heating plate 36 form the cooking zone of said heating plate 36.

The heating element 37 is arranged against the face of the heating plate 36 which is opposite to the face with the imprints. The shape of the element heater 37 is adapted to the surface of the cooking zone and to the width and the length of the heating plate 36, forming a loop.

The diffusion plate 38 has a housing 32 which follows the shape of the heating element 37 and is adapted to receive it. So the heating element 37 is sandwiched between the heating plate 36 and the diffusion plate 38.

The latter is shaped so that it matches over a predetermined height e , at least part of all of the cavities of the cavities of the heating plate 36.

Thus, the diffusion plate 38, comprises, in addition to the housing 32 receiving the heating element 37, a cavity 35 receiving the heating plate 36 at the predetermined height e . In this way, the heat exchanges between the heating plate 36 and the diffusion plate 38 are encouraged.

Preferably, the heating plate 36 is made of a material which is a poor thermal conductor, for example stainless steel, and its thickness, substantially constant, is between 0.6 and 0.8 mm. Such a heating plate 36 can be easily produced by stamping and then by cutting a sheet. Prior to stamping, the steel plate stainless can be coated with a non-stick material on the side to be contact with the food to be cooked.

Preferably, the diffusion plate 38 is made of a material which is a good thermal conductor, for example aluminum, and its thickness, substantially constant, is between 0.8 and 2 mm, and preferably between 0.8 and 1 mm. Such a diffusion plate 38 can be produced by stamping.

The diffusion plate 38 thus plays the role of thermal diffuser in distributing by thermal conduction the thermal energy coming from the element heater 37 on the part of the heating plate 36 which is in contact with the diffusion plate 38.

In the opposite direction, the diffusion plate 38 promotes the feedback of information to the heating element 37 of the thermal state of the heating plate 36, which improves the responsiveness of regulation. This aspect is all the more important when the heating plate is made of steel which has a low conductivity thermal generating significant thermal inertia.

The assembly of the heating plate 36 with the diffusion plate 38 can be done by welding, gluing, or, preferably for cost reasons, by riveting or screwing. Preferably, the heating plate 36 and the heating plate diffusion 38 are fixed to each other by clinching, that is to say by deformation mutual following a common stamping: the embedding of the plate heater 36 in the diffusion plate 38 provides better hold the rise and fall in temperature despite the difference in expansion between the two metals used.

The present invention is not limited to the only example presented where the element heating equips a croque-monsieur. Such a heating element and its associated self-regulation capabilities can also find applications in other electrical heating / broiling appliances in contact with food, such as barbecues, crepe makers, grills, as well as in water heating such as coffee makers, kettles, or even irons, to also avoid dry heating.

Claims (17)

Heating element (2, 20, 37) for an electrical appliance for heating food or cooking food, comprising a metallic tubular casing (6) inside which is housed a resistive wire (4, 40) surrounded by 'insulator (5), the two main elements constituting said wire (4, 40) being nickel and iron, said wire having a temperature coefficient α greater than 1500 ppm / ° C and preferably greater than 3000 ppm / ° C , characterized in that the wire is wound in a spiral whose outside diameter is greater than 0.7 times the inside diameter of the tubular envelope. Heating element (2, 20, 37) according to the preceding claim, characterized in that the proportion of nickel is greater than 40%. Electric apparatus for heating food or cooking food, comprising at least one heating plate (8, 36, 80, 82, 86) of said food, said plate being in connection with a heating element (2, 20, 37 ), comprising a metallic tubular envelope (6) inside which is housed a resistive wire (4, 40) surrounded by insulation (5), characterized in that the two main elements constituting said wire (4, 40) are nickel and iron, and in that the wire has a temperature coefficient α greater than 1500 ppm / ° C and preferably greater than 3000 ppm / ° C. Electric appliance for heating food or cooking food according to the preceding claim, characterized in that the wire of the heating element (2, 20, 37) is wound in a spiral whose outside diameter is greater than 0, 7 times the inside diameter of the tubular casing. Electric appliance for heating food or cooking food according to the preceding claim, characterized in that the heating element comprises a proportion of nickel greater than 40%. Electrical appliance for heating food or cooking food according to the preceding claim, characterized in that the resistance of the wire (4, 40) is adjusted so that the heating generated by the electrical supply to the heating element ( 2, 20, 37) causes an increase in the resistance of the wire (4, 40) up to an equilibrium value corresponding to a temperature of the heating plate (8, 36, 80, 82, 86) which is the operating temperature of said heating plate (8, 36, 80, 82, 86) for heating food or cooking food. Electric appliance for heating food or cooking food according to one of claims 3 to 6, characterized in that the adjustment of the resistance of the wire (4, 40) is obtained by varying its length and / or variation of its diameter. Electrical appliance for heating food or cooking food according to one of Claims 3 to 7, characterized in that the power (P _c ) of the heating element (2, 20, 37) at the required temperature of the plate (8, 36, 80, 82, 86) for heating or cooking food is between 0.4 and 0.7 times the power (P _f ) of the heating element (2, 20, 37) at ambient temperature, under the same supply voltage of said heating element (2, 20, 37), said variation in power (ΔP) being solely due to the thermal variation in resistance (R) of the wire resulting from the value of the coefficient of temperature α. Electric appliance for heating food or cooking food according to one of claims 3 to 8, characterized in that it includes means promoting heat exchange between the heating element (20, 37) and the plate heating (36, 80, 82, 86). Electric appliance for heating food or cooking food according to the preceding claim, characterized in that the heating plate (36, 80, 82, 86) has a groove (12) for housing the heating element (20 , 37). Electric appliance for heating food or cooking food according to the preceding claim, characterized in that the groove (12) surrounds the heating element (20, 37) on at least half a perimeter of the tubular casing ( 6) of said heating element (20, 37). Electrical appliance for heating food or cooking food according to one of claims 10 or 11, characterized in that the heating element (20, 37) undergoes a compression step in the groove (12) in order to increasing the contact surface between said element and said groove. Electrical appliance for heating food or cooking food according to one of Claims 10 to 12, characterized in that the parts of the heating element in contact with the heating plate (8, 36, 80, 82, 86) have a higher surface emissivity than the parts which are not in contact with the heating plate (8, 36, 80, 82, 86). Electrical appliance for heating food or cooking food according to one of Claims 10 to 13, characterized in that the parts of the heating element (37) which are not in contact with the heating plate (36 ) are covered with a diffusion plate (84) in a material which is a good thermal conductor, such as aluminum or copper. Electrical appliance for heating food or cooking food according to the preceding claim, characterized in that the diffusion plate (84) is also in contact with the heating plate (36) extending over a significant surface. Electrical appliance for heating food or cooking food according to one of Claims 10 to 15, characterized in that , inside the tubular casing (6), the resistive wire (40) is eccentrically direction of the heating plate (82). Electrical appliance for heating food or cooking food according to one of Claims 9 to 16, characterized in that , by improving the heat exchanges between the heating element (20, 37) and the heating plate (36, 80, 82, 86), said heating element detects any variation in temperature of the plates, automatically causing a change in its power.

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