WO2017009809A1

WO2017009809A1 - Induction heating element

Info

Publication number: WO2017009809A1
Application number: PCT/IB2016/054238
Authority: WO
Inventors: Ennio Corrado
Original assignee: E-Wenco S.R.L.
Priority date: 2015-07-16
Filing date: 2016-07-15
Publication date: 2017-01-19
Also published as: ITUB20152251A1

Abstract

An induction heating plate-like element comprises a discoid frame (11) made of electrically insulating material and a spiral electric conductor (12) arranged in the plane of said discoid frame (11), supported by the latter and having ends (13', 13") extending towards the outside of the discoid frame. The discoid frame (11) has an upper face (14) on one side and a lower face (15) on the other side, with the spiral electric conductor (12) facing both sides of the discoid frame (11). The first magnetic elements (16) are arranged on the side of the upper face (14) housed in seats obtained in a corresponding upper portion (11') of the discoid frame (11), whereas second magnetic elements (17) are arranged on the side of the lower face (15) of the frame (11) and housed in seats obtained in a corresponding lower portion (11") of the discoid frame (11). The upper and lower faces (14, 15) are covered by respective closing discs (18, 19) made of material provided with ferromagnetic or ferrimagnetic properties and spaced from the spiral electric conductor (12). The heating element (10) can be inserted inside a container (20) having walls made of a material suitable for the induction heating and being intended to be filled with water.

Description

INDUCTION HEATING ELEMENT

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Field of the invention

The present invention relates to a heating element and a multi-function heating system, operating by electromagnetic induction.

State of the Art

At present, the induction heating technology is widely used, particularly in the kitchen appliance sector for cooking food. Induction cookers are essentially constituted by a coil hidden below the cooktop, inside which an oscillating electric current (inductive circuit) flows and produces a magnetic field oscillating too as a function of the current generating it.

Because of the Faraday's law, variations over time of the flow of the magnetic field produce an induced electromotive force. If a metal conductive object (for example a pot) is placed inside such an oscillating magnetic field, the aforementioned induced electromagnetic force generates, in its turn, induced currents (or eddy currents) circulating inside the metal conductive material (induced circuit) . Because of the Joule effect, these induced currents dissipate energy as heat, thus causing the heating of the metal material constituting the pot.

In induction cookers, this is obtained by placing the coils and adjusting the magnetic field so that the latter extends above the cooktop, affecting the area within which the pot is laid on the cooktop itself. A series of permanent magnets can be arranged on the upper face of the coil so to improve and make more uniform the distribution of the magnetic field towards the induced circuit.

However, the induction heating devices described above constitute an integral part of a kitchen appliance and thus suffer from poor flexibility of use: for example it is not possible displacing, even temporarily, the generator of the magnetic field (coil) in another position that should be appropriate, if necessary.

Documents JP H05 251167, EP 0565186 and WO 2015/029441 are indicative of the state of the art.

Not all of the materials with metallic behavior are adapted to make objects of practical interest which exploit the phenomenon of induction heating.

For example for making pots for induction cooktops, it is necessary to use a metal having sufficiently low electrical resistance to efficiently drive the induced eddy currents, but beyond a certain lower limit of the electrical resistance, the sufficient dissipation of energy cannot be obtained for heating the pot by the Joule effect cooperating with the dissipative effect of re-orientation of the magnetic domains, which is known in literature as hysteresis loop typical and characteristic of the ferromagnetic materials .

Therefore, over time some metals have been preferred to others, so that de facto standards have been created. For example, in the pot field the cast iron and some steels have been preferred to aluminium, although the latter has a lower specific weight - this is an aspect that would allow making lighter and cheaper pots - and a high thermal conductivity which makes it more suitable for cooking food.

In other words, to some metals having more suitable physical-chemical properties for a particular use, other metals have been preferred featuring lower performance from these points of view, but that better respond to the magnetic fields generated at powers compatible with the civil or industrial use within the scope of the phenomenon described hereinabove.

In general, metals having high values of thermal conductivity also boast high electrical conductivity but sometimes excessive for obtaining an effective heat production caused by the induction. For example silver, gold and aluminium are characterized by excellent thermal and electrical conductivities, but are poorly reactive to oscillating magnetic fields with civil and industrial powers and/or frequencies.

Notoriously, metals can be classified based on the aptitude to be magnetized in the presence of a magnetic field. On a quantitative and practical level, metals are classified as ferromagnetic, diamagnetic and paramagnetic depending on the value of the relative magnetic permeability, in its turn corresponding to the ratio:

(1) μ_Γ = μ/μ₀,

between the absolute magnetic permeability of the metal and the magnetic permeability μο of vacuum. The absolute magnetic permeability is defined as the ratio between the magnetic induction B and the intensity H of the magnetizing field, i.e.:

(2) μ = B/H.

The magnetic permeability of vacuum μ₀ is one of the fundamental physical constants; its value is expressed in Henry/meter in the International System: (3) μ₀ = 4π· 10 ' H/m.

The relative magnetic permeability is constant in the diamagnetic metals (μ < μ₀) and slightly lower than the unit. In the paramagnetic metals the relative magnetic permeability is slightly higher than the unit and is inversely proportional to the temperature. In the ferromagnetic metals the relative magnetic permeability is much higher than the unit (μ >> μ_ο) and varies, in addition to the temperature, also upon varying the magnetizing field.

There are a few metals presenting ferromagnetic and ferrimagnetic properties at room temperature, such as for example iron, cobalt and nickel. Some rare earth elements are ferromagnetic at temperatures even much lower than room temperature.

The following table 1 summarizes the classification.

Table 1

The difference between the values of the relative magnetic permeability of the paramagnetic metals, with respect to the diamagnetic metals, is minimal and often negligible for practical purposes, particularly for what concerns the induction heating.

Independently from the just summarized classification, for simplicity in the following description the paramagnetic metals and the diamagnetic metals will be simply defined amagnetic or non-magnetic metals, the same way as metals that in general are not appreciably interacting with the magnetic fields, among which, for example, is possible to mention aluminium, copper, titanium, tungsten.

As mentioned above, some amagnetic metals have excellent physical properties and particularly thermal conductivity, but are not directly used in applications providing for the heating by eddy currents, precisely because instead of these other metals are preferred such as iron, cast iron or some specific steels having more effective response to the magnetic fields. The use of the amagnetic metals is only possible in combination with ferromagnetic metals, for example by assembling parts made of different metals as often occurs in pots made of aluminium of the known art.

For example, (laminated) aluminium has thermal conductivity equal to 190 kcal/m°C - i.e. at least seven times higher than a common stainless steel, and (electrolytic) copper has thermal conductivity equal to 335 kcal/m°C - i.e. at least twelve times higher than the stainless steel. Therefore in an application that provides for heating, either by induction or any other system, and for which is important to have the maximum thermal conductivity, copper will be preferable to aluminium and the latter to steel.

Objects and SuiranarY of the Invention

General object of the present invention is to overcome the drawbacks mentioned above by providing an induction heating element that could be placed at user's convenience. Another object of the present invention is to provide a multi-function heating system wherein the heating element could further serve as induction element for the induction heating of a container it is placed in.

Further object of the present invention is to provide an induction heating element allowing a particularly high recovery of thermal energy.

However, it is important to underline that the heating element according to the invention is thought to optimize the heat generation with absorbed power lower than 3 kW and, more preferably, lower than 1 kW.

In view of such purposes it has been thought to implement an induction heating plate-like element according to the invention, comprising a discoid frame made of electrically insulating material and a spiral electric conductor arranged in the plane of said discoid frame, supported by the latter and having ends extending towards the outside of the discoid frame, said discoid frame having an upper face on one side and a lower face on the other side, with the spiral electric conductor facing both sides of the discoid frame, first magnetic elements being arranged on the side of the upper face and housed in seats obtained in a corresponding upper portion of the discoid frame, characterized in that second magnetic elements are arranged on the side of the lower face of the frame, housed in seats obtained in a corresponding lower portion of the discoid frame and in that said upper and lower faces are covered by respective closing discs spaced from the spiral electric conductor.

The closing discs are made of a material provided with ferromagnetic or ferrimagnetic properties. This characteristic is obtainable in different ways, as it will be now explained, also by exploiting amagnetic metals.

For example according to a first mode, the closing discs can be made of an alloy of aluminum and iron comprising an amount of aluminium in the range 97% - 99% by weight, referred to the total weight of the alloy, and an amount of iron in the range 1% - 3%.

Alternatively, the closing discs can be made of a metal alloy containing a first metal or a first mixture of metals in a percentage between 90% and 99% by weight to the total weight and containing a second metal or a second mixture of metals in a percentage between 1% and 10% by weight to the total weight. The first metal is an amagnetic metal, for example diamagnetic or paramagnetic or antiferromagnetic metal, or else the first mixture of metals is amagnetic and/or can comprise only non-magnetic metals. The second metal is a ferromagnetic or ferrimagnetic metal, or else the second mixture of metals comprises only ferromagnetic or ferrimagnetic metals.

Alternatively to the metals it is possible to use materials with metallic behavior, such as for example the electrically conductive engineering plastics.

In some embodiments, the alloy can contain less than 1% by weight of one or more rare earth elements, where the rare earth elements are identified according to IUPAC definition, or an oxide thereof, or else MishMetal, in its turn composed of 50% cerium, 25% lanthanum and a little percentage of neodymium and praseodymium; non-metals, such as carbon, and/or semimetals, such as silicon. Thanks to this embodiment, it is possible to have an induced element having excellent physical and/or chemical characteristics.

In other embodiments, the content by weight of the first metal or first mixture of metals, with respect to the alloy total weight, can be in the range 95% - 99%, and the content by weight of the second metal or second mixture of metals, with respect to the alloy total weight, can be in the range 1% - 5%, preferably in the range 1% - 2%. Thanks to this embodiment, it is possible to have an induced element having excellent physical and/or chemical characteristics .

In still other embodiments, the first metal can be one among gold, silver, copper, aluminium, platinum, boron, or where the first mixture can be a mixture of two or more among gold, silver, copper, aluminium, platinum, boron, and the second metal can be one among nickel, iron, cobalt, and the second mixture can consist of two or more among nickel, iron, cobalt. Thanks to this embodiment, it is possible to have an induced element having excellent physical and/or chemical characteristics.

In further embodiments, the titanium content in the alloy, if present, can be lower than 0.5% by weight to the total weight, and can be preferably in the range 0.1% - 0.2%; the boron content in the alloy, if present, can be lower than 0.5% by weight to the total weight, and can be preferably in the range 0.1% - 0.2%; the iron content in the alloy, if present, can be lower than 3% by weight to the total weight, and can be preferably in the range 1% - 3%. Thanks to this embodiment, it is possible to have an induced element having excellent physical and/or chemical characteristics .

According to the invention, it has been further made an induction heating system comprising a container having walls made of a material suitable for the induction heating, intended to be filled with water or other liquid, said heating element being placed inside the container, the ends of the spiral electric conductor of the heating element constituting a connection with an external electric current supply for the power supply of the heating element with an oscillating electric current adapted to generate a magnetic field being distributed on both sides of the heating element corresponding to the two faces which are provided with said first and second magnetic elements and being directed towards the walls of the container in order to generate, in its turn, induced currents circulating in the material of said walls of the container .

Brief Description of the Drawings

In order to make clearer the explanation of the innovative principles of the present invention and the advantages thereof with respect to the known art, a possible exemplary implementation applying such principles will be described herein below, with the aid of the attached drawings . In the drawings :

- figure 1 shows a plan view of the upper face of a heating element according to the invention, with the respective closing disc partially removed;

- figure 2 shows a plan view of the lower face of the heating element of figure 1, with the respective closing disc partially removed; - figure 3 shows a view of the heating element taken according to the section III-III of figure 1 ;

figure 4 schematically shows a section of a possible application of the heating element according to the invention in a water heating system;

- figure 5 shows a partial view of a section of the closing disc according to a first preferred embodiment;

- figure 6 shows a partial view of a section of the closing disc according to a further preferred embodiment. Detailed Description of the Invention

With reference to the figures, in figure 1 a heating element 10 comprising a discoid frame 11 made of electrically insulating material and a spiral electric conductor 12 supported by the frame 11 and arranged in the plane of said discoid frame 11, are shown.

The heating element 10 has substantially plate-like or discoidal structure, where with such a definition it is meant that the element extent 10 in radial direction is significantly higher than its thickness. A similar consideration is made for the discoid frame 1.

Consistently with this definition, in the present description simple reference will be made to the "plane of the discoid frame 11", meaning the plane being in practice substantially identifiable with the median plane of the element 11.

The discoid frame 11 is advantageously made of borosilicate glass (also known with the trade name of Pyrex) , as it has excellent electrical insulation and thermal-shock resistance properties, it resists without any problem at temperatures up to 550-600°C, it has low expansion coefficient and involves a substantially negligible ecological impact in terms of environmental pollution .

The spiral electric conductor 12 has respective ends 13', 13" extending towards the outside of the discoid frame 11 to make a connection with an external supply of oscillating electric current, not shown in the figures. In the discoid frame 11 of the heating element 10, for the purposes of the following description, an upper face 14 (shown in figure 1) on one side and a lower face 15 (shown in figure 2) on the other side can be identified. As visible in these figures, the spiral electric conductor 12 is facing both sides of the heating element 10 with respect to the discoid frame 11.

First magnetic elements 16 are arranged on the side of the upper face 14 of the discoid frame 11 and housed in specific seats obtained in a corresponding upper portion 11' of the frame 11. On the side of the lower face 15 of the discoid frame 11, second magnetic elements 17 housed in specific seats obtained in a corresponding lower portion 11" of the frame 11, are further arranged.

Advantageously, the magnetic elements 16, 17 are arranged offset on the two faces of the discoid frame 11, so that each magnetic element 16 of one of the two faces is at the space between two adjacent magnetic elements 17 of the other face. Such an arrangement looks clear by observing the section of figure 3.

In fact it has been found that, with this arrangement of magnetic elements on the two faces of the discoid frame 11 and thus of the heating element 10, on both sides of the heating element 10 an optimal and uniform distribution is obtained of the magnetic field generated by the oscillating electric current flowed in the spiral electric conductor 12.

The magnetic elements 16, 17 can be made of various types of permanent magnets, for example magnets made by samarium-cobalt, magnets made by neodymium-iron-boron or ceramic magnets.

It has been found that an optimal operation is when the magnetic elements 16, 17 cover at least 30%, preferably at least 50%, of the surface of the spiral electric conductor 12 at each face 14, 15.

In the embodiment described and illustrated herein, seven magnetic elements per each face 14, 15 of the discoid frame 11 are used and radially arranged with respect to the spiral electric conductor. However, it is clear that a different number or arrangement of the magnetic elements can be provided, for example as annular magnetic elements. The two faces 14, 15 of the discoid frame 11 are further covered by respective closing discs 18, 19 to close the heating element 10 (visible in figure 3 and only partially depicted, for grater graphic clarity, in figure 1 and in figure 2), which are made of material provided with ferromagnetic or ferrimagnetic properties and arranged spaced from the spiral electric conductor 12.

Advantageously, in order to provide the adequate ferromagnetic properties, the discs 18, 19 can be made of aluminium and iron alloy comprising an amount of aluminium in the range 97% - 99% by weight, referred to the total weight of the alloy, and an amount of iron in the range 1% o ·

Alternatively the discs 18, 19 are made of, as anticipated above, a metal alloy containing a first metal or a first mixture of metals in a percentage between 90% and 99% by weight to the total weight and containing a second metal or a second mixture of metals in a percentage between 1% and 10% by weight to the total weight. The first metal is an amagnetic metal, for example, diamagnetic or paramagnetic or antiferromagnetic metal, or else the first amagnetic mixture of metals and/or can comprise only non-magnetic metals. The second metal is a ferromagnetic or ferrimagnetic metal, or else the second mixture of metals comprises only ferromagnetic or ferrimagnetic metals.

Herein below three examples are provided.

Example 1

Alloy constituted by silver, copper, nickel and rare earth elements in the percentages by weight shown in the table below.

In its turn the rare earth silicide is composed of 40%-45%, 8%-10% rare earth elements and iron for remainder; MishMetal is typically composed of 50% cerium, 25% lanthanum and a little percentage of neodymium and praseodymium.

Example 2

Alloy constituted by copper, nickel and rare earth elements in the percentages by weight shown in the table below .

In its turn the rare earth silicide is composed of Si = 40%-45%, 8%-10% rare earth elements and iron for the remainder; MishMetal is typically composed of 50% cerium, 25% lanthanum and a little percentage of neodymium and praseodymium.

Example 3

Alloy constituted by aluminium and iron in the percentages by weight shown in the table below.

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In an embodiment of the present invention, the closing discs 18 and 19 are made of an aluminium and iron alloy, with aluminium present in amounts in the range 97% - 99% by weight (wt%) and iron present in amounts in the range 1% - 3% (wt%), advantageously in the range 1% - 1.5% (wt%) . The alloy can further comprise titanium and/or boron, each in an amount not higher than 0.5%, advantageously in the range 0.1% - 0.2%. These metals have the purpose to make a satisfactory refining of the alloy, allowing the formation of smaller and substantially spherical-shaped granules and improving its overall mechanical characteristics. Furthermore, there can be traces of other (metal and non-metal) elements, generally in an overall amount lower than 0.1%.

In other embodiments, the closing discs 18 and 19 are made of a metal alloy containing a first metal or a first mixture of metals in a percentage between 90% and 99% by weight to the total weight and containing a second metal or a second mixture of metals in a percentage between 1% and 10% by weight to the total weight. The first metal is an amagnetic metal, for example, diamagnetic or paramagnetic or antiferromagnetic metal, or the first mixture of metals is amagnetic and/or can comprise only non-magnetic metals. Also, still as previously described, the second metal is a ferromagnetic or ferrimagnetic metal, or the second mixture of metals comprises only ferromagnetic or ferrimagnetic metals. Alternatively to the metals it is possible to use materials with metallic behavior, such as for example the electrically conductive engineering plastics.

The spiral electric conductor 12 constitutes the inductive circuit of the induction heating element 10, whereas the closing discs 18, 19 (made of material provided with ferromagnetic or ferrimagnetic properties) are constituting the induced circuit. In fact, once an appropriate oscillating electric current is applied to the spiral electric conductor 12, a magnetic field (itself also oscillating as a function of the current generating it) is generated and uniformly distributed on both sides of the element 10 towards the aforementioned discs 18, 19 made by ferromagnetic material, the whole aided by the presence of magnetic elements at each of its faces 14, 15.

The variations over time of the flow of the magnetic field produce an induced electromotive force in its turn generating electrical currents induced in the ferromagnetic material constituting the discs 18, 19. Because of the Joule effect, these induced currents dissipate energy in the form of heat, thus causing the heating of the discs 18, 19, which in their turn can transfer heat to an object laid on the heating element 10 (for example a pot, or other suitable container containing food or liquids to be heated) or else to a fluid (for example water) wherein the heating element 10 is immersed.

In this regard, in figure 4 a possible application of the heating element 10 in a water heating system is schematically shown. Particularly, a container 20 filled with water or other liquid to be heated is shown in the figure, in which the heating plate-like element 10 is immersed. Naturally, in order to guarantee effective waterproofing and electrical insulation of the heating element 10 and elements 13', 13" thereof, which connect with the external electric current supply, the appropriate shrewdness, well known to the technician, must be taken.

The heating element 10 is preferably arranged with its faces 14, 15 facing the sidewalls 21 of the container 20, advantageously in a position equidistant from them. By applying an appropriate oscillating electric current to the spiral electric conductor 12, as described above, the discs 18, 19 (constituting the induced circuit of the induction heating element 10) are able to directly heat the water of the container 20 wherein the heating element 10 is immersed.

In figure 5 a detail of one of the closing discs 18, 19 according to a preferred embodiment, is shown, wherein they are constituted by a film made of an aluminium and iron alloy as defined above, having thickness in the range 3 μπι - 100 μπι and folded like a concertina so as to form a plurality of fins 22 adapted to increase the effective heat exchange surface of the discs themselves; this solution is particularly advantageous in the operation by direct immersion in water described hereinabove.

It is evident how the heating element according to the invention offers great versatility of use with respect to the induction devices currently present on the market, that are an integral part (and thus fixed) of a kitchen appliance and therefore suffer from poor flexibility of use .

If the sidewalls 21 of the container 20 (and, if it is the case, also its bottom) are made of material provided with ferromagnetic properties, adapted to be induction heated, the magnetic field generated by the spiral electric conductor 12 can be extended (with appropriate adjustment of the circulating current in the conductor 12 and thanks to the enhancement of the field itself, guaranteed by the presence of the magnetic elements 16, 17 on the two sides of the discoid frame 11) until reaching the aforementioned sidewalls of the container 20 with sufficient intensity, thus also inducing in them a current adapted to cause their heating because of the Joule effect, so that to transfer also a further amount of heat from the walls 21 to the water contained in the container 20.

At this point it is clear how the intended purposes have been achieved with a heating element according to the invention. In fact, such a heating element is able to generate a magnetic field evenly distributed on both sides and can be arranged in the position the user believes more appropriate, thus allowing a great versatility of use also up to obtain a multi-function heating system, wherein the heating element 10 makes a double function of direct heating element (with induction heat generation on its closing discs 18, 19), and induction element for the induction heating of the ferromagnetic walls of a container 20 inside which the element 10 is arranged.

Typically, the multi-function heating system according to the invention can be used for the production of hot water or steam, by arranging the plate-like element 10 directly inside a water container 20, so that the water receives heat both by the closing discs 18, 19 of the heating element 10 and by the container walls 21 which are induction heated, in their turn, by the magnetic field generated by the spiral electric conductor 12 of the heating element 10.

A further increase of the efficacy of the heating element 10 can be obtained by making the spiral electric conductor 12 by means of a metal tube 23 (for example a copper tube) provided with an internal passage 24 for the circulation of a fluid between the ends 13', 13" of the spiral electric conductor, where said ends 13', 13" respectively constitute inlet and outlet ends of the fluid in a spiral circuit identified by the aforementioned internal passage 24 of the metal tube.

Advantageously, the ends 13', 13" of the metal tube 23 constituting the spiral electric conductor 12 extend radially towards the outside of the discoid frame 11.

The aforementioned spiral circuit can be part of a cooling circuit of the spiral electric conductor 12 (obviously subject to heating, since electric current flows therein) , so that the fluid circulating therein removes heat from the metal material constituting the conductor, thus heating itself in turn. If water is used as the fluid circulating along the aforementioned spiral circuit, it is possible to obtain a further production of hot water (in addition to that heated inside the container 20) and in case also steam production, if temperatures into play allow this.

Also, a further increase of the efficacy of the heating element 10 can be obtained by making the closing discs 18, 19 (or at least one of them) by means of a spiral duct 26 provided with an internal passage 27 adapted to be path for a further fluid. A detail of a closing disc 18, 19 according to this alternative embodiment is shown in figure 6.

The fluid circulated inside the duct 26, conveniently connected with an external circuit, acts as heat exchanger contributing to remove the induction heat generated in the disc 18, 19 by the magnetic field generated by the oscillating electric current applied to the spiral electric conductor 12. Such a so-heated fluid (for example water) can in its turn be used in the most appropriate manner .

Thus, it is clear how the multi-function heating system according to the invention allows achieving the intended purposes of positioning versatility and use of the heating element and maximizing the produced heat exploitation .

Naturally, the description above of an implementation applying the innovative principles of the present invention is reported by way of example of such innovative principles and should not therefore be understood as a limitation of the scope of the herein claimed Patent.

For example, the heating element 10 can be arranged freely inside the container 20 or else specific supports 25 (depicted schematically in figure 4) can be provided inside the container itself, to which the heating element 10 has to be fixed so that it is not accidentally displaced as a result of convective flows generating during the water heating and boiling or also following a possible transport of the container.

Furthermore, the container 20 (showed herein in as an open container) can also be a closed container, for example a boiler for the production of hot water of a domestic or industrial system. In such a case, appropriate sealed coupling must be provided for the passage of the connecting elements 13', 13" with external electric current supply through the walls of the container itself, as it is well clear to the skilled person.

Claims

1. Induction heating plate-like element comprising a discoid frame (11) made of electrically insulating material and a spiral electric conductor (12) arranged in the plane of said discoid frame (11), supported by the latter and having ends (13', 13") extending towards the outside of the discoid frame, said discoid frame (11) having an upper face (14) on one side and a lower face (15) on the other side, with the spiral electric conductor (12) facing both sides of the discoid frame (11), first magnetic elements (16) being arranged on the side of the upper face (14) and housed in seats obtained in a corresponding upper portion (11') of the discoid frame (11) ,

characterized in that second magnetic elements (17) are arranged on the side of the lower face (15) of the frame (11), housed in seats obtained in a corresponding lower portion (11") of the discoid frame (11)

and in that said upper and lower faces (14, 15) are covered by respective closing discs (18, 19) made of material provided with ferromagnetic properties, spaced from the spiral electric conductor (12) .

2. Induction heating element according to claim 1, characterized in that said closing discs (18, 19) are made of a metal alloy containing a first metal or a first mixture of metals in a percentage comprised between 90% and 99% by weight to the total weight and containing a second metal or a second mixture of metals in a percentage comprised between 1% and 10% by weight to the total weight, and wherein the first metal is an amagnetic metal, for example, diamagnetic or paramagnetic or antiferromagnetic metal, or else the first mixture of metals is amagnetic and/or comprises only non-magnetic metals,

and the second metal is a ferromagnetic or ferrimagnetic metal, or else the second mixture of metals comprises only ferromagnetic or ferrimagnetic metals.

3. Induction heating element according to claim 1, characterized in that said closing discs (18, 19) are made of a material with metallic behavior, such as for example the electrically conductive engineering plastics.

4. Induction heating element according to any one of the preceding claims, characterized in that said first and second magnetic elements (16, 17) are arranged offset on the two faces (14, 15) of the discoid frame (11), each magnetic element (16) of one of the two faces being at the space comprised between two adjacent magnetic elements (17) of the other face.

5. Induction heating element according to any one of claims 1-4, characterized in that said first and second magnetic elements (16, 17) are constituted by single magnetic elements radially arranged with respect to the spiral electric conductor (12) .

6. Induction heating element according to any one of the preceding claims 1-5, characterized in that said first and second magnetic elements (16, 17) cover at least 30%, preferably at least 50%, of the surface of the spiral electric conductor (12) at each face (14, 15) of the discoid frame (11) .

7. Induction heating element according to any one of the preceding claims 1-6, characterized in that the spiral electric conductor (12) is constituted by a metal tube (23) provided with an internal passage (24) for the circulation of a fluid between its ends (13', 13"), said ends (13', 13") respectively constituting fluid inlet and outlet ends in a spiral circuit identified by the internal passage (24) of said metal tube (23) .

8. Induction heating element according to claim 7, characterized in that the metal tube (23) that forms the spiral electric conductor (12) is a copper tube.

9. Induction heating element according to claim 7 or claim 8, characterized in that said ends (13', 13") of the metal tube (23) that forms the spiral electric conductor

(12) extend radially towards the outside of the discoid frame (11) .

10. Induction heating element according to any one of claims 7-9, characterized in that the fluid circulating along the spiral circuit identified by the internal passage (24) of the metal tube (23) is water.

11. Induction heating element according to claim 1 or any one of claims 4-10, characterized in that said closing discs (18, 19) are made of aluminium and iron alloy, comprising an amount of aluminium comprised between 97% and 99% by weight, referred to the total weight of the alloy, and an amount of iron comprised between 1% and 3%.

12. Induction heating element according to claim 1, characterized in that said closing discs (18, 19) made of aluminium and iron alloy are made with a film having thickness comprised between 3 um and 100 um folded like a concertina so as to form a plurality of fins (22) adapted to increase the effective surface of the discs themselves.

13. Induction heating element according to claim 1, characterized in that at least one of said closing discs (18, 19) is made by means of a spiral duct (26) provided with an internal passage (27) for the circulation of a heat exchange fluid between said at least one closing disc (18, 19) and the external environment .

1 . Induction heating element according to any one of the preceding claims, characterized in that the discoid frame (11) is made of borosilicate glass.

15. Induction heating element according to any one of the preceding claims, characterized in that the alloy of the closing discs (18, 19) contains less than 1% by weight of:

- one or more rare earth elements, where the rare earth elements are identified according to IUPAC definition, or an oxide thereof, or else

- MishMetal, in its turn composed of 50% cerium, 25% lanthanum and a little percentage of neodymium and praseodymium;

- non-metals, such as carbon, and/or semimetals, such as silicon.

16. Induction heating element according to any one of the preceding claims, characterized in that:

- the content by weight of the first metal or first mixture of metals, with respect to the alloy total weight, is comprised between 95% and 99%, and

- the content by weight of the second metal or second mixture of metals, with respect to the alloy total weight, is comprised between 1% and 5%, preferably between 1% and ?.9o- ·

17. Induction heating element according to any one of the preceding claims, characterized in that:

- the first metal is one among gold, silver, copper, aluminium, platinum, boron, or where the first mixture is a mixture of two or more among gold, silver, copper, aluminium, platinum, boron, and

- the second metal is one among nickel, iron, cobalt, and the second mixture is of two or more among nickel, iron, cobalt.

18. Induction heating element according to any one of the preceding claims, characterized in that, in the closing discs (18, 19) :

- the titanium content in the alloy, if present, is lower than 0.5% by weight to the total weight, and is preferably comprised in the range 0.1% - 0.2%;

- the boron content in the alloy, if present, is lower than 0.5% by weight to the total weight, and is preferably in the range 0.1% - 0.2%;

- the iron content in the alloy, if present, is lower than 3% by weight to the total weight, and is preferably in the range 1% - 3%.

19. Induction heating system comprising a container (20) having walls made of a material suitable for induction heating, intended to be filled with water or other liquid, a heating element (10) according to one of claims 1-17 being placed inside the container (20), the ends (13', 13") of the spiral electric conductor (12) of the heating element (10) constituting a connection with an external electric current supply for the power supply of the heating element (10) with an oscillating electric current adapted to generate a magnetic field being distributed on both sides of the heating element corresponding to the two faces (14, 15) which are provided with said first and second magnetic elements (16, 17) and being directed towards the walls of the container (20) in order to generate, in its turn, induced currents circulating in the material of said walls of the container.

20. Induction heating system according to claim 19, characterized in that the heating element (10) is arranged with said faces (14, 15) facing the sidewalls (21) of the container (20) .

21. Induction heating system according to claim 20, characterized in that the heating element (10) is arranged in a position equidistant from the sidewalls (21) of the container (20) .

22. Induction heating system according to claim 19, characterized in that the heating element (10) is fixed to supports (25) present inside the container (20) .