CN114303035A

CN114303035A - Induction furnace comprising an additional resonant circuit

Info

Publication number: CN114303035A
Application number: CN202080060804.4A
Authority: CN
Inventors: P·布伦; E·索瓦吉
Original assignee: Commissariat a lEnergie Atomique et aux Energies Alternatives CEA
Current assignee: Commissariat a lEnergie Atomique et aux Energies Alternatives CEA
Priority date: 2019-08-30
Filing date: 2020-08-21
Publication date: 2022-04-08
Also published as: KR20220075219A; FR3100421A1; FR3100421B1; JP2022546446A; WO2021038163A1

Abstract

A crucible comprising at least two electric induction heating circuits, one of the two electric induction heating circuits being located around a side wall and the other being located below or in a floor of the crucible. One of the circuits (18) comprises an electric generator (15) connected to the inductor (3), while the other circuit (19) does not have an electric generator, but is electromagnetically coupled to the circuit (18) and consists of an auxiliary inductor (7) and a capacitor (17) with an adjustable total capacitance. The coupling conditions, the respective power and heating profile through the two inductors (3, 7) can be modified to homogenize the heating or to concentrate the heating on the floor of the crucible. Good thermal efficiency is expected.

Description

Induction furnace comprising an additional resonant circuit

The invention relates to an induction furnace comprising an additional resonant circuit.

The invention relates to a cold crucible furnace with electromagnetic induction heating for melting, in particular, for example, at least one electrically conductive material, such as an oxide or oxide mixture, or such a mixture with a metal proportion of up to approximately 30% by weight, which represents a molten melt. The invention is also particularly suitable for furnaces used in the glass industry for producing enamels or other high purity materials produced at high temperatures, for example 1200K to more than 3000K. The induction frequency is typically between 100kHz and 1000kHz, depending on the size of the crucible.

The invention entails improvements to cold crucible furnaces, in particular by better homogenisation of the charging temperature, so as to reduce the thickness of the crust at the bottom of the crucible, without involving a considerable overheating of the bath or of the molten material or the generation of induced currents, which are harmful and may interfere with the components of the electric circuit constituting the inductor or inductors, in particular, for example, current generators. The advantages obtained are associated with better efficiency and better thermal uniformity; they make it possible to simplify the installation, improve the mixing of the liquid bath by convection and promote the pouring of the liquid by gravity through the floor.

Induction furnaces are well known in the field of casting or metallurgy or in the nuclear industry for the production of materials or for the formation of homogeneous mixtures. Such an induction furnace comprises a crucible containing the charge and at least one inductor, the excitation of which generates an electric current in the charge, thereby causing heating and melting of the charge. This heating process is simple to carry out and avoids any contact between the source of thermal energy and the crucible.

The crucible typically includes side walls joined to a floor, which may be flat and form a bottom. If the crucible has an electrically conductive material, the electrically conductive material usually forms sectors, i.e. consists of parts extending over the angular sectors, which parts are separated by electrically insulating partitions, in order to avoid eddy currents and corresponding energy losses around the charge arising therein.

Some crucibles are made of refractory materials, such as ramming mass or graphite. Their advantage is that the melting temperature is high, but some charges, in particular those consisting of oxides, must melt at higher temperatures or at least temperatures sufficient to physically or chemically attack them.

Furthermore, a very common design adopted by the present invention is that of a cold crucible equipped with a cooling circuit cooled by water or another liquid and extending inside the crucible. Thus, during operation of the process, the crucible is much cooler than the core of the charge, and a layer of solidified material, which may be considered to constitute the inner wall of the crucible and which is referred to as a skull crucible, may protect the crucible from corrosion or other physical or chemical attack from the charge it contains. These cold crucibles are capable of melting highly reactive materials at high temperatures above 1500K or even above 3000K, which may be metals such as titanium, steel or various alloys composed of oxides (such as glass, titanium oxide, rare earth metals or mixtures); or for example a material of low conductivity such as silicon or enamel.

However, a defect in heating uniformity in the crucible can be observed. The hottest part of the charge tends to rise to the free surface by convection. However, the induced currents responsible for heating tend to concentrate in the portion of highest conductivity, especially corresponding to the hot portion of the oxide charge. This convection may be enhanced by uneven heating and by reducing electromagnetic induction near the floor (if the floor is metallic), with the result that the charge is more difficult to heat at the bottom of the crucible, where the temperature remains low, and the solidified layer is generally thicker at the top of the floor (sometimes tens of millimeters compared to a few millimeters of the crucible side wall). This makes melting more difficult to achieve, with high electrical losses of the cooling structure, which is detrimental in itself and can affect the quality of the process. A greater obstacle is still encountered when discharging the molten charge from the bottom of the crucible by opening the plug at the center of the floor, rather than by inverting the crucible, because inverting the crucible is delicate to operate and is often prohibited. In fact, the solidified crust prevents pouring when the stopper is removed. It is conceivable to increase the power to melt the crust at the bottom of the crucible and concentrate the heat in the center; but elsewhere there is a risk of overheating while increasing the volatility of some oxides, thereby changing the composition of the charge to melt, while melting the charge solidified layer covering the sidewalls or while damaging some structure of the crucible, such as the insulation between the sectors. In processes that have been characterized by low efficiency, heat losses can also be multiplied by factors that may be 1.5 or 2, where radiation from the free surface of the molten bath, conduction on the crucible walls, and convection to the surrounding atmosphere are all high.

The induction frequency can also be acted on in an attempt to obtain melting of the charge at the bottom of the crucible: if the inductor is arranged around the side wall (which is most common), the heating profile inside the crucible depends radially on the excitation frequency of the inductor: the higher the frequency, the more concentrated the heat in the wall; the excitation frequency can thus be selected or adjusted to adjust the volume of the molten bath and to some extent the temperature distribution in the crucible; but temperature non-uniformities in the molten bath may become unacceptable. In practice, the circuitry and cooling system are often oversized and this burden on the device is not necessarily satisfactory.

This drawback is particularly evident in the present case of cold crucibles with electrically conductive materials, such as copper or certain grades of stainless steel, in particular considering the increase of electrical losses in these structures, which are present regardless of the material constituting the crucible and its structure.

A final drawback of the simplest induction furnaces described so far is that they do not adapt well to variable volume charges and therefore have different heights inside the crucible if the height of the inductors is significantly different.

There is a large body of prior art describing such induction furnaces. Certain specific designs will now be described in detail, provided they can improve the uniformity of heating.

In addition to the main inductor surrounding the side wall of the crucible, document EP 1045216B 1 describes a supplementary inductor provided around the pouring area below the floor, surrounding a stopper that can be poured from the crucible. It can generate additional induced currents around the pouring aperture through the floor to prevent the charge from solidifying there and to ensure that pouring can take place when the stopper is removed. However, difficulties in implementation still remain, and the device is not necessarily suitable for melting materials with very high melting points. Its effect is only local for a limited time, so the more general problem of lack of homogeneity when heating the charge to melt is not solved. The same applies if, as already proposed, an uncooled metal tube is provided between the lower inductor and the pouring aperture, by means of which tube a greater heat gain is desired; furthermore, the latter also risks being rapidly corroded, especially at high temperatures.

In certain designs, such as the one of document US 6185243B 1, the more conventional inductors disposed around the crucible side walls are located below the floor and may be in the shape of one or more spiral inductors instead. This design is only suitable for melt pools having a small height relative to the diameter, and it may be associated with high heat losses due to convection at the free surface of the melt pool and conduction on the side walls. The power supplied is still not uniform, is typically higher at the inductor turns, but much weaker near the sidewall and crucible center (if there are no turns).

It has also been proposed to provide a single inductor (US-1645526-a) extending around the side walls and under the floor, or two separate inductors each driven by a power supply (US 4609425, JP 10253260, US 4687646). However, the conflict between the electric fields generated by the two inductors or inductor components counteracts the possibility of heating the charge more uniformly, which disturbs their operation and may even damage the control electronics. Finally, an improvement to these designs is proposed in document WO 2017/093165 a1, in which a portion called the flux concentrator is located at the junction of the side wall and the bottom plate, thereby separating the two inductors. This part is a static part of the material with high permeability, thus reducing the interaction between the magnetic fields. The above disadvantages are reduced and better efficiency is expected. Unfortunately, particularly for the operating range contemplated herein (with respect to the excitation frequency of the power supply), it is necessary to cool the flux concentrator so that it retains its performance, and this may be technically difficult in some cases. The concentrators must be placed in areas that are difficult to access and that may require the inductor to move relative to the charge; finally, in the case of radioactive charges, it is very susceptible to radiation. One disadvantage of another placement sequence is that the device cannot distribute the heating power between the two inductors.

The present invention relates to an improved induction furnace, the most important of which is that it makes it possible to increase the efficiency of heating the molten charge and to adjust the heating profile in the heating volume as required by means of a more advantageous arrangement of two inductors, one of which is located around the side wall of the crucible and the other of which is therefore located below (or in) the floor. It is expected to achieve higher electrical efficiency than that of earlier devices and processes. Since the heating of the bottom of the crucible is more efficient, it is expected that the pouring is also easier.

In particular, it relates to an induction furnace comprising a crucible having a side wall cooled by circulation of a fluid and a bottom plate located below the side wall, a first induction heating circuit comprising an inductor and an electric generator, and a second induction heating circuit comprising a second inductor, wherein one of the inductors is located around the side wall and the other one is located below or in the bottom plate, characterized in that the second circuit has no electric generator but is electromagnetically coupled to the first circuit and is provided with a capacitor arrangement having an adjustable capacitance, said capacitor arrangement being connected to the second inductor.

Such an electrical device with dual circuits exploits the phenomenon of electrical resonance of the oven, which benefits from the free electromagnetic interaction of the two circuits. Contrary to the known devices, in which the inductors surrounding the side walls and the inductors assigned to the bottom plate are simultaneously connected to each other, excited by the same power supply, or are excited by two different power supplies and then separated from each other, no interference is observed between the induced magnetic fields at their junction and no interference is caused to the circuit. This promotes better electrical efficiency and lower losses.

Another advantage of the invention is the facility for better distributed heating, in particular the heating can be added at the centre and at the bottom, just above the floor, to improve the uniform process and possibly facilitate pouring. This heating profile depends on the setting of the total capacitance (adjustable during melting), and moreover has a very high sensitivity to variations in this capacitance, to the extent that it is possible to obtain a good level of temperature uniformity or, conversely, to the heat substantially at the periphery, in the centre or at the bottom of the melting charge.

Since the first inductor constitutes the main inductor, the first inductor is usually arranged around the side walls and then the second circuit is assigned to the bottom plate. However, it goes without saying that the opposite arrangement may be encountered, in particular for crucibles having a large surface area and a low height.

The inductor at the bottom of the crucible can form a spiral covered with an electrical insulator, which is a very good thermal conductor: it is then possible and advantageous for the bottom plate of the crucible bottom to be constituted by the inductor itself, thus simplifying the design of the crucible and increasing the heating efficiency.

The induction furnace may comprise at least one additional circuit without any electrical generator and comprising an inductor and capacitor arrangement having an adjustable total capacitance and being electromagnetically coupled to at least the first circuit; thus, this additional circuit or these additional circuits have the same characteristics as the mentioned second circuit. Additional circuitry may prove useful at the top of the furnace to improve heating of the newly added charge. An additional circuit can then be placed over the apex of the crucible, facing and parallel to the indicator located on or in the floor.

The one or more additional circuits may then advantageously be equipped with circuit breakers that enable them to be opened at will.

The various aspects, features and advantages of the present invention will now be described with the aid of the following drawings, which represent some specific embodiments of the invention, which are intended to be illustrative only.

FIG. 1 is an overall view of an induction furnace;

FIG. 2 is a view of a lateral inductor;

FIG. 3 is a view of a bottom inductor;

FIG. 4 is a circuit diagram;

FIG. 5 is an embodiment of a capacitor bank;

FIG. 6 shows another embodiment;

fig. 7 shows a first heating state;

FIG. 8 is a first heating state from a bottom up perspective;

fig. 9 shows a second heating state;

FIG. 10 is a second heating state from a lower and upper perspective;

fig. 11 shows a third heating state;

FIG. 12 is a third heating state from a lower and upper perspective;

FIG. 13 shows another embodiment of a crucible;

FIG. 14 shows a third embodiment of the crucible;

FIG. 15 shows a base plate of a third embodiment;

fig. 16 is a cross section of the base plate of the first embodiment.

An embodiment will be described with the aid of fig. 1 to 3. This embodiment of the induction furnace comprises a side wall 1 or cylindrical sleeve consisting of vertical tubes 2 of copper or stainless steel, which are adjacent and connected by an electrically insulating barrier to prevent the occurrence of annular induced currents in the side wall 1. According to a known arrangement, the tube 2 is crossed by a cooling liquid, such as water. The cooling circuit 12 circulates liquid alternately upwards and downwards, wherein the connections between the tubes 2 are alternately located at the upper and lower ends. Another possible configuration is, among others, that of US-6996153-B2, in which each tube is independently cooled by a circuit that reciprocates vertically.

The lateral inductor 3 surrounds the side wall 1. Here it is a single turn consisting of parallel strands 4 that overlap along the height, and each strand is wound around the crucible for a single turn. Their ends are joined together by two

vertical connectors

5 and 6, which in addition provide electrical and hydraulic connections to the ac generator, the strands 4 are also cooled by the circulation of a cooling fluid, and the strands are connected to a cooling circuit 13. Furthermore, other arrangements of transverse inductors, in particular turns arranged in a continuous spiral, may be possible.

The crucible is completed by a bottom plate 8 mainly consisting of an auxiliary inductor 7 in the form of a spiral (fig. 3 and 16). The auxiliary inductor 7 is also a water-cooled tube and its

ends

9 and 10 also lead to a cooling circuit 14. The turns 31 of the helix are joined by a separator electrical insulator 32, which is an electrically insulating, very good heat conductor, separating them all and providing a continuous structure for the base plate 8. Furthermore, the auxiliary inductor 7 is covered on its upper surface with a thin layer 33, which is an electrically insulating, very good heat conductor, for example consisting of aluminum oxide. This design makes it possible to eliminate the conventionally used floor and to directly preserve the charge of the crucible by means of the auxiliary inductor 7.

The cooling of the bottom plate 8 and the side pairs 1 also results in the formation of skull crucibles, i.e. the charge layers in contact with them remain solid and protect them from corrosion.

A pouring stopper 11 is arranged in the centre of the spiral. It is cooled by conventional means, not shown here. The cooling

circuits

12, 13 and 14 of the side wall 1, the main inductor 3 and the auxiliary inductor 7 are here only schematically represented and may, according to known embodiments and each, in particular comprise, for example, a pump and a heat exchanger.

The electrical device is shown in fig. 4. The main inductor 3 is connected to a terminal of an alternating current generator 15. A capacitor bank 16 is also connected to the terminals of the electrical generator 15 in parallel with the main inductor 3. The components form a closed main circuit 18. The resistance of this main circuit 18 is mainly due to the resistance of the crucible structure, the charge contained therein and the resistance of the main inductor 3. The auxiliary inductor 7 together with the adjustable capacitor bank 17 forms a closed auxiliary circuit 19 which is physically separated from the electrical generator 15 and does not have its own electrical generator. The resistance of this auxiliary circuit 19 is mainly due to the resistance of the auxiliary inductor 7. The proximity of

inductors

3 and 7 means that in operation an electromagnetic coupling occurs between

circuits

18 and 19, although no electrical generator is present in auxiliary circuit 19. This coupling varies in particular according to the power input and the charge in the crucible and the setting of the total capacitance of the capacitor bank 17, which is composed of individual capacitors, which can be connected to the auxiliary circuit 19 or separated by the switch 21. As shown in fig. 5, the various capacitors 20 may be placed in parallel, or otherwise. As a variant (fig. 6), the capacitor bank 17 can be replaced by an adjustable capacitor 22, the effect of which will be the same.

The invention is based on the electromagnetic coupling between the

circuits

18 and 19 through the

inductors

3 and 7 to adjust the current induced by them and to act on the distribution of heat in the charge contained in the crucible, in particular to homogenize the temperature of the charge during melting and mixing, or (among other possibilities) to increase the heating of the bottom to release the pouring stopper 11 and to facilitate the pouring operation.

The effects obtained may be those given in table 1 below:

the first row represents the operation of the device with the auxiliary circuit 19 open. The voltage at the terminals of the main inductor 3 and the current passing therethrough is high and the voltage at the terminals of the auxiliary inductor 7 is low and therefore heating is mainly performed by the main inductor 3, but the losses in the side walls 1 are high and the efficiency is relatively low. This mode of operation, similar to known conditions, is neither representative of the present invention nor is the mode commonly sought. Fig. 7 and 8 are representations of the temperature distribution of the molten charge, from top to bottom and from bottom to top (light tones indicating greater local heating), showing higher heating at the periphery of the charge and weaker at the center, so that the charge is cooler there, in particular the electric generator 15 immediately above the floor is heated very weakly. The choice of different excitation frequencies may result in a variation of the heating distribution in the radial direction, but in any case the heating of the crucible bottom is still weak and temperature uniformity is not achieved.

The following row of the table below shows the effect of increasing the capacitance of the capacitor bank 17 or the adjustable capacitor 22. The electromagnetic resonance increases, the voltage and current values at the terminals of the main inductor 3 decrease, while the voltage at the terminals of the auxiliary inductor 7 and the current values passing therethrough increase. The losses in the side wall 1 decrease substantially and increase in amplitude, the losses in the auxiliary inductor 7 increase but remain at a much lower value, which means that the efficiency of the furnace is much higher, reaching a maximum of 85% at a capacitance value estimated as 69.45 nF. Thus, the typical distribution diameter of the heating in the charge is shown in fig. 9 and 10, also viewed from above and from below respectively: excellent heating uniformity was obtained in the charge and this time it was observed that a portion of the bottom crucible was heated while being at almost the same temperature as the portion located above.

By further increasing the capacitance of the capacitor bank 17 or the adjustable capacitor 22, the heating of the crucible bottom may be exacerbated, thereby damaging the rest: fig. 11 and 12 show the conditions obtained with a capacitance of 90nF, in this case experimentally corresponding to the maximum electrical resonance, where the voltage and current values at the terminals of the main inductor 3 are minimum, while the voltage at the terminals of the auxiliary inductor 7 and the current flowing therethrough are maximum. As shown in fig. 12, the apex of the charge is less heated and the heating is concentrated at the bottom of the crucible. The high total capacitance and maximum resonance condition of this group 17 is maintained, in which the auxiliary circuit 19 works the most when supplying maximum power, making it possible to concentrate the heating near the auxiliary circuit 19 (i.e. here at the bottom of the crucible) and to promote the melting of the solid crust while the bath continues to mix. Pouring can then be effected by gravity by opening the stopper 11.

Fig. 13 shows the use of a second auxiliary circuit 23 comprising an inductor 24 and a capacitor 25 placed at the terminals of the inductor 24, thereby closing the circuit. The capacitor 25 may be tunable or may be non-tunable. The inductor 24 is located above the apex of the crucible and is of a similar form to the ancillary circuit 19 facing it. The second auxiliary circuit 23 has no electrical generator. It also operates by resonant electromagnetic coupling with the main circuit 18 and by judicious adjustment or selection of the capacitance of the capacitor 25, so that additional heating can be established in the upper part of the crucible, suitable for preheating a portion of the charge newly introduced into the crucible, for example in particular in processes in which the charge is introduced gradually, or for example in processes in which the charge volume is greater and its volume is high. The second auxiliary circuit 26 may be deactivated by opening the circuit breaker 26.

The invention may be embodied in many other forms. Fig. 14 and 15 show an embodiment in which the bottom plate with reference number 27 consists of circular sector tanks 28, which are joined by arc pipes 29 for circulating the cooling fluid from one tank 28 to the next. The end 30 of the cooling circuit is also used for connecting an adjustable capacitor bank as in the previous embodiments. The device is otherwise identical to the previous device and operates similarly; this embodiment is not preferred because the electromagnetic coupling that facilitates the auxiliary circuitry is much less favorable.

The auxiliary inductor can still be placed above the fire-resistant and electrically inert bottom plate, as in the known arrangement, again at the cost of reduced efficiency.

The inductor may be of any known type. The auxiliary inductor primarily envisaged has a spiral form, but inductors with a single turn formed by several concentric strands or a single strand arranged in parallel and which will give good results are conceivable. The main inductor with several strands of parallel single strands as envisaged herein may be replaced by a multi-turn spiral inductor, respectively.

The total capacitance of the capacitor bank 17 must be selected to be adjustable to achieve different modes of operation corresponding to the desired heating profile to be obtained (such as good uniformity in the largest part of the volume) and heating sufficiently concentrated at the bottom of the crucible to achieve dumping. It will generally be noted that a maximum resonance state of the circuit can be reached, so that maximum power can be transmitted into the auxiliary circuit 19. The state corresponding to the highest efficiency will be preferred during most of the process, but this is not essential, since the efficiency is still improved over a large range of the total capacitance value of the capacitor bank 17.

The positions of the main inductor (connected to the electrical generator) and the auxiliary inductor (equipped with the adjustable capacitor) can be interchanged without having to change in particular the structure of the arrangement of the crucible and of the circuit represented in figures 1 to 4.

If desired, other auxiliary circuits can be added at different locations of the crucible, which are electromagnetically coupled to the main circuit, consist of inductors and capacitors, and do not have their own electrical generator.

Claims

1. An induction furnace comprising crucibles (1, 8) with side walls (1) cooled by fluid circulation and a floor (8) below the side walls (1), a first induction heating circuit (18) comprising a first inductor (3) and an electric generator (15), and a second induction heating circuit (19) comprising a second inductor (7), one of the inductors being located around the side walls and the other being located below or in the floor, characterized in that the second circuit (19) has no electric generator but is electromagnetically coupled with the first circuit (18) and is provided with a capacitor arrangement (17) with an adjustable capacitance, which is connected to the second inductor (7).

2. Induction furnace according to claim 1, characterized in that the soleplate (8) is constituted by one of the inductors (7) which is in the form of a spiral constituted by turns (31) joined by an electrical insulator (32).

3. Induction furnace according to any of claims 1 or 2, characterized in that the capacitor means comprise a capacitor (22) with variable capacitance.

4. Induction furnace according to any of claims 1 or 2, characterized in that the capacitor means comprise a capacitor (20) connected to the second electric circuit (19) by a circuit breaker (21).

5. The induction furnace according to any of the claims 1 to 4, characterized in that the first inductor (3) is located around the side wall (2).

6. The induction hob according to any one of the claims 1 to 4, characterized in, that the first inductor is located below or in the floor.

7. The induction hob according to any one of the claims 1 to 6, characterized in, that the induction hob comprises at least one additional induction heating circuit (23) without any electrical generator and comprising an inductor (24) and a capacitor arrangement with an adjustable total capacitance and being electromagnetically coupled with at least the first circuit (18).

8. Induction furnace according to claim 7, characterized in that at least one of said additional circuits is equipped with a circuit breaker (26) capable of opening said additional circuit.

9. The induction furnace according to any one of claims 7 or 8, characterized in that the inductor of at least one additional circuit is located above the crucible and facing the second circuit.

10. The induction hob according to any one of the claims 1 to 9, characterized in, that the total capacitance of the capacitor means of the second circuit is adjustable to a maximum electrical resonance state of the hob.