Induction furnace with additional resonant circuit

Abstract

The crucible is equipped with at least two electric induction heating circuits, one of which is arranged around the sidewall and the other below or on the crucible floor. One of these circuits (18) includes a generator (15) connected to an inductor (3), while the other circuit (19) lacks a generator but is electromagnetically coupled to the former and consists of an auxiliary inductor (7) and a capacitor (17) with an adjustable total capacitance. The coupling conditions, the power passing through each of the two inductors (3, 7), and the heating distribution can be varied to achieve uniform heating or concentrated heating on the crucible floor. Good thermal efficiency can be expected.

Description

[Technical field]

[0001] The present invention relates to a furnace with an additional resonant circuit. [Background technology]

[0002] It concerns cold crucible furnaces with electromagnetic induction heating for melting, in particular at least one electrically conductive material, such as, for example, oxides or oxide mixtures or such mixtures with a proportion of metals of up to about 30% by weight, which mixtures are representative of molten core melts. It also applies in particular to furnaces used in the glass industry, for example for the production of enamels or other high purity materials produced at high temperatures from 1200 K to more than 3000 K. The induction frequency is generally comprised between 100 kHz and 1000 kHz, depending on the size of the crucible.

[0003] It must be improved, in particular, by allowing a good uniformity of the temperature of the charge without significant overheating of the liquid bath or molten material or without generating induced currents, which may be harmful and interfere with the components that make up the electric circuit of an inductor or inductors, in particular the current generators, so that the thickness of the crust of the sole at the bottom of the crucible can be reduced without significant overheating. The advantages obtained are related to better efficiency and better thermal uniformity, which simplify the installation, improve the mixing of the liquid bath by convection and make it possible to facilitate the pouring of the liquid from the bed by gravity.

[0004] Induction ovens are well known in the fields of foundry or metallurgy or in the nuclear industry for the production of materials or the formation of homogeneous mixtures. They comprise a crucible in which the charge is held and at least one inductor, the excitation of which generates a current in the charge, which leads to its heating and melting. This heating process is easy to implement and makes it possible to avoid contact between the thermal energy source and the crucible.

[0005] The crucible typically comprises side walls joined to a floor which may be flat and constitutes the bottom. If it is of an electrically conductive material, in order to avoid the appearance of induced currents swirling around the charge and the corresponding energy losses, it is often made in sectors, i.e. made up of parts extending over angular sectors, the parts being separated by electrically insulating separators.

[0006] Some crucibles are constructed of refractory materials such as ramming mass or graphite. Their advantage is their high melting temperature, but certain charges, especially those constructed of oxides, must be melted at even higher temperatures, or at least at temperatures sufficient to physically or chemically corrode them.

[0007] A very popular design, which the invention also adopts, is that of a cold crucible, cooled by water or another liquid and equipped with a cooling circuit extending inside the crucible. The crucible therefore remains much cooler than the centre of the charge during the operation of the process and remains protected from corrosion or other physical or chemical attack by the solidified layer of material from the charge present against it, which can be considered as constituting the inner wall of the crucible, and which is called a skull crucible. These cold crucibles allow high temperature melting, for example here 1500K, or even 3000K or higher, of highly reactive materials, which can be metals such as titanium, steel, or oxides such as glass, titanium oxide, various alloys composed of rare earth metals or mixtures, or materials with low electrical conductivity, such as silicon or enamel.

[0008] However, defects can be found in the uniformity of heating in the crucible. The hottest parts of the charge tend to rise to the free surface by convection. However, the induced currents that cause the heating tend to concentrate in the parts of maximum electrical conductivity, which correspond to the hot parts of the charge, especially of oxides. The consequences of this convection can be intensified by the uneven heating and, in the case of metals, the reduction of electromagnetic induction near the floor, which makes it more difficult to heat the charge at the bottom of the crucible, where the temperature remains lower and the solidified layer is usually much thicker on the floor (sometimes tens of millimeters instead of a few millimeters opposite the side wall of the crucible). This greater difficulty to achieve melting, associated with high electrical losses in the cooled structure, can be harmful in itself and affect the quality of the process. It is an even greater obstacle if it is provided to drain the charge molten by the bottom of the crucible, not by inverting the crucible, which is delicate to carry out and often prohibited, but by opening a plug in the middle of the floor. The solidified crust can in fact prevent pouring when the plug is removed. It may be envisaged to increase the power to melt that crust at the bottom of the crucible and concentrate the heating in the center; however, there is a risk of overheating elsewhere, increasing the volatility of certain oxides and thereby modifying the composition of the molten charge, melting the solidified layer of the charge covering the side walls, or even damaging certain structures of the crucible, such as the insulation between the sectors. Heat losses can also be multiplied by a factor of 1.5 or 2 in a process already characterized by low efficiency, with high radiation from the free surface of the bath, conduction in the crucible walls, and contact with the surrounding atmosphere.

[0009] It is also possible to act on the induction frequency so that the filling melts at the bottom of the crucible: if the inductors are arranged around the side walls, which is the most common, the distribution of heating in the crucible in the radial direction depends on the excitation frequency of the inductors: the higher it is, the more concentrated the heating is on the walls; it is therefore possible to select or adjust that excitation frequency to adjust the volume of the molten bath and, to some extent, the temperature distribution in the crucible; however, the lack of uniformity of the temperature of the molten bath may become intolerable. In practice, the electrical circuits and cooling systems are generally over-dimensioned and such a burden on the devices is not always satisfactory.

[0010] This failure is particularly evident in the current case where cold crucibles are made of conductive materials such as copper or certain grades of stainless steel, due to increased electrical losses in these structures, but they are present whatever the material from which the crucible is made and its structure.

[0011] The last drawback to be mentioned here of the simplest induction furnaces described so far is that they do not adapt well to variable amounts of charge if the inductors are of considerably different heights and therefore different heights in the crucible.

[0012] There is a wealth of prior art describing such induction furnaces. Here, we will detail a particular design that is supposed to improve heating uniformity.

[0013] In addition to the main inductor surrounding the side wall of the crucible, EP 1 045 216 describes an auxiliary inductor, placed around the injection area below the floor, around the plug that allows injection from the crucible. This creates an additional induced current around the injection opening that passes through the floor, preventing the solidification of the charge there and ensuring that injection occurs when the plug is removed. However, difficulties of implementation remain and the device is not always suitable for melting materials with very high melting points. Its effect is purely local for a limited time, so it does not solve the more general problem of lack of uniformity in heating and melting the charge. The same applies if, as already proposed, an uncooled metal tube is provided between that lower inductor and the injection opening in the hope of obtaining a greater heating by the tube, and moreover the latter runs the risk of corroding quickly, especially at high temperatures.

[0014] In certain designs, such as that of US Patent No. 6,185,243, the conventional inductor placed around the side walls of the crucible is replaced by an inductor placed under the floor and whose shape is one or more spirals. This design is only suitable for baths with a small height compared to the diameter and may involve high heat losses due to convection at the free surface of the molten bath and conduction at the side walls. The power delivered is still non-uniform, usually above the windings of the inductor, but much weaker near the side walls and in the center of the crucible if there are no windings.

[0015] Furthermore, it has been proposed to place a single inductor that extends both around the side wall and under the floor (US Patent Application Publication No. 1,645,526), ​​or two separate inductors, each operated by a power source (US Patent Application No. 4,609,425, JP Patent Publication No. 10-253,260, US Patent No. 4,687,646). However, the possibility of a more uniform heating of the charge is opposed by the competition between the electric fields generated by the two inductors or inductor components, which can disrupt their operation and even damage the control electronics. Finally, an improvement of these designs has been proposed in WO 2017 / 093165, where a component called a flux concentrator is placed at the junction of the side wall and the floor, thereby separating the two inductors. This component is a static piece of material with high magnetic permeability, so that the interaction between the magnetic fields is reduced. The aforementioned drawbacks are reduced and better efficiency may be expected. Unfortunately, especially in the operating range envisaged here (operating range with respect to the excitation frequency of the power supply), the flux concentrator needs to be cooled to maintain its performance, which may be technically difficult in some circumstances. It would have to be located in an area that is difficult to access, it may require movement of the inductor relative to the fill, and finally, in the case of a radioactive fill, it is vulnerable to irradiation. One disadvantage of the different ordering is that the device does not allow for the distribution of the heating power between the two inductors. [Prior art documents] [Patent documents]

[0016] [Patent Document 1] European Patent No. 1045216 [Patent Document 2] U.S. Patent No. 6,185,243 [Patent Document 3] US Patent Publication No. 1,645,526 [Patent Document 4] U.S. Pat. No. 4,609,425 [Patent Document 5] Japanese Patent Application Publication No. 10-253260 [Patent Document 6] U.S. Pat. No. 4,687,646 [Patent Document 7] International Publication No. 2017 / 093165 Summary of the Invention [Means for solving the problem]

[0017] The present invention concerns an improved induction furnace, the most important aspect of which is that it makes it possible both to increase the effectiveness of heating the molten charge and to adjust the heating distribution of the heated volume, if necessary, by a more favorable structure of two inductors, one of which is placed around the side wall of the crucible and the other one below (or in) the bed. A higher electrical efficiency is expected than in previous devices and processes. Easier pouring is also expected due to a more effective heating at the bottom of the crucible.

[0018] In particular, it relates to an induction furnace comprising a crucible with a side wall cooled by circulation of a fluid and a floor arranged below the side wall, a first induction heating electric circuit comprising a first inductor and a generator, and a second induction heating electric circuit comprising a second inductor, one of the inductors being arranged around the side wall and the other of the inductors being below or in the floor, characterized in that the second circuit does not have a generator but is electromagnetically coupled to the first circuit and comprises an electric capacitor device of adjustable capacitance, the electric capacitor device being connected to the second inductor.

[0019] This electrical device with a double circuit utilizes the phenomenon of electric resonance of the furnace, which is facilitated by the free electromagnetic interaction of the two circuits. Unlike known devices, in which the inductors surrounding the side walls and the inductors assigned to the floor are excited by the same source while connected to each other, or by two different sources while isolated from each other, no conflict is observed between the induced magnetic fields at their junction, and there are no disturbances to the electrical circuit. This increases the electrical efficiency and reduces losses.

[0020] Another advantage of the present invention is the provision for better distribution of heating, which can be increased, especially in the center and at the bottom, just above the bed, in order to improve the process and possibly facilitate pouring. This distribution of heating depends on the setting of the total capacitance (adjustable during melting) and is also very sensitive to variations in that capacitance, to the extent that one can obtain a good level of temperature uniformity or, conversely, heat essentially either at the periphery, or at the center, or at the bottom of the molten charge.

[0021] It is common to arrange the first inductor around the side wall, since it constitutes the main inductor, and the second electrical circuit is assigned to the floor. However, the opposite arrangement may be encountered without criticism, especially in the case of crucibles with large surface area and low height.

[0022] The inductor at the bottom of the crucible can form a spiral covered with an electrical insulator that is a very good thermal conductor: it is then possible, and advantageous, for the floor at the bottom of the crucible to consist of said inductor itself, which improves the efficiency of the design and heating of the crucible.

[0023] The induction furnace does not have a generator and may include at least one additional circuit, comprising an inductor and a capacitor device with an adjustable total capacitance, electromagnetically coupled to at least the first circuit; this additional circuit or these additional circuits therefore have the same characteristics as the second circuit to which they refer. They may prove useful in the upper part of the furnace to improve the heating of the newly added quantity of charge. They may then be placed above the apex of the crucible, facing and parallel to an indicator located on or in the floor.

[0024] The additional circuit or circuits can then advantageously be equipped with disconnectors allowing them to be opened freely.

[0025] Various aspects, features and advantages of the present invention are illustrated by the following drawings, which represent, purely by way of example, specific embodiments thereof. [Brief description of the drawings]

[0026] [Figure 1] FIG. 1 is a schematic view of an induction furnace. [Diagram 2] FIG. 13 is a diagram of a lateral inductor. [Diagram 3] FIG. 1 is a diagram of a bottom inductor. [Figure 4] FIG. [Diagram 5] 1 is an embodiment of a bank of capacitors. [Figure 6] Another embodiment is shown. [Figure 7] 1 shows a first heating state. [Figure 8] FIG. 13 shows the first heating state, as viewed from below. [Figure 9] 2 shows a second heating state. [Figure 10] FIG. 13 shows a second heating state, as viewed from below. [Figure 11] 3 shows a third heating state. [Figure 12] FIG. 13 shows the third heating state, as viewed from below. [Figure 13] 1 shows another embodiment of a crucible. [Figure 14] 3 shows a third embodiment of the crucible. [Figure 15] 3 shows a third embodiment of the floor. [Figure 16] FIG. 2 is a cross-sectional view of a floor of the first embodiment. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0027] An embodiment is described with the aid of figures 1 to 3. This embodiment of the induction furnace comprises a side wall 1 or a cylindrical sleeve consisting of vertical tubes 2 of copper or stainless steel adjacent and connected by an electrically insulating separator to prevent the occurrence of circular induction currents in the side wall 1. The tubes 2 are crossed by a cooling liquid, such as water, according to known arrangements. The cooling circuit 12 can circulate the liquid alternately up and down, using connectors between the tubes 2 alternately at the top and bottom ends. Other possible constructions are, among others, those of US Pat. No. 6,996,153, in which each tube is cooled independently by a circuit with vertical reciprocating trips.

[0028] The lateral inductor 3 surrounds the lateral wall 1. Here, parallel strands 4 are stacked in the height direction, each a single winding that passes once around the crucible. Their ends are joined together by two vertical connections 5 and 6, which further provide both the electrical and hydraulic connections to the alternator, the strands 4 being also cooled by the circulation of a cooling liquid, which are connected to a cooling circuit 13. Furthermore, other arrangements of the lateral inductors are also possible, in particular windings arranged in a continuous spiral.

[0029] The crucible is completed by a floor 8, which mainly consists of a spiral auxiliary inductor 7 (Figs. 3 and 16). The auxiliary inductor 7 is also a tube cooled by water, whose ends 9 and 10 also lead to the cooling circuit 14. The windings 31 of the spiral are joined by a separator electrical insulator 32, which is electrically insulating and a very good thermal conductor, separating them all and giving a continuous structure to the floor 8. Furthermore, the auxiliary inductor 7 is covered on its upper surface with a thin layer 33, which is electrically insulating and a good thermal conductor, for example made of alumina. Such a design makes it possible to do away with the floor used up to now and to hold the crucible charge directly by the auxiliary inductor 7.

[0030] The cooling of the bed 8 and the lateral pair 1 also leads to the formation of a skull crucible, i.e. the layers of the filling when in contact with them remain solid, protecting them from corrosion.

[0031] An injection plug 11 is arranged in the center of the spiral. It is cooled by conventional devices, not shown here. The cooling circuits 12, 13 and 14 of the side walls 1 of the main inductor 3 and the auxiliary inductor 7 are represented here only diagrammatically and are according to already known embodiments, each of which may in particular include, for example, a pump and a heat exchanger.

[0032] The electric device is shown in FIG. 4. The main inductor 3 is connected to the terminals of the alternator 15. A bank of capacitors 16 is also connected to the terminals of the generator 15 in parallel with the main inductor 3. The assembly forms a closed main electric circuit 18, which has a resistance due, among other things, to the structure of the crucible, the filling contained therein and the resistance of the main inductor 3. The auxiliary inductor 7 together with an adjustable bank of capacitors 17 forms a closed auxiliary electric circuit 19, which is physically separated from the generator 15 and does not have its own generator. This auxiliary electric circuit 19 has a resistance due, among other things, to the auxiliary inductor 7. The proximity of the inductors 3 and 7 means that an electromagnetic coupling occurs between the electric circuits 18 and 19 during operation, even though the auxiliary electric circuit 19 does not have a generator. This coupling varies, among other things, depending on the incoming power and the filling of the crucible, as well as the setting of the total capacitance of the bank of capacitors 17, which consists of individual capacitors that can be connected to the auxiliary electric circuit 19 or isolated by a switch 21. The individual capacitors 20 may be arranged in parallel, as shown in Figure 5, or otherwise. As a variant (Figure 6), the bank of capacitors 17 may be replaced by an adjustable capacitor 22, the effect of which is the same.

[0033] The invention is based on the electromagnetic coupling between the electric circuits 18 and 19 by means of the inductors 3 and 7, in order to regulate the current induced by them in order to homogenize its temperature, in particular during the melting and mixing processes, to affect the distribution of the heating of the charge contained in the crucible or (among other possibilities) to release the injection plug 11 and increase the heating at the bottom to facilitate the injection operation.

[0034] The effect obtained may be that of Table 1 below.

[0035] [Table 1]

[0036] 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 flowing therein are high, the voltage at the terminals of the auxiliary inductor 7 is low, therefore the heating is mainly performed by the main inductor 3, but with high losses in the side wall 1 and relatively low efficiency. This mode of operation is similar to known conditions and is not representative of the invention, nor is it usually required. Figures 7 and 8 show the temperature distribution of the molten charge from above and below (lighter shades indicate greater local heating), thus showing high heating at the periphery, here weaker in the center, therefore the charge is much cooler there, and especially very weak heating by the generator 15 just above the floor. The choice of different excitation frequencies can change the distribution of heating in the radial direction, but in any case the heating at the bottom of the crucible is still weak and temperature uniformity is not achieved.

[0037] The next row of the table shows the effect of increasing the capacitance of the bank of capacitors 17 or the adjustable capacitor 22. The electromagnetic resonance becomes stronger, the values ​​of the voltage and current at the terminals of the main inductor 3 decrease, and the values ​​of the voltage at the terminals of the auxiliary inductor 7 and the current flowing therethrough increase. The losses in the side walls 1 decrease significantly, the losses in the auxiliary inductor 7 increase, but remain much lower, i.e. the efficiency of the furnace is much higher, reaching up to 85% at a capacitance whose value estimated here is 69.45 nF. Thus, the diameter of a typical distribution of the heating in the charge is that of Figures 9 and 10, seen from above and below respectively: it is observed that an excellent uniformity of heating is obtained in the charge and this time the bottom of the crucible is heated at almost the same temperature as the part above.

[0038] By further increasing the capacitance of the bank of capacitors 17 or the adjustable capacitor 22, the heating of the bottom of the crucible can be accentuated to the detriment of the rest: Figures 11 and 12 show the state obtained with a capacitance of 90 nF, which in this case corresponds experimentally to the maximum electric resonance, where the values ​​of voltage and current 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 therein are maximum. As shown in Figure 12, the top of the charge is hardly heated, the heating being concentrated at the bottom of the crucible. By maintaining this state of high total capacitance of the bank 17 and maximum resonance, which works best when the auxiliary electric circuit 19 delivers maximum power, it is possible to concentrate the heating close to the auxiliary electric circuit 19, i.e. here at the bottom of the crucible, facilitating the dissolution of the solid crust while continuing the mixing of the molten bath. Gravity pouring can then be obtained by opening the plug 11.

[0039] FIG. 13 shows the use of a second auxiliary electric circuit 23 comprising an inductor 24 and a capacitor 25 placed at the terminals of the inductor 24 to close the circuit. The capacitor 25 may be adjustable or not. The inductor 24 is placed above the apex of the crucible and is of a shape similar to that of the auxiliary electric circuit 19 facing its opposite side. The second auxiliary electric circuit 23 does not have a generator. It also works through a resonant electromagnetic coupling with the main circuit 18 and, by a judicious adjustment or selection of the capacitance of the capacitor 25, makes it possible to establish an additional heating of the crucible on its upper part, which may be useful, for example, to preheat a part of the charge newly introduced in the crucible, especially in processes where the charge is introduced progressively or where, for example, the volume of the charge is larger and rises to its height. The second auxiliary circuit 26 may be deactivated by opening the disconnector 26.

[0040] The invention can be implemented in many other ways. Figures 14 and 15 show an embodiment in which the floor, which in turn supports the datum 27, is made up of water boxes 28 of circular sectors connected by curved tubes 29 for the circulation of cooling fluid from one water box 28 to the next. The end 30 of the cooling circuit also serves as a connection for an adjustable bank of capacitors, as in the previous embodiment. Moreover, this device is the same as the previous one, and the operation is similar; however, this embodiment is not preferred, since the electromagnetic coupling in favor of the auxiliary circuits is much less good.

[0041] As in the case of known devices, it is still possible to place the auxiliary inductor on a fireproof and electrically inert floor, at the expense of reduced efficiency.

[0042] The inductor may be of any known type. The auxiliary inductor mainly envisaged is helical, but single-winding inductors formed from multiple concentric strands arranged in parallel or a single strand may be envisaged with good results. The main inductor with a single multiple strand in parallel envisaged here may be replaced accordingly by a helical inductor with several turns.

[0043] The total capacitance to which the bank of capacitors 17 can be adjusted must be chosen to allow different operating modes corresponding to the heating distribution one wishes to obtain, such as good uniformity in most of the volume and heating well concentrated at the bottom of the crucible to allow pouring. In general, it is shown that a maximum resonant condition of the circuit can be reached in order to be able to deliver maximum power to the auxiliary circuit 19. Since the efficiency is still improved within a wide range of values ​​of the total capacitance of the bank of capacitors 17, the condition corresponding to the highest efficiency is preferred during most of this process, even if this is not essential.

[0044] The positions of the main inductor (connected to the generator) and the auxiliary inductor (with adjustable capacitor) can in particular be interchanged without modifying the structure of the crucible and the layout of the electrical circuit represented in Figures 1 to 4.

[0045] Other auxiliary circuits, consisting of inductors and capacitors and not having their own generators, electromagnetically coupled to the main circuit, can also be added as required at various locations in the crucible. [Explanation of symbols]

[0046] 1 Side wall, crucible 3. Inductor 7 Inductors 8 beds, crucible 15. Generator 17 Electrical Capacitor Devices 18 Induction heating electric circuit 19 Induction heating electric circuit 20 Capacitor 21 Disconnector 22 Capacitor 23 Induction heating electric circuit 24 Inductor 26 Disconnector 31 Windings 32 Electrical insulators

Claims

1. A crucible (1, 8) with a side wall (1) cooled by the circulation of a fluid and a floor (8) arranged below said side wall (1), a first induction heating electric circuit (18) comprising a first inductor (3) and a generator (15); a second induction heating electric circuit (19) comprising a second inductor (7); one of the inductors is disposed around the side wall and the other of the inductors is below or in the floor, 1. An induction furnace, characterized in that the second circuit (19) does not have a generator but is electromagnetically coupled to the first circuit (18) and comprises an electric capacitor device (17) of adjustable capacitance, the electric capacitor device being connected to the second inductor (7).

2. 2. An induction furnace according to claim 1, characterized in that said floor (8) is constituted by one of said inductors, which is in the form of a spiral made up of windings (31) bounded by electrical insulators (32).

3. 3. An induction furnace according to claim 1 or 2, characterized in that the electrical capacitor device comprises a variable capacitance capacitor (22).

4. 3. An induction furnace according to claim 1 or 2, characterized in that the electrical capacitor device comprises a capacitor (20) connected to the second circuit (19) by a disconnector (21).

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

6. 5. An induction furnace according to claim 1, characterized in that the first inductor is arranged under or in the floor.

7. 7. An induction furnace according to any one of claims 1 to 6, characterized in that it comprises at least one additional induction heating electric circuit (23) which does not have a generator but comprises an inductor (24) and a capacitor device with an adjustable total capacitance and is electromagnetically coupled to at least the first circuit (18).

8. 8. Induction furnace according to claim 7, characterized in that at least one of said additional electric circuits comprises a disconnector (26) making it possible to open it.

9. 9. An induction furnace according to claim 7 or 8, characterized in that the inductor of at least one additional circuit is arranged above the crucible and faces the second electric circuit.

10. 10. An induction furnace according to claim 1, characterized in that the total capacitance of the capacitor devices of the second circuit is adjustable to maximum electrical resonance conditions of the furnace.

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