JP2022546446A - Induction furnace with additional resonant circuit - Google Patents

Abstract

The crucible is equipped with at least two electric induction heating circuits, one of which is arranged around the side wall and the other one of which is arranged under or on the floor of the crucible. One of said circuits (18) comprises a generator (15) connected to an inductor (3) and one other circuit (19) lacks a generator but is electromagnetically coupled with the former. and consists of an auxiliary inductor (7) and a capacitor (17) with adjustable total capacitance. The coupling conditions, the respective power passing through the two inductors (3, 7) and the heating distribution can be changed to homogenize or concentrate the heating on the floor of the crucible. Good thermal efficiency can be expected.

Description

The present invention relates to a conducive furnace with an additional resonant circuit.

It is a cold crucible furnace 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 up to about 30% by weight. In terms of materials, these mixtures are representative of molten core melt. It also applies in particular to furnaces used in the glass industry, for example in the production of enamels or other high purity materials produced at high temperatures of 1200K to over 3000K. The induction frequency typically consists of 100 kHz to 1000 kHz, depending on the size of the crucible.

It does not involve significant overheating of the liquid bath or molten material, which is harmful and can interfere with the components making up the electrical circuit of an inductor or inductors, especially the current generator, or an inductive Cold crucible furnaces in particular must be improved by allowing good uniformity of the temperature of the filling without generating an electric current, so that the thickness of the crust in the sole at the bottom of the crucible is , can be reduced without significant overheating. The benefits obtained are related to better efficiency and better thermal uniformity, which simplify installation, improve mixing of the liquid bath by convection, and facilitate the injection of liquid from the floor by gravity. allows you to

Induction ovens are well known in the field 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 filling is held and at least one inductor whose excitation produces an electric current in the filling which leads to its heating and melting. This heating process is simple to implement and can avoid contact between the thermal energy source and the crucible.

The crucible typically comprises side walls which may be flat and which are joined to a floor forming the bottom. If it is a conductive material, it is often formed in sectors, i.e. composed of sections extending over angular sectors, in order to avoid the appearance of induced currents swirling around the filling and the corresponding energy losses. and the portions are separated by electrically insulating separators.

Some crucibles are constructed of refractory materials such as ramming mass and graphite. Their advantage is the high melting temperature, although certain fillings, especially those composed of oxides, can be melted at even higher temperatures, or at least at temperatures sufficient to physically or chemically corrode them. There must be.

Also, a very popular design that the present invention employs is that of a cold crucible cooled by water or another liquid and with a cooling circuit extending into the crucible. The crucible thus remains much cooler than the center of the charge during operation of the process and is protected against corrosion or other physical or chemical attack by a solidified layer of material from the charge present against it. As it is, it can be considered to form the inner wall of the crucible, which is called the skull crucible. These cold crucibles are made of metals such as titanium, steel or various alloys composed of oxides such as glass, titanium oxide, rare earth metals or mixtures or low electrical conductivity materials such as e.g. silicon or enamel. Allows high temperature melting of highly reactive materials, which may be materials, for example melting here at 1500K, or 3000K or higher.

However, there may be deficiencies in the uniformity of heating within the crucible. The hottest parts of the packing tend to rise to the free surface due to convection. However, the induced current that causes heating tends to be concentrated in the areas of greatest conductivity, especially those corresponding to the hot portions of the oxide fill. The result of this convection may be enhanced by non-uniform heating and, in the case of metals, reduced electromagnetic induction near the floor, making it more difficult to heat the filling at the bottom of the crucible and The temperature is kept low and the solidified layer is usually much thicker on the floor (sometimes tens of millimeters instead of millimeters on the opposite side of the crucible sidewall). This greater difficulty in achieving melting, associated with high electrical losses in the cooled structure, is detrimental in itself and can affect process quality. It provides for discharging the melted charge by the bottom of the crucible by opening a plug in the center of the floor rather than by inverting the crucible, which is delicate and often prohibitive to perform. If so, it becomes an even bigger obstacle. The hardened crust can actually 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; Risk of overheating elsewhere while altering the composition, melting the solidified layer of the filling covering the sidewalls, or damaging certain structures of the crucible, such as the insulation between sectors. be. Heat loss can also be by a factor of 1.5 or 2 in processes already characterized by low efficiencies, with high radiation from the free surface of the bath, conduction at the crucible walls, and convention with the surrounding atmosphere. may be multiplied.

It is also possible to act on the induction frequency to cause the filling to melt at the bottom of the crucible: radially within the crucible, although this is most common if the inductor is placed around the sidewalls. The heating distribution of depends on the excitation frequency of the inductor: the higher it is, the more the heating is concentrated on the walls; It is possible to adjust the temperature distribution; however, the lack of temperature uniformity in the molten bath can become unacceptable. In practice, electrical circuits and cooling systems are generally oversized and such straining of devices is not always satisfactory.

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

The final drawback mentioned here of the simplest induction furnaces so far described is that if the heights of the inductors are significantly different, they do not adapt well to variable amounts of filling and thus the height inside the crucible. It is the difference.

There is a wealth of prior art describing such induction furnaces. Specific designs that are supposed to improve heating uniformity are now detailed.

EP 1 045 216 describes, in addition to a main inductor surrounding the side walls of the crucible, auxiliary inductors arranged around the injection area under the floor and around a plug allowing injection from the crucible. This creates an additional induced current around the injection port through the bed to prevent solidification of the fill there and ensure that injection occurs when the plug is removed. However, implementation difficulties remain and this device is not always suitable for melting materials with very high melting points. Since its effect is purely local for a limited time, it does not solve the more general problem of lack of uniformity in heating and melting the filling. The same applies if, as already suggested, an uncooled metal tube is provided between its lower inductor and the injection opening in hopes of obtaining greater heating by the tube, and furthermore the latter are at risk of rapid corrosion, especially at high temperatures.

In certain designs, such as U.S. Pat. No. 6,185,243, conventional inductors placed around the side walls of the crucible are replaced with inductors placed under the floor and whose shape is one or more spirals. . This design is suitable only for baths that are small in height relative to their diameter and can be associated with 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 sidewalls and in the center of the crucible if there are no windings.

In addition, either a single inductor extending both around the sidewalls and under the floor (US Pat. -253260, US Pat. No. 4,687,646). However, the potential for more uniform heating of the fill is countered by the competition between the electric fields generated by the two inductors or inductor components, which interfere with their operation and damage the control electronics. may even give Finally, an improvement to these designs is proposed in WO2017/093165, in which a component called a flux concentrator is placed at the junction of the sidewall and floor, thereby separating the two inductors. Since this part is a static part of a highly permeable material, the interaction between magnetic fields is reduced. The aforementioned shortcomings may be mitigated and better efficiency may be expected. Unfortunately, especially in the operating range assumed here (with respect to the excitation frequency of the power supply), the flux concentrator must be cooled to maintain its performance, which can be technically challenging in some situations. be. The baffle plate has to be placed in areas that are difficult to access, may require movement of the inductor relative to the fill, and finally, in the case of radioactive fills, is vulnerable to irradiation. One of the drawbacks of the different order is that this device cannot distribute the heating power between the two inductors.

EP 1045216 U.S. Pat. No. 6,185,243 U.S. Patent Application Publication No. 1645526 U.S. Pat. No. 4,609,425 JP-A-10-253260 U.S. Pat. No. 4,687,646 WO2017/093165

The present invention relates to an improved induction furnace, the most important aspect of which is that the more favorable structure of the two inductors increases the effectiveness of heating the molten charge, if desired, and heats the It is possible both to adjust the heating distribution of the volume, one inductor being placed around the side wall of the crucible and the other under (or in) the floor. Higher electrical efficiency is expected than previous devices and processes. Easier pouring is also expected due to more efficient heating at the bottom of the crucible.

Specifically, it comprises a crucible with a side wall cooled by fluid circulation and a floor located below the side wall, a first induction heating electrical circuit with a first inductor and a generator, a second a second induction heating electrical circuit comprising an inductor of does not have a generator but comprises an electrical capacitor device of adjustable capacitance electromagnetically coupled to the first circuit, the electrical capacitor device being connected to the second inductor; , relating to induction furnaces.

This electrical device with dual circuits exploits the phenomenon of electrical resonance of the furnace facilitated by the free electromagnetic interaction of the two circuits. Unlike known devices in which the inductors surrounding the sidewalls and the inductors assigned to the floor are energized by the same power supply while connected to each other, or by two different power supplies while isolated from each other, their junctions No competition was observed between the induced magnetic fields at the part and there was no disturbance to the electrical circuit. This improves electrical efficiency and reduces losses.

Another advantage of the present invention is the provision for better distribution of heating, especially in the center and bottom, just above the floor, in order to improve the process and possibly facilitate pouring. can be increased by The distribution of this heating depends on the setting of the total capacitance (adjustable during melting) to further obtain a good level of temperature uniformity or, conversely, to be essentially peripheral, or central, or To the extent that it can be heated at any of the bottoms of the fill, it exhibits a very high sensitivity to variations in its capacitance.

The first inductor constitutes the main inductor and is generally arranged around the sidewalls, while the second electrical circuit is assigned to the floor. However, especially in the case of crucibles with large surface area and low height, the opposite arrangement can be encountered without being subject to criticism.

The inductor at the bottom of the crucible can form a helix covered with an electrical insulator that is a very good heat conductor; Advantageously, it improves crucible design and heating efficiency.

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

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

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

1 is a general view of an induction furnace; FIG. Fig. 3a is a diagram of a lateral inductor; Fig. 10 is a diagram of a bottom inductor; It is an electric circuit diagram. 4 is an embodiment of a bank of capacitors; 4 shows another embodiment. A first heating state is shown. FIG. 11 shows a first heating state, viewed from below; A second heating state is shown. FIG. 11 shows a second heating state, viewed from below; A third heating state is shown. FIG. 10 is a view from below showing a third heating state; Figure 3 shows another embodiment of a crucible; Figure 3 shows a third embodiment of the crucible; Figure 3 shows the floor of the third embodiment; 1 is a sectional view of the floor of the first embodiment; FIG.

An embodiment will be described with reference to FIGS. 1 to 3. FIG. This embodiment of the induction furnace comprises a side wall 1 or cylindrical sleeve consisting of vertical tubes 2 of copper or stainless steel adjacent and connected by electrically insulating separators to prevent the generation of circular induced currents in the side walls 1 . The tube 2 is traversed by a cooling liquid such as water according to known arrangements. The cooling circuit 12 can circulate the liquid alternately up and down using the connectors between the tubes 2 alternately at the top and bottom ends. Other possible constructions are, inter alia, that of US Pat. No. 6,996,153, in which each tube is independently cooled by circuits that make vertical reciprocating trips.

A lateral inductor 3 surrounds the sidewall 1 . Here, parallel strands 4 are stacked in height, each being a single winding with one pass around the crucible. Their ends are joined together by two vertical connections 5 and 6 which further provide both electrical and hydraulic connections to the alternator, the strands 4 also being cooled by circulation of the coolant. are connected to the cooling circuit 13 . Furthermore, other arrangements of the lateral inductors, in particular windings arranged in a continuous spiral, are also possible.

The crucible is completed with a floor 8 consisting mainly of a helical auxiliary inductor 7 (Figs. 3 and 16). The auxiliary inductor 7 is also a water-cooled tube whose ends 9 and 10 also lead to the cooling circuit 14 . The spiral windings 31 are joined by separator electrical insulators 32 which are electrically insulating and very good thermal conductors, separating them all and giving the floor 8 a continuous structure. 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 allows the filling of the crucible to be held directly by the auxiliary inductor 7 without the floor conventionally used.

The floor 8 and lateral pair 1 cooling also results in the formation of skull crucibles, i.e. the layers of packing when in contact with them remain solid and protect them from corrosion.

The injection plug 11 is arranged in the center of the helix. Not shown here, it is cooled by conventional devices. 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 schematically and are according to already known embodiments, each in particular for example a pump A heat exchanger may be included.

An electrical device is shown in FIG. The main inductor 3 is connected to the terminals of the alternator 15 . A bank of capacitors 16 is also connected in parallel with the main inductor 3 to the terminals of the generator 15 . The assembly forms a closed main electrical circuit 18 . This main electrical circuit 18 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 the adjustable bank 17 of capacitors form a closed auxiliary electrical circuit 19 which is physically separated from the generator 15 and has no generator of its own. This auxiliary electrical circuit 19 has a resistance due inter alia to the auxiliary inductor 7 . The close proximity of inductors 3 and 7 means that there is electromagnetic coupling between the working electrical circuits 18 and 19 even though the auxiliary electrical circuit 19 is free of a generator. This coupling is notably the total capacitance of the input power and the crucible filling and the bank of capacitors 17 which are connected to the auxiliary electrical circuit 19 or made up of individual capacitors which can be isolated by switches 21. setting. Individual capacitors 20 may be arranged in parallel as shown in FIG. 5 or otherwise. As a variant (FIG. 6), the bank of capacitors 17 can be replaced by adjustable capacitors 22 with the same effect.

The present invention is based on the electromagnetic coupling between electrical circuits 18 and 19 by means of inductors 3 and 7 to regulate the currents induced by them, especially in order to equalize its temperature during the melting and mixing process, and to or (among other possibilities) open the injection plug 11 to increase the heating at the bottom to facilitate the injection operation.

The effects obtained may be those in Table 1 below.

The first row represents the operation of the device with auxiliary circuit 19 open. The voltage at the terminals of the main inductor 3 and the current through it is high, the voltage at the terminals of the auxiliary inductor 7 is low, so the heating is mainly done by the main inductor 3, but the losses in the sidewall 1 are large and the efficiency is comparatively low. Low. This mode of operation resembles known conditions and is neither representative of the present invention nor generally required. Figures 7 and 8 show top and bottom views of the temperature distribution of the molten fill (brighter tones indicate greater localized heating); shows high heating at , here weaker in the middle, so the charge is much cooler there, showing very weak heating especially by the generator 15 just above the floor. Choosing 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.

The next row of the table shows the effect of increasing the capacitance of the bank of capacitors 17 or adjustable capacitor 22 . The electromagnetic resonance is strengthened, the voltage and current values at the terminals of the main inductor 3 decrease and the voltage and current values at the terminals of the auxiliary inductor 7 increase. The side wall 1 losses are significantly reduced and the auxiliary inductor 7 losses are increased, but the values remain much lower, i.e. the efficiency of the furnace is much higher, estimated here at 69.45 nF. A maximum of 85% is reached at a certain capacitance. Thus, the diameter of a typical distribution of heating within the packing is that of Figures 9 and 10, when viewed from above and below respectively: excellent heating uniformity within the packing is obtained, This time it is observed that the bottom of the crucible is heated to approximately the same temperature as the upper part.

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 exacerbated to damage the rest: FIGS. , which corresponds experimentally to the maximum electrical resonance in this case, with the smallest voltage and current values at the terminals of the main inductor 3, whereas the auxiliary inductor The voltage at the terminal of 7 and the current flowing there are maximum. As shown in Figure 12, the top of the fill is barely heated and the heating is concentrated at the bottom of the crucible. By maintaining this state of high total capacitance and maximum resonance of the bank 17, where the auxiliary electric circuit 19 works best when delivering maximum power, the , allowing the melting of the solid crust to proceed while continuing to mix the molten bath. Gravity injection can then be obtained by opening the plug 11 .

FIG. 13 shows the use of a second auxiliary electrical circuit 23 comprising an inductor 24 and a capacitor 25 placed at the terminals of the inductor 24 to close the circuit. Capacitor 25 may or may not be adjustable. An inductor 24 is positioned above the apex of the crucible and has a shape similar to that of the auxiliary electrical circuit 19 facing the opposite side thereof. The second auxiliary electrical circuit 23 does not have a generator. It also operates via resonant electromagnetic coupling with the main circuit 18, allowing judicious adjustment or selection of the capacitance of capacitor 25 to establish additional heating of its upper crucible, which For preheating a portion of the charge newly introduced into the crucible, e.g., especially in processes where the charge is progressively introduced, or, e.g., when the volume of the charge is larger and rises to its height can be useful in the process. The second auxiliary circuit 26 can be deactivated by opening the disconnector 26 .

The invention can be implemented in many other ways. Figures 14 and 15 show a circular sector water box in which the floors supporting the datums 27 in turn are joined by curved tubes 29 for the circulation of cooling fluid from one water box 28 to the next. 28. FIG. The cooling circuit end 30 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 device and the operation is similar; however, this embodiment is not preferred as the electromagnetic coupling in favor of the auxiliary circuit is much poorer.

As in the known device, it is still possible to place the auxiliary inductor on a refractory, electrically inert floor, at the expense of reduced efficiency.

The inductor can be of any known type. The main conceivable auxiliary inductor is helical, but single winding inductors formed from multiple concentric strands arranged in parallel or from a single strand are conceivable and give good results. be done. The main inductor with single multiple strands in parallel, which is assumed here, can accordingly be replaced by a spiral inductor with several turns.

The total capacitance that can be adjusted for the bank of capacitors 17 is desired to obtain, such as good uniformity over the bulk of the volume, and heating that is well concentrated at the bottom of the crucible so that it can be poured. It must be chosen to allow different modes of operation corresponding to the heating distribution. In general, it is shown that maximum resonant conditions of the circuit can be achieved in order to be able to deliver maximum power to the auxiliary circuit 19 . Conditions corresponding to the highest efficiency are preferred during most of this process, even though this is not essential, as efficiency is still improved within a wide range of values of the total capacitance of the bank of capacitors 17. .

The positions of the main inductor (connected to the generator) and the auxiliary inductor (comprising an adjustable capacitor) can be interchanged without, among other things, changing the structure of the crucible and the layout of the electrical circuits represented in FIGS. can do.

Other auxiliary circuits electromagnetically coupled to the main circuit, consisting of inductors and capacitors and not having their own generator, can also be added at various locations in the crucible if desired.

1 sidewall, crucible 3 inductor 7 inductor 8 floor, crucible 15 generator 17 electrical capacitor device 18 induction heating circuit 19 induction heating circuit 20 capacitor 21 disconnector 22 capacitor 23 induction heating circuit 24 inductor 26 disconnector 31 winding 32 electrical insulator

Claims (10)

a crucible (1, 8) with a side wall (1) cooled by fluid circulation and a floor (8) arranged below said side wall (1);
a first induction heating electrical circuit (18) comprising a first inductor (3) and a generator (15);
a second induction heating electrical circuit (19) comprising a second inductor (7);
wherein one of said inductors is arranged around said sidewall and the other of said inductors is below or in said floor,
said second circuit (19) having no generator but comprising an electrical capacitor device (17) of adjustable capacitance electromagnetically coupled to said first circuit (18); An induction furnace, characterized in that a capacitor device is connected to said second inductor (7). CHARACTERIZED IN THAT said floor (8) is constituted by one of said inductors (7), which is in the form of a spiral composed of windings (31) joined by an electrical insulator (32). Item 1. The induction furnace according to item 1. 3. Induction furnace according to claim 1 or 2, characterized in that the electrical capacitor device comprises a capacitor (22) of variable capacitance. An induction according to any one of claims 1 or 2, characterized in that said electrical capacitor device comprises a capacitor (20) connected to said second circuit (19) by means of an isolator (21). Furnace. 5. An induction furnace as claimed in any one of the preceding claims, characterized in that the first inductor (3) is arranged around the side wall (2). 5. An induction furnace as claimed in any one of the preceding claims, characterized in that the first inductor is arranged under or in the floor. at least one additional generator without a generator but comprising an inductor (24) and a capacitor device with adjustable total capacitance electromagnetically coupled to at least said first circuit (18) 7. An induction furnace according to any one of the preceding claims, characterized in that it comprises an induction heating electric circuit (23). 8. Induction furnace according to claim 7, characterized in that at least one said additional electrical circuit comprises a disconnector (26) enabling it to be opened. 9. 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 electrical circuit. . 10. Induction furnace according to any one of the preceding claims, characterized in that the total capacitance of the capacitor devices of the second circuit is adjustable to the maximum electrical resonance condition of the furnace.

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