RU2822212C1 - Induction furnace containing additional resonant circuit - Google Patents

Abstract

FIELD: electrical engineering.

SUBSTANCE: invention relates to induction furnaces. For this purpose, the induction furnace crucible contains at least two electric circuits of induction heating, one of which is located around the side wall, and the other one is under the floor or in the floor of the crucible. One of the circuits contains an electric power generator connected to the inductor, and the other circuit has no electric power generator, but is in electromagnetic connection with the first one and consists of an auxiliary inductor and capacitors with adjustable total capacitance.

EFFECT: increased efficiency of melted charge heating and provision of possibility to control distribution of heating in heated volume.

10 cl, 16 dwg, 1 tbl

Description

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

It relates to cold-crucible furnaces with electromagnetic induction heating for melting, in particular, for example, at least one electrically conductive material, for example an oxide or mixtures of oxides, or such mixtures with a metal content of up to about 30% by weight, these mixtures being molten corium . It also applies in particular to furnaces used in the glass industry, in the production of enamels or other high purity material, produced at high temperatures, for example from 1200 K to more than 3000 K. The induction frequency is generally from 100 kHz to 1000 kHz according to crucible size.

This should improve cold crucible furnaces, particularly by increasing batch temperature uniformity, which can result in a thinner skin at the bottom of the crucible without significantly overheating the liquid bath or molten materials or generating induced currents that will damage and impair the operation of components. , forming an electrical circuit of an inductor or inductors, in particular, for example, current generators. The benefits achieved are related to increased efficiency and increased thermal uniformity; they allow for easier installation, improved convective mixing of the liquid bath, and easier pouring of liquid through the floor by gravity.

Induction furnaces are widely known in the field of casting or metallurgy or in the field of nuclear energy for the production of materials or the formation of homogeneous mixtures. They contain a crucible in which the charge is held, and at least one inductor, the excitation of which leads to the generation of currents in the charge, which ensure its heating and melting. This heating process is easy to implement and avoids any contact between the heat source and the crucible.

The crucible typically includes a side wall connected to a floor, which may be flat and forms the bottom. If it consists of electrically conductive material, it is often divided into sectors, that is, it consists of sections covering corner sections, the sections being separated by electrically insulating separators, in order to avoid the occurrence of induced currents circulating around the charge and the corresponding energy losses.

Some crucibles are made of a refractory material, such as padding or graphite. Their advantage is a high melting point, but some types of charge, in particular those consisting of oxides, must melt at even higher temperatures or temperatures at least sufficient for their physical or chemical destruction.

The invention also uses a very common design called a cold crucible, equipped with a cooling circuit that is cooled by water or other fluid that passes through the crucible. In this way the crucible remains much cooler than the central part of the charge during the process, and remains protected from corrosion or other physical or chemical attack by the solidified layer of material from the charge that exists opposite it, which can be considered as forming the inner wall of the crucible and which is called the crucible with a garnissage. Modern cold crucibles allow high temperature melting, over 1500 K, or even over 3000 K, for example, highly reactive materials, which can be metals, such as titanium, steel or various alloys consisting of oxides, such as glass, titanium oxide, rare earth metals or mixture; or, for example, materials with low electrical conductivity, such as silicon or enamels.

However, disturbances in the uniformity of heating in the crucible may be observed. The hottest section of the charge tends to rise to the free surface due to convection. However, induction currents responsible for heating tend to concentrate in areas of highest electrical conductivity, which correspond to hot areas, in particular, for the oxide charge. The consequence of this convection, possibly enhanced by non-uniform heating and a decrease in electromagnetic induction near the floor, if it is metallic, is that it is more difficult to heat the charge at the bottom of the crucible, that its temperature there remains lower, and that the solidified layer is usually much more thicker on top of the floor (sometimes several tens of millimeters rather than a few millimeters from the side wall of the crucible). This significant difficulty in achieving melting, due to the high electrical losses in the cooled structures, is damaging in itself and can negatively affect the quality of the process. An even more serious difficulty arises when it is necessary to drain the molten charge through the bottom of the crucible by removing a plug in the center of the floor instead of tilting the crucible, which requires care and is often prohibited. The hardened cake may actually prevent pouring when the plug is removed. It can be assumed to increase the power to melt the crust at the bottom of the crucible and concentrate the heat in the center; however, there is a danger of overheating elsewhere, by increasing the volatility of some oxides and thus changing the composition of the melting charge, by melting the solidified layer of charge covering the side wall, or by damaging some crucible structures, such as insulators between sectors. Heat losses can also increase by 1.5 or 2 times in processes whose efficiency is reduced due to high losses due to radiation from the free surface of the bath, heat conduction through the walls of the crucible and heat exchange with the surrounding atmosphere.

To achieve melting of the charge at the bottom of the crucible, you can also change the induction frequency: if the inductor is located around the side wall, which is most typical, the heating distribution inside the crucible in the radial direction depends on the excitation frequency of the inductor: the higher it is, the more heating is localized in the wall; this allows this excitation frequency to be selected or adjusted to regulate the volume of the melt pool, and, to some extent, the temperature distribution within the crucible; but the lack of temperature uniformity in the melt pool may be unacceptable. In practice, the electrical circuit and cooling system are usually too large, but you don’t have to put up with such a heavy device.

These disadvantages are especially pronounced in this case when the cold crucible is made of an electrically conductive material, such as copper or some grades of stainless steel, in particular taking into account the increase in electrical losses in these structures, but they are present regardless of the material forming the crucible and its structure.

The last of the disadvantages of the simplest induction furnaces described above to be mentioned here is that they are poorly adapted to variable volumes of the charge, which, therefore, have different heights inside the crucible if the height of the inductor is significantly different.

Such induction furnaces are well known in the art. Let us now dwell on some specific designs that supposedly increase heating uniformity.

EP 1 045 216 B1 describes an auxiliary inductor located around the casting area under the floor, around a plug allowing casting from the crucible, in addition to the main inductor surrounding the side wall of the crucible. It allows additional induced currents to be generated around the casting hole through the floor to prevent the charge from solidifying there and to allow casting when the plug is removed. However, implementation difficulties remain; this device is not always suitable for melting materials with a very high melting point. This effect is purely local for a limited time, so the more general problem of lack of homogeneity of heating of the melting charge is not solved. The same will apply if, as has already been suggested, an uncooled metal tube is provided between this lower inductor and the casting hole with the aim of achieving greater heating due to the tube; in addition, the latter is susceptible to rapid corrosion, especially at high temperatures.

In some designs, such as those presented in US 6,185,243 B1, the more traditional inductor located around the side wall of the crucible is replaced by an inductor located under the floor and configured as one or more helices. This design is only suitable for baths of small height relative to the diameter and can be associated with high heat losses due to convection on the free surface of the melt bath and thermal conductivity of the side wall. The power provided remains non-uniform, usually high above the inductor turns, but significantly weaker near the side wall and in the center of the crucible in the absence of turns.

It has also been proposed to use a single inductor running around both the side wall and under the floor (US-1645526-A), or two separate inductors each driven by a power supply (US 4,609,425, JP 10,253,260, US 4,687,646). However, more uniform heating of the charge is prevented by the mutual cancellation of electric fields created by two inductors or sections of an inductor, which disrupts their operation and can even damage the control electronics. Finally, an improvement on these designs was proposed in WO 2017/093165 A1, where a part called a flux concentrator is located at the junction of the side wall and the floor, thus separating the two inductors. This part is a static part made of a material with high magnetic permeability, which thus reduces the interaction between magnetic fields. This can neutralize the above-described disadvantages and create conditions for increasing efficiency. Unfortunately, it is necessary, in particular for the operating range assumed here (which concerns the drive frequency of the power supply), to cool this magnetic flux concentrator in order to maintain its functionality, and in some cases this can be technologically difficult. The concentrator should be located in a hard-to-reach area, which may require movement of the inductors relative to the charge; finally, it can be exposed to radiation in the case of a radioactive charge. One drawback of another order is that this device does not allow the heating power to be distributed between two inductors.

The invention relates to an improved induction furnace, the most important feature of which is that it allows not only to increase the heating efficiency of the molten charge, but also to regulate the heating distribution in the heated volume as necessary, thanks to a more convenient design of two inductors, one of which is located around the side wall of the crucible, and the other under (or in) the floor. Increased electrical efficiency is expected compared to earlier devices and processes. It is also expected that casting will be simplified due to more efficient heating of the crucible bottom.

In particular, it relates to an induction furnace comprising a crucible, the side wall of which is cooled by a circulating fluid, and a floor located under the side wall, a first induction heating electrical circuit comprising a first inductor and an electrical power generator, and a second induction heating electrical circuit comprising a second inductor, wherein one of the inductors is located around the side wall, and the other is located under the floor or in the floor, characterized in that the second circuit does not have an electrical power generator, but is electromagnetically coupled to the first circuit and is equipped with an electrical capacitor device of adjustable capacitance, and said electrical the capacitor device is connected to the second inductor.

This double circuit electrical device utilizes the furnace electrical resonance phenomenon, which is facilitated by the free electromagnetic interaction of the two circuits. Unlike known devices in which the inductor surrounding the side wall and the inductor intended for the floor are excited by the same power source when connected to each other, or two different sources when separated from each other, mutual cancellation of the induced magnetic no fields are observed in the area where they are applied, and electrical circuits are not subject to disturbance. This improves electrical efficiency and reduces losses.

Another advantage of the invention is the ability to better distribute the heat, which is in particular increased in the middle and at the bottom, directly above the floor, to improve the uniformity of the process and possibly facilitate casting. This heating distribution depends on the setting of the total capacitance (adjustable during melting), in addition with a very high sensitivity to changes in this capacitance, to the extent that it is possible to either obtain a good level of temperature uniformity, or, on the contrary, to carry out heating that is essentially either at the periphery, or in the middle, or at the bottom of the molten charge.

The first inductor is usually located around the side wall as it forms the main inductor, while the second electrical circuit is dedicated to the floor. However, the opposite configuration can occur without being criticized, particularly for crucibles with a large surface area and low height.

The crucible bottom inductor can form a spiral coated with an electrical insulator that conducts heat very well: in this case it is possible and preferable for the crucible bottom floor to be formed by said inductor itself, which simplifies the crucible design and increases the heating efficiency.

The induction furnace may comprise at least one additional circuit that does not have any electrical power generator and contains an inductor and a capacitor device having an adjustable total capacitance, and electromagnetically coupled to at least the first circuit; thus, this additional chain or these additional chains have the same properties as said second chain. They can be useful in the upper part of the furnace to improve the heating of newly added batches. In this case, they can be placed on top of the crucible, oriented towards and parallel to the indicator located on or in the floor.

In such a case, it is preferable to provide the additional circuit or circuits with a disconnector allowing them to be opened at will.

Various aspects, features and advantages of the invention will be described below with reference to the following drawings, which represent certain specific embodiments thereof, for illustrative purposes only.

Fig. 1 - general view of the induction furnace;

fig. 2 - view of the transverse inductor;

fig. 3 - view of the lower inductor;

fig. 4 - electrical circuit diagram;

fig. 5 - embodiment of a capacitor bank;

fig. 6 - another embodiment;

fig. 7 - first heating state;

fig. 8 - the same thing in bottom view;

fig. 9 - second heating state;

fig. 10 - the same thing in bottom view;

fig. 11 - third heating state;

fig. 12 - the same thing in bottom view;

fig. 13 - another embodiment of the crucible;

fig. 14 - third embodiment of the crucible;

fig. 15 shows the floor of this third embodiment;

fig. 16 is a sectional view of the floor of the first embodiment.

The embodiment will be described with reference to FIG. 1-3. This embodiment of an induction furnace includes a side wall 1 or cylindrical sleeve consisting of vertical copper or stainless steel tubes 2 that are adjacent and electrically connected by insulating separators to prevent the occurrence of circular induced currents in the side wall 1. Along the tubes 2, according to known configurations, coolant, such as water, circulates. The liquid can circulate in the cooling circuit 12 alternately up and down, and connectors are installed between the tubes 2 alternately at the upper end and the lower end. Another possible design, among others, is presented in US - 6 996 153 - B2, where each tube is independently cooled by a circuit running vertically back and forth.

The transverse inductor 3 surrounds the side wall 1. It is formed by a single turn consisting of parallel cores 4 stacked vertically on top of each other, each of which makes one circuit around the crucible. Their ends are connected to each other by two vertical connectors 5 and 6, which additionally provide both an electrical connection to the alternator and a hydraulic connection, the cores 4 being further cooled by circulating coolant while connected to the cooling circuit 13. Other configurations of the transverse inductor are possible, in particular, successive helical turns.

The crucible ends with a floor 8, mainly consisting of an auxiliary inductor 7 of a spiral shape (Fig. 3 and 16). The auxiliary inductor 7 is also a water-cooled tube, and its ends 9 and 10 are also connected to the cooling circuit 14. The turns 31 of the spiral are connected by an electrically insulating separator 32, which is made of a material that is an electrical insulator and a very good conductor of heat, separating them all and providing a continuous structure of the floor 8. In addition, the upper surface of the auxiliary inductor 7 is covered with a thin layer 33, which is made of a material which is an electrical insulator and a good conductor of heat, for example, aluminum oxide. This design makes it possible to replace the floor, traditionally used to hold the charge in the crucible, directly with the auxiliary inductor 7.

Cooling of the floor 8 and side wall 1 also leads to the formation of a crucible with a scull, that is, to the fact that the layer of charge in contact with them remains solid and protects them from corrosion.

The pouring plug 11 is located in the center of the spiral. It is cooled by a conventional device, 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 shown here only schematically, in accordance with already known embodiments, and each of them may 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 the terminals of the alternating current generator 15. The bank of capacitors 16 is also connected to the terminals of the electric generator 15, in parallel with the main inductor 3. The unit forms a closed main electrical circuit 18. The resistance of this main electrical circuit 18 mainly consists of the resistance of the crucible structure, the charge contained in it and the main inductor 3. The auxiliary inductor 7 forms together with a regulated bank of capacitors 17, a closed auxiliary electrical circuit 19, which is physically separated from the electric generator 15 and does not have its own electric generator. The resistance of this auxiliary electrical circuit 19 is determined mainly by the auxiliary inductor 7. The proximity of inductors 3 and 7 means that an electromagnetic coupling is established between the electrical circuits 18 and 19 during operation, despite the absence of an electric generator in the auxiliary electrical circuit 19. This coupling changes in in particular according to the power input and the charge in the crucible, as well as the settings of the total capacitance of the capacitor bank 17, consisting of individual capacitors, which can be connected to the auxiliary electrical circuit 19 or separated from it by switches 21. The individual capacitors 20 can be arranged in parallel, as shown in FIG. 5, or vice versa. Alternatively (FIG. 6), the bank of capacitors 17 may be replaced by a variable capacitor 22, which operates in an identical manner.

The invention is based on electromagnetic communication between electrical circuits 18 and 19 through inductors 3 and 7, to regulate the currents induced by them, and to influence the distribution of heat in the charge contained in the crucible, in particular, to homogenize its temperature during the melting and mixing process, or (among other possibilities) to increase the heat at the bottom to release the pouring plug 11 and facilitate the casting operation.

The results obtained are shown below in Table 1:

Table 1

Bake Voltage supplied by generator 15 to main inductor 3 [V] Current through the main inductor 3 [A] Voltage at the terminals of the auxiliary inductor 7 [V] Current through auxiliary inductor 7 [A] Furnace active power [kW] Furnace efficiency [%] Side wall losses 1 [kW] Losses in auxiliary inductor 7 [kW] Losses in the main inductor 3 [kW] 1 - spiral floor not connected 1254 2008 1191 - 267.8 74.0 64.7 1.4 2.5 2 - half 60 [nF] 1049 1444 2474 265.8 246.5 81.0 39.5 3.7 1.3 3 - floor 69.45 [nF] 958.0 1216 2941 365.8 241.6 85.0 32.6 5.3 1 4 - floor 75 [nF] 887 1045 3309 444.4 241.8 84.0 28.9 6.9 0.8 5 - half 90 [nF] 527.0 507.0 4518 728.1 234.1 82.0 24.9 14.2 0.7

The first line 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 through it are high, the voltage at the terminals of the auxiliary inductor 7 is low, heating is thus carried out mainly by the main inductor 3, but with high losses in side wall 1 and relatively low efficiency. This mode of operation is similar to known conditions and is usually neither an illustration of the invention nor the subject of the search. Fig. 7 and 8, which show top perspective and bottom perspective views of the temperature distribution in the molten charge (lighter shades indicate stronger local heating) show high heating at the periphery, weaker in the center, causing the charge to be much cooler there, and , in particular very weak heating by the electric generator 15 directly above the floor. The choice of different excitation frequencies may lead to a change in the heating distribution in the radial direction, but the heating in any case will remain weak at the bottom of the crucible, and temperature uniformity will not be achieved.

The following rows of the table demonstrate the result of increasing the capacitance of the capacitor bank 17 or the adjustable capacitor 22. Electromagnetic resonance increases, the voltage at the terminals of the main inductor 3 and the current strength decrease, while the voltage at the terminals of the auxiliary inductor 7 and the current flowing through it increase. The losses in the side wall 1 are reduced greatly and increasingly, and the losses in the auxiliary inductor 7 increase, remaining at much lower values, and this means that the efficiency of the furnace is much higher and reaches a maximum of 85% with an estimated capacitance value of 69.45 nF. Typical diameters of the heating distribution in the charge are the same as in Fig. 9 and 10, again in perspective from above and below, respectively: high uniformity of heating in the charge is achieved, and this time heating of the bottom of the crucible is observed, and at almost the same temperature as the areas located above it.

By further increasing the capacitance of the capacitor bank 17 or the adjustable capacitor 22, the heating at the bottom of the crucible can be increased to the detriment of the rest of the crucible: FIG. 11 and 12 show the state reached at a capacitance of 90 nF, which in this case experimentally corresponds to maximum electrical resonance, in which the voltage at the terminals of the main inductor 3 and the current flowing through it are minimal, while the voltage at the terminals of the auxiliary inductor 7 and the current flowing through it are maximum. The upper part of the charge is heated very little, and the heat is concentrated at the bottom of the crucible, as shown in Fig. 12. Maintaining this state of high total capacity of the bank 17 and maximum resonance, in which the auxiliary electrical circuit 19 operates most when applying maximum power, allows the heat to be concentrated near the auxiliary electrical circuit 19, that is, in this case, at the bottom of the crucible, and to enhance the melting of the solid crust, while stirring of the melt bath continues. Then casting under the influence of gravity can be carried out by opening the plug 11.

Fig. 13 demonstrates the use of a second auxiliary electrical circuit 23 comprising an inductor 24 and a capacitor 25 connected to the terminals of the inductor 24 to form a closed loop. Capacitor 25 may or may not be adjustable. The inductor 24 is located on top of the crucible, similar in shape to the auxiliary electrical circuit 19 and located opposite it. The second auxiliary electrical circuit 23 does not contain an electric generator. It also provides resonant electromagnetic coupling with the main circuit 18 and, with proper adjustment or selection of the capacitance of the capacitor 25, allows additional heating of the crucible in its upper part, which can be useful either for preheating a portion of the charge newly introduced into the crucible, e.g. in particular, in processes with sequential introduction of the charge or, for example, for processes in which the volume of the charge is larger and grows to its height. The second auxiliary circuit 26 can be deactivated by opening the disconnect switch 26.

The invention can be implemented in many other ways. Fig. 14 and 15 show an embodiment in which the floor, designated 27, consists of water chambers 28 in sectors of a circle connected by curved tubes 29 for circulating coolant from one water chamber 28 to another. The ends 30 of the cooling circuit also serve to connect an adjustable bank of capacitors, as in the previous embodiment. In addition, the device is identical to the previous one and operates in a similar way; but this embodiment is not preferred since the electromagnetic coupling supporting the auxiliary circuit will not be as good.

Still, the auxiliary inductor can be placed above a refractory and electrically inert floor, as in known devices, but again at the cost of reduced efficiency.

Inductors of any known type can be used. It is generally assumed that the auxiliary inductor is of a helical form, but it may be assumed that an inductor with a single turn formed from several concentric strands arranged in parallel or as a single strand will give good results. It is assumed that the main inductor having several parallel cores and a single core can be correspondingly replaced by a spiral inductor with several turns.

It will be necessary to select a total capacitance to which the bank of capacitors 17 can be adjusted to obtain different modes of operation corresponding to the desired heating distributions, for example, good uniformity over the largest portion of the volume, and heat sufficiently concentrated at the bottom of the crucible to ensure casting. In general, it will be stated that the state of maximum resonance of the circuit can be achieved to allow maximum power to be sent to the auxiliary circuit 19. The state corresponding to the highest efficiency will be preferred during the largest part of the process, but is not necessary as efficiency continues to increase over a large range of values. the total capacity of the capacitor bank is 17.

The positions of the main inductor (connected to the electrical power generator) and the auxiliary inductor (equipped with an adjustable capacitor) can be swapped without changing the design of the crucible and the configuration of the electrical circuits, shown in particular in FIG. 1-4.

If necessary, other auxiliary circuits, electromagnetically connected to the main circuit, consisting of an inductor and a capacitor and not having their own electric generator, can also be added to various places in the crucible.

Claims (10)

1. An induction furnace containing a crucible (1, 8) with a side wall (1) cooled by a circulating fluid, and a floor (8) located under the side wall (1), a first induction heating electrical circuit (18) containing a first inductor (3) and an electrical power generator (15), and a second induction heating electrical circuit (19), comprising a second inductor (7), one of the inductors located around the side wall, and the other located under the floor or in the floor, characterized in that only the first circuit (18) contains an electrical power generator, and the second circuit (19) is electromagnetically coupled to the first circuit (18) and is provided with an electrical capacitor device (17) of adjustable capacitance, said electrical capacitor device being connected to the second inductor (7). 2. An induction furnace according to claim 1, characterized in that the floor (8) is formed by one of the mentioned inductors (7), made in the form of a spiral consisting of turns (31) connected by an electrical insulator (32). 3. Induction furnace according to any one of paragraphs. 1 or 2, characterized in that the electrical capacitor device contains a variable capacitor (22). 4. Induction furnace according to any one of paragraphs. 1 or 2, characterized in that the electrical capacitor device contains capacitors (20) connected to the second circuit (19) by disconnectors (21). 5. Induction furnace according to any one of paragraphs. 1-4, characterized in that the first inductor (3) is located around the side wall (2). 6. Induction furnace according to any one of paragraphs. 1-4, characterized in that the first inductor is located under the floor or in the floor. 7. Induction furnace according to any one of paragraphs. 1-6, characterized in that it contains at least one additional electrical circuit (23) of induction heating, containing an inductor (24) and a capacitor device having an adjustable total capacitance, and electromagnetically connected to at least the first circuit (18). 8. An induction furnace according to claim 7, characterized in that at least one of said additional electrical circuits is equipped with a disconnector (26) allowing it to be opened. 9. Induction furnace according to any one of paragraphs. 7 or 8, characterized in that the inductor of at least one additional circuit is located above the crucible and faces the second electrical circuit. 10. Induction furnace according to any one of paragraphs. 1-9, characterized in that the total capacitance of the capacitor device of the second circuit is adjusted to the state of maximum electrical resonance of the furnace.