US 3107268 A
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Description (OCR text may contain errors)
Oct. 15, 1963 P. a. KRAUS MELTING FURNACE Fi1edDec.9,1960 F I6. I
qaanaafiifaa 7/ W auuuuubduuuo INVENTOR PHILIP B. KRAUS United States Patent 3,107,268 MELTING FURNACE Philip B. Kraus, Landeuberg, Pa., assignor to E. I. du Pont de Nemours and Company, Wilmington, Del., a corporation of Delaware Filed Dec. 9, 1960, Ser. No. 74,875 6 Claims. (Cl. 13--26) This invention relates to electrical induction furnaces.
Several devices for heating and melting metals by electrical induction are well known. One type of conventional apparatus consists of a non-conducting container which is encircled by an induction coil. The electromagnetic flux from the induction coiicouples with the charge within the non-conducting container, and the energy absorbed by the charge raises its temperature. Since many metals and metalloids are reactive with the container at elevated temperatures contamination of the charge is often the result of inductive heating. One method for overcoming contamination involves cooling the container wall sutficiently to cause some of the metal charge to solidify thereon, thus forming a protective liner or skull of the metal within the container. However, the establishment of such a protective liner has the disadvantage of absorbing much of the energy from an encircling induction coil, thus resulting in poor heating efficiency.
The present invention is an improvement in an inductive type of heating furnace. In the apparatus of this invention, a magnetic core is provided within the heating vessel, and this core receives or concentrates the electromagnetic fiux from a primary coil and effects the inductive coupling with the metal charge. Such an arrangement makes for a more efficient transfer of energy to the charge and substantially reduces the loss of energy due to absorp tion by the container walls. Moreover, a rotatable heating vessel is employed in this invention which makes it possible to utilize centrifugal force to hold the charge in annular form, such as a ring or hollow cylinder, against the walls of the heating vessel. As a result of this configuration, there is more efficient coupling of the elec trical energy.
The furnace of this invention is essentially a combination of four elements. The first is a primary induction coil located outside but not encircling the second element which is a symmetrically rotatable vessel. This vessel serves as a container for the charge to be heated, and it has an axially located aperture at at least one end. The
third element is ring-like magnetic core, common to and interlinking both the primary coil and the vessel. The fourth element is a means for rotating the vessel about its axis at various speeds, including those at or above the critical speed (the speed at which there is sufficient centrifugal force to hold the charge against the sides of the vessel). The core segments in the coil and vessel are usually positioned axially; however, slight deviation from this position can be tolerated. When current is supplied to the induction coil, an electromagnetic flux is established within the rotatable vessel by means of the magnetic core, and when the vessel is rotating at critical speed or greater, the metallic charge is distributed around the inside of the vessel forming a ring which acts as a turn of a secondary coil in the inductive system. The charge is thus uniquely coupled through the core to the source of power and heated by induction to the desired temperature.
The present invention is illustrated by the accompanying drawings in which- FIG. 1 is a side elevation of a preferred embodiment of the induction furnace of this invention. FIG. 2 is a cross section of the furnace of FIG. 1 along the section line 2, 2.
3,107,268 Patented Oct. 15, 1963 ice In these drawings, there is the rotatable, hollow heating vessel with a shell 1, usually made of steel, and having a lining 2 made of a suitable non-metallic substance such as the usual metallurgical furnace-lining materials, including silica, magnesia, alumina, clay, etc., in brick or troweled form. The lining material selected for .a particular heating operation should be one which exhibits resistance to the chemical action of the molten charge 3 which is being heated. The vessel is provided with riding rings 4 which rest on wheels 5, and is connected to a suitable means for rotating the vessel. The vessel is also provided with central apertures at each end to admit the magnetic core 6 which also passes through the induction coil 7 in the manner shown. The core 6 is made of magnetic materials, preferably soft iron, and laminated to reduce inductive'heating therein. The core is cooled, particularly in the section passing through the heating vessel, by circulation of a fluid coolant through pipe 8, and this core is protected by insulating cover 9 to reduce heat absorption from the charge. The core is preferably made in sections so that it can be dismembered and removed from the furnace when it becomes no longer necessary to supply heating energy to the charge. If desired, a hollow ceramic or non-magnetic tube or sleeve may be placed in an annular-spaced relationship over that portion of core 6 within the heating vessel and water or other fiuid coolant circulated through such a sleeve to replace or supplement the cooling accomplished by pipe 8. Alternatively, the insulating cover 9 may be extended outside the furance and the core cooled by the passage of fluid, preferably a gas, through a space between the core and the cover. Various other well-known auxiliary devices may be used in the furnace described. These include a feed chute as shown by the dashed line 10; a circular pouring spout 11 made of a suitable non-contaminating material such as graphite or-silica, depending on the melt; suitable means for maintaining an inert atmosphere over the charge; and external cooling means such as an air blast or a liquid shower on the external wall of the cylindrical vessel for the purpose of creating a protective liner 12 for containing the melt. It is also contemplated that the apparatus can be equipped with conventional temperature measuring instruments. 7
ln operating a horizontal unit such as that just described, the cylindrical vessel is rotated at at least the critical speed prior to the introduction of the metallic charge. A portion of the charge is then fed into the vessel from chute 10 in a powdered or particulate form. As a result of the centrifugal force created by the rotation of the vessel, the particles will distribute themselves in a ring near the end of the feed chute. When a suitable alternating current is applied to coil 7, and becomes magnetically coupled through the core 6 to the charge, the latter can be heated to a desired temperature. Once the furnace is started in this manner, the remainder of the charge may be gradually introduced into the vessel from chute 10 until the vessel is loaded. By continuing the flow of current to coil 7, heat will be generated in the charge due to inductive coupling with the electromagnetic fiux from core 6. Although melting is the usual objective when using the apparatus of this invention, other heat treatments of the charge below the melting point may be accomplished. When the furnace is being used for the purpose of melting, the metal charge may be added while the molten metal fills the vessel and overflows the pouring spout 11. Such a procedure provides a means for operating the furnace in a continuous manner. The overflow from spout 11 may be collected in a mold or other suitable collecting means. Another way in which the metal may be withdrawn from the vessel is by slowing the speed of rotation so that the charge will recede to the bottom of the vessel where it will overflow through spout II. If the furnace is emptied in this manner, it may be desirable to-remcve the core 6 so that it does not come in contact with the molten charge as the speed of rotation is decreased. The vessel may also be provided with a tilting mechanism to empty the remaining portiomof' the charge which is below the rotating level ofspout 11. In shutting down the apparatus of this invention, it is recommended that the vessel be rotated at least at critical speed until the residual molten metal in the vessel is solidified. Such a procedure evenly distributes this residual metal on the walls of the vessel to facilitate subsequent starting.
The vessel should be symmetrical about its axis of rotation for the obvious reasons of balance and stability during rotation. Any axially symmetrical shape may be used, but the preferred ones are cylindrical, spherical, conical, double conical, etc. The shell of the vessel must withstand the forces encountered. For small units, the stronger ceramic materials will serve. However, it is usually necessary to employ steel or other high-strength metal. It is preferable to use a shell of laminated sheets or one which is constructed in segments with interposed insulation between the segments to reduce heating by induced currents. Such methods of fabrication are known and are employed here to provide a vessel of low inductivity.
If for any reason it becomes difiicult to start heating in the furnace due to poor inductive coupling of the initial portion of the charge, auxiliary heating means may be employed for the start-up. For example, the initial portion of the charge may be are melted by insertion of two or more electrodes therein. This are melting can be accomplished when the furnace is rotating or a pool of metal may be formed while the vessel is stationary and once the pool is established, the furnace may be rotated. Heating by flame or other chemical means is also feasible. If titanium sponge is to be melted, the initial charge may be warmed to 290 C. with hot air and then it may be rapidly heated to melting by introducing a halogen such as chlorine gas, which reacts exothermically with The resulting titanium chlorides can be swept out by a flow of inert gas.
The furnace shown in the drawings may, with minor changes in the rotating means and auxiliary fittings, be operated with its axis in a vertical position or at any angle between vertical and horizontal. The start-up and operating procedures are based on the same principles. Discharge of the furnace can be accomplished by slowing down the speed of rotation until metal starts to flow through the discharge end. One advantage of the vertical or near-vertical axis is that molten metal may be completely discharged or a portion may be retained for subsequent start-up. Batch and continuous methods are applicable to all furnace positions.
The drawings show only one primary coil. This, however, may be augmented by other coils arranged outside the melting vessel and surrounding other core members magnetically linked with the central core-section passing through the vessel.
The primary coil is of standard construction. A watercooled copper coil is especially suitable for this purpose. The size of the coil as well as the voltage and frequency of the current are matters of electrical engineering which can be worked out by one skilled in that art for the particular furnace being constructed according to this invention.
The core may be so designed that its external parts are relatively remote from external metallic furnace parts so as to prevent undue heating of the latter. The cooling of the core section within the furnace is preferred, particularly when operating in the higher temperature ranges. Other parts of the inductive system may be cooled, but usually the coupling is quite efiicient and any heat generated is removed by normal convection cooling by the surrounding air. External cooling of the lip 11 by a 4 blast of air or inert gas may be used. or internal passages for this cooling gas will improve the heat transfer at this point.
Another embodiment of the furnace of this invention employs a central aperture in only one end of the vessel. To do this and still maintain the essential core feature, the opposite end is closed with a non-metallic material, preferably not over an inch thick, such as silica, asbestoscement, clay, or the like. This, of course, necessitates a physical discontinuity of the core member, but by placing its adjacent segments in juxtaposition on either side of the closure, the ring-like structure is maintained. With this type of construction, the magnetic flux will efiiciently bridge the gap, and the inductive coupling is established when the charge is in position. The portion of the core in the vessel may be supplied with cooling media through the single aperture. Feed and discharge are also accomplished through the same aperture. Certain advantages accrue from this arrangement such as relative ease in maintaining an inert or special atmosphere in the vessel. Other small gaps or simple contact joints may be present in the core to facilitate its assembly and removal, but these are preferably kept at a minimum.
There are various ways of mounting the core section within the single aperture furnace. It may be supported within the vessel to rotate with it while the magnetic flux or circuit is completed at the closed end across the aforementioned gap and at the opposite end by, for example, a sliding contact with the adjacent core section just outside the aperture. The internal support may be, for example, a socket at the closed end, thus permitting withdrawal of the core.
In an alternative embodiment of FIG. 1, the insulating tube or cover 9 may be afiixed to the vessel, and the aperture on the right side may be sealed. The core section would extend through the cover 9, as shown in FIG. 1, and the annular space between the core and the interior walls of the cover could be utilized for the purpose of carrying a non-conducting fluid coolant.
The arrangement of furnace parts according to this invention has several advantages not previously found in any one device. The combination'of advantages provides for certain operations heretofore unknown or impractical. For example, it permits the large-scale continuous melting of titanium out of contact with contaminating containers and with a relatively cool, non-contaminating energy source. Such melting conditions have only been found before in the so-called levitation melting which at most has handled only a fraction of a pound at a time.
Other structural advantages are also present. Due to the unique combination of the centrifugally formed annular charge interlinked by the common core to the primary induction coil, the melting vessel can be charged and discharged, with wide control over the amount of molten metal in the chamber. Another advantage of this combined structural relationship is found in the highly efiicient coupling of the electrical energy, supplied at the primary, to the load or charge in the chamber. This efficiency is further enhanced by the fact that the metallic parts and a graphite discharge lip may be so situated, constructed (e.g., segmented), or shielded that they absorb very little of the magnetic flux intended for melting the charge.
Since it is obvious that many changes and modifications can be made in the above-described details without departing from the nature and spirit of the invention, it is to be understood that the invention is not to be limited to said details except as set forth in the appended claims.
The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:
1. In a centrifugal electrical induction furnace, the combination of elements comprising at least one primary induction coil located outside, but not encircling, a symmetrically rotatable heating vessel of low inductivity at least one end of which has an axially located aperture, a
- combination of elements comprising a primary induction coil located outside, but not encircling, a rotatable, cylindrical heating vessel of low inductivity having a central aperture at each end, a ring-like magnetic core common to and interlinking both said coil and said vessel, and means for rotating said vessel about its axis at at least critical speed.
3. In the furnace of claim 1, means for internally cooling the segment of said core situated within the vessel.
4. In the furnace of claim 2, means for internally cooling the segment of said core situated within the vessel.
5. In a centrifugal electrical induction furnace, the combination of elements comprising a primary induction coil located outside but not encircling a symmetrically rotatable, low inductivity heating vessel having a central aperture at one end communicating with the exterior, a
non-inductive refractory tube aflixed axially in the vessel, a ring-like magnetic core common to and in annularspaced relationship with said coil and said vessel and intcrlinking both of these latter elements, and means for rotating said vessel.
6. In the furnace of claim 5, means for flowing a cooling fluid through the annular space between the tube and the core.
References Cited in the file of this patent UNITED STATES PATENTS 1,444,584 Clamer et al. Feb. 6, 1923 1,821,530 Spire Sept. 1, 1931 2,501,393 Kendall Mar. 21, 1950 FOREIGN PATENTS 683,393 France Mar. 3, 1930 471,708 Great Britain Sept. 9, 1937 819,884 Germany Nov. 5, 1951 167,787 Australia June 6, 1956