US 3495019 A
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J. D. SANTI Feb. 10, 1970 INDUCTION FURNACE FOR MELTING ALUMINUM AND SIMILAR METALS Filed June 12, 1968 2 Sheets-Sheet 1 INERT GAS \SHIJM John I7. 5522217.
United States Patent 3,495,019 INDUCTION FURNACE FOR MELTING ALUMINUM AND SIMILAR METALS John D. Santi, West Allis, Wis., assignor to Briggs &
Stratton Corporation, Milwaukee, Wis., a corporation of Delaware Filed June 12, 1968, Ser. No. 736,441 Int. Cl. H05b 5/02 U.S. Cl. 13-31 7 Claims ABSTRACT OF THE DISCLOSURE This invention relates to induction furnaces for melting aluminum and similar metals, such as are used with die casting machines, and the invention relates more particularly to improvements in such furnaces by which their useful life is greatly increased.
In general, an induction furnace of the type to which this invention relates comprises a crucible of refractory cementitious material with a substantially deep base and with upright walls that cooperate with the base to define a melting chamber. In the base of the crucible is a coil that is wound on a rectangular core and is adapted to be energized with alternating current. A generally U-shaped molten metal passage in the base has upright legs which open to the melting chamber and which are located at opposite sides of the coil, and has its bight portion beneath the coil and extending transversely to it.
When the melting chamber and the U-shaped passage are filled with metal to be melted, such metal defines a single-turn short circuited winding or loop that is in flux linking relationsihip with the energized coil. The high current induced in that loop is relied upon to heat the metal to its melting temperature, and eddy currents in the loop, together with natural convection, normally maintain a circulation of molten metal to and from the melting chamber through the U-shaped passage.
Of course a substantial amount of heat is produced directly in the energized coil, due to its resistance, and therefore the furnace must ordinarily have means for cooling the coil to prevent its destruction. Heretofore it has been customary to provide air passages in the base of the furnace through which cooling air from a fan or blower could be circulated around the coil. Such forced air cooling was generally undesirable in a die casting shop because the exhaust air stream from the furnace, if not conducted to the outside, stirred up the dirt and soot that is almost always present in such a plant. Often, too, dust and dirt picked up by the fan or blower would settle out of the cooling air and accumulate in the cooling passages, blocking them and causing burn-out of the coil. Frequently in such cases the intense heat produced by the nncooled coil cracked the refractory material, permitting molten metal to pour out of the crucible.
The possibility of failure of the air cooling system posed a severe problem, but another and more serious objection to the air cooling arrangement existed, although the true nature of its seems not to have been appreciated heretofore. The cementitious refractory material of which the crucible of such a furnace is made is sufliciently porous so that it could be permeated by the cooling air being circulated through the base portion. Under the high temperatures existing in the furnace, the molten metal in the U-shaped passage in the base was readily oxidized by such air, and in time the molten metal passage became filled with the oxidized metal. Since the oxide remained solid at the temperatures for which the furnace was intended, it blocked circulation of molten metal through the passage. When this happened the furnace became unuseable and had to be completely rebuilt.
Such blocking occurred with remarkable frequency and regularity, and oxidization of the metal in the molten metal passage was therefore the cause of a substantial amount of expense in a die casting shop. One or more melting furnaces were always undergoing reconstruction in a shop that had any substantial number of them.
While it was known that oxidized metal was a frequent cause of breakdown in induction furnaces for melting aluminum and the like, it was apparently not appreciated that the air for such oxidization was brought to the metal by way of the pores in the refractory material of the crucible, and that cooling the coil with forced air circulation greatly encouraged air to permeate the pores of the refractory material and thereby materially hastened oxi dization of metal in the molten metal passage. The present invention is thus predicated upon the discovery of the basic cause of many expensive failures in induction melting furnaces, and resides in the provision of simple and inexpensive means for preventing oxidization of metal in the U-shaped molten metal passage.
It follows that it is the general object of the present invention to provide means in an induction melting furnace of the character described for substantially eliminating oxidization of metal melted in the furnace and thereby greatly prolonging the useful life of the furnace.
It is also a general object of this invention to eliminate the objectionable features attendant upon the use of forced air cooling of induction type melting furnaces of the character described, to thereby afford cleaner, more pleasant and safer working conditions in die casting plants and the like in which such furnaces are employed, and to eliminate a major cause of burned out coils in such furnaces.
A more specific object of the invention is to provide a method and means for excluding air from the pores of the cementitious refractory material of the crucible of a melting furnace of the character described, to thereby prevent oxidization of metal in the molten metal passage of the crucible.
With these observations and objects in mind, the manner in which the invention achieves its purpose will be appreciated from the following description and the accompanying drawing. The disclosure is intended merely to exemplify the invention. The invention is not limited to the particular method and structure disclosed, and changes can be made therein which lie within the scope of the appended claims without departing from the invention.
The drawings illustrate several complete examples of physical embodiments of the invention constructed according to the best modes so far devised for the practical application of the principles thereof, and in which:
FIGURE 1 is a vertical sectional view of an induction furnace for melting aluminum and the like that embodies the principles of this invention;
FIGURE 2 is a horizontal sectional view taken on the plane of the line 2-2 in FIGURE 1;
FIGURE 3 is a diagrammatic top view of a melting furnace incorporating a modified embodiment of the invention; and
FIGURE 4 is a more or less diagrammatic vertical sectional view of an induction melting furnace embodying another version of the invention.
Referring now to the accompanying drawings, the numeral 4 designates generally an induction furnace comprising a crucible 5 of refractory cementitious material having substantially upright side walls 6 and a substantially deep base 7 which cooperate in defining a melting chamber 8. Housed in the base is a laminated core 10 which is rectangular as viewed from above and which has windings 9 on a pair of its opposite legs that comprise a coil connectable with a source of alternating current (not shown).
In the base 7 there is also a generally U-shaped molten metal passage 11. The upright legs 12 of the passage 11 open at their tops to the melting chamber 8, and its bight portion 13 is beneath the coil and extends transversely to its windings, intermediate their ends. The upright passage portions 12 are spaced to opposite sides of the core 10, and in this case there is, as in many such furnaces, a third upright passage 14 which communicates the melting chamber 8 with the transverse passage portion 13 and which is substantially coaxial with the core.
It will be seen that when metal is present in the passage 11 and the melting chamber 8, such metal defines a pair of closed loops around the core, in flux linking relation with the coil, which loops have a common leg defiined by the central upright passage portion 14. This arrangement is a common one in induction melting furnaces, and in fact the structure described to this point is generally conventional.
Since the crucible is made of cementitious material, it is substantially porous. Heretofore air has been permitted to permeate the pores of the refractory material, and of course the hot metal in the crucible was readily oxidized by such air. Such oxidization occurred at all of the interfaces between the molten metal and the crucible, but it was most prevalentand most destructivein the molten metal passage 11. Actually, the prior practice of air cooling the coil materially contributed to the rapidity of oxidization of metal in the molten metal passage because cooling air was literally being forced through the pores of the cementitious refractory material, more or less directly to the neighborhood of the molten metal passage.
According to the present invention, air is excluded from the pores of the refractory material by forcing inert gas under pressure into surface portions of the crucible that are remote from those of its surfaces which are normally in contact with molten metal, to thereby cause such inert gas to permeate the refractory material to the exclusion of air.
One means for thus forcing inert gas into the pores of the crucible material comprises a jacket 16 of steel or the like which surrounds at least the lower portion of the crucible, and which has its interior communicated, as by means of a duct 17, with a source 18 of an inert gas under above-atmospheric pressure. The jacket 16 and the exterior crucible surface surrounded thereby thus cooperate to define a plenum chamber 24 from which the inert gas escapes through the pores of the refractory material. As it permeates the refractory material the inert gas drives air out of its pores and then continues to prevent air from permeating the refractory.
Nitrogen is a very suitable inert gas for the purpose because it is readily available in pressurized containers, does not react with aluminum and similar metals and is completely harmless as it escapes into the air.
The pressure in the plenum chamber 24 can be permitted to vary rather widely so long as it is maintained high enough so that some of the inert gas is always permeating the refractory material and is not permitted to gos so high as to cause bubbling of the molten metal in the crucible.
With cooling air excluded from the base portion of the furnace, other means must be provided for cooling the coil 9, or it must be of such nature as to be capable of withstanding substantial heat. Preferably means are provided for circulating cooling liquid in heat exchange relation with the coil. Since liquid cooled coils are in themselves well known to those skilled in the art, although not heretofore used in induction melting furnaces for aluminum and similar metals, the liquid cooled coil 9 is in this case shown rather diagrammatically.
In general, the conductor 19 of which the coil is wound is tubular so that water or other liquid coolant can be circulated through it. Its end portions extend out of the base of the furnace, as at 20, and are connected with hoses 21 or the like that lead to suitable means (not shown) for cooling the liquid and circulating it through the coil. Also connected to the projecting end portions of the coil are conductors 22 by which the coil can be connected with a current source.
The tubular conductor comprising the coil has a suitable insulation coating on its exterior. The generally rectangular core 10 can be of conventional construction, comprising generally C-shaped laminations that can be inserted axially into the windings from opposite ends thereof.
Surrounding the coil is a coolant jacket 23 through which cooling liquid is also circulated. The outer wall of this coolant jacket serves as a core mold that defines the coil receiving channel in the base of the crucible when the cementitious refractory is cast, and the liquid cooling of this jacket prevents heating of the windings by radiation from the interior of the crucible. The cooling jacket has an inlet 25 and an outlet 26 that extend to the exterior of the furnace. Through suitable Ts and insulating ducts or hoses the coolant jacket inlet and outlet can be connected in a parallel cooling liquid flow circuit with the coil.
In the modified embodiment of the invention illustrated in FIGURE 3 a jacket 16 of steel or the like again surrounds at least the lower portion of the crucible 5' to define a plenum chamber which is kept filled with a pressurized inert gas that permeates the refractory material and excludes air from its pores. In this instance the laminated rectangular core 10' has two substantially elongated legs that have their medial portions more or less embedded in the base portion of the crucible and their end portions projecting outside the jacket 16. The other two legs 2.19 of the core are thus external to the jacket 16, and the windings 9 of the coil are on those external core legs. Hence the windings can be readily cooled by means of fans 27 or the like blowing across them.
It will be understood that the inner portions of the legs 110' of the core 10 in the FIGURE 3 embodiment of the invention will be subjected to substantial heating by conduction and radiation from the interior of the crucible, and will attain a temperature above the Curie temperature of most magnetic materials, the Curie temperature, of course, being that at which the material loses its ferro-magnetic properties. Hence it will 'be recognized that a material will have to be selected for the core 10' that has a very high Curie temperature.
In the embodiment of the invention illustrated in FIG- URE 4 the crucible 5 per se and the core 10 and windings 9" in its base are like those that have heretofore been conventional in induction melting furnaces in which the coil is intended for forced air cooling. In this case, however, the jacket 16' of steel or the like which surrounds at least the lower portion of the crucible is somewhat larger than in the previously'described embodiments of the invention, so that it can accommodate in its interior a coolant radiator or heat exchanger 29 and a fan or blower 30.
As in the previously described embodiments of the invention, the plenum 24 conjointly defined by the crucible and the jacket 16' isv kept filled with an inert gas, such as nitrogen, at a pressure somewhat above atmospheric. The fan or blower 30 circulates such inert gas across the core 10 and windings 9 and through the radiator 29. A suitable coolant liquid (e.g., cold water) is circulated through the radiator to abstract heat from the inert gas circulated therethrough. Thus the core and coil are directly cooled by the circulating inert gas in the jacket and are indirectly cooled by the liquid circulating through the radiator 29, to which the inert gas gives up its heat.
In the embodiments of the invention disclosed in FIG- URES 1-3 some problems with overheating of the crucible may be encountered unless the jacket 16 is arranged to radiate a substantial amount of heat, since the plenum chamber 24 tends to serve as an insulation around the crucible. This problem can be readily overcome with the embodiment of the invention illustrated in FIGURE 4, provided the heat exchanger 29 has sufficient capacity to accommodate both the resistance heating of the coil and a substantial amount of the heat imparted to the crucible by the molten metal within it.
It will be appreciated that in some cases the crucible could be surrounded by a jacket filled with inert gas, as in the embodiments hereinabove described, and the core and coil could be disposed in the base of the crucible, with no provision for their cooling. In that event the core material would have to have a very high Curie temperature and the coil would have to be capable of withstanding substantially high heat, comprising, for example, a conductor with ceramic insulation.
From the foregoing description taken with the accompying drawings it will be apparent that this invention provides improvements in an induction melting furnace for aluminum and the like by which oxidization of metal in the molten metal passage of such a furnace is prevented and the furnace is afforded a very long useful life.
What is claimed as my invention is:
1. An induction furnace for melting aluminum and similar metals, comprising a crucible of refractory material, and a coil which is in the body of the crucible and which is adapted to be energized with alternating current for inducing a current in metal in the crucible to effect heating of such metal, said induction furnace being characterized by:
(A) means defining a jacket around the exterior of the crucible, spaced from the outer surface of the crucible and cooperating therewith to define a plenum chamber; and
(B) means for maintaining pressurized inert gas in said plenum chamber to cause such gas to permeate the refractory material of the vessel and thereby exclude air from the pores thereof.
2. The induction melting furnace of claim 1, further characterized by:
means for circulating liquid coolant in heat exchange relation with the coil to cool the same.
3. The induction melting furnace of claim 1, further characterized by:
means for circulating inert gas in heat exchange relaship with the coil.
4. The induction melting furnace of claim 1, further characterized by:
(A) means for circulating inert gas within said jacket and in heat exchange relation with the coil; and
(B) heat exchanger means in the jacket for cooling inert gas being circulated therein.
5. An induction furnace for melting aluminum and the like, comprising a coil adapted to be energized with alternating current and a crucible of cementitious material having a substantially deep base in which the coil is located, said crucible also having a melting chamber above the base and a molten metal passage in the base that has opposite end portions which open upwardly to the melting chamber so that metal in the melting chamber and said passage defines a closed loop in which a current can be induced in consequence of energization of the coil, to effect heating of the metal, said furnace being characterized by:
(A) means defining a jacket embracing a substantial portion of the exterior surface of the crucible and cooperating therewith to define a plenum chamber; and
(B) means for maintaining inert gas in said plenum chamber at above atomspheric pressure to cause such gas to permeate the refractory material and thereby exclude air from its pores.
6. An induction furnace for melting aluminum and similar metals, comprising a body of refractory material that defines a melting chamber and a passage that has opposite ends opening to the melting chamber, and a coil in the body of refractory material disposed to have flux linking relationship with metal in the melting chamber and said passage, for inducing a current in such metal by which the metal is heated, said furnace being characterized by:
(A) means for substantially preventing air from permeating the refractory material and oxidizing metal in said passage, comprising (1) a jacket embracing the body of refractory material, and (2) means for maintaining pressurized inert gas in said jacket; and
(B) means for circulating cooling liquid into and out of the jacket for abstracting heat given off by the coil.
7. In an induction furnace for melting aluminum and the like, comprising a crucible of cementitious refractory material,
means for preventing air from permeating the pores of the refractory material, to thereby prevent oxidization of metal being melted in the interior of the cruible, said means compising:
(A) jacket means cooperating with a substantial portion of the exterior surface of the crucible to define a plenum chamber; and
(B) means for maintaining inert gas in said plenum chamber at above-atmospheric pressure, so that such gas is caused to permeate the pores of the refractory material and thereby exclude air therefrom.
References Cited UNITED STATES PATENTS 1,680,595 8/1928 Davis. 3,092,682 6/1963 Tama et al 13-29 2,503,621 4/1950 Lindner et al l326 X 3,039,864 6/1962 Hess et al. -68 X FOREIGN PATENTS 341,949 12/ 1959 Switzerland.
BERNARD A. GILHEANY, Primary Examiner H. B. GILSON, Assistant Examiner U.S. CL X.R. 1326; 75-68