US1660209A - Electric furnace - Google Patents

Description

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AIIYALWYAJ AIV IJTVALTY TVIV Feb. 21, 1928.

Feb. 21, 192s.

E. C. SASNETT ELECTRIC FURNACE Original Filed Sept. 24, 1919 5 Sheets-Sheet 2 Feb. 21, 1928.

E. c. SASNETT ELECTRIC FURNAE Original Filed Sept. 24, 1919 5 Sheets-Sheet 5 Patented Feb. 21, 1928.

UNITED STATES 1,660,209| PATENT OFFICE.

EIOWARD C. SASNETT, F WSHINGTON, DISTRICT OF COLUMBIA, ASSIGNOR, .'BY MESNE ASSIGNMENTS, T0 CEAS. B. FOLEY, INCL, A. CORPORATION OF NEW YORK.

ELECTRC FURNACE.

Application filed September 24, 1919, Serial No. 325,838. Renewed July 7, 1927.

My invent-ion relates-to electric inductionfurnaces. Specifically, it relates to that type of crucible induction furnace which employs a loop of molten metal connected with the pool of metal held by the crucible well below the surface thereof, the heat being generated by electric currents induced in the loop by aprimary winding and core linked therewith. .Y

The objects of my invention are: to produce unidirectional and thorough circulation of the metal constituting the secondary loop; to provide a form of secondary loop which can withstand a relatively high current density without being disrupted by the so-called pinch effect; and to improve the general efficiency of the above-mentioned type of furnace. y

Referring to the drawings:

Figs. 1 to 4, inclusive, are diagrams illustrating the theory upon which my inventionis based;

Fig. 5 is a central vertical section of a crucible furnace embodying my invention;

Fig. 6 is a horizontal section on the line 6-6 of Fig. 5;

Fig. 7 is a central vertical section of another form of crucible induction furnace embodying my invention;

Fig. 8 is a plan view of the furnace shown in Fig. 7.;

Figs. 9 and 1 0 are a vertical section and plan view, respectively, of a further modification; and i Figs. 11 and l2 are a vertical section and plan of another modification.

When an electric current flows through a fluid conductor, the magnetic field of the current exerts radial pressure on the conductor tending to squeeze it into a smaller compass. This pressure is concentrated at the central axis of the conductor; and, under certain conditions, produces an axial flow of the metal constituting the conductor. Fig. 1 is a cross-section of a current-bearing conductor, showing the circular field of the current, the axially-directed arrows indicating' the radial forces created by the current field.

Fig. 2 illustrates a 'longitudinal section of edportion connected at its ends with portions having a much larger'cross-sectionala conductor consisting of a cent-ral constrictnitude of the forces created by the current field. The forces vary directly with the curi rent density in the conductor, and are, therefore, considerably more intense in the constricted part, Where the current density is'f .great as compared with the current density in the enlarged parts. Because of the fiuid character of the conductor, the radial forces are transmitted along the longitudinal axis thereof. Since these axially-directed forces are greater in the constricted part of the conductor, there will be an axial fiow from the points of greater to the points of less pressure; that is, the Huid will move axially from the constricted towards the 'enlarged 70 parts, there being a returnflow from the enlarged partsalong the periphery of the constri'cted part. This circulation is representd inga general way by the arrow heads in The activity of the circulation'will, of course, vary with the strength of the forcesthat is, with the current density. When the fluid is squirted out of the restricted part .l with such rapidity that there is not sufii- 30 cient time ,to replace it by a return flow of fluid from'the enlarged portions, the conductor will contract, as Shown in Fig. 3, resulting in an increase in lthe current density in the contracted portion and a corresponding increase in the rate of outflow of meta-l therefrom. This process will continue until the conductor is disrupted. The factors determining the disruption of the conductor are: the current density in the conduct-or, the o viscosity of the material of which the conductor is formed, the frictional resistance offered by the walls confining the conductor, and the hydrostatic head of metal which the disrupting forces have to overcome.

In crucible induction furnaces employing a loop of metal connected with the bottom of the pool held by the crucible, the molten metal constituting the loop has, ordinarily,

a rather high degree of viscosity, and the lo@ channel walls confining the loop offer considerable friction to the peripheral flow of metal. It has been found, therefore, in the olgierationof these furnaces that the loop of metal is disrupted at certain critical values of'current density below the value required to furnish that rate of heataddition necessary., to employ if the furnace is to be operated at vits highest efficiency. This is true even Where the disruptive forces are` op- 310 maximum cross-section respectively.

tion of the metal forming the loop. To this end, I use a loop for the secondary current `such as shown in Fig. 4;. This figure represents diagrammatically a development of the loop, the enlarged end portions of the iigure representing the bottom of the pool. As shown, the loop consists of a tapering conductor having its ends of minimum alillld ic axial forces created by the current ield in this form .of conductor decrease uniformly from a maximum value at the constricted end to a minimum value in the end of max imum width, in/accord with the decrease 1n the current density. The direction of the circulation of the metal flowing axially in both directions from the point of maximum pressure is represented by thev full lines and arrow heads. As shown, the metal flows along the loop centrally thereof from the point of' maximum pressure-that is, the point where the constricted part of the loop joinsthe pool. It is obvious that the flow of metal along the loop will be fed by a flow from the pool through the restricted mouth of the loop, for this is the'path of least resistance. There will be no substantial peripheral flow from the other end of the loop along the path indicated by the dotted lines, this path offering great resistance-as compared with the path through the restricted part of the loop.

The forces created by the current field cannot disrupt the conductor at the point of maximum"pressure-that is, at the restricted mouth of the loop, for here the loop directly joins the pool. The loop, of course, could contract at some intermediate point if the axial forces are of sufiicient' magnitude to. squirt the metal out from that point faster than metal from the pool can flow in throughv the contracted mouth of the loop. But since the flow is unidirectional, the metal flowing directly from the pool to feed the mid-current in the loop, a very excessive current density would be required to cause contraction. There is no point in the loop where the metal is squirted out axially in both directions to be, replaced by a peripheral flow along the Walls of the channel conlining the loop,.as is the case in forms of loops heretofore used. I

In Figs. 5 and 6. I have illustrated a crucible furnace utilizing the form of secondary loop described above. Here the loop 1s curved on the arc of a circle concentric with the primary coil 10. The loop, 'as shown, has an yelongated rectangular crosssection with the major axis thereof parallel to the axis of the primary coil. The loop has a uniform depth and a width tapering from a minimum at the point a to a maximum at the point b. The arrows indlcate the circulation of the molten metal.

In the operation of this furnace, the oppositely-flowing primary and secondary currents `create mutually repulsive forces which act repulsively on the primary and secondary coils; These forces havev components radiating from the axis of the primary coil and components parallel to said axis. The lines of action of the latter components are inclined at acute angles to the side walls of the channel owing to the tapering form 0f the latter, and, therefore, have components tending to move the metal along said side walls in a clockwise direction, assisting the axial flow caused by the pressure exerted by the field of the secondary current.

Figs. 7 and 8 illustrate a crucible furnace having a pair of transverse openings 1l and 12 through which pass the legs of a laminated core 15,-theprimary windings 13 and 14 being located in the openings. The crucible has interior walls forming a pair of circular channels partly surrounding the primary coils. 'The channels have corresponding ends al of minimum cross-section opening into the-bottom of the crucible at the sides thereof. The other ends have a maximum cross-section and converge upwardly, opening at b into aspassageway connected with the bottom of the pool. In the operaf tion of this furnace, the currents induced in the metal held by the channels How in relatively opposite directions, and, hence, flow upwardly through the converging portions in the same direction. The motor action between the currents flowing upwardly through the converging portions, therefore, tends to move the metal upwardly through said portions. The currents flowing in the same direction attract each other, and the lines of action have components directed upwardly in said converging portions. This motor effect in conjunction with the motor effect between the primary and secondary currents previously'discussed and the pinch effect causes the metal to iiow in through the contracted mouths a of the channels and to be ejected centrally into the bottom of the pool.

Figs. 9 and l0 illustrate a furnace adapted'y to withdraw metal from the.. bottom of the pool centrally thereof and to eject relatively hot metal into the bottom ofthe pool at the sides. Here the inner portions of the channels converge upwardly and open into a channel connected with the bottom of the pool, `as in Figs. 7 and 8. The channels have their minimum cross-section, however, at the ends of the converging portions. The other ends of the channels have flaring'conneotions with the bottom Sides of the pool. The furnace is adapted to beoperated with regular Ilo two-phase currents or with currents of the same phase. When operated with currents having the regular ninety degree phase displacement, there will be no motor action between the currents flowing in the converging portions, for the reason that half the time the currents attract and the other half repel each other, whereby their 'motor action is neutralized. On the other hand, where the currents have the same phase and iiow in the same relativedirection, the currents in the converging portions flow in opposite directions and repel each other at all times, the lines of action having components directed downwardly in the converging portions. In either case the repellent motor action between the primary and secondary currents drives the metal upwardly through the flaring mouths d.

Figs. 1l and 12 illustrate a furnace also adapted for operation by two-phase currents or by currents having the same phase. This furnace is like the furnace shown in Figs. 9 and 10, except that the lower portions of the channels are in alinement. It is to be noted that when thisfurnace is operated by currents of the same phase-flowing in the same direction, there will be no current flow in the metal contained in the vertically-extending channel opening centrally into the bottom of the pool. Relatively cold metal flows down through this vertical channel to replacethe metal flowing out from the horizontal contracted portion of the secondary channel.

My invention is not limited to the exact form of loop shown, but contemplates broad-- ly the'use of a loophaving a m1nimum crosssection adjacent one of its connections with the pool.

Having fully described my invention, I claim:

l. An electric induction furnace comprising a struct-ure lined with refractory material adapted to hold a body of molten metal in the shape of a superposed pool with a j subjoined loop, the cross sectional area of which loop is a minimum at one pointat which the loop joins the pool and increases gradually and continuously to a maximum at the other point at which said loop joins the pool, said furnace having a transverse opening through it within the loop, and electrical induction means threaded through said opening.

2. An induction furnace, comprising a body of refractory material adapted to hold a pool of molten' metal and having interio1 walls adapted to form a part of themetal into a circular loop joined with the pool and having its minimum cross-sectional area adjacent one of its points of connection with the pool, said furnace having a transverse opening therethrough Within the loop, and induction means threaded through sai opening.

3. An induction furnace, comprising a body of refractory material adapted to hold a pool of molten metal and having interior -walls adapted to form a part of the metal verse opening therethrough within the loop,

and .induction means threaded through said opening.

5. The process of stirring a pool of molten metal held in an electric induction furnace whichconsists in forming a loop of said metal depending from separate points lin the bottom of said pool and applying pinch effect in gradually andA continuously decreasing intensity from one extreme end to the other extreme end of said loop.

6. An electric induction furnace, comprising a body of refractory material adapted to hold a pool of molten metal and having interior walls adapted to form a part of the metal into a pair of loops having corre- 1 spending ends connected at opposite sides of the pools bottom, the other ends of the loops connected with the bottom of the pool centrally thereof through a common connection, said loops having a cross-section increasing uniformly from a minimum at their first-mentioned ends, said furnace having a pair of openings extending therethrough within the embrace of the loops, and induction means in said openings for inducing oppositely-lowing currents in the loops.

7 An electric induction furnace, comprising a body of refractory material adapted to hold a pool of molten metal and Vhaving interior walls forming a pair of curved channels opening into the bottom of the pool, said channels having corresponding ends opening into thebottom of the pool at 0plofi posite sides thereof and the other two ends ing a body of refractory material adapted to hold a pool of molten metal and havlng interior Walls forming a pair of curved chan. nels opening into the bottom of the pool and located substantially in the same vertical plane, twocorresponding ends of the channels opening into the 'bottom of the poolat opposite sides thereof, and the other two ends of the channels converging upwardly and opening into a channel connected With the bottom of the pool centrally thereof, the

cross-sectional area of the channels decreasf' surrounded by said channels, and induction means threaded through said openinvs.

- EDWARD c. sAsNrrr.

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