US1272077A - Electric furnace. - Google Patents

Description

W. E. MOORE.

ELECTRIC FURNACE.

APPLICATION FILED SEPT. 11. 1912.

Patented July 9, 1918.

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WILLIAM E. MOORE, OI! PITTSBURGH, PENNSYLVANIA,

ELECTRIC FURNACE.

Specification of Letters Patent.

Patented July 9, 1918.

Original application filed September 7, 1916, Serial N 0. 118,890. Divided and this application filed September 11, 1917. Serial No. 1.903%,

To all whom it may concern:

Be it known that I, WILLIAM E. Moore, a citizen of the United States, residing at Pittsburgh, in the county of Allegheny and State of Pennsylvania, have invented certain new and useful Improvements in Electric Furnaces, of which the following is a specification.

This invention relates more especially to improvements in are type electric steel melting and refining furnaces.

In this class of furnace it has heretofore been customary to melt down the cold scrap with the same voltage which is used for re fining, and on the refining operation to reduce the current, that is, the power in the furnace to keep the temperature within the limits which the refractory linings and roof of the furnace will stand without too much deterioration.

I find that when melting steel, it is possible to run with much higher energy input and longer arcs without damage to the refractories so long as there is unmelted steel in the furnace which keeps the temperature constant, but when refining it is best to materially reduce the power and shorten the arcs to keep within safe temperature limits onthe refractories. It is, however, desirable to run at much higher power for melting than for refining in order to reduce the heat losses from radiation per ton melted and obtain a greater output per day from a given size furnace.

I have also found it extremely important when refining to run with a very low voltage and short are, say 50 volts, to prevent radiation at are temperature from injuring the furnace walls, whereas, when melting a much longer are is permissible and also desirable to prevent the fragments of scrap caving in to the cavities moltened by the arcs and contacting with the electrodes thus causing short circuits; and also to get increased power into the furnace without in creasing the already enormously large currents, or requiring conductors and electrodes of unwieldy size in cross section.

I have found too, that in an are playing from a carbon or graphite electrode to scrap metal or to a slag containing lime, silica, etc., that there is a rectifying efi'ect on the alternating currents tending to permit heavier current to flow in one direction for one-half wavelength than in the other, and

that if this rectified current be caused to flow down through the metal bath and furnace bottom, it tends to agitate the metal and prevent hot spots or a cold bottom by circulating the bath and brings it into more rapid circulation with the overlying highly heated slags which are depended upon in a large measure to refine steel.

The primary object of this invention may be stated generally to expedite or shorten the time involved in melting and refining steel, allowing the use of smaller conductors and electrodes and more thorough heating and mixing the bath, in this type of furnace, and this I accomplish first by providing means for obtaining a large variation of the power input in the furnace by varyng the arc voltage without materially varymg the current through the arcs, thus permitting rapid melting with electrodes and secondary leads of minimum size. Second, by effecting an electrical connection between the conducting bottom of the furnace and a neutral point in the secondary windings of the transformers so as to permit a heavy fiow of rectified or direct current for better heating and mixing the bottom of the bath, and third, by including in the secondary circuit of the transformer a reversing switch by which still greater current flow may be effected through the bath in refining.

This application is a division of my application Serial Number 118,890, filed September 7, 1916.

In order to more particularly describe my invention, reference will be had to the accompanying drawings in which;

Figure 1, is a diagrammatic view illus trating one form of my invention;

Fig. 2, a diagram showing the star or Y connection of the transformer primaries when refining, and

Fig. 8, a diagram showing delta connections of the transformer primaries when melting.

In the accompanying drawings, the furnace proper illustrated is of the three phase are type in which 1, 2 and 3 represent the three arcing electrodes. These electrodes extend through the furnace top 4: and may be supported in the usual or any desired way. They are usually mounted upon mechanism for adjusting them vertically. The arched top 4 of the furnace is supported upon side walls 5, composed of so-called acid brick down to a point several inches above the slag level, and preferably of so-called basic brick below this point. The bottom lining of the furnace is formed preferably in two or more layers 6 and 7, the former composed ofmagnesite, dolomite, or other refractory material, and the latter of a mixture of refractory material and such better conducting material as carbon, graphite or metal turnings or similar conducting material progressively increasing in conductivity toward the bottom and rammed or sintered'in. I do not, however, limit my invention to this particular way of making the furnace bottom conducting. The sides and bottom of the furnace are inclosed in a metal shell 8. In this drawing, 9 represents the metal bath, and 10 the slag.

Current is supplied to the furnace from three transformers 11, 12 and 13, in which the secondaries connected to the electrodes are Y connected. Tapped on to the neutral point in this Y connection is a conductor 1 l- V electrically connected to the furnace shell 8.

. Connected in the secondary circuit of transformer 12 is a reverslng switch 15, the

function of which will later be described.

The primaries of the three transformers are connected to the power mains 16, 17 and 18 through two switches 19 and 20, the former of which when closed connects the primaries in Y or star formation, and the latter when closed connects them in delta formation. These two switches may be inter-connected by a rod 21 or other suitable means whereby when one is closed, the other is always open.

When melting, switch 20 is closed and switch 19 open. This connects the primaries of the transformers in delta formation. In refining, switch 19 is closed, and switch 20 open, thus connecting the primaries in a or star formation. By means of this arrangement a large variation of the power input to the furnace may be secured by varying the arc voltage without materially varying the current through the arcs, thus permitting more rapid melting with given size electrodes and secondary leads.

By connecting the neutral point of the'sec ondaries of the transformers tothe metal shell of the furnace 8, a direct current flow, due to the rectifying elfect hereinbefore re ferred to, is effected from the electrodes through the metal bath, which causes a bottom heating and agitation of the bath, as

above pointed out.

When the upper layers of the lining of the bottom of this furnace become highly heated, they become electrically conducting and therefore form a path for current from the metal bath to the top layer which is rendered electrically conducting by the heat of the furnace, thence to the lower layers of the bottom which are made conducting at lower temperatures by reason of their being intermixed with conducting materials such as carbon, graphite or other metal in the manner above described, thence to metal shell 8.

If switch 15 be thrown in such direction as to reverse the normal flow of current in secondary of one transformer 12, a portion of the alternating as well as the rectified current is caused to flow through the bottom in the manner stated, and this further expedites the refining process by heating and mixing the bottom of the bath.

What I claim is 1. An electric furnace having three arcing electrodes, and a refractory bot-tom rendered electrically conducting, an electrically conducting shell, transformers for supplying electric power to said electrodes and having secondaries connected to said electrodes in Y formation, an electrical connection from a neutral point in said secondary connection to the conducting shell of said furnace, and switching means for connecting the primaries of said transformer in delta formation or in Y formation at will.

2. An electric furnace having electrodes and a bottom which becomes electrically conducting when heated, an electrically con-- and a refractory bottom which becomes electrically conducting when heated, an electrically conductingshell, adapted to form with said bottom a part of an electric circuit, transformers connected to said electrodes,an electrical connection from a neutral point in said secondary connectionto the furnace shell, in combination with transformer switches connected for varying the voltages supplied to said furnace by changing the connections from delta to star.

4. An electric furnace having electrodes, and a refractory bottom of a progressively increasing electrically conductive mixture, transformer secondaries connected to said electrodes in low voltage formation, and an electrical connection from a neutral point in said secondary connection tosaid conducting bottom. 7

5. An electric furnace having arcing electrodes, and provided with a bottom composed of a mixture of refractory material increasing in electrical conductivity toward the lower face thereof, transformers for supplying power to said electrodes and having a neutral point connected to said furnace bottom, and switching means in the primary circuit of said transformers for varying the voltage of the electrode arcs at will.

6. .An electric furnace having electrodes and a stratified bottom the lower stratum of which is normally electrically conductive and the upper stratum thermo-electrically conductive, an electrically conductive shell adapted to form with said bottom a part of an electric circuit, transformer secondaries connected to said electrodes, an electrical connection from an equipotential point in said secondary connection around the electrodes to the furnace shell, and switching means for changing the transformer connection from high voltage formation to low voltage formation and vice versa.

7. In the manufacture of steel and other metals in an electric furnace, the method which consists in causing two or more electric currents of different phase to pass from electrodes to a molten bath, and varying the amount of current passing through said bath by unbalancing the resistance of the arcing circuits.

S. In the manufacture of steel and other metals in an arc type electric furnace, the method which consists in causing two or more electric arcs of different phase to pass from electrodes to the molten bath, and varying the amount of current passing through said bath by unbalancing the voltage of said arcs.

9. In the treatment of steel and other meta-ls in an electric furnace having three electrodes supplied with \alternating current power from a three-phase circuit, the method which consists in passing current rectified from said alternating current, through a molten bath within the furnace and to a neutral point in the three-phase circuit.

10. In the treatment of steel and other metals in an electric furnace having electrodes supplied with power from a 1nultiphase circuit, the method which consists in producing a circulation of the molten bath by a rectified current from the current of separate phases.

In testimony whereof I affix my signature.

WILLIAM E. MOORE.

Copies of this patent may be obtained for five cents each, by addressing the Commissioner of Eatents,

Washington, D. 0.

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