recently the largest three-phase Héroult furnaces were the two in this country, one at South Chicago, Ill., and the other at Worcester, Mass., which are of identical design, but recently there has been erected a 25-ton three-phase Héroult furnace at Bruckhausen, Germany, and a 22-ton furnace is almost completed at the same plant.

The essential difference between the 15-ton, three-phase furnace at South Chicago, Ill.," and the single phase 2.5-ton Héroult furnace is that in place of two electrodes there are three let down through the roof so that their ends form the vertices of an equilateral triangle, each side of which is 5 feet 2 inches long, one vertex of this triangle pointing toward the back of the furnace. The center of this triangle Each electrode is connected

coincides with the center of the furnace.

to a phase of a three-phase circuit.

A steel overhead structure supports the electrodes, the weight being directly supported by chains which run back over pulleys on the framework to the drums at the back of the furnace. The electrodes are kept in alignment by vertical guides. The chains are attached to three separate solid copper holders, which are bolted directly to the bus bars. In front these holders are split and joined with a rightand-left screw, which enables the holder to be opened or closed at will. The holders can be made to carry any size of electrode up to 24 inches diameter by use of contact blocks.

The electrode may be regulated by three hand regulators about 4 feet back of the furnace, or by automatic regulators. There is an individual motor for each electrode and a complete automatic device similar to the Thury regulator.

The furnace proper has a shallow hearth, as in the single-phase furnace, but is circular instead of rectangular. The outside dimensions are approximately that of a complete circle 13.5 feet in diameter, with two flattened portions at the front and back. The furnace shell is of plate steel 1 inch thick, riveted together.

The furnace bottom is made of one row of magnesite brick laid on edge across the steel shell, over which is rammed dead-burned Spaeter magnesite to a depth of 12 inches at the center-its thinnest point. The side walls consist of two rows of magnesite bricks laid on end, giving a thickness of 18 inches up to the furnace roof. The roof is made of silica brick and is 12 inches thick. There is an 8-inch rise in the 10-foot span across the furnace.

The furnace has five doors, two on each side and one in front over the pouring spout, of cast iron lined with fire brick, 4 inches thick. They are operated by steam pressure, with the exception of the one over the pouring spout, which is operated by hand with a counterbalance.

a Osborne, G. C., The 15-ton Héroult furnace at the South Chicago works of the Illinois Steel Co.: Trans. Am. Electrochem. Soc., vol. 19, 1911, p. 205.

The foundation is of concrete and extends 5 feet above the ground. On the foundation is a stationary rack 8 feet 9 inches long, upon which the furnace proper rests on a floating pinion fastened to the shell by rivets. The arc of this floating pinion has a radius of 10 feet, which gives the furnace a tilting angle of 29°. Attached to the extreme back of the furnace is an 18-inch plunger with a 4-foot stroke working in a cylinder attached to a hydraulic line of 500 pounds pressure per square inch, which gives a lifting power of about 45 tons. The furnace rights itself by its own weight after tilting.

For operation the furnace takes 1,200 to 1,500 kw., supplied by a three-phase current at about 90 volts, and having a frequency of 25 cycles.

THE GIROD FURNACE.

The main difference between the Girod electric steel furnace and the Héroult furnace is that the former has a hearth which conducts the electric current. There are in operation 16 Girod furnaces of 2.5 to 12 tons capacity and 5 are in course of construction. This type of furnace seems to be especially satisfactory in the refining of cold scrap steel because of the slight fluctuations in power demand.

SINGLE-PHASE GIROD FURNACE.

The single-phase furnace at Ugine, France" (fig. 28), has a shallow conducting hearth of dolomite with pieces of soft steel embedded in the dolomite near the periphery, and a carbon electrode passes through the roof. In the operation of the furnace current passes through the carbon electrode and through the steel bath, which touches the tops of the steel poles embedded in the hearth. The furnace is mounted on rollers and tilted by an electric motor. This furnace has a capacity of 2.5 to 3 tons and is operated with 300 kw. The voltage is 60 to 65 volts, and the frequency of the current is 25 cycles.

The hearth of the furnace is 3 feet square at the bottom and 6 feet square at the top. The roof is 31 inches above the hearth. The water-cooled steel electrodes (fig. 29), embedded in the hearth when new, project beyond the bottom of the hearth a short distance. There are 6 steel poles set on the circumference of a 31-inch circle. The floor of the hearth is made of dolomite and tar rammed in. The walls are magnesite brick; the roof is silica brick. The roof is insulated from the walls by a thin layer of asbestos. The whole hearth is encased in a g-inch steel shell and is set on a concrete foundation that extends 6 feet above the ground level. The furnace has one

a Borchers, W., Electric smelting with the Girod furnace: Trans. Am. Inst. Min. Eng., vol. 41, 1910, p. 120.

charging door at the rear and a pouring spout in front, and resembles the open-hearth furnace even more than the Héroult furnace does.

The 2.5-ton furnaces at Ugine are operated with with carbon electrodes 14 inches in diameter and 5 feet long, some of which are threaded for continuous feeding. The means of supporting the electrode differs from the Héroult scheme in that the supporting steel structure is built over the furnace from the sides (fig. 22), making it possible to have a door at the rear. The crosspiece holds a water cooled holder attached to the electrode. There is also a water jacket around. the electrode where it passes through the roof. The crosspiece is raised or lowered by a screw at each end, the side structural work serving as a guide. The electrode may be adjusted by hand or by automatic control.

With a conductinghearth furnace there is a considerable tendency to the presence of

FIGURE 28.—Plan and elevation of 2.5 to 3 ton, single- induced currents in the steel shell which reduce

phase Girod steel furnace, Ugine, France.

the power factor. Hence the arrangement of the electrical conductors is of great importance in the Girod furnace. Three meth

ods have been used: (1) The shortest path from the motor-generator set to the carbon and steel electrode is used, so that all of the cables are on the side of the furnace that faces the motor generator. (2) The cable to the electrode is in two parallel sections, and the steel electrode is connected by the shortest path to the motor-generator set, so that each steel pole has a direct cable connection with the generator. (3) The third method is now being adopted for the latest furnaces of this type. The current is conducted to the carbon electrode in a manner similar to method 2, but whereas in methods 1 and 2 the bottom steel electrodes are insulated from the furnace body, in this method the steel electrodes are electrically connected to the furnace body; also the con

ductors are bus bars instead of cables attached to the steel shell of the furnace. These bus bars are arranged symmetrically around the furnace. This arrangement is shown in figure 22. The advantages of this arrangement are: A better agitation of the bath due to the arc circling around the periphery of the carbon electrode, greater durability of roof and lining, a saving of 10 per cent of energy consumption, the use of copper bus bars instead of cables, lack of current interruptions due to rupture of the arc, and reduction of electrode consumption.

Copper plate

Sheet iron

Cooling water

FIGURE 29.-Section of water-cooled steel electrode used in Girod furnace.

The cost of the metallic parts of a 2.5-ton Girod furnace, including electrode regulators, measuring instruments, tilting device, and conductors from the furnace to a dynamo or transformer near the furnace room, not including transforming or generating machinery and license fee, is estimated at about $3,000. The cost of a plant consisting of one 2.5-ton furnace for regular running and one furnace for reserve, with all appliances and smelter building, but without dynamo or transformer, is estimated to be approximately $40,000 to $50,000. The license fee is not included in this estimate.

THREE-PHASE GIROD FURNACE.

A section of the three-phase 10 to 25 ton Girod furnace is shown in figure 30. There are four upper carbon electrodes 14 inches in

a Mueller, A., The manufacture of steel in the Girod electric furnace: Metall. Chem. Eng., vol. 9, 1911, p. 581.

diameter, which with their connections constitute the essential difference between the single-phase and the three-phase furnace. On a three-phase circuit the Girod furnace is connected by the star con

SECTION A-A

A

FIGURE 30.-Plan and elevation of 10 to 12.5 ton, three-phase Girod steel furnace, Ugine, France.

nection. Two of the carbon electrodes are each connected to a phase. The other two are connected in parallel with the third phase, while the hearth is connected so as to form the neutral point of the system.