Construction of The furnace is built on the regenerative principle. The waste heat of the spent gases from fuel sumption being absorbed and stored in the walls and partitions of chambers placed below the furnace body and at each end of it, can be utilized for preheating the air necessary to promote complete combustion of the fuel. Without such preheating it is not possible to depend upon flame temperature alone for melt ing and preserving the desired fluidity of the charge.
Reference to Figs. 1 and 2 will outline the general form of a stationary open-hearth fur nace. The furnace body or melting chamber is lined with high-grade refractory bricks. The whole is braced and supported by girders, tie rods and plates. In an open-hearth furnace for the production of acid steel, the roof, hearth sides and end walls are constructed of silica bricks. The roof is very thin and of the purest silica bricks set with a little lime. It is arched and suspended from above so as not to weigh upon the side walls. The regeneration chambers are lined with fire bricks. The bottom of the hearth is lined with 18 to 24 inches in depth of fire brick over which is spread about half an inch of silica sand, the surface being dished or so formed that the metal when leav width of the furnace is usually between 12 and 15 feet and does not exceed 16 feet. The length and depth of hearth are not fixed, but the area of both closely follows certain val ing the furnace through the tapping hole will flow and drain toward it. When a new furnace is ready• to receive a charge the gradual pre liminary heating up is of sufficient degree to harden the sand by slight fusion or sintering so that the hearth will preserve its shape and not be broken down by rough usage or attri tion when receiving a charge of stock. In a furnace for the manufacture of basic steel, the roof walls, sides and end walls above the slag line are formed of silica bricks, but the hearth is lined in the pan with first quality fire brick followed by bricks composed of mag nesite, and at the junction of the silica bricks and the magnesite a neutral parting of chrome ore is placed to prevent fusion of the two kinds of bricks at furnace-working temperatures. The upper courses of magnesite bricks will stand about 6" to 8" above the slag line. In other particulars of brick-work the construction is the same in both classes of furnaces.
The various dimensions, areas and volumes of body, down-takes and regenerator chambers vary mainly with the capacity of the furnace in regard to tons per heat or operation. The ues per ton. The length and depth of regener ator chambers will vary also, but the volumes as a rule closely follow standard figures per ton. Generally stated the following cover the various dimensions, etc.: Hearth area. 9 sq. ft. per ton.
Chamber volume, 90 cu. ft. (I for gas, I for air, per ton.) Air port area. 16 sq. ft.
GO port area. 7; sq. ft.
Stack, diameter 4-5 ft., 100'460' height.
Producer area, 3f sq. ft. per ton.
Fuel.
Coal. 700 to 1,000 lbs. per ton.
Natural gas. 800 to 1,000 cu. ft. per ton. Oil, 60 to 100 gals. per ton.
The following figures represent hearth meas urements for representative capacities: Capacity Length x Width Hearth Area 30 tons 27 x 10 ft. 270 sq. ft.
50 tons 34x 13 ft. 438 sq. ft.
200 tons 40x 16 ft. 640 sq. ft.
The last item covers the dimensions of a Talbot furnace which has a deeper bath than other furnaces. The depth of bath varies from 12 inches for a very small furnace •(5 to 15 tons) to 24 inches for a 50-ton furnace. The Talbol bath is over 36 inches deep.
Fuel and Accessories.— The choice of fuel varies with the location of plants. In and around Pittsburgh natural gas is extensively used and is really the ideal fuel. It is not passed through the regenerator chambers but is fed directly into the furnace port (Fig. 3). It is uniform in composition, possesses the high est calorific value and does not contaminate the bath. Next in heating value is crude petroleum or residuum, a by-product in the distillation of petroleum. Both have advantages in uniformity of composition and .ease of control. Oil is pumped, under pressure, from storage tanks delivered to a suitable oil-burning device where it is atomized by compressed air or steam be fore ignition in the furnace (Figs. 4 and 5) The next preferable source of fuel is the gas producer (Fig. 6). In this apparatus bituminous coal is gasified and the gaseous matter conveyed through mains controlled by regulating valves (Fig. 7) before entering the regenerator cham bers. The principle of operation is to force • through a mass of incandescent carbon air and steam. On top of the glowing coals fresh fuel is regularly fed to keep the bed at a constant level. The bed must be frequently poked to process and maintain it to the end of the op eration. Low-carbon stock, such as steel or wrought-iron scrap, cannot be used singly be cause if exposed to flame action it would not fuse, but would be pasty and viscous, causing loss. The charge may be all pig-iron or a mixture of 20 per cent pig-iron and 80 per cent prevent holes forming, allowing air and steam to pass through un-decomposed. The bottom of the producer rests in a tank of water to pre vent too much air from entering, since the air necessary to form a good working gas must be under control by compelling it to pass through the blower. The gas is apt to vary in composi tion and is not always constant in volutfie. A ton of coal treated in a producer will give about 160,000 cubic feet of gas. The average com position is as follows: Carbon Monoxide 27 per cent by volume Carbon Di-Oxide 5 per cent by volume Hydroen 10 per cent by volume etc., by diff 58 per cent by volume Furnace Operation — Acid Practice.— In order to produce steel by any open-hearth proc ess it is necessary that a certain amount of carbon be introduced with the metal to be con verted. For that reason ordinary pig-iron which will contain 2.5 to 3.5 per cent, of total carbon is required. The presence of carbon lowers the fusing point of iron and when charged in an open-hearth furnace it is comparatively easy to liquify the bath at an early stage of the scrap. The usual American practice is 50 per cent of each.