Page:The American Cyclopædia (1879) Volume IX.djvu/408

 394 IRON MANUFACTURE ing the sheet-iron casing. The essential acces- sories of the blast furnace are the blowing en- gines, hot-blast ovens, and hoist. The blowing engines are of three kinds, the vertical beam, the horizontal, and the upright engine. The latter has been generally introduced of late years, owing to its compactness and efficiency. One of the largest blowing engines ever erected is a beam engine at Dowlais in Wales. The blowing cylinder is 12 ft. diameter with 12 ft. stroke. With 19 strokes per minute it dis- charges 51,528 cub. ft. per minute at a pressure of 3 Ibs. to the square inch, capable of supply- ing six large furnaces and four fineries. From the blowing cylinders the air passes to the hot- blast ovens. These consist of a series of cast- iron pipes, arranged in a fire-brick chamber, and FIB. 4. heated by the combustion of the gases drawn from the top of the furnace. The gases are generally burnt in a special combustion cham- ber, and the products of combustion only pass into the chamber containing the pipes. Equa- ble heating without danger of injury to the pipes is thus effected. This arrangement is shown in fig. 4. Recently Siemens's system of regenerative heating (see FUKSTACE) has been applied to hot-blast stoves, and a much higher temperature of blast attained than could be produced by the simple combustion of the gas. Whitwell's and Cowper's stoves are both con- structed on this system. The former are being extensively adopted. They consist of two " re- generators" of fire brick, which are heated alternately by the combustion of the furnace gases. While one is heating, the blast passes through the other, and the currents of air and gas are changed at intervals of about half an hour. The temperature of the blast ordinarily employed varies greatly. A few furnaces are still blown with cold blast, where it is desired to produce an iron of superior quality; but usually the blast is heated from 500 to 1000 F. The hotter the blast, the sooner the pipes in the ovens burn out, and therefore the tem- perature rarely reaches 1000 with iron pipes. In Whitwell's stoves a temperature of 1550 may be obtained; but it does not generally rise above 1200 or 1400. The pressure of blast varies from 1 Ib. per inch to 5 or 6 Ibs. Charcoal furnaces are usually blown with the lowest pressure, anthracite furnaces with the highest, and coke furnaces with a pressure of 3 or 4 Ibs. Oc- casionally furnaces are situated on a hillside, and the stock of ore and fuel is on a level with the mouth of the furnace ; but ordinarily lifts or hoists are re- quired to raise the materials of the charge from the ground to the top of the furnace. There is a great variety of lifts, em- bracing among others the hy- draulic, the pneumatic, and the steam lift. The blast-furnace process, expressed in its sim- plest form, is as follows : The furnace is charged with ore, fuel, and limestone, which gradually descend the shaft as the smelting proceeds. The air of the blast, on coming in contact with the incandescent fuel, is converted into carbonic acid gas, but, speedily taking up another atom of carbon, is re- duced to carbonic oxide, which, together with the inert nitro- gen of the air, rises through the descending charge, abstracts the oxygen of the ore, and pass- es out of the mouth as carbonic acid. When the reduced iron reaches the vicinity of the tuyeres, it takes up carbon, melts, and drops down into the crucible of the furnace, while the earthy ingredients of the ore, flux, and fuel unite and form a fluid cinder, which likewise drops into the cru- cible and floats on the top of the molten iron. At regular intervals the iron is tapped off into moulds of sand or iron, where it cools in "pigs." The foregoing expresses merely the general progress and final results of the blast-furnace process. In practice it has been found that the changes which take place are very complicated and involved, and depend on a great number of conditions. The phenomena of iron smelting have recently been the sub- ject of searching investigation ; and we are mainly indebted for our present knowledge