PRINCIPLE AND OPERATIOI OF THE BLAST-FURNACE.
Figure 180 represents the interior of the blast furnace. The form is that which is general where anthracite coal is used as a fuel. The height and diameter of these anthracite furnaces vary con- • siderably. The most available dimensions are twenty feet diameter above the "boshes," by fifty to sixty feet in height. The dimensions and form of the hearth are matters on which but few "founders" agree. There can be no doubt, however, in regard to the two most important considerations; first, the hearth should never be too wide to admit the passage of the blast to its centre ; and second, it should always be deep enough to pre vent the exposure of the fluid metal to the action of the blast. In a furnace of twenty feet bosh, the hearth should not be less than five nor more than seven feet deep.
The form of the hearth we think of more importance than is generally accredited to this item in the construction of furnaces.
Figure 181 represents the general form of hearth, which answers the best purpose in prac tical operation. It is not important, however, that it should be precisely like the diagram, since it may be parallel, elliptic, or oblong, with the corners rounded in all cases. In a furnace of twenty feet bosh, this hearth should be from 2 to 21 feet wide, and from 10 to 12 feet long. The bottom of the boshes should conform, of course, to the general form of the hearth.
The advantages of this style of hearth are several: the blast penetrates beyond the centre, and may be made to reach nearly to the opposite side ; the tuyers enter from opposite sides the entire length of the hearth, and the blast is evenly distributed; there is less danger from " scaffolding," and the burden comes down more regularly.
The height of the boshes will depend on their slope. An inclination or angle of 75° to 80° may be used in anthracite furnaces; but in coke, bituminous coal, or charcoal furnaces, the angle ranges from 45° to 75°. The walls or body of the furnace reach from the top of the boshes to the bottom of the flues. The height of this portion depends on
the depth of the hearth, the slope of the boshes, and the position of the flues. In furnaces of this size, about 20 feet is the common elevation of the body from the boshes to the flues, or from one-third to one-half the height of anthracite stacks generally. In all old furnaces the walls inclined from the boshes to the throat, or the body had the form of a bottle, the neck answering to the form of the throat, with a gradual decrease in diameter from about the middle to the top. But in all well-built and successful anthra cite, and perhaps we might include all kinds of furnaces, the body is of nearly equal diameter from the boshes to the flues, or with but a slight decrease towards the top.
The flues are distributed around the top of the furnace, between the body and the mouth. The whole space may be denominated the "throat," though that term is gene rally applied to the month or head. The height of this portion is from 8 to 10 feet; and in this distance the diameter decreases from 18 or 19 feet to 6 feet. This part of modern furnaces has an important part to perform in the operations of smelting. The increase in diameter from the throat or head to the flues admits of the expansion of the materials or charges, which are thrown in cold, and their enlargement between these points is considerable; while the decrease in diameter from the body to the throat acts as a check to the ascending gases, and diverts them through the flues to the steam-boilers and hot air chamber,—aided, however, by the draft through the hot-blast chimney.
The principal gas which escapes from blast-furnaces is carbonic oxide, which is com posed of one equivalent of carbon and one of oxygen. This gas, when combined with a certain amount of air, burns with great heat, and furnishes a sufficiency of caloric to generate steam for the operation of the blowing-machinery, and to heat the air thrown into the furnace from 600° to 1500° of temperature, according to the mechanical arrange ment of the hot-blast oven.