Fig. 6 shows the essential parts of a blast furnace for smelting iron. It is given the name of blast furnace because combustion is maintained by forcing a blast of air through the mass of fuel, ore and flux which completely fills the furnace. This type of furnace is vertical so that gravity may assist in disposing of the fused products. The main point of difference from the reverberatory furnace is that ore and fuel are mixed in the blast furnace and are kept separate in the reverberatory furnace.
Figs. 6a and 6b are two views of the outside of a blast furnace. They are lettered to agree with the description given for Fig. 6.
Fig. 6. - Blast Furnace.
The parts of this furnace, shown in Fig. 6, are designated as follows, viz.:
S. Shaft. This extends from the top down to the part of largest diameter.
B. Boshes. This is the tapered portion below the shaft. Built in with the brick work of the boshes are many hollow wedge-shaped segments of copper. These segments are placed with alternate segments of bricks to encircle the boshes, and many layers of this construction make up the walls of the boshes. These copper segments are known as "bosh plates." Some of them are marked P in Fig. 6a. Water is circulated through each bosh plate, by a system of external piping, to allay the intense heat at that part of the furnace and thus prevent injury to the bosh walls. The bosh plate edges do not extend in quite so far as does the brick work, and are protected by a covering of clay. If a plate becomes clogged by sediment from the circulating water, or if its inner edge is melted or damaged, it can be pulled out and replaced by a new plate by reducing the blast pressure while this work is in progress.
Y. Tuyere holes. These openings conduct into the furnace the air of the blast. Fig.' 6 has eight tuyere holes. The tuyeres are metal tubes leading from the blast main into the furnace through the tuyere holes. Tuyeres are not shown in Fig. 6, and the horizontal portion (or tuyere proper) is disconnected and removed in Fig. 6a. Each tuyere is surrounded by a helix of pipe through which water circulates to keep the end of the tuyere from melting in the furnace. Fig. 6a shows at E an opening, covered by a mica door when the furnace is in use, which enables the furnace man to see the interior of the furnace through the tuyere when in blast.
M. Hot blast main. This is a pipe 3 ft. or more in diameter, made of iron plates and lined inside with refractory brick. It encircles the furnace, delivering highly heated air from the blast stove to the tuyeres.
H. Hearth. This is the part of the furnace below the line of tuyere holes which receives molten slag and metal, and when the slag, which floats on the metal, reaches the height of the cinder notch C, it is drawn off.
T. Tapping hole. Metal is tapped from the hearth through this hole. In tapping, an iron bar is used to dig out the clay plug stopping the hole, and when the metal has run out, another plug is forced in to stop the hole, by means of the ram U (known as the "mud gun") swung from the small crane W.
G. Gas main or "down comer," a brick-lined pipe leading away the gaseous products of the blast and delivering them, through the dust catcher, to the gas main, Q, which distributes them to the blast stoves.
J. Shaft lining. This lining is of highest quality refractory brick (fire clay) laid in a mortar of fire clay. The lining can be renewed when worn out. The double hatched lining below the tuyere holes in Fig. 6 is not subject to excessive wear.
D. Silica brick body, in Fig. 6. The furnace body is encased in plate steel marked D in Fig. 6b.
K. Cone. This is a cone-shaped hollow cast iron ring built into the top or "throat" of the furnace.
L. Bell, a cast iron cone suspended by a heavy chain from the lever N. This cone closes the throat of the furnace and is opened for admitting materials of the furnace charge.
R. Stand pipe. Before lowering the bell to admit a new charge to the furnace, the lid of the stand pipe is raised to relieve the gas pressure at the top of the furnace. This obviates the escape of gas and flame from the furnace throat.