In blast furnace smelting of copper and lead, the gases passing from the top of the furnace do not contain much gas which will burn, hence they are allowed to escape, but in the iron smelting furnace not enough oxygen reaches the upper part of the furnace to unite with all the incandescent carbon of the fuel. As a result of this, much Co2 is reduced to CO, which passes from the furnace unburned. To let this gas escape into the atmosphere would be a considerable loss of fuel, hence it is conveyed through the "down-comer" and dust catcher to a large stove called the blast or regenerative stove, where it is burned to heat the 4 brickwork of the stove. After the brickwork has become very hot, the gas is shut off from this stove, and air is blown through to absorb the heat on its way to the blast furnace through the tuyeres.
Fig. 8. - Hot Blast Stove (Vertical Section).
In this way the blast is supplied with air at about 1400° F., and the intensity of the furnace heat is much greater in the fusion zone than if cold air were blown in.
Fig. 8 shows a form of blast stove used for this purpose, known as the Calder Stove. Several other forms of stoves are used, all operated on the same principle and differing only in interior arrangement. Fig. 7 shows a group of five stoves which are connected to their common chimney P by an underground smoke conduit. The stoves are made of an outside casing of steel plates and an interior lining of refractory brick. Brick partitions are so placed as to check the passage of air and gases.
Fig. 8a. - Hot Blast Stove (Horizontal Sections).
The stove in Fig. 8 is operated as follows: Valves connecting the interior space of the stove with the hot and cold blast mains having been closed, the chimney damper is opened and gas from the dust catcher is admitted to the stove through the gas inlet marked in section B-B, of Fig. 8a. This gas is hot enough to burn just as soon as air is admitted through the various air inlets, which are regulated by dampers. Combustion takes place in the two large vertical spaces in the right half of the stove, the burned gases are drawn downward by the chimney draft through the small vertical spaces into which the left hand part of the stove is divided, and, following the small arrows, enter the circular conduit through openings at the bottom of the stove. In passing downward, the gases give up much heat to the brick partitions, and finally they escape through the chimney which surmounts the stove. After about an hour of this operation, the brickwork has reached its maximum heat. The gas from the dust catcher is then shut off, the chimney damper is closed, the valves connecting the stove to the cold and hot blast mains are opened, and atmospheric air is forced by blowers through the stove in a direction opposite to that taken by the gases which were burned to heat the brickwork. This part of the operation continues for a half hour or more, during which the air is delivered very hot to the hot blast main. The operation is again reversed as soon as the brickwork begins to become appreciably cooled.
At least two stoves, and usually four, are installed for each blast furnace, so that at least one stove may be constantly in use for each part of the reverse operation just described.