Mechanical draft has never been applied to hot-blast stoves so far as I know, their draft always being supplied by a stack.

The three-pass stoves each have their individual stack resting on top of them. In the case of the two-pass stoves the choice of two styles of construction is possible. One is to have a single stack for a battery of three or more stoves, the entire complement of one or two furnaces. The other alternative is to have a single stack for each stove directly connected with it.

The former practice is very much the more general, and in my judgment much to be preferred, since one large stack is undoubtedly cheaper than three or more small ones of the same total capacity. Moreover, if it be desired to cool off one stove for repairs and it be connected only with its own individual stack, no air can be drawn through it after it has cooled off a little, because there is no difference in temperature to produce a draft, although the stove may be far too hot to work in or even to enter. But if all the stoves are connected to a common stack then the chimney valve may be opened to some extent on the stove which it is desired to cool off without seriously impairing the draft of the other stoves and the cold air thus drawn through it will cool it down to a temperature at which it can be entered and repaired, in a small fraction of the time in which it would cool to the same extent without this assistance. .

Fig. 161. Cylindrical stove top.

The Domes Of Stoves

In the early days many stoves were fitted with a cylindrical top as shown by Fig. 161 and this shape conforms with the mechanical requirement that all sections of the stove shall be circular in order to resist deformation, but this style of top is difficult and expensive to fit and seems particularly liable to give out along the square corners where the horizontal cylindrical surface of the top meets the vertical surface of the stove body, and stoves are now almost universally fitted with either conical or spherical tops.

The Bottom And Base Ring Of Stoves

The brickwork within bottom sheet of the stove must of necessity carry all the rest and this ring is, therefore, not capable of repair except by tearing out practically the whole lining of the stove. The principal point of corrosion about stoves appears to be where the cylindrical surface meets the bottom plate, and owing to this corrosion old stoves are liable to fail at this point sooner than almost any other, the base ring contains nearly all the openings made into the stove for cleaning-out doors, valves, etc., and its strength is thereby greatly reduced. For these reasons the bottom ring should be made of heavier material than the rest of the stove.

The stove base instead of being set down flat on a masonry foundation, as formerly, now generally rests on a set of T-rails placed close together; this carries it up off the ground and prevents corrosion to a large extent.

The joint where the bottom sheet meets the side sheets should always be made with the utmost care and almost without regard to expense. It is very doubtful if flanging is a good construction at this point. A heavy angle bent to the circle and riveted to the bottom and side sheets furnishes a better construction.