In coke furnaces using a bosh of this type I have several times used only nine inches of brick with very satisfactory results, and in the furnace shown by Fig. 167 which was to use charcoal and therefore much easier on the lining than a coke furnace, I used only four and one-half-inch bricks made up specially as circle bricks to fit the curvature of the bosh. This furnace was blown out on account of commercial conditions after a campaign of about a year and a half, and two inches of the original brick was still left. The judgment of the furnace-men was that it was as good as new.

A point about this construction which must not be overlooked is that the lining in this portion of the furnace is practically self-renewing. If the brick were all scoured off the jacket clean, by bad slag or bad work of the furnace, and the water-cooling were maintained, within a few hours the furnace would have built back for itself a lining of slag, carbon dust, etc., just as resistant for its purposes, if not more so, than the firebrick initially used.

This condition arises from the fact that this portion of the furnace converges toward the bottom, and that any material chilled on it tends to stay because of its conical shape. This is a feature of much importance which does not exist in any other portion of the furnace.

Owing to the small quantity of brick in the lining of the bosh in this construction it is without much compressive strength and differs in that respect from the cooling plate construction of Fig. 164 in which a considerable portion of the weight of the furnace structure is carried on the hearth and bosh walls when the latter are heated up by expansion. With the steel-plate bosh construction this must not be allowed to happen, as the thin brick lining would quickly crush and the steel jacket buckle under compressive stress, with disastrous consequences. It is necessary, therefore, to provide a sort of expansion joint in which the expansion due to the heating of the brick can be taken up without objectionable consequences. That is accomplished in the present construction by making the lower portion of the bosh jacket proper, cylindrical, and putting inside it a set of cast-iron water-cooled housings or blocks, B, through low openings in which project cooling-plates as shown at A. These cast-iron housings and the cooling-plates are supported on the heavy brick structure around the tuyeres which extends up from the bottom. The bosh jacket and its lining on the other hand are practically suspended from the mantle.

Fig. 167. Charcoal furnace, showing steel bosh jacket construction.

When expansion takes place the cylindrical portion of the bosh jacket slides down over the water-cooled housings, and if the lining is cracked or broken by this action, the crack is at once sealed by cinder running down over the surface and chilling against the housing and the bosh jacket.

In order to avoid the necessity of lugs and fastenings on the housings they are held together by the band, B, around their tops, this band being protected from burning by the cold bosh jacket just outside it and the water-cooling within the housing.

The water pipes for the cooling blocks A were brought out at the bottom so that the maximum of expansion could take place without shearing off these pipes, which has been known to happen in a construction of this kind where this provision was not made for it. Even in this case it would have been better if the space between the bottom of the trough and the top of the solid brick wall had been made three or four inches higher.

The water-cooled housing and the cooling plates projecting through them perform a double function: first, they protect the tuyeres and coolers below them from the scouring action of the streams of iron and cinder and prevent the spaces between the coolers from being scoured out and so weakening the tuyere zone; secondly, they form a water-cooled shelf at the foot of the bosh slope which supports the lining of the latter and forms a ledge on which a new lining can start to form in case the old one is scoured off by irregular furnace work.

Both these functions are of great importance and some element of construction must be supplied to perform them if this type of bosh is to give the long and useful service of which it is capable. When properly designed there is no doubt that this type of bosh construction will give absolutely satisfactory results in furnace operation on account of the perfectly smooth slope of its working surface and the complete absence of steps on shelves, at a cost of construction which is literally only a small fraction of that of the cooling plate or water-cooled stave construction.