The hot blast is delivered from the stoves through the brick-lined hot-blast main to a circular distributing pipe surrounding the furnace which is called the bustle pipe. This is a complete annulus of heavy steel pipe lined with firebrick, surrounding the furnace concentrically (see Fg. 175). It is commonly supported directly on brackets on the furnace columns or suspended by hanger bolts from heavy brackets riveted to the shell. The latter method is much to be preferred as the pipe is a massive structure and when it expands and contracts from changes in temperature is capable of exerting great stresses. If it is supported on swinging hangers, it can move without doing any damage, but if supported in the furnace columns, it not only puts an eccentric load on them by its weight, which is bad enough, but may exert a direct lateral thrust of much more serious amount.

It is well to note here that the bustle pipe is a far heavier structure than its appearance would lead one to expect. The one here shown for a small furnace weighs about 50 tons and one for a large furnace might easily go to 150 tons. The brick lining of the bustle pipe is laid in two courses of circle brick made for the purpose. On account of the double curvature and the brick cutting which this necessitates, the labor of laying up the lining is very great; nevertheless it is of the utmost importance that the work be well done.

The heat losses by conduction and radiation are very serious in spite of the firebrick lining, and in recent years several furnacemen have tried making the outer portion of the lining of bricks or blocks made of infusorial earth or kieselguhr, whose conductivity is only one-tenth that of firebrick. This material is too soft to withstand the mechanical wear on the inside of the pipe so must be protected by a course of firebrick next the blast. Conduction losses have been greatly reduced by this practice, and it is destined to grow in favor. This applies of course to the hot-blast main as well as the bustle pipe, which is only the extension of this main.

Fig. 176. Bustle pipe penstock and blow pipe.

The blast is delivered from the bustle pipe to the individual tuyeres by connecting pipes called the "penstock" and "blow pipe." The design of these is very important in the practical operation of the furnace for the reason that whenever a tuyere is to be changed - a more or less normal occurrence even in the best practice, and an onerously frequent one when the furnace is in trouble - the connecting pipes must be removed before, and replaced after, the tuyere is changed, and it is vital that this should be done quickly.

Many designs have been used from time to time, but certain general principles have been developed, and a very satisfactory design in which these were applied is shown by Fig. 176. The walls of the hearth and tuyere zone, with the bands and jackets surrounding them, are clearly shown on the drawing. The cooler is a hollow, conical, bronze casting shown in section; water circulates vigorously through the hollow space, being delivered to it and taken from it by pipes entering at holes tapped in the butt, not shown on the drawing. The inside of the cooler has a slightly raised strip at its inner end cast to a true surface and into this fits the tuyere, which is a hollow bronze casting similar to but smaller than the cooler, and provided with a raised strip at its outside end, which is a close fit in the nose of the cooler, as shown. The tuyere is cooled by circulating water exactly the same as the "cooler".

The purpose of the cooler is to supply a protective, more or less permanent support for the tuyere, which projects, as shown, bodily into the furnace from 6 to 15 inches, depending upon the practice, the size and condition of the furnace, and the opinion of the furnaceman.

The blow pipe is of cast iron turned to a ball joint at both ends. At the front end, the ball fits in the butt of the tuyere, which is bored to a ball joint to fit it, while at the back end is a female ball which takes the projecting ball on the end of the penstock or "bootleg" as it is sometimes called.

Ball joints are universally used because the tuyeres do not always go into the furnace the same distance, and even if they did, the expansion of the hot-blast main is more than likely to throw the bustle pipe more or less off the center of the furnace. By having ball joints, these variations in the relative position of the tuyeres and bustle pipe, both vertical and horizontal, are taken up and a perfect joint quickly and easily made.

The penstock is in the general form of the pipe-fitting called a "sweep tee." The sweep turns towards the tuyere and the opposite (smaller) end is closed by a cap. The penstock is separate from, but supported by and makes a tight joint with, a nozzle riveted to the bustle pipe. The planes of the ball joints should always be at a right angle to one another so that swinging the penstock in or out to compensate for a difference in the location of the tuyere may affect the vertical alignment as little as possible and swinging the blow pipe up or down affect the horizontal alignment as little as possible. In many cases the penstock is designed to run on a diagonal, but this is exceedingly bad practice because an adjustment in one direction cannot be made without simultaneously affecting that in the other direction, and this greatly increases the difficulty of making both joints quickly and perfectly.