This boiler, which is extensively used by the manufacturers of New England, is, as will be seen by the illustration, of the horizontal tubular class, and is essentially different from the well known type only in the arrangement of the tubes. The method secures the passage of the products of combustion through the same shell twice; forward through a part of the tubes, and backwards through the remaining ones. The manner of accomplishing this result can be best described by explaining how a common tubular boiler may be remodelled so as to carry out this principle.
Fig. 35.
A cylindrical shell, as shown in [Fig. 34]—of sufficient size to encircle about one-half of the tubes, is attached to the outside of the rear head below the water line, and extended backward to the back end of the setting. The encircled tubes are lengthened and carried backward to the same point; the extension is closed in and made to communicate with the boiler proper; the inner tubes emerge to the flue leading to the chimney and the old connection from the smoke arch is cut off. With this arrangement, the outer tubes of the boiler—a cluster on each side of the supplementary shell carry the products of combustion forward to the front smoke arch, and the inner tubes carry them backward to the chimney.
[Fig. 35] exhibits the boiler in half section and shows the course of the heat products through one of the outer tubes and returning through the boiler by one of the inner cluster.
[Fig. 36] (page 84) shows the boiler sectionally, over the bridge wall; the shaded tube ends exhibit the cluster which return the heat products to the rear of the boiler, after being brought forward by the two outer clusters which are left unshaded.
This arrangement of the tubes gives several advantages:
1. It enables an exceedingly high furnace temperature, without loss at the chimney.
2. By dividing the heat into these currents a more equal expansion and contraction is secured. This is an important point secured.