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.
[Illustration: 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.