It follows, too, that _the horizontal tubular boiler_, substantially
as shown in fig. 30, is the standard steam boiler; engineers and steam
power owners cling with great tenacity to this approved form, which is
an outgrowth of one hundred years’ experience in steam production.
In the plain horizontal tubular boiler shown in cuts, the shell is
filled with as many small tubes varying from two inches to four inches
in diameter as is consistent with the circulation and steam space. In
firing this type of boiler the combustion first takes place under the
shell, and the products, such as heat, flame, and gas, pass through
the small tubes to the chimney, although in the triple draught pattern
of the tubular boiler, the heat products pass, as will hereafter be
explained, a second time through the boiler tubes, making three turns
before the final loss of the extra heat takes place.
The illustrations on pages 78 and 80 exhibit the gradual advances to
the horizontal tubular by the two-flued boiler (fig. 31) of the six
flues (fig. 32) and of the locomotive Portable Boiler (fig. 33). The
vertical or upright tubular boiler is but another modification of the
horizontal tubular.
[Illustration: THE LOCOMOTIVE PORTABLE BOILER.—Fig. 33.]
In parts of the vertical boiler there is very little circulation and
the corrosion on the inner side is such as to wear the boiler rapidly.
In the ash pit, ashes and any dampness that may be about the place also
causes rapid corrosion. The upper part of the tubes and tube sheet are
frequently injured; for instance, if the tubes pass all the way through
to the upper tube sheet, providing there is no cone top, when the fire
is first made under the boiler, combustion at times does not take place
until the gases pass nearly through the tubes. The water usually being
carried below the tube sheet there is a space left above the water
line, where there is neither steam nor water, and the heat is so great
that the ends of the tubes are burned and crystalized, and the tube
sheet is often cracked and broken by this excessive heat before the
steam is generated. The first difficulty is experienced in “the legs”
of the Portable Locomotive boiler—hence the general verdict of steam
users in favor of the round shell, many-tubed boiler.