_The pumps_ may be of the reciprocating or rotary type, and are
generally placed in front of the boiler. If of the reciprocating type,
two pumps are placed alongside each other, and are operated either by a
double slide valve or piston valve engine.
_The piston rods_ connect directly with the plunger rods and are
also connected to a crank shaft by means of either connecting rods
or yokes, the cranks being set at right angles, so that one pump is
always acting, while the other passes the dead center, thus giving a
practically steady stream.
_The engine exhausts into the stack_, which gives the necessary draft.
Some engines are equipped with a boiler feed pump, others only depend
upon an injector, or feed directly from the main pump. _The coal box_,
which also forms a platform for the engineer to stand upon while under
way, is placed back of the boiler.
_All engines are equipped with two suctions and two discharge
openings_, so that either side may be connected up. The tool box and
driver’s seat are in front of the engine. The frame rests upon springs,
to make the machine easy running.
[Illustration: FIG. 396.]
_The Fox Boiler_ with which the Metropolitan and other engines are
equipped deserves an extended notice. It is shown in vertical section
in Fig. 395, the arrows indicating the steam and water circulation.
Its design, while simple, embodies some original ideas as to the
arrangement of the tube surface method of circulation, etc.; it is
a steam generator of _the water tube type_ designed to meet the
requirements peculiar to the fire service. The steam take-off and
sectional view of shell with the tube system removed is shown in Fig.
397.
NOTE.—Working pressure can be generated in this boiler in six minutes
from cold water, and the provisions for expansion are so near perfect
that no bad effect is noticeable from such severe treatment. The
manifold tube sections are tested to 600 pounds pressure, and are put
together with great care; _the manifolds are counter-bored_ to admit
the full diameter of the tube, leaving none of the threaded portion
exposed.
[Illustration: Plan showing Steam Take-Off.—FIG. 397.]
[Illustration: Top View of Empty Shell, showing manifold Beam.—FIG.
398.]
The boiler consists primarily of a simple annular shell heavily
stay-bolted throughout, and constitutes a water-legged fire-box and
steam reservoir; the principal heating surface of the boiler consists
of straight water tubes, manifolded in sectional form and housed within
the shell, the general scheme providing arrangements to make all
connections readily accessible, and permitting the withdrawal from the
boiler of any one or all of the several tube sections; the shell, being
practically a permanent feature, need seldom be disturbed by reason of
subsequent repairs or renewals of the tube systems.
It may be noted that the lower part, or water leg, of the shell is
contracted for the purpose of facilitating the rapid generation of
steam, and also providing the maximum grate area; at a point somewhat
below the water line of the boiler, the inner shell is flanged inward,
thereby enlarging the annular space between the inner and outer sheets
for the purpose of providing a more copious reservoir.
The water line being carried in this larger part of the shell, tends
to prevent the rapid fluctuation of the water level, and the increased
area of its surface at this point is favorable to the disengagement of
the steam.
[Illustration: Sectional Unit for Outer-Tube System.