To illustrate the operations of a steam generator, we give the details
of an appliance, which may be compared to the letter A of the alphabet,
or the figure 1 of the numerals, so simple is it.
Fig. 11, is an elevation of boiler, fig. 12 a vertical section through
its axis, and fig. 13 a horizontal section through the furnace bars.
[Illustration: Fig. 11.]
[Illustration: Fig. 12.]
The type of steam generator here exhibited is what is known as a
vertical tubular boiler. The outside casing or shell is cylindrical
in shape, and is composed of iron or steel plates riveted together.
The top, which is likewise composed of the same plates is slightly
dome-shaped, except at the center, which is away in order to receive
the chimney _a_, which is round in shape and formed of thin wrought
iron plates. The interior is shown in vertical section in fig. 12.
It consists of a furnace chamber, _b_, which contains the fire. The
furnace is formed like the shell of the boiler of wrought iron or
steel plates by flanging and riveting. The bottom is occupied by the
grating, on which rests the incandescent fuel. The grating consists of
a number of cast-iron bars, _d_ (fig. 12), and shown in plan in fig.
13, placed so as to have interstices between them like the grate of an
ordinary fireplace. The bottom of the furnace is firmly secured to the
outside shell of the boiler in the manner shown in fig. 12. The top
covering plate _cc_, is perforated with a number of circular holes of
from one and a half to three inches diameter, according to the size of
the boiler. Into each of these holes is fixed a vertical tube made of
brass, wrought iron, or steel, shown at _fff_ (fig. 12). These tubes
pass through similar holes, at their top ends in the plate _g_, which
latter is firmly riveted to the outside shell of the boiler. The tubes
are also firmly attached to the two plates, _cc_, _g_. They serve to
convey the flame, smoke, and hot air from the fire to the smoke box,
_h_, and the chimney, _a_, and at the same time their sides provide
ample heating surface to allow the heat contained in the products of
combustion to escape into the water. The fresh fuel is thrown on the
grating when required through the fire door, A (fig. 11). The ashes,
cinders, etc., fall between the fire bars into the ash pit, B (fig.
12). The water is contained in the space between the shell of the
boiler, the furnace chamber, and the tubes. It is kept at or about the
level, _ww_ (fig. 12), the space above this part being reserved for
the steam as it rises. The heat, of course, escapes into the water,
through the sides and top plate of the furnace, and through the sides
of the tubes. The steam which, as it rises from the boiling water,
ascends into the space above _ww_, is thence led away by the steam
pipe to the engine. Unless consumed quickly enough by the engine, the
steam would accumulate too much within the boiler, and its pressure
would rise to a dangerous point. To provide against this contingency
the steam is enabled to escape when it rises above a certain pressure
through the safety-valve, which is shown in sketch on the top of the
boiler in fig. 11. The details of the construction of safety-valves
will be found fully described in another section of this work, which
is devoted exclusively to the consideration of boiler fittings. In the
same chapters will be found full descriptions of the various fittings
and accessories of boilers, such as the water and pressure gauges,
the apparatus for feeding the boiler with water, for producing the
requisite draught of air to maintain the combustion, and also the
particulars of the construction of the boilers themselves and their
furnaces.
[Illustration: Fig. 13.]