After the first crude forms, such as that used in connection with the
Baranca and Newcomen engine, and numerous others, the steam boiler
which came into very general use was _the plain cylinder boiler_. An
illustration is given of this in figures 14 and 15.
It consists of a cylinder A, formed of iron plate with hemispherical
ends B. B. set horizontally in brick work C. The lower part of this
cylinder contains the water, the upper part the steam. The furnace D
is outside the cylinder, being beneath one end; it consists simply of
grate bars _e e_ set in the brick work at a convenient distance below
the bottom of the boiler.
[Illustration: Fig. 14.]
[Illustration: Fig. 15.]
The sides and front of the furnace are walls of brick work, which,
being continued upwards support the end of the cylinder. The fuel is
thrown on the bars through the door which is set in the front brick
work. The air enters between the grate bars from below. The portion
below the bars is called the ash pit. The flame and hot gases, when
formed, first strike on the bottom of the boiler, and are then carried
forward by the draft, to the so-called bridge wall _o_, which is a
projecting piece of brick work which contracts the area of the flue _n_
and forces all the products of combustion to keep close to the bottom
of the boiler.
Thence the gases pass along the flue _n_, and return part one side
of the cylinder in the flue _m_ (fig. 15) and back again by the other
side flue _m_ to the far end of the boiler, whence they escape up the
chimney. This latter is provided with a door or damper _p_, which can
be closed or opened at will, so as to regulate the draught.
This boiler has been in use for nearly one hundred years, but has two
great defects. The first is that the area of heating surface, that is
the parts of the boiler in contact with the flames, is too small in
proportion to the bulk of the boiler; the second is, that if the water
contains solid matter in solution, as nearly all the water does to a
greater or less extent, this matter becomes deposited on the bottom
of the boiler just where the greatest evaporation takes place. The
deposit, being a non-conductor, prevents the heat of the fuel from
reaching the water in sufficient quantities, thus rendering the heating
surface inefficient; and further, by preventing the heat from escaping
to the water, it causes the plates to become unduly heated, and quickly
burnt out.
There is another defect belonging to this system of boiler to which
many engineers attach great importance, viz.: that the temperature
in each of the three flues _n_, _m_, _m´_ is very different, and
consequently that the metal of which the shell of the boiler is
composed expands very unequally in each of the flues, and cracks
are very likely to take place when the effects of the changes of
temperature are most felt. It will be noted that the flames and gases
in this earliest type of steam boiler make three turns before reaching
the chimney, and as these boilers were made frequently as much as 40
feet long it gave the extreme length of 120 feet to the heat products.
THE CORNISH BOILER is the next form in time and excellence. This is
illustrated in figures 16 and 17.
It consists also of a cylindrical shell _A_, with flat ends as
exhibited in cuts. The furnace, however, instead of being situated
underneath the front end of the shell, is enclosed in it in a second
cylinder _B_, having usually a diameter a little greater than half that
of the boiler shell. The arrangement of the grate and bridge is evident
from the diagram. After passing the bridge wall the heat products
travel along through the internal cylinder _B_, till they reach the
back end of the boiler; then return to the front again, by the two side
flues _m_, _m_´, and thence back again to the chimney by the bottom of
flue _n_.
In this form of boiler the heating surface exceeds that of the last
described by an amount equal to the area of the internal flues, while
the internal capacity is diminished by its cubic contents; hence for
boilers of equal external dimensions, the ratio of heating surface
to mass of water to be heated, is greatly increased. Boilers of this
sort can, however, never be made of as small diameters as the plain
cylindrical sort, on account of the necessity of finding room inside,
below the water level, for the furnace and flue.
[Illustration: Fig. 16.]
[Illustration: Fig. 17.]
The disadvantage, too, of the deposits mentioned in the plain cylinder
is, to a great extent got over in the Cornish boiler, for the bottom,
where the deposit chiefly takes place, is the coolest instead of being
the hottest part of the heating surface.
But the disadvantage of unequal expansion also exists in this type
of boiler, as the internal flue in the Cornish system is the hottest
portion of the boiler, and consequently undergoes a greater lengthways
expansion than the flues. The result is to bulge out the ends, and when
the boiler is out of use, the flue returns to its regular size, and
thus has a tendency to work loose from the ends to which it is riveted
and if the ends are too rigid to move, a very serious strain comes on
the points of the flue.
Even while in use the flue of a Cornish boiler is liable to undergo
great changes in temperature, according to the state of the fire;
when this latter is very low, or when fresh fuel has been thrown on,
the temperature is a minimum and reaches a maximum again when the
fresh fuel commences to burn fiercely. This constant expansion and
contraction is found in practice to also so weaken the tube that
it frequently collapses or is pressed together, resulting in great
disaster.