The method, either ill or good in which steam boilers are “set” or
arranged in their brick work and connections, will vary the quantity of
fuel used by as much as one-fifth; hence the importance of knowing the
correct principles upon which the work should be done.
[Illustration: Fig. 106.]
The portion of the steam plant called “the boiler” is composed of two
parts—the boiler and _the furnace_, and the latter may be considered a
part of the “setting” as it is mainly composed of brick work.
Two kinds of brick are used in boiler setting—the common brick
for walls, foundations and backing to the furnace, and so-called
fire-brick, which should be laid at every point where the fire operates
directly upon the furnace and passages.
Fire brick should be used in all parts of the setting which are exposed
to the hot gases. It is better to have fire brick lining tied in with
red brickwork, unless the lining is made 13-1/2 inches thick, when it
can be built up separate from outside walls. This arrangement will
require very heavy walls. As usual, but 9 inches fire brick lining is
used in the fireplace and 4-1/2 inches behind the bridge wall. Joints
in the fire brick-work should be as thin as possible.
Fig. 106 represents some of the different shapes in which fire brick
are made to fit the side of the furnace. They are called by special
names indicated by their peculiar form, circle-brick, angle-brick,
jamb-brick, arch-brick, etc. The common fire brick are 9″×4-1/2″×2-1/2″
in size, as shown in the figure.
The peculiar quality in fire bricks is their power to resist for a long
time the highest temperatures without fusion; they should be capable of
being subjected to sudden changes of temperature without injury, and
they should be able to resist the action of melted copper or iron slag.
Fire brick are cemented together with fire clay which is quite unlike
the ordinary mortar which is most suitable for common brick.
The setting as well as construction of boilers differs greatly, but in
all the end to be sought for is _a high furnace heat_, with as little
_waste as possible, at the chimney end_. To attain this there must
be (1) a sufficient thickness of wall around the furnace, including
the bridge, to retain as nearly as may be every unit of heat. (2) A
due mixture of air admitted at the proper time and temperature to the
furnace. (3) A proportionate area between the boiler and the surface of
the grates for the proper mixing of the gases arising from combustion.
(4) A correct proportion between the grate surface, the total area of
the tubes and the height and area of the chimney.
The principal parts and appendages of a furnace are as follows:
_The furnace_ proper or fire box, being the chamber in which the
solid constituents of the fuel and the whole or part of its gaseous
constituents are consumed.
_The grate_, which is composed of alternate bars and spaces, to support
the fuel and to admit the air.
_The dead-plate_, that part of the bottom of the furnace which consists
of an iron plate simply.
_The mouth piece_, through which the fuel is introduced and often some
air. The lower side of the mouth piece is the dead plate.
_The fire door_: Sometimes the duty of the fire door is performed by a
heap of fuel closing up the mouth of the furnace.
_The furnace front_ is above and on either side of the fire door.
_The ash pit._ As a general rule the ash pit is level, or nearly so,
with the floor on which the fireman stands, and as for convenient
firing, the grate should not be higher than 28 to 30 inches, the depth
of ash pit is thereby determined.
_The ash pit door_ is used to regulate the admission of air.
_The bridge wall._
_The combustion or flame chamber._
[Illustration: Fig. 107.]
[Illustration: Fig. 108.]
[Illustration: Fig. 109.]
[Illustration: Fig. 110.]
The arrangement of the space behind the bridge wall is found usually to
be in some one of the following forms: Level from bridge wall to back
(Fig. 107). A square box, depth ranging from 15 inches to 6 feet (Fig.
108). A gradual rise from bridge to back end of boiler, where only six
inches is found and generally circular in form (Fig. 109). A gradual
slope toward back, leaving a distance of about 36 inches from boiler
(Fig. 110).
The advocates of Fig. 107 claim that the office of the flame is to
get into as close contact with the bottom as possible, and this form
compels the flame to do so. In burning soft coal this form is found to
soot up the bottom of the boiler very badly.
Fig. 108 is followed more extensively than any other, the variations
being the depth of chamber; with depth generally from 36 to 40 inches.
Fig. 109 has nothing to commend it, except in cases where bridge is too
low.
Fig. 110 is followed a great deal and gives very good satisfaction.
This form allows for the theory of combustion, namely, the expansion of
the gases after leaving bridge wall.
Space behind the bridge wall should be enlarged, as it will reduce the
velocity of fire gases, and thus have them give up more of their heat
to the boiler.
The bridge wall should not be less than 18 inches at bottom, but may be
tapered off toward top to 9 or 13 inches.