[Illustration: Fig. 22.]
[Illustration: Fig. 23.]
It will be seen from these drawings that there are three internal
cylindrical furnaces at each end of these boilers, making in all six
furnaces per boiler. The firing takes place at both ends. The flame and
hot gases from each furnace, after passing over the bridge wall enter a
flat-sided rectangular combustion chamber and then travel through tubes
to the front uptake (_i.e._ the smoke bonnet or breaching), and so on
to the chimney.
The sides of the combustion chambers are stayed to each other and to
the shell plate of the boiler; the tops are strengthened in the same
manner as the crowns of locomotive boilers, and the flat plates of the
boiler shell are stayed together by means of long bolts, which can be
lengthened up by means of nuts at their ends. Access is gained to the
uptakes for purposes of cleaning, repairs of tubes, etc., by means of
their doors on their fronts just above the furnace doors. The steam is
collected in the large cylindrical receivers shown above each boiler.
The material of construction is mild steel.
The following are the principal dimensions and other particulars of one
of these boilers:
Length from front to back, 20 feet.
Diameter of shell, 15 feet 6 inches.
Length of furnace, 6 feet 10 inches.
Diameter of furnace, 3 feet 10 inches.
Length of tubes, 6 feet 9 inches.
Diameter of tubes, 3-1/2 inches.
No. of tubes, 516.
Thickness of shell plates, 15/16.
Thickness of tube plates, 3/4.
Grate area, 126-1/2 square feet.
Heating surface, 4015 square feet.
Steam pressure, 80 lbs. per sq. inch.
Fig. 24 is a sketch of a modern marine boiler, which is only fired
from one end, and is in consequence much shorter in proportion to its
diameter than the type illustrated in figs. 22 and 23.
Marine boilers over nine feet in diameter have generally two furnaces,
those over 13 to 14 feet, three, while the very largest boilers used
on first-class mail steamers, and which often exceed fifteen feet in
diameter, have four furnaces.
In the marine boiler the course taken by the products of combustion
is as follows; the coal enters through the furnace doors on to the
fire-bars, the heat and flames pass over the fire bridge into the flame
or combustion chamber, thence through the tubes into the smoke-box, up
the up-take and funnel into the air.
[Illustration: Fig. 24.]
The fittings to a marine boiler are—funnel and air casings, up-takes
and air casings, smoke boxes and doors, fire doors, bars, bridges,
and bearers, main steam stop valve, donkey valve, safety valves and
drain pipes, main and donkey feed check valves, blow-off and scum
cocks, water gauge glasses on front and back of boiler, test water cock
for trying density of water, steam cock for whistle, and another for
winches on deck.
A fitting, called a blast pipe, is sometimes placed in the throat of
the funnel. It consists of a wrought iron pipe, having a conical nozzle
within the funnel pointing upwards, the other end being connected to
a cock, which latter is bolted on to the steam space or dome of the
boiler. It is used for increasing the intensity of the draft, the
upward current of steam forcing the air out of the funnel at a great
velocity; and the air having to be replaced by a fresh supply through
the ash-pits and bars of the furnaces, a greater speed of combustion is
obtained than would otherwise be due to simple draft alone.
Boilers are fitted with dry and wet uptakes, which differ from each
other as follows:—The dry uptake is wholly outside the boiler, and
consists of an external casing bolted on to the firing end of the
boiler, covering the tubes and forming the smoke-box, and is fitted
with suitable tube doors. A wet uptake is carried back from the firing
ends of the boiler into its steam space, and is wholly surrounded
by water and steam. The dry uptake seldom requires serious repair;
but the wet uptake, owing to its exposure to pressure, steam, and
water, requires constant attention and repair, and is very liable to
corrosion, being constantly wetted and dried in the neighborhood of the
water-line. The narrow water space between both front uptakes is also
very liable to become burnt, owing to accumulation of salt. The flue
boilers of many tugs and ferry boats are fitted with wet uptakes.
A superheater is a vessel usually placed in the uptake, or at the base
of the funnel of a marine boiler, and so arranged that the waste heat
from the furnaces shall pass around and through it prior to escaping up
the chimney. It is used for drying or heating the steam from the main
boiler before it enters the steam pipes to the engine. The simplest
form of superheater consists of a wrought iron drum filled with tubes.
The heat or flame passes through the tubes and around the shell of
the drum, the steam being inside the drum. Superheaters are usually
fitted with a stop valve in connection with the boiler, by means of
which it can be shut off; and also one to the steam pipe of the engine;
arrangements are also usually made for mixing the steam or working
independently of the superheater.
A safety-valve is also fitted and a gauge glass; the latter is to show
whether the superheater is clear of water, as priming will sometimes
fill it up.
The special fittings of the marine boiler will be more particularly
described hereafter as well as the stays, riveting, strength, etc.,
etc.
The use of the surface condenser in connection with the marine boiler
was an immense step toward increasing its efficiency. In 1840 the
average pressure used in marine boilers was only 7 or 8 lbs. to the
square inch, the steam being made with the two-flue pattern of boiler,
sea water being used for feed; as the steam pressure increased as now
to 150 to 200 lbs. to the square inch, greater and greater difficulty
was experienced from salt incrustation—in many cases the tubes did not
last long and frequently gave much trouble, until the introduction of
the surface condenser, which supplied fresh water to the boilers.
[Illustration: Fig. 25]