THE SHELL. This is the round or cylindrical structure which is commonly
described as the boiler, in which are inserted the braces and tubes,
and which sustains the internal strain of the pressure of the steam,
the action of the water within, and the fire without.
THE DRUM. This part is sometimes called the dome, and consists of an
upper chamber riveted to the top of the boiler for the purpose of
affording more steam space.
THE TUBE SHEETS. These are the round, flat flanged sheets forming the
two ends of the boiler, into which the tubes are fastened.
THE MANHOLE COVER. This is a plate and frame commonly opening inwards
and large enough to admit a man into the interior of the boiler. These
openings are sometimes made on the top and sometimes at the end of the
boiler. Manhole openings in steam boilers should invariably be located
in the head of the boiler, except in rare cases that may arise, when
circumstances require it to be placed in the shell. The manhole, so
placed, will not materially reduce the strength of the boiler, and
from this position it can more readily be seen that the boiler is kept
in proper condition. The proper sizes for manholes are 9×5 and 10×16,
according to circumstances. These are amply large for general use and
no material advantage is gained by increasing them.
THE HAND HOLE PLATES. These are similar arrangements to the manhole
cover, except as to size. They are made large enough to admit the hand
into the boilers for the purpose of removing sediment and they are also
used for the purpose of inspecting the interior of the boiler. Two are
usually put in each boiler, one front and one in the rear.
THE BLOW OFF. This consists of pipes and a cock communicating with the
bottom of the boiler for the purpose of blowing off the boiler or of
running off the water when the former needs cleaning.
[Illustration: THE TRIPLE DRAUGHT TUBULAR BOILER.—Fig. 34.]