In fig. 27, F B represents the fire box or furnace; F D, fire door;
D P, deflector plate; F T P, fire box tube plate; F B R S, fire box
roof stays; S T P, smoke box tube plate; S B, smoke box; S B D,
smoke box door; S D, steam dome; O S, outer shell; R S V, Ramsbottom
safety-valve; F, funnel or chimney.
[Illustration: Fig. 28.]
The crown plate of the fire-box being flat requires to be efficiently
stayed, and for this purpose girder stays called fox roof stays are
mostly used, as shown in the figure. The stays are now made of cast
steel for locomotives. They rest at the two ends on the vertical plates
of the fire-box, and sustain the pressure on the fire-box crown by a
series of bolts passing through the plate and girder stay, secured by
nuts and washers. Fig. 28 is a plan and elevation of a wrought-iron
roof stay.
Another method adopted in locomotive types of marine boilers for
staying the flat crown of the fire-box to the circular upper plate is
shown in fig. 29—namely, by wrought-iron vertical bar stays secured by
nuts and washers to the fire-box with a fork end and pin to angle-iron
pieces riveted to the boiler shell.
[Illustration: Fig. 29.]
The letters in this figure refer to the same parts of the boiler as do
those in fig. 27, _i.e._, F B to the fire-box, etc., etc.
It was formerly the custom to make the tubes much longer than shown
in the fig., with the object of gaining heating surface; but modern
experience has shown that the last three or four feet next the smoke
box were of little or no use, because, by the time the products of
combustion reached this part of the heating surface, their temperature
was so reduced that but little additional heat could be abstracted from
them. The tubes, in addition to acting as flues and heating surface,
fulfil also the function of stays to the flat end of the barrel of the
boiler, and the portion of the fire box opposite to it.
In addition to the staying power derived from the tubes, the smoke box,
tube plate and the front shell plate are stayed together by several
long rods.
[Illustration: THE HORIZONTAL TUBULAR BOILER.—Fig. 30.]