ranging from 100.000 to 1,000,000 pounds of steam per
hour. In case of tube failure, there is little danger of a
disastrous explosion of the water-tube boiler. The
furnace not only can carry a high overload, it can also be
modified for tiring by oil or coal. Still another advantage
is that it is easy to get into sections inside the furnace to
clean and repair them.
There are also several
disadvantages common to water-tube boilers. One of the
main drawbacks of water-tube boilers is their high
construction cost. The large assortment of tubes required
of this boiler and the excessive weight per unit weight of
steam generated are other unfavorable factors.
There are four types of fire-tube boilersthe Scotch
marine boiler. the vertical-tube boiler, the horizontal
return tubular boiler. and the firebox boiler. These four
types of boilers are discussed in this section.
Scotch Marine Boiler
The Scotch marine tire-tube boiler is especially
suited to Seabee needs. Figure 1-1 is a portable Scotch
marine tire-tube boiler. The portable unit can be moved
easily and requires only a minimal amount of foundation
As a complete self-contained unit. its design
includes automatic controls. a steel boiler. and burner
equipment. These features are a big advantage because
no disassembly is required when you must move the
boiler into the field for an emergency.
The Scotch marine boiler has a two-pass (or more)
arrangement of tubes that run horizontally to allow the
heat inside the tubes to travel back and forth. It also has
an internally fired furnace with a cylindrical combustion
chamber. Oil is the primary fuel used to fire the boiler;
however. it can also be fired with wood, coal, or gas. A
major advantage of the Scotch marine boiler is that it
requires less space than a water-tube boiler and can be
placed in a room that has a low ceiling.
The Scotch marine boiler also has disadvantages.
The shell of the boiler runs from 6 to 8 feet in diameter, a
detail of construction that makes a large amount of
The fixed dimensions of the
internal surface cause some difficulty in cleaning the
sections below the combustion chamber.
drawback is the limited capacity and pressure of the
Scotch marine boiler.
An important safety device sometimes used is the
fusible plug that provides added protection against
low-water conditions. In case of a low-water condition.
the fusible plug core melts, allowing steam to escape, and
a loud noise is emitted which provides a warning to the
operator. On the Scotch boiler the plug is located in the
crown sheet, but sometimes it is placed in the upper back
of the combustion chamber. Fusible plugs are discussed
in more detail later in this chapter.
Access for cleaning, inspection, and repair of the
boiler watersides is provided through a manhole in the
top of the boiler shell and a handhold in the water leg. The
manhole opening is large enough for a man to enter the
Figure 1-1.Scotch marine type of fire-tube boiler.