NUMBER OF FURNACES
CONTROL OF SUPERHEAT
All boilers commonly used in the propulsion
A boiler that provides some means of
plants of naval ships may be classified as either
controlling the degree of superheat independently
SINGLE-FURNACE BOILERS or DOUBLE-
of the rate of steam generation is said to have
FURNACE BOILERS. The D-type boiler is a
CONTROLLED SUPERHEAT. A boiler in
single-furnace boiler; the M-type boiler is a
which such separate control is not possible is said
double-furnace (divided-furnace) boiler.
to have UNCONTROLLED SUPERHEAT.
Normally, the term superheat control boiler
BURNER LOCATION
is used to identify a double-furnace boiler. The
term uncontrolled superheat boiler is used to
Naval boilers are also classified on the basis
identify a single-furnace boiler.
of where their burners are located. Most burners
in naval propulsion plants are located at the front
of the boiler. These are called FRONT-FIRED
OPERATING PRESSURE
BOILERS. Other ships, such as the AO-177 and
LKA-113 class ships, have their burners on the
top of the boilers. These are called TOP-FIRED
BOILERS.
FURNACE PRESSURE
Another convenient boiler classification is
based on the air pressure used in the furnace. Most
boilers in use in naval propulsion plants operate
with a slight air pressure (seldom over 5 psig) in
the boiler furnace. This slight pressure is not
enough to justify calling these boilers pressurized-
furnace boilers. However, some boilers installed
on naval ships are truly pressurized-furnace
boilers. They are called PRESSURE-FIRED or
SUPERCHARGED BOILERS. These furnaces
are maintained under a positive air pressure of
about 65 psia (about 50 psig) when operated at
full power. The air pressure in these boiler
furnaces is maintained by special air compressors
called superchargers.
TYPE OF SUPERHEATERS
On almost all boilers used in the propulsion
plants of naval ships, the superheater tubes are
protected from radiant heat by water screen tubes.
The water screen tubes absorb the intense radiant
heat of the furnace, and the superheater tubes are
heated by convection currents rather than by
direct radiation. These superheaters are called
CONVECTION-TYPE SUPERHEATERS.
In a few older ships, the superheater tubes are
not screened by water screen tubes but are exposed
directly to the radiant heat of the furnace.
Superheaters of this design are called RADIANT-
TYPE SUPERHEATERS.
For some purposes, it is convenient to classify
boilers according to operating pressure. Most
classification of this type are approximate rather
than exact. Header-type boilers and some older
drum-type boilers are often called 400-PSI
BOILERS even though their operating pressures
range from about 435 psi to 700 psi.
The term high-pressure boiler is at present used
rather loosely to identify any boiler that operates
at a substantially higher pressure than the so-
called 600-PSI BOILERS. In general, we will con-
sider any boiler that operates at 751 psi or above
as a high-pressure boiler. Many boilers in naval
ships operate at about 1200 psi. These boilers are
referred to as 1200-PSI BOILERS.
As you can see, classifying boilers by operating
pressure is not very precise since actual operating
pressure may vary widely within any one group.
Also, any classification based on operating
pressure may easily become obsolete. What is called
a high-pressure boiler today may well be called
a low-pressure boiler tomorrow.
BOILER COMPONENTS
Boilers used onboard naval ships have
essentially the same components: steam and water
drums, generating and circulating tubes, super-
heaters, economizers, and accessories and fittings
for controlling steam pressure and temperature
and other aspects of boiler control and operation.
Figure 4-1 shows a cutaway view of a D-type
boiler. You should refer to this figure as a guide
to the arrangement of the boiler components. As
we discuss the boiler and its components, imagine
that you are assembling a similar boiler. As you
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