Page 94 - Introduction to Marine Engineering

Boilers 81

Firetube boilers
The firetube boiler is usually chosen for low-pressure steam production
on vessels requiring steam for auxiliary purposes. Operation is simple
and feedwater of medium quality may be employed. The name 'tank
boiler* is sometimes used for firetube boilers because of their large water
capacity. The terms 'smoke tube' and 'donkey boiler* are also in use.

Package boilers
Most firetube boilers are now supplied as a completely packaged unit.
This will include the oil burner, fuel pump, forced-draught fan, feed
pumps and automatic controls for the system. The boiler will be fitted
with all the appropriate boiler mountings.
A single-furnace three-pass design is shown in Figure 4.7. The first
pass is through the partly corrugated furnace and into the cylindrical
wetback combustion chamber. The second pass is back over the furnace
through small-bore smoke tubes and then the flow divides at the front
central smoke box. The third pass is through outer smoke tubes to the
gas exit at the back of the boiler.
There is no combustion chamber refractory lining other than a lining

Main steam Double spring
stop valve
safety valve
Forced draught
Access tackier fan compartment
and platform
Pressure gauge
Gas exit flange
\
Forced
draught fan
Hinged door

Pressurized
plenum chamber

Combustion
appliance

Water inlet Control panel
strainer Feed pump
Figure 4,7 Package boiler