Page 86 - Introduction to Marine Engineering

Chapter 4
Boilers

A boiler in one form or another will be found on every type of ship.
Where the main machinery is steam powered, one or more large water-
tube boilers will be fitted to produce steam at very high temperatures
and pressures. On a diesel main machinery vessel, a smaller (usually
firetube type) boiler will be fitted to provide steam for the various ship
services. Even within the two basic design types, watertube and firetube,
a variety of designs and variations exist.
A boiler is used to heat feed water in order to produce steam. The
energy released by the burning fuel in the boiler furnace is stored (as
temperature and pressure) in the steam produced. All boilers have a
furnace or combustion chamber where fuel is burnt to release its energy.
Air is supplied to the boiler furnace to enable combustion of the fuel to
take place. A large surface area between the combustion chamber and
the water enables the energy of combustion, in the form of heat, to be
transferred to the water.
A drum must be provided where steam and water can separate. There
must also be a variety of fittings and controls to ensure that fuel oil, air
and feedwater supplies are matched to the demand for steam. Finally
there must be a number of fittings or mountings which ensure the safe
operation of the boiler.
In the steam generation process the feedwater enters the boiler where
it is heated and becomes steam. The feedwater circulates from the steam
drum to the water drum and is heated in the process. Some of the
feedwater passes through tubes surrounding the furnace, i.e. waterwall
and floor tubes, where it is heated and returned to the steam drum.
Large-bore downcomer tubes are used to circulate feedwater between
the drums. The downcomer tubes pass outside of the furnace and join
the steam and water drums. The steam is produced in a steam drum and
may be drawn off for use from here. It is known as 'wet' or saturated
steam in this condition because it will contain small quantities of water,
Alternatively the steam may pass to a superheater which is located within
the boiler. Here steam is further heated and 'dried', i.e. all traces of

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