Page 104 - Introduction to Marine Engineering

Boilers 91

any condensed steam which might otherwise collect above the valve and
stop it opening at the correct pressure.

Combustion

Combustion is the burning of fuel in air in order to release heat energy.
For complete and efficient combustion the correct quantities of fuel and
air must be supplied to the furnace and ignited. About 14 times as much
air as fuel is required for complete combustion. The air and fuel must be
intimately mixed and a small percentage of excess air is usually supplied
to ensure that all the fuel is burnt. When the air supply is insufficient the
fuel is not completely burnt and black exhaust gases will result.

Air supply
The flow of air through a boiler furnace is known as 'draught'. Marine
boilers are arranged for forced draught, i.e. fans which force the air
through the furnace. Several arrangements of forced draught are
possible. The usual forced draught arrangement is a large fan which
supplies air along ducting to the furnace front. The furnace front has an
enclosed box arrangement, known as an 'air register', which can control
the air supply. The air ducting normally passes through the boiler
exhaust where some air heating can take place. The induced draught
arrangement has a fan in the exhaust uptake which draws the air
through the furnace. The balanced draught arrangement has matched
forced draught and induced draught fans which results in atmospheric
pressure in the furnace.

Fuel supply
Marine boilers currently burn residual low-grade fuels. This fuel is
stored in double-bottom tanks from which it is drawn by a transfer
pump up to settling tanks (Figure 4.15). Here any water in the fuel may
settle out and be drained away.
The oil from the settling tank is filtered and pumped to a heater and
then through a fine filter. Heating the oil reduces its viscosity and makes
it easier to pump and filter. This heating must be carefully controlled
otherwise 'cracking' or breakdown of the fuel may take place. A supply
of diesel fuel may be available to the burners for initial firing or
low-power operation of the boiler. From the fine filter the oil passes to
the burner where it is 'atomised', i.e. broken into tiny droplets, as it
enters the furnace. A recirculating line is provided to enable initial
heating of the oil.