Bukh DV36/48, Workshop Manual

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The engine can be c ooled by direct seawater cooling which is supplied as standard cooling
system. Alternatively freshwater cooling can be supplied as extra equipment and this c an be
arranged in three different ways: either with heat exchanger, keel cooler or with radiator
cooling. However, the latter will n ot be of i mmediate importance when using the engine as
marine engine and therefore it is not dealt with separately in this section.
Generally the basic design of engine as a marine engine does that indirect cooling will not be
benefitial unless the engine is us ed for more than 500 hours of r unning per year. However
other circumstances like use in proluted rive r and narrow waters can do that i.e. a keel
cooling system is recommended. Keel cooling is always used for Lifebo at Engines due to
SOLAS requirements.
In case of direct seawater cooling the seawater is drawn through the external strainer and the
lubricating oil cooler to the cooling water pump (Johnson type F5B9) from where the water is fed
through the cooling jackets of the engine before it – via the thermostate – is lead overboard
through the exhaust system.
In case of freshwater cooling with heat exchanger the engine is cooled by the freshwater which is
circulating in a closed system. The freshwater is cooled in the heat exchanger by seawater. With
freshwater cooling with heat exchanger it is necessary – out of considerationfor the capacity of the
seawater pump – to equip the exhaust system with a by-pass hose, as stated in section Y,
pages 8 and 9.
In case of freshwater cooling with keel cooler the freshwater also circulates in a closed system.
The freshwater is cooled by the freshwater cooling pipes being led out somewhere through the
bottom of the boat and fitted on the outside of the hull. Thus the pipes are cooled by the seawater.
The seawater is always circulated by a Johnson pump type F5B9, whereas the freshwater is
always circulated by a circulation pump with metal rotor. The Johnson pump must never be used
for circulating the freshwater as the temperature can be considerably higher when using freshwater
cooling than in case of seawater cooling. This causes braking down of and damage to the impeller
of the Johnson pump, the impeller being made from neoprene.
The advantages of freshwater cooling are:
A higher operating temperature can be reached, having the effect that the running parts of the
engine will be less worn.
The engine has a somewhat better economy.
The engine block is not exposed to the corrosive actions of the seawater.
It is possible to add an anti-freeze solution to the freshwater in winter periods where you want to
use the engine.
The reason why the engine is not run at a higher temperature when seawater cooled is that the
salt in the seawater starts crystallizing at 70°C and this salt will settle in the engine block round the
cylinder liners and result in bad heat conduction and corrosions.