Diesel
engines become hot during operation and are cooled by fresh water circulating
throughout the engine.
The
heated fresh water is then cooled in the heat exchanger by salt water.
The
two most common heat exchanger systems are:
1. A tube nest cooler
2. A keel
cooler
Fresh
water circulation through the engine is common to both systems. When the engine
is cold, a thermostat circulates the fresh water by passing the tube nest
cooler and keel cooler. When the engine heats up the thermostat, allows the
fresh water to circulate through the tube nest or keel cooling system.
Tube Nest Cooling System
Two
pumps are used and both are powered by the main engine. The first pump circulates
fresh water through the engine water jackets and through the tube nest cooler
when the engine is hot. The second pump circulates sea water.
Heat Exchanger from a tube nest cooling system.
Internals of a heat exchanger from
a tube nest cooling system.
Internals
of a heat exchanger from a tube nest cooling system. (Courtesy: Caterpillar)
The second pump draws sea
water through a rose or grid on the vessel’s hull to prevent large pieces
of foreign matter entering or blocking the flow. A sea cock or valve is attached
directly inside of the hull so that the sea water can be shut off during maintenance
whilst the vessel remains in the water.
A strainer is fitted into
the pipe work on the suction side of the pump, to capture small foreign matter
before they can enter the system. The strainer may be fitted with a sight
bowl and must be easily accessible for frequent cleaning and inspection.
The sea water then flows
through the tubes in the tube nest cooler and then overboard. The cooler is
located after the sea water pump.
Keel Cooling System
In this system, fresh water
is circulated through the engine. When hot, the thermostat directs water flow
through a pipe mounted externally on the hull below the waterline. The sea water on the outside of the keel cooling pipes cools the
fresh water on the inside. The sea water pump is eliminated,
however a pump is required to circulate the fresh water within the system.
As the fresh water remains
in the system, keel cooling has the advantage that there is no possibility
of the system becoming clogged by foreign matter which could enter the sea
water cooling system.
The disadvantage is that
marine growth on the external pipe will impair the transfer of heat. The vessel
would have to be slipped to clean the keel cooling pipe.
Types of external piping mounted on the hull in a keel cooling system
Ranger Hope © 2008 (contains
reworked material courtesy of ANTA)