Cooling
As working temperatures rise, the
strength of metals used reduces.
This in turn will force
manufacturers to use superior
metals/alloys, which can easily
withstand these high temperatures
and/or use thicker materials to
withstand stresses with reduced
strength. Superior metals/alloys
will increase the cost.
Thicker materials will increase weight and
cost of engines and will also increase
thermal stresses, which may result in
failures of working parts. Hence, cooling of
heated parts of the diesel engine is
arranged, so that the strength of materials
can be kept within safe limits and
manufacturing of engine is economical
and hence competitive and affordable.
Although there is abundance of sea
water available at sea, marine diesel
engines do not use sea water directly
for cooling the hottest parts of engine.
This will result in corrosion of cooling
water spaces and also deposit of
salts on the cooling surfaces, which
will result in interfering with heat flow,
as salts have insulating properties.
Instead the water circulated around the
engine is fresh water (better still distilled
water) and this fresh water in turn is
cooled by sea water. This fresh water is
treated chemically to keep slightly alkaline
(to reduce corrosion) and prevent scale
deposits. If distilled water (most of the
ships generate distilled water from sea
water for use on board ship) is used than
risk of scale deposits is reduced.
The cooling water pump - may be engine
driven or be a separate electrical driven
pump – pushes the cooling water around
cylinder liners, cylinder heads, exhaust
valves, turbochargers, etc and removes
the heat. The heated fresh water is then
cooled by sea water in a heat exchanger
and than returns back to cool the engine
again. Thus the cooling fresh water is in a
closed circuit.
The temperature of cooling fresh water
is closely controlled by a three way
valve control valve. If the cooling
water temperature falls down then
• it may cause thermal stress, which
may lead to component failure
• lead to decrease in thermal efficiency
of engine by reducing temperatures of
gases in the cylinder
• if temperatures go below dew point of
sulphuric acids, then sulphuric acids and
water vapours in the product of
combustion will go beyond the
neutralization effect of alkaline cylinder oil
and lead to corrosion (clover leafing) of the
liners.
• will result in difficulty in initial starting of
diesel engine.
If the cooling water temperature
rises then
• it will not remove the heat
effectively, thus leading to
excessive wear down
• it will lead to higher scale
formation
For this reason, the cooling water
temperature is maintained between 78 to
82 degree C. (on board ships, follow the
temperature specified by the
manufacturers operating manual –
normally referred to as manual).
Because the temperature of this
cooling water is much higher than
the cooling water used for other
purposes (known as low
temperature cooling – LT cooling),
the water for cooling diesel
engine is known as HT (high
temperature) cooling water.
• To make up for leaks from the system,
there is a header tank (also known as
expansion tank). Vents from the system
are also led to this header tank to allow for
expansion and also to get rid of trapped
air.
Header tank is placed high in the
engine room and may be
manually replenished and fitted
with low water level alarm, so as
to know major leaks immediately.
Header tank level is checked
once at least during watch
keeping and level topped if
required.
The system also contains a heater, which
is to keep the water hot when engine is not
working and to raise the water
temperature to required level before
starting of engine. Some of the ships have
a common fresh water cooling system
which cools main engine as well as
generator engines. This has an advantage
whereby engines which are stopped, are
kept warm and ready for starting by
engines which are working.
A fresh water generator is also
included in the system. The
system may also have a drain
tank incorporated to retain the
chemically treated for reuse after
the engine is drained of fresh
water for maintenance purpose.
A single medium speed diesel engine with
engine driven main pump and electrically
driven auxiliary pump and heater for keeping
engine warm when engine stopped.
Engine circulation system with only HT cooler
Two HT cooling pump
Steam heater to keep the system hot when
ship in port. Water is pumped through a
heater using a small pump.
Fresh water generator using heat from Diesel engine.
It operates under vacuum.
Temperature is controlled by a 3-way
automatically
Air is removed in a Dearation vessel.
To make up any leak there is a header tank. Vents
also are led here to allow for expansion.
LT cooler. Sea water cools fresh water in a plate type
of cooler