15-09-2016, 04:48 PM
1454708723-14BPWaterCoolingSystem.pdf (Size: 1.28 MB / Downloads: 72)
Although there is an abundance of free sea water available,
marine diesel engines do not use it directly to keep the
hottest parts of the engine cool. This is because of the
corrosion which would be caused in the cooling water
spaces, and the salts which would be deposited on the cooling
surfaces interfering with the heat flow.
Instead, the water circulated around the engine is fresh water
( or better still, distilled water) which is then itself cooled
using sea water. This fresh water is treated with chemicals to
keep it slightly alkaline ( to prevent corrosion) and to prevent
scale formation. Of course, if distilled water, which some
ships can make from sea water using evaporators, is used then
there is a reduced risk of scale formation.
The cooling water pump which may be engine driven or be a separate
electrically driven pump pushes the water around the circuit. After passing
through the engine, where it removes the heat from the cylinder liners, cylinder
heads, exhaust valves and sometimes the turbochargers, it is cooled by seawater
and then returns to the engine. The temperature of the cooling water is closely
controlled using a three way control valve. If the water is allowed to get too
cold then it will cause thermal shocking which may lead to component failure
and will also allow water and acids to condense on the cylinder bores washing
away the lubricating film and causing corrosion. If it gets too hot then it will not
remove the heat effectively causing excessive wear and there is a greater danger
of scale formation. For this reason the cooling water outlet temperature is
usually maintained at about 78-82°C. Because it is at a higher temperature than
the cooling water used for other purposes (known as the LT cooling), the water
for cooling the engine is known as the HT (High Temperature) cooling water.
The temperature of the cooling water is
controlled automatically by a 3 way valve.
If no cooling is required (when the engine
is stopped) then the valve allows water to
circulate through the valve back to the
pumps.
If cooling is required, then water is diverted
to the Low Temperature (LT) system and
replaced with cool water from the LT
system. The LT water is cooled using sea
water in plate coolers.
Cooling can be achieved by using a dedicated cooler or by
mixing in some of the water from the LT cooling circuit.
The LT cooling water is then cooled in the sea water coolers.
The temperature is controlled using cascade control which
monitors both the inlet and outlet temperatures from the
engine. This allows a fast response to any change in
temperature due to a change in engine load.
To make up for any leaks in the system there is a header tank,
which automatically makes up any deficiency. Vents from the
system are also led to this header tank to allow for any
expansion in the system and to get rid of any air (if you are
familiar with a domestic central heating system then you will
see the similarities). The header tank is relatively small, and
usually placed high in the engine room. It is deliberately
made to be manually replenished, and is fitted with a low
level alarm. This is so that any major leak would be noticed
immediately. Under normal conditions, the tank is checked
once per watch, and if it needs topping up, then the amount
logged.