It is recommended to divide the engines into several circuits in multi-engine installations. One
reason is of course redundancy, but it is also easier to tune the individual flows in a smaller
system. Malfunction due to entrained gases, or loss of cooling water in case of large leaks
can also be limited. In some installations it can be desirable to separate the HT circuit from
the LT circuit with a heat exchanger.
The external system shall be designed so that flows, pressures and temperatures are close
to the nominal values in Technical data and the cooling water is properly de-aerated.
Pipes with galvanized inner surfaces are not allowed in the fresh water cooling system. Some
cooling water additives react with zinc, forming harmful sludge. Zinc also becomes nobler
than iron at elevated temperatures, which causes severe corrosion of engine components.
Ships (with ice class) designed for cold sea-water should have provisions for recirculation
back to the sea chest from the central cooler:
●
For melting of ice and slush, to avoid clogging of the sea water strainer
●
To enhance the temperature control of the LT water, by increasing the seawater temperature
9.3.1
Stand-by circulation pumps (4P03, 4P05)
Stand-by pumps should be of centrifugal type and electrically driven. Required capacities and
delivery pressures are stated in Technical data.
NOTE
Some classification societies require that spare pumps are carried onboard even
though the ship has multiple engines. Stand-by pumps can in such case be worth
considering also for this type of application.
9.3.2
Sea water pump (4P11)
The capacity of electrically driven sea water pumps is determined by the type of coolers and
the amount of heat to be dissipated.
Significant energy savings can be achieved in most installations with frequency control of
electrically driven sea water pumps. Minimum flow velocity (fouling) and maximum sea water
temperature (salt deposits) are however issues to consider.
9.3.3
Temperature control valve for central cooler (4V08)
When it is desired to utilize the engine driven LT-pump for cooling of external equipment, e.g.
a reduction or a generator, there must be a common LT temperature control valve in the
external system, instead of an individual valve for each engine. The common LT temperature
control valve is installed after the central cooler and controls the temperature of the water
before the engine and the external equipment, by partly bypassing the central cooler. The
valve can be either direct acting or electrically actuated.
The set-point of the temperature control valve 4V08 is 38 ºC in the type of system described
above.
Engines operating on HFO must have individual LT temperature control valves. A separate
pump is required for the external equipment in such case, and the set-point of 4V08 can be
lower than 38 ºC if necessary.
9.3.4
Fresh water central cooler (4E08)
The fresh water cooler can be of either plate, tube or box cooler type. Plate coolers are most
common. Several engines can share the same cooler.
It can be necessary to compensate a high flow resistance in the circuit with a smaller pressure
drop over the central cooler.
9-12
Wärtsilä 20 Product Guide - a13 - 6 September 2016
Wärtsilä 20 Product Guide
9. Cooling Water System
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