The various cooling liquids which circulate the engine are themselves cooled by seawater. The usual arrangement uses individual coolers for lubricating oil, jacket water, and the piston cooling system, each cooler being circulated by seawater. Some modern ships use what is known as a 'central cooling system' with only one large sea-water-circulated cooler.
[ Also check diagram: central cooling system]
This cools a supply of fresh water, which then circulates to the other Individual coolers. With less equipment in contact with seawater, this system's corrosion problems are much reduced.
A seawater cooling system From the sea suction one of a pair of sea-water circulating pumps provides sea water which circulates the lubricating oil cooler, the jacket water cooler and the piston water cooler before discharging overboard. Another branch of the seawater main provides seawater to directly cool the charge air (for a direct-drive two-stroke diesel).
One arrangement of a central cooling system. The seawater circuit is made up of high and low suctions, usually on either side of the machinery space, suction strainers and several seawater pumps. The sea water is circulated through the central coolers and then discharged overboard. A low-temperature and high-temperature circuit exists in the freshwater system.
The fresh water in the high-temperature circuit circulates the main engine and may if required, be used as a heating medium for an evaporator. The low-temperature circuit circulates the main engine air coolers, the lubricating oil coolers and all other heat exchangers. A regulating valve controls water mixing between the high-temperature and low-temperature circuits.
A temperature sensor provides a signal to the control unit which operates the regulating valve to maintain the desired temperature setting. A temperature sensor is also used in a similar control circuit to operate the regulating valve which controls the bypassing of the central coolers.
With appropriate control equipment, it is also possible to vary the quantity of seawater circulated by the pumps to almost precisely meet the cooler requirements.
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