ENGINE COOLING PROCESS

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Ref : Introduction marine engineering


Engine cooling process of engine divide into:

1. Fresh water cooling system
2. Sea water cooling system

Cooling of engines is achieved by circulating a cooling liquid around internal passages within the engine. The cooling liquid is thus heated up and is in turn cooled by a sea water circulated cooler. Without adequate
cooling certain parts of the engine which are exposed to very high temperatures, as a result of burning fuel, would soon fail. Cooling enables the engine metals to retain their mechanical properties. The usual coolant used is fresh water: sea water is not used directly as a coolant because of its corrosive action. Lubricating oil is sometimes used for piston cooling since leaks into the crankcase would not cause problems. As a result of its lower specific heat however about twice the quantity of oil compared to water would be required.

Fresh water cooling system

A water cooling system for a slow-speed diesei engine is shown in Figure 2.18. It is divided into two separate systems: one for cooling the cylinder jackets, cylinder heads and turbo-blowers; the other for piston cooling.

The cylinder jacket cooling water after leaving the engine passes to a sea-water-circulated cooler and then into the jacket-water circulating pumps. It is then pumped around the cylinder jackets, cylinder heads
and turbo-blowers. A header tank allows for  expansion and water make-up in the system. Vents are led from the engine to the header tank for the release of air from the cooling water. A heater in the circuit facilitates warming of the engine prior to starting by circulating hot water. The piston cooling system employs similar components, except that a
drain tank is used instead of a header tank and the vents are then led to high points in the machinery space. A separate piston cooling system is used to limit any contamination from piston cooling glands to the piston cooling system only.


Sea water cooling system

The various cooling liquids which circulate the engine are themselves cooled by sea water. The usual arrangement uses individual coolers for lubricating oil, jacket water, and the piston cooling system, each cooler being circulated by sea water. Some modern ships use what is known as a 'central cooling system' with only one large sea-water-circulated cooler. This cools a supply of fresh water, which then circulates to the32 other Individual coolers. With less equipment in contact with sea water the corrosion problems are much reduced in this system. A sea water cooling system is shown in Figure 2.19. 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 sea water main provides sea water to directly cool the charge air (for a direct-drive two-stroke diesel). One arrangement of a central cooling system is shown in Figure 2.20. The sea water circuit is made up of high and low suctions, usually on either side of the machinery space, suction strainers and several sea water pumps. The sea water is circulated through the central coolers and then discharged overboard. A low-temperature and high-temperature circuit exist in the fresh water 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 the mixing of water 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. It is also possible, with appropriate control equipment, to vary the quantity of sea water circulated by the pumps to almost precisely meet the cooler requirements.