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Cooling of ships engine - how it works , requirement of fresh water & 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.

Water carried in pipes is used to cool machinery. The main engine is cooled by two separate but linked systems: an open system (sea-to-sea) in which water is taken from and returned to the sea (seawater cooling), and a closed system where freshwater is circulated around an engine casing (freshwater cooling).

Freshwater is used to cool machinery directly, whereas seawater is used to cool freshwater passing through a heat exchanger. The particular feature of an engine cooling system is continuous fluid flow. Fluid in motion causes abrasive corrosion and erosion. To reduce the effects of turbulent flows, seawater systems incorporate large diameter mild steel pipes, the ends of which open to the sea through sea chests where gate valves are fitted.

If a seawater cooling pipe bursts, both suction and discharge valves will have to be closed to prevent engine room flooding. In order to make sure the valves operate correctly when you need them to, open and close them at regular, say monthly, intervals. Seawater pipes are usually mild steel, but galvanised steel, copper or copper alloy are also used. Freshwater cooling pipes are generally made of mild steel.

Fresh water cooling system

Fresh water cooling system for marine diesel engine

A water cooling system for a slow-speed diesel engine is shown in Figure . 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 the other Individual coolers. With less equipment in contact with sea water the corrosion problems are much reduced in this system.

Fresh water cooling system for marine diesel engine

A sea water cooling system is shown in Figure . 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).

The upper sea suction valve is used while in port to prevent any mud or sand entering the cooling system. Its also used while sailing in shallow waters. The lower sea suction valve is used when sailing in deep water to avoid air entering the cooling system while the ship is rolling or pitching.

Central cooling system

In a central cooling system 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

The advantages of a central cooling system are;
  1. Less maintenance, due to fresh water system having cleaned treated water
  2. fewer salt water pumps with attendant corrosion and fowling problems
  3. simplified and easier cleaning of coolers higher water speeds possible with a fresh water system,
  4. resulting in reduced pipe dimensions and installation costs
  5. the number of valves made of expensive material is greatly reduced, also cheaper materials can be used throughout the system
  6. constant level of temperature is maintained, irrelevant of seawater temperature, also no cold startings, reduced cylinder liner wear, etc.

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