Fresh Water & Sea Water Cooling System For Marine Diesel Engine

Cooling System for Fresh and Salt Water for Marine Diesel Engine

Engines are cooled by circulating a cooling liquid through internal passageways within the engine. As a result, the cooling liquid is heated and chilled by a sea water circulating cooler. Without appropriate cooling, certain engine parts exposed to extremely high temperatures as a result of fuel combustion might fail prematurely.

Cooling allows engine metals to retain mechanical characteristics. Fresh water is the most commonly used coolant; salt water is not utilized directly as a coolant due to its corrosive impact. Since leaks into the crankcase would not cause difficulties, lubricating oil is sometimes utilized for piston cooling. Yet, due to its lower specific heat, approximately twice as much oil than water would be required.

Water circulated through pipes is used to cool machinery. The primary engine is cooled by two different but related systems: an open (sea-to-sea) system that takes water from and returns it to the sea (seawater cooling), and a closed system that circulates freshwater around an engine casing (freshwater cooling).

Freshwater is used directly to cool machinery, whereas seawater is utilized to cool freshwater after it has passed through a heat exchanger. Continuous fluid flow is a distinguishing property of an engine cooling system. Abrasive corrosion and erosion are caused by fluid in motion. Seawater systems feature large diameter mild steel pipes, the ends of which exit to the sea through sea chests with gate valves to decrease the impacts of turbulent flows.

To prevent engine room flooding if a seawater cooling line bursts, both suction and discharge valves must be closed. Open and close the valves at regular, say weekly, intervals to ensure they function properly when needed. Seawater pipes are often made of mild steel, however galvanized steel, copper, or copper alloy can also be found. Mild steel is commonly used for freshwater cooling pipes.

Fresh water cooling system
Figure depicts a water cooling system for a slow-speed diesel engine. It is separated into two systems: one for cooling the cylinder jackets, cylinder heads, and turbo-blowers, and another for cooling the pistons.

After exiting the engine, the cylinder jacket cooling water is sent to a seawater-circulated cooler and then to the jacket-water circulating pumps. The fluid is then circulated through the cylinder jackets, cylinder heads, and turbo-blowers. A header tank enables for system expansion and water make-up. Vents are connected from the engine to the header tank to allow air to escape from the cooling water. A heater in the circuit circulates hot water to warm the engine before starting it.

The piston cooling system uses similar components, but instead of a header tank, a drain tank is utilized, and the vents are then sent to high locations in the machinery space. To limit pollution from piston cooling glands to the piston cooling system exclusively, a separate piston cooling system is employed.

Sea water cooling system

Sea water cools the numerous cooling substances that pass through the engine. Individual coolers are often used for lubricating oil, jacket water, and the piston cooling system, with each cooler being circulated by sea water. Several modern ships employ a 'central cooling system,' which consists of a single enormous seawater-circulated cooler. This chills a supply of fresh water, which is subsequently circulated to the remaining Individual coolers. Corrosion concerns are greatly decreased with this system because there is less equipment in touch with sea water.
One of a pair of sea-water circulating pumps draws sea water from the suction and circulates it through the lubricating oil cooler, jacket water cooler, and piston water cooler before dumping it overboard. Another branch of the sea water main supplies sea water straight to the charge air to chill it (for a direct-drive two-stroke diesel).

While in port, the upper sea suction valve is utilized to prevent dirt or sand from entering the cooling system. It is also employed when sailing in shallow waters. When traveling in deep water, the lower sea suction valve is utilized to prevent air from entering the cooling system while the ship is rolling or pitching.

Central cooling system

The sea water circuit in a central cooling system is made up of high and low suction lines, usually on either side of the machinery space, suction strainers, and many sea water pumps. The seawater is pumped through the central coolers before being dumped overboard.

The fresh water system has both a low-temperature and a high-temperature circuit. The fresh water in the high-temperature circuit circulates the main engine and can be used as a heating medium for an evaporator if necessary. The main engine air coolers, lubricating oil coolers, and all other heat exchangers are circulated through the low-temperature circuit. A regulating valve regulates the mixing of water between the high and low temperature circuits. A temperature sensor sends out a signal.

The benefits of a central cooling system include:

less maintenance due to the fresh water system's pure treated water;

fewer salt water pumps with attendant corrosion and fowling concerns;

and simpler and easier cooler cleaning.

With a fresh water system, higher water speeds are feasible, resulting in smaller pipe dimensions and lower installation costs.
The number of valves made of expensive materials is considerably decreased, and cheaper materials may be utilized throughout the system to maintain a constant level of temperature, regardless of seawater temperature, as well as no cold startings, reduced cylinder liner wear, and so on.