How charge air coolers of marine engines works

Charge air coolers or scavenge air coolers are used to cool down the charge air, usually after supercharger or turbocharger compressor in diesel engines. In marine engines, the charge air temperature after the T/C compressor can rise upto 145-160 deg Celsius.

The increase in temperature of air is not at all a good thing for the efficiency of the plant, as efficiency increases proportionally with charge air density. Thus, this air needs to be cooled to the lowest possible temperatures, to increase the density of the air, and thus the efficiency of the plant.

The volume of air handled is so huge that the LT system of the ship ( central fresh water system ) cannot handle the heat transfer. A LT system which can cool the charge air supplied to ME needs huge plate coolers, which are not worth it. So, a conventional tube type cooler with sea water as cooling medium is used for this purpose.And please note that the lowest charge air temp here possible is also sea water temperature , which is always less than the LT temperatures.

Direct cooling using sea water onboard ships is used only in Main Charge Air coolers for Scavenging due to this.

Sea water is very corrosive and is not usually used for cooling other engine parts directly.