As discussed in the previous chapter on central cooling systems, all the working machinery on ships is cooled down using circulating fresh water. This system comprises three different circuits:
The seawater is used as a cooling media in large seawater-cooled heat exchangers to cool the freshwater of the closed circuit. They are the central coolers of the system and are normally installed in a duplex.
The low-temperature circuit is used for low-temperature zone machinery and this circuit is directly connected to the main seawater central cooler; hence its temperature is lower than that of high temperature (H.T circuit). The L.T. circuit comprises all auxiliary systems.
The total quantity of low-temperature or L.T. fresh water in the system is maintained in balance with the H.T. freshwater cooling system by an expansion tank which is common to both systems. The expansion tank used for these circuits is filled and makeup from the hydrophore system or from the distilled water tank using the F.W. refilling pump.
High-temperature circuit (H.T)
The H.T. circuit in the central cooling system mainly comprises the jacket water system of the main engine where the temperature is quite high. The H.T. water temperature is maintained by low-temperature fresh water and the system normally comprises the jacket water system of the main engine, FW generator, DG during standby condition, and a Lube oil filter for the stuffing box drain tank.
The HT cooling water system is circulated by electrical cooling water pumps, one in service and one on standby. During standby, the DG is kept warm by the circulating system from the DG in service. When ME is stopped, it is kept warm by HT cooling water from DG. If this is insufficient, the water may be heated by a steam-heated FW heater.
The loss in the closed circuit of the central cooling freshwater system is continuously compensated by the expansion tank which also absorbs the increase in pressure due to thermal expansion.
Temperature Control Valves
The heat absorbed by the H.T circuit is transferred to L.T circuit at the temperature control valve junction.
The outlet temperature of the main engine cooling water is kept constant at 85-95 by means of temperature control valves by mixing water from the two central cooling systems i.e. LT system into the HT system.
Check this video lesson on how the Main Engine Cooling System works –