The machineries fitted on board ships are designed to work with maximum efficiency and run for long running hours. The most common and maximum energy loss from machineries is in the form of heat energy. This loss of heat energy has to be reduced or carried away by a cooling media to avoid malfunctioning or breakdown in the machinery. For this reason, cooling water systems are fitted on board ships.
Types of Cooling Systems
There are two cooling systems used on board for the cooling purpose:
1. Sea Water cooling system: Sea water is directly used in the machinery systems as a cooling media for heat exchangers.
2. Fresh water or central cooling system: Fresh water is used in a closed circuit to cool down the engine room machineries. The fresh water returning from the heat exchanger after cooling the machineries is further cooled by sea water in a sea water cooler.
Understanding Central Cooling System
As discussed above, in central cooling system, all the working machineries on ships are cooled down using circulating fresh water. This system comprises of three different circuits:
1. Sea water circuit: The sea water is used as a cooling media in large sea water cooled heat exchangers to cool the fresh water of the closed circuit. They are the central coolers of the system and are normally installed in duplex.
2. Low temperature circuit: The low temperature circuit is used for low temperature zone machineries and this circuit is directly connected to the main sea water central cooler; hence its temperature is low than that of high temperature (H.T circuit). The L.T circuit comprises of all auxiliary systems.
3. High temperature circuit (H.T): The H.T circuit mainly comprises of 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.
4. Expansion tank: The loss in the closed circuit of fresh water is continuously compensated by the expansion tank which also absorbs the increase in pressure due to thermal expansion.
Advantages of central cooling System
-Low maintenance cost: As the system runs with fresh water, the cleaning, maintenance and component replacement reduces.
-Less corrosion: Since the sea water system is only in the central part, the corrosion of pipes and valves decreases.
-Higher speed of fluid hence better heat exchange: Higher speed is possible in the fresh water system which results in reduced piping and low installation cost.
-Use of cheaper materials: Since the corrosion factor decreases, expansive materials are not required for valves and pipelines.
-Constant temperature level maintained: Since the temperature controlled is irrespective of sea water temperature, stable temperature is maintained which helps in reducing machinery wear down.
You may also like to read-Types of Heat Exchangers Used on Ship
Marine Aux Machinery by H.D.McGeorge