3.3 Central cooling system
The machinery systems fitted onboard ships are
designed to work with maximum efficiency and run for long hours. The most
common and maximum energy loss from machinery is in the form of heat energy. This
loss of heat energy has to be reduced or carried away by a cooling medium, such
as a central cooling water system.
There are two cooling systems used onboard 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 machinery. The fresh water returning from the heat exchanger after cooling the machinery is further cooled by sea water in a sea water cooler.
Sea water is used as a cooling medium 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 a duplex. The HT circuit in the central cooling system mainly comprises of the jacket water system of the main engine where the temperature is quite high. The HT water temperature is maintained by low-temperature fresh water and the system normally comprises of the jacket water system of the main engine, FW generator, DG during standby condition, lube oil filter for stuffing box drain tank. The low-temperature circuit is used for low-temperature zone machinery and this circuit is directly connected to the main sea water central cooler; hence its temperature is lower than that of high temperature (HT circuit). The LT circuit comprises all auxiliary systems.
Important things to remember
- The cooling water in the system to be treated with chemicals
- The make-up of the central cooling system normally takes place from the FW expansion tank, which is filled with a drinking and wash water system, or, from the distilled water tank using the FW refilling pump
- If it is necessary to refill the system with a larger amount of water, this shall be supplied from the distilled water tank by the FW refilling pump via the LT system connection
- During overhaul/repair of the main engine, which requires FW cooling water inlet and outlet valves to be closed, the FW cooling pump and high-temperature circulation must be stopped and the air to control valve must be closed
- In standby conditions,
the transfer of surplus heat from diesel generators can be used for FW
generator service. In the jacket water inlet to the FW generator evaporating
section the high load inlet is to be replaced with the low load inlet.
- The circulating rate will depend on sea water temperature, engine load, the pressure drop across pumps and required heat removal from the system. The circulating rate can be adjusted by operating one or more pumps.
- Generally, two fresh water pumps are installed and one complete spare pump is stored close to the area of the working pump for quick installation in case one of the working pumps should fail.
- The central coolers in the fresh water system are generally of plate type with plates of titanium material.
- In the case of manual cleaning, the FW inlet and outlet valves shall be closed. In the case of chemical cleaning of the SW side the FW side may be kept open.