There are two HT cooling pumps fitted. These are centrifugal pumps which maintain the cooling water circulating pressure at about 4 Bar.

To keep the system hot when the ship is in port, a steam heater is used. The water is pumped through the heater using a small circ pump.

The fresh water generator (or evaporator) uses the heat from the main engines in the cooling water to produce fresh water from sea water. The pressure in the evaporator is below atmospheric (i.e. a vacuum) so that the water boils at a lower temperature (about 65°C). On a large passenger vessel the evaporators can produce 500 tons/day. This version is for a container ship and can produce 30 tons/day.

The temperature of the cooling water is controlled automatically by a 3 way valve. If no cooling is required (when the engine is stopped) then the valve allows water to circulate through the valve back to the pumps.

If cooling is required, then water is diverted to the Low Temperature (LT) system and replaced with cool water from the LT system. The LT water is cooled using sea water in plate coolers.

Any air is removed in the dearation vessel before the water is led to the HT pumps

 To make up for any leaks in the system there is a header tank, which automatically makes up any deficiency. Vents from the system are also led to this header tank to allow for any expansion in the system and to get rid of any air. The header tank is relatively small, and usually placed high in the engine room. It is deliberately made to be manually replenished, and is fitted with a low level alarm. This is so that any major leak would be noticed immediately. Under normal conditions, the tank is checked once per watch, and if it needs topping up, then the amount logged.