The document describes the central cooling water system used on ships. It has two interconnected freshwater circuits called the high temperature (HT) and low temperature (LT) circuits. The HT circuit cools the main and auxiliary engines while the LT circuit cools other machinery. Temperature is controlled through the use of 3-way valves that mix water from the LT and HT circuits. A seawater circuit with heat exchangers and pumps provides cooling to the freshwater circuits. The central cooling system provides both heating and cooling to ship systems in a simplified configuration compared to individual cooling arrangements.

1. CENTRAL COOLING
WATER SYSTEM (HT & LT)
Dr. S. Thangalakshmi

2. ▪ The amount of maintenance required for any cooling system depends mainly on quality of water and
water treatment used in the system.
▪ When using seawater at temperatures above 50 degrees Celsius, corrosion and scale formation
become unacceptably high.
▪ The corrosion problem can to some extent be avoided by material selection although the cost
increases when using special alloys. Some of the materials are copper based alloys. Material
selection does not really help with scale formation and the only way to reduce this is to use better
quality water in the systems.
▪ This is the main reason for the development of central cooling systems in which much of the
traditional seawater system is replaced by freshwater circuits. This is now preferred option and is
likely that such system is employed on present vessels.

3. In Engine room the basic central cooling system has two interconnected freshwater circuits; one high
temperature circuit and the other a low temperature circuit and a simple sea water circuit. The freshwater
circuits are normally referred to as the HT and LT circuits to denote high and low temperature
respectively.

4. The HT circuit serves the diesel engine jacket cooling water systems for the main and auxiliary
engines. It can also be used for heating the freshwater generator. Typically the system operates to
give an outlet temperature from the engine of between 75 and 85 degrees Celsius.

5. The HT circuit usually consists of two circulating pumps which circulate the water through the main engine and
auxiliary engine jackets. A 3-way temperature control valve is provided which connects to the LT system. Provision is
also made to pass the HT water through the fresh water generator heat exchanger when required with a suitable
bypass facility when it is offline. The header tank and deaerator complete the system. For maintenance purposes,
suitable valves for isolating the engines and pumps are provided.

6. The LT circuit takes care of all cooling needs other than
the main and auxiliary engine jackets. This includes
cooling for the main and auxiliary engine lubricating oil
and charge air. Other machinery served by the LT
system includes air compressors and refrigeration and
air conditioning compressors and condensers. Typically
the LT system operates with a temperature of about 37
degrees Celsius the system usually comprises two
circulating pumps and a number of heat exchangers. The
heat exchangers include those for cooling lubricating oil
and air for the diesel engines, the air compresses,
refrigerating & air conditioning equipment and 2
seawater-cooled central coolers. The three-way
temperature control valve used in bypass diverter mode
controls the freshwater flow through the central coolers.
The LT system is connected to the HT system via the 3-
way mixing valve and a return connection.

8. Temperature control of the HT system is achieved
by a 3-way valve used as a mixing valve. The
sensor for controlling this valve is usually
positioned at the main engine jacket water outlet or
the return line to the circulating pumps. Constant
engine outlet temperature is achieved by mixing
water from the LT circuit into the HT circuit via this
3-way valve. As the HT water temperature rises at
the engine outlet due to increased engine power,
the sensor detects the rise and sends a signal to
the controller. It is then compared to the set-point,
and a corrective signal is issued to the control
valve, which opens to allow some LT water to mix
with the HT water, lowering the temperature. There
may be additional individual temperature control for
the auxiliary engines using thermostatically-
controlled valves.

10. Temperature control of the LT circuit is achieved by a 3-
way valve used to control the flow through the central
coolers in a bypass arrangement. The sensor for the valve
is positioned downstream from the point at which the return
from the central coolers re-enters the LT circuit. As the
temperature of the system starts to rise due to increased
thermal load, the sensor detects the rise and sends a signal
to the controller. When this value is compared to the set-
point, a corrective signal is given to the control valve, which
opens to allow some of the LT water to pass through to the
coolers, lowering the temperature. Any further increase in
thermal load on the system will cause the valve to divert
more flow to the coolers. In some systems there are
additional 3-way valves to control temperatures of the
individual services by bypassing the primary fluids at the
relevant heat exchangers. Minimum temperature for the LT
circuit may be achieved by using “split range control” to
control a heating unit in the system.

12. The sea-water circuit for a central cooling system is very simple. It usually consists of two seawater suction strainers,
two or more circulating pumps, and two heat exchangers for central coolers. These heat exchangers are usually of the
plate type; the material used for the pipe work is generally high-quality copper based alloy although some systems
use stainless steel. These materials produce maintenance requirements as they are resistant to corrosion and as
there is so little pipe work in the system; the cost is moderate. The plates for the heat exchanger are also made from
high-quality material which is resistant to corrosion. Stainless steel or titanium is often chosen since the high-strength
allows very thin plates to be used which improve heat transfer.

14. ✔ The central cooling system can be adapted to provide cooling or heating for most of the
machinery on board.
✔ The LT system can be used to provide the secondary fluid for many of the cooling
requirements such as air compressor, heat exchanges, and oil coolers.
✔ The HT system can be used as a source of heating. It can be used as the heat source for
the freshwater generator. It can also be used to heat domestic water and a preheat
machinery such as the main and auxiliary diesel engines.
✔ The seawater circuit can be used to provide preheated feed for the freshwater generator
and also the cooling requirements for any systems requiring direct seawater cooling to
achieve lower temperatures than can be achieved by the LT water. By doing this the
number of subsystems can be reduced and the overall plant is simplified.