Cooling system

Purpose and applications

The cooling system maintains engine component temperatures and removes heat from lubricating oil, charge air and other machinery.

Inside the engine, water flows through cooling jackets — spaces around cylinder liners and passages in cylinder heads. Metal separates these spaces from the working space containing combustion gases and the moving piston. Heat travels from the hot component surface through the metal into the water.

Three separate central-cooling circuits

The main example is a central system with three separate water circuits:

• HT — high-temperature freshwater. Cools the engine and transfers heat to LT water in a separate jacket-water cooler.
• LT — low-temperature freshwater. Collects heat from HT, oil coolers and charge-air coolers, then transfers it to seawater in the central cooler.
• SW — seawater. Passes through the seawater side of the central cooler and carries heat overboard.

HT and LT describe the different operating temperature levels of the freshwater circuits; SW stands for seawater. Water stays within its own circuit, while heat passes between circuits through exchanger walls.

An alternative: individual seawater-cooled exchangers

In an arrangement with individual seawater-cooled exchangers, seawater passes directly through the cooling side of the oil, air and engine-freshwater coolers. The fluid being cooled travels on the other side of each exchanger. Engine freshwater circulates between its jackets and its own cooler. In central cooling, the intermediate LT circuit concentrates the main seawater flow at the central coolers and reduces the extent of seawater pipework.

Main components

Seawater intake and supply

• A sea chest is a space beside the hull plating with an inlet grating, from which the pump draws seawater. High and low inlets, where fitted, allow selection according to draught, sea conditions, ice and water contamination.
• The seawater strainer intercepts larger debris before the pump and cooler. Accumulating debris increases resistance, affecting the pressure difference across the strainer and the water supply.
• Seawater pumps drive water through the central coolers to the overboard discharge. A standby pump can restore supply after a duty-pump failure. Installations with variable-speed drives adjust delivery within their permitted operating range.

Heat exchange and freshwater circulation

• The central cooler contains freshwater and seawater channels separated by plates or tube walls. Heat passes through the metal, while seals and connections keep the channels watertight.
• The LT pump supplies cooled freshwater to the jacket-water cooler, lubricating-oil coolers and charge-air coolers. Parallel branches divide the total flow between these exchangers.
• The HT pump maintains circulation through the engine jackets and jacket-water cooler. Water cools liners, cylinder heads and the exhaust-valve cooling spaces incorporated in the engine design.

Regulation, preheating and monitoring

• A three-way temperature-control valve divides water between the cooler and a bypass, a route around it. A temperature-sensitive element, or a sensor and actuator, changes the distribution in response to water temperature.
• The expansion tank accommodates water expansion and provides make-up. An open tank installed above the engine supplies static head, the pressure of the water column. Vent lines carry gas away from high points; an air separator removes free bubbles from the water flowing through it.
• A preheater and circulation pump maintain the temperature of a stopped engine. A freshwater generator connected to HT uses jacket-water heat to evaporate seawater in a separate space inside the unit.
• Thermometers, pressure sensors and level indicators provide information for observation and regulation. Alarms report deviations, while the installed protection restricts equipment operation under dangerous conditions.

How water and heat move

1. Before starting, the preheater warms HT water and the circulation pump carries heat to the engine jackets. Circulation warms the components and keeps the engine ready for operation.
2. With the engine running, the HT pump supplies its cooling passages. Water absorbs heat from the metal and leaves through the common outlet pipe. If a freshwater generator is connected, it uses part of this heat to produce distillate.
3. The temperature-control valve sends part of the water through the jacket-water cooler and part through the bypass. The reunited streams return to the pump suction and enter the engine again, completing the cycle.
4. In the jacket-water cooler, hot HT water transfers heat to LT water across the separating wall. The LT pump also circulates water through the other connected coolers; the warmed streams meet in the return main.
5. In the central cooler, LT water transfers the collected heat to seawater. Cooled freshwater returns to the LT pump for redistribution among the consumers.
6. The seawater pump draws through a sea chest and strainer, sends water through the seawater side of the central cooler and discharges it overboard. Engine and machinery heat ultimately passes to the sea.

The regulator maintains the target temperature by adjusting the proportion passing through the cooling path. During warm-up, much of the water returns through the bypass; as heat input rises, more passes through the cooler. The valve may divide streams before the exchanger or mix them afterwards. The installation diagram identifies the port connections and temperature-measurement point.

After stopping, the engine continues releasing stored heat. Continued circulation and subsequent preheating follow the engine instructions. Taking the freshwater generator out of service reduces useful heat recovery, so the jacket-water cooler takes a greater share of the duty. During filling and warm-up, designated vent lines release air and the expansion tank accommodates changes in water volume.

Measurements and their meaning

The transferred heat rate equals water mass flow multiplied by specific heat capacity and temperature change. Mass flow describes how many kilograms pass per second. At steady operation with small heat losses, the hot fluid releases approximately the same heat rate as the cold fluid receives.

For the same specific heat capacity, a lower mass flow rate corresponds to a larger temperature change. At the same engine load, an increasing difference between engine inlet and outlet temperatures can therefore indicate reduced flow. Assess cooler condition using both fluids’ temperatures, flow rates and pressure differences under comparable operating conditions.

Pressure affects water’s boiling temperature: higher absolute pressure, measured relative to a perfect vacuum, raises the boiling point. Adequate pressure at the pump inlet helps maintain stable delivery. When investigating circulation problems, check pressure at several points together with flow and temperature; local resistance or trapped air can restrict an individual branch.

Maintenance and observation

Work can be grouped into watchkeeping observations, periodic checks and maintenance shutdowns. The vessel’s planned maintenance system, PMS, and manufacturer instructions specify the intervals. Water analysis has its own schedule, which may require weekly checks. Pumps, strainers and coolers are opened after the relevant section has been taken out of service, isolated, cooled and depressurized.

Protecting the water spaces requires checks of make-up quality, pH, chlorides and corrosion-inhibitor concentration. This additive helps protect metal in the coolant. The test method and target concentration depend on the product; use a formulation approved by the engine manufacturer. Distillate is used after quality checks and addition of the specified treatment.

Record make-up quantities: increasing demand helps reveal leakage. A leak removes both water and additive, whereas evaporation predominantly removes water. Account for this distinction when restoring coolant composition. Confirm product compatibility and any mixing procedure with the manufacturer; establish dosage from system volume and test results.

Strainer blockage and cooler fouling require different tasks: cleaning the strainer element and cleaning exchanger channels or heat-transfer surfaces. Select the flushing method for the equipment’s materials and construction. Where reverse flushing is provided, follow its valve arrangement and permitted pressure difference. After maintenance, check tightness and restoration of normal flow.

Symptoms, causes and actions

Common questions

Conclusion

In the central system described here, heat passes from engine components to HT water, then to LT water and through the central cooler to the sea. Reliable operation requires stable circulation, effective temperature regulation, clean heat-transfer surfaces and suitable coolant chemistry.

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