How does a marine diesel engine heat exchanger work, and how should it be maintained?
My yacht's diesel has a freshwater cooling circuit and a seawater heat exchanger, but I'm unclear how the two work together. What should I check routinely, and when should I remove the end caps or clean the tube stack?
Answer
A marine diesel heat exchanger transfers heat from the engine's closed coolant circuit to seawater, keeping the two liquids separate. Maintain it by checking coolant level and seawater flow, inspecting hoses and seals, and cleaning the tube bundle at the engine maker's specified interval or when inspection reveals fouling.
How the cooling circuits work
The engine's circulating pump moves coolant through passages around the cylinders and cylinder head. As the engine warms up, a thermostat controls the coolant's route, allowing heat to pass to the exchanger once operating temperature is reached.
In a typical shell-and-tube exchanger, seawater flows through narrow tubes surrounded by engine coolant inside the housing. Heat passes through the tube walls. The closed circuit normally contains an approved mixture of antifreeze and corrosion inhibitor rather than plain fresh water.
A separate seawater pump, usually fitted with a flexible rubber impeller, draws water through the inlet seacock and strainer. Depending on the installation, it may also feed gearbox or oil coolers before or after the main exchanger. On most wet-exhaust installations, the warmed seawater then enters the exhaust mixing elbow and leaves through the exhaust outlet.
Some engines combine the exchanger with the exhaust manifold; others have a separate cylindrical housing. Tube material, end-cap design and the presence of sacrificial anodes vary by engine and year.
First checks for overheating
Check the seawater supply before dismantling the exchanger: a shut seacock, blocked strainer or damaged impeller can cause the same symptoms as blocked tubes.
Stop the engine before investigating overheating. Let it cool before opening the pressure cap, and close the seawater inlet before opening any part of the raw-water circuit.
- Confirm that the inlet seacock is fully open and the strainer is clear, with its lid seal seated properly.
- Check the exhaust outlet for its usual water discharge. The Moody 36 manual specifies stopping and investigating if no water appears within 20 seconds of starting.
- Check the cold coolant level at the filling point specified for your engine. Inspect hoses and clamps, and look for leaks.
- Inspect the seawater impeller. If blades are missing, recover the fragments from downstream hoses, coolers and the exchanger inlet.
- If flow remains poor, inspect the exchanger and exhaust mixing elbow for restrictions.
If seawater discharge appears normal but temperature rises under load, investigate coolant circulation, the thermostat and exchanger fouling. Coolant loss, an oily film or unexplained level changes need further investigation and may call for separate pressure tests of the two circuits.