How does a marine heat exchanger work?

A heat exchanger transfers heat from a hotter fluid to a cooler one through a solid wall, without the fluids mixing, and a motor ship is essentially a network of them: jacket water coolers, lube oil coolers, charge air coolers, central seawater coolers, fuel oil heaters, condensers, refrigeration evaporators and the fresh water generator. The principle is conduction and convection: heat flows through the thin metal wall at a rate set by the surface area, the temperature difference between the fluids, and the overall heat transfer coefficient. Counterflow arrangement, with the fluids moving in opposite directions, gives the best mean temperature difference and is used wherever practical.

Two constructions dominate at sea. The shell-and-tube type carries one fluid through a bundle of tubes and the other around them inside a cylindrical shell, guided by baffles; seawater conventionally goes through the tubes, which can be cleaned by removing the end covers, and sacrificial anodes in the water boxes protect against galvanic corrosion. The plate type stacks thin corrugated plates, usually titanium for seawater duty, with gaskets directing the fluids into alternate gaps; the corrugations create turbulence and a very large area in a small volume, which is why plate coolers dominate central cooling systems. They can be opened, cleaned and re-gasketed aboard, and extended with extra plates if more capacity is needed.

The enemy of every exchanger is fouling. Marine growth, scale, mud, oil films and sludge coat the surfaces, insulate them and choke the passages. The operational signs are unmistakable: for a given load, the temperature difference between the two fluids grows, the cooled fluid runs hotter than its normal figure, and the pressure drop across the unit rises. Trend these numbers; a slowly rising lube oil temperature with unchanged seawater temperature is a fouling curve, not a mystery.

Practical care follows:

• Vent air on filling and after maintenance, since an air blanket kills heat transfer.
• Keep seawater velocities in the design band: too slow silts up, too fast erodes the tube inlets.
• Renew anodes and inspect for erosion whenever the unit is opened.
• On plate coolers, tighten the stack to the specified dimension, never further, and renew hardened gaskets rather than reusing them.

An exchanger that is clean, vented and correctly flowed holds its design temperatures for years; almost every cooling complaint in service traces back to one of those three.