Seawater Systems

A marine raw water cooling circuit is a chain, and its capacity is set by the weakest link, not by the exchanger. Water enters through a grating and a sea chest, passes a seacock, a strainer, a pump with an elastomer impeller, some length of reinforced hose, the exchanger tubes, and leaves through the exhaust or an overboard fitting. Growth on the grating, a strainer full of weed, a five-season impeller, a hose that has gone soft on the suction side: each of these reduces flow, and reduced flow looks precisely like a cooler that is too small.

Every method of making fresh water at sea concentrates salt somewhere, and that is the hard part. A flash evaporator takes seawater, heats it with engine jacket water or steam, drops the pressure so part of it boils, condenses the vapor into distillate and discharges the remainder as brine that is saltier than what came in. A reverse osmosis watermaker does the same job with pressure instead of heat, and its energy recovery and cooling exchangers still handle concentrated feed. Either way there is a stream hotter and saltier than seawater passing through tubes.

A condenser's job is to turn hot high pressure refrigerant vapor back into liquid, and how well it does that sets the whole plant's efficiency. Condensing temperature is seawater temperature plus the approach the condenser achieves. Every degree the condenser fails to achieve raises condensing pressure, which raises the compression ratio, which costs the compressor work and therefore fuel. Capacity falls at the same time, so the plant runs longer to do the same job and the compressor accumulates hours it did not need to.

Air conditioning is the load that runs when everything else is stopped. A vessel at anchor in the tropics has its main engines off, a genset running, and a chilled water plant working flat out through the afternoon and most of the night. The condenser is rejecting the whole cooling load plus the compressor work into seawater that is warm, still, and often drawn in a harbour rather than offshore. There is no forward speed helping the intake and no cool water passing the hull.

A fresh water isolation heat exchanger is the one cooler on the vessel that seawater is allowed to touch. Raw water from the sea chest passes through its tubes or plates and goes back over the side. On the other side of the wall runs a closed loop of treated fresh water, usually with glycol, that is pumped around every other cooling duty on board: main engine jacket water, lube oil, reduction gear, hydraulics, the chiller condenser, the refrigeration plant and the generator sets. Those coolers never see salt water, so they can be plain steel and copper, run without zincs, and last the life of the vessel.

Brackish water is not diluted seawater from a corrosion point of view; it is a different and harsher environment. Estuary, harbour and river-mouth water carries low and variable salinity, a heavy silt load, warm summer temperatures, and in working harbours a share of sewage, industrial run-off and chlorination from nearby intakes. Copper-nickel depends on a stable protective film that forms in clean, oxygenated, steady-salinity seawater. Brackish water gives it none of those conditions, so the film never settles and the tube wall goes on being consumed underneath it.

Putting seawater inside straight tubes means every surface it touches can be reached with a rod and a brush from the bonnet end. It also confines the corrosion problem: only the tubes, the tubesheet face and the bonnets have to be built for salt water, while the shell can be steel and the shell-side fluid can be whatever the duty needs.

Below roughly three feet per second, seawater drops its sediment and gives marine organisms somewhere comfortable to settle, so low-velocity tubes silt and foul. Above the upper limit for the alloy, the flow strips the protective film off copper-nickel faster than it can reform, and the tube thins from the inside starting at the inlet end.

Chlorination, elevated temperature, high velocity and concentrated brine are the four conditions that defeat copper alloys. Titanium is unaffected by any of them at the concentrations found in marine service, which is why evaporators, watermaker exchangers and many condensers are built with titanium tubes as a matter of course rather than as an upgrade.