Seawater Systems

Read this page before buying any cooler on this one, because most overheating complaints begin upstream of the exchanger. Raw water cooling treats intake, seacock, strainer, impeller pump and hose as a single circuit whose delivered flow at working RPM is the number that matters. A worn impeller or a collapsing suction hose gives the same gauge reading as an undersized unit, and a bucket-and-stopwatch flow check settles the argument in twenty minutes. The page also weighs a direct arrangement, seawater in every cooler, against central cooling, and shows how pass count holds tube velocity inside the alloy's safe window.

Freshwater generation is the one seawater duty where the fluid is hotter and saltier than the sea itself. A flash evaporator boils seawater under vacuum using jacket water heat and discharges concentrated brine; a reverse osmosis plant still passes concentrated feed through its cooling and energy recovery exchangers. Elevated temperature plus high chloride plus velocity is the exact combination that strips the protective film from copper alloys, so titanium evaporators and condenser sections are the standard build rather than an option. Ratings depend on brine concentration and operating vacuum, so these units are quoted per duty, not from a table.

A hold that takes longer to pull down each season is a condenser problem, and this page explains how to prove it. Refrigeration condensing temperature is seawater temperature plus the approach the condenser achieves, so lost surface shows up as rising head pressure, more compressor hours and more fuel long before anything leaks. The diagnostic is a logged gap between condensing pressure and seawater inlet temperature. Condensers run higher tube velocity than jacket coolers and scale on their hottest surfaces, which is why titanium condensers are specified for warm, fast or chlorinated water while small cold-water plants stay on copper-nickel.

Air conditioning is sized for the stopped vessel, which makes its condenser different from every refrigeration plant that runs underway. A chiller works hardest at anchor in a warm, still harbor with the main engines off and a genset carrying the load, so its condenser is selected for the warmest water the boat will ever sit in, not for cruising conditions. The symptom is warm cabins rather than an alarm, and the condenser water pump, strainer and through-hull are checked before the condenser is blamed. Copper-nickel serves temperate cruising boats; titanium is the choice for tropical, silty or chlorinated berths.

If you want to stop maintaining five or six seawater coolers and look after one instead, this is the architecture. A fresh water isolation exchanger takes the sea through a single unit built for it, with copper-nickel or titanium tubes and bronze or 316 bonnets, and circulates a closed treated loop to every other duty aboard. Engines, gears, hydraulics and condensers then run in clean fresh water with no zincs. The page covers how the loop is arranged, what sizes or confirms a unit, and where a titanium gasketed plate unit suits a tight space or a large load.

Boats that live on rivers and in harbors need a different metal, not a shorter cleaning interval. Variable salinity, silt at the inlet ends, warm sulfide-rich summer water and nearby chlorination keep the protective film copper-nickel depends on from ever stabilizing, so a bundle that should last a decade pits through in two or three years. The brackish water unit is the same shell-and-tube frame, most often the HC Series with the T option, built with titanium tubes and tubesheet on the identical footprint and connections. It is the page for a replacement that keeps failing on the same schedule.

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.