Heat exchangers in treated cooling water service should be designed and operated consistent with the…
Untreated Sea Water Cooling
Heat exchangers using untreated sea water should be designed and operated to avoid inorganic fouling, and to avoid or accommodate biological fouling. By operating exchangers near economic velocities and suppressing water vaporization, inorganic fouling can usually be avoided. Periodic cleaning is usually required to accommodate biological fouling.
Solubility of calcium sulfate usually limits the applications of sea water cooling. Calcium sulfate hemi-hydrate determines the limits in nonboiling applications; it precipitates at surfaces over 310°F. Calcium sulfate anhydrite determines the limits where subcooled boiling occurs; it precipitates on boiling surfaces over 230°F.
Alkaline scaling (e.g., CaCO3 or Mg(OH)2 precipitation) may occur in some coastal waters where the pH exceeds about 7.6. Acidification of such water is necessary to make it suitable for cooling purposes. Particulates do not cause fouling unless velocities are less than about half the economic velocity.
Biological fouling can be prevented by chlorination or by maintaining surfaces over 120°F. Chlorination is not always environmentally acceptable or practical. Maintaining heat transfer surfaces over 120°F has been used in subsea natural convection crude oil coolers needed to protect nonmetallic subsea production hoses. Keeping tube surface temperatures over 120°F is not practical in most cases.
Biological fouling occurs spontaneously in natural sea water between 32°F and 120°F. Particulates make up a major portion of the deposit and are bonded to each other and to the tube by biological material. Observed average fouling rates for titanium and 90-10 Cu-Ni tubes in sea water are shown in Figure 300-3.
The lower average fouling rate for 90-10 Cu-Ni tubes reflects the tubes periodic sloughing of deposit from itself. Copper slowly dissolves in sea water (corrodes) through the deposit and is toxic to lower forms of aquatic life. Copper corrosion and toxicity, however, are not sufficient to break the bond at the deposit-metal interface. Titanium is immune to corrosion, is not toxic, and therefore, is generally recommended for sea water service.
Larvae and small marine creatures pass through line filters (screens) and can grow large enough to block heat exchanger tubes. Back flushing lines once a shift eliminates this possibility.
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