Abnormal films that form in polluted waters and under some startup conditions are less protective and more easily disrupted. These films are replaced by normal films once the units are returned to normal operation in clean waters. The inherent resistance of copper-nickel alloys to biofouling enables units to operate for several months between the mechanical cleanings needed to restore original heat transfer capability. Chlorine injection will extend the intervals between mechanical cleanings to a year or more without detrimental effect on the corrosion resistance of copper-nickel alloys. Failures of tubing downstream of lodgments are really screen failures and must be treated as such in order to prevent recurrence.
Low velocity can be as damaging, or perhaps more so, than high velocity.
Units that are drained should be blown dry to remove water left in the bottom of tubing between low spots. Copper-nickel alloys are galvanically compatible with copper alloy tube sheets and waterboxes. Coated waterboxes require supplemental galvanic protection to prevent severe corrosion of the substrate steel that becomes exposed where the coating is damaged. Such units also require chlorine injection as the galvanic protection needed for the waterbox negates the inherent biofouling resistance of copper-nickel.