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Scaling only occurs on surfaces where the seawater is flashing, leaving behind scale if not treated. In a system that is being operated correctly, flashing only occurs in the flash chamber and not in any of the heat exchangers. This eliminates the possibility of scaling on the heat exchanger surfaces. If excessive scaling occurs in the flash chamber, you need to increase the seawater flow to reduce brine concentration.
An inlet strainer (which should be on anything that uses seawater) will remove most marine life/flotsam and the chemical dosing combined with the conditions of the evaporator will kill anything else and keep it from building up.
I ran an evap for 6 months continuously on my ship with only a strainer and a small chemical dosing pump for pretreatment. Worked perfectly fine and upon shutdown, there was very little fouling on some parts, none on most. The zincs needed to be replaced, but that was pretty much it!