In a jacket water heated submerged tube evaporator, what is the primary factor that determines the rate of scale build-up on the submerged tube bundle?

Correct Answer: Option D — The jacket water inlet temperature to the submerged tube bundle.

Scale formation in evaporators is primarily driven by the temperature of the heating medium. Higher jacket water temperatures increase the surface temperature of the tube bundle, accelerating the precipitation of calcium and magnesium salts from the seawater, which adhere to the tubes as hard scale, reducing heat transfer efficiency.