For stubborn, hard scale deposits on the inside of the tubes of a seawater-cooled shell-and-tube heat exchanger, what is typically the preferred method of cleaning?

Correct Answer: Option D — Chemically cleaning with descaling compounds.

Hard, stubborn scale requires chemical descaling compounds to dissolve the mineral deposits that mechanical brushes cannot penetrate. These chemicals are formulated to react with the scale while minimizing corrosion to the base metal of the tubes, restoring the heat transfer efficiency of the exchanger effectively.