For removal of sludge from in the inside of the shell of a fuel oil heater or lube oil cooler, what is typically the preferred method of cleaning?

Correct Answer: Option A — Circulating hot distillate oil.

Sludge in fuel or lube oil coolers is typically composed of heavy oil residues. Circulating hot distillate oil acts as a solvent, effectively thinning and flushing the sludge from the shell side without the need for mechanical disassembly or the use of harsh, potentially damaging chemical cleaners.