When securing a jacket-water heated submerged tube evaporator for an extended period of time, you should __________.

Correct Answer: Option C — completely drain the unit

When securing a submerged tube evaporator for an extended period, it must be completely drained to prevent corrosion and the hardening of scale deposits. Leaving water in the unit promotes galvanic corrosion and allows residual salts to settle and solidify on the tube surfaces, complicating future maintenance.