Other than taking a scale thickness through a zinc plug opening of the water box of a feed water heater of a flash evaporator, what would be the BEST indication for a need to remove scale build-up?

Correct Answer: Option D — A rise in the feed water heater shell temperature and a rise in the feed water heater shell absolute pressure (decrease in vacuum).

Scale buildup on heat exchanger surfaces acts as an insulator, reducing heat transfer efficiency. This causes the feed water heater shell temperature to rise as the heat source struggles to transfer energy, while the absolute pressure rises (vacuum drops) due to reduced steam production, signaling the need for descaling.