Which type of blowdown is used to remove impurities that increase the surface tension of boiler water?

The correct choice, surface blowdown, is specifically designed to remove impurities from the surface of the boiler water that can affect the water quality and efficiency of the boiler system. These impurities often include oils, scum, and other debris that can accumulate at the surface and increase the surface tension of the water, leading to problems such as foaming and carryover. By performing surface blowdown, operators can maintain water quality, ensure effective heat transfer, and prevent operational issues in the boiler system.

Continuous blowdown typically manages the overall content of dissolved solids in the boiler water but does not specifically target contaminants at the surface level. Bottom blowdown, on the other hand, is aimed at removing sludge and other heavier deposits that settle at the bottom of the boiler, while blowoff refers to a more general practice of expelling water and potentially steam from the system to manage pressure or for maintenance purposes, rather than addressing surface impurities.