Which of the following is NOT a method for treating boiler water externally?

The correct choice represents a method that does not involve the treatment of boiler water externally. Cooling fans are typically used for heat dissipation or cooling purposes and do not play a role in the conditioning or treatment of boiler water.

In contrast, reverse osmosis is a filtration process that removes impurities from water by pushing it through a semipermeable membrane, making it effective for treating boiler feed water. A water softener is specifically designed to reduce hardness in water by removing calcium and magnesium ions, preventing scaling in boilers. Distillers operate by heating water to create steam and then condensing it back into a liquid, effectively removing contaminants and providing purified water.

The other methods are all relevant techniques for treating boiler water externally, ensuring that the water entering the boiler system is free of impurities and minerals that could cause issues such as scaling or corrosion.