The primary function of a boiler is to transfer heat from hot gases generated by the combustion of fuel into water until it becomes hot or turns to steam. The steam or hot water can then be used in building or facility processes.

Except for a small number of specialty models, boilers generally fit into one of the two common categories: fire-tube boilers and water-tube boilers.
Fire-tube boilers pass hot combustion gases through tubes submerged in water. Water-tube boilers, on the other hand, circulate water inside the tubes in a closed vessel filled with hot combustions gases. In either category the boiler feed water and fuel often contain impurities, which impairs boiler operation and efficiency. Chemical additives can be used to correct the problems caused by these impurities. To improve feed water quality, fuel oil condition, and steam purity, these chemicals can be injected directly into the feed water, steam or fuel oil.