Boiling is the addition of latent heat at constant temperature. It is of two types. Consider heating water in a pan. When the heating is relatively low, bubbles form at the bottom and rise up, carrying heat with them. The bottom surface is always covered by the water film, which keeps the surface cool and close to the liquid temperature. This is nucleate boiling, and all efforts are made to stay in this regime in a boiler as the water absorbs its latent heat.

When the heating is too high and heat flux is large, the water film at the bottom quickly evaporates and is replaced by a steam film with low conductance. As a result, the surface is no longer cooled adequately and tends to attain a temperature close to the heat source, becoming severely overheated in no time. This is film boiling, which must be avoided in boiler practice under all conditions. Water moving at velocities of 2-3 m/s at the entry to a tube can effectively prevent film boiling, such as in the evaporator surfaces of boilers.