Nucleate and film boiling

Boiling of liquid occurs as nucleate or as film boiling. Figure below illustrates a typical flux curve for water. In the region 1-2 the liquid is being heated by natural convection; in 2-3 the nucleate pool boiling occurs with bubbles forming at active sites on the heat transfer surface, natural convection currents set up. 

Q/A varies as Δt. Where n is 3-4, and the peak flux is at point 3 corresponding to the critical Δt for nucleate boiling; in 3-4 film boiling begins; and at 4-5-6 film boiling occurs. In film boiling heat us transferred by conduction and radiation through a film in the heating surface.

Note that the rate of effective heat transfer decreases beyond point 3 and it is for this reason that essentially all process heating-boiling equipment is designed to operate to the left of point 3.