REACHED at that pressure, but bubbles do not form until the liquid is heated few degrees above the saturation temperature, therefore the liquid is evaporated when it rises to the free surface, this FLUID motion is due to the natural convection currents and heat transfer is taking place between the heated surface and the liquid is due to the natural convection.At first, the number and size of bubbles are small, and bubbles rise up and condense in the liquid before reaching the interface.NUCLEATE Boiling: In nucleate boiling the bubbles start forming at an increasing rate and at an increasing number of nucleation sites.This regime can be separated into two regions, between A-B isolated bubbles are formed and dissipated into the liquid after they separate from the surface.The SPACE vacated by the bubbles is filled by the liquid in the vicinity of the heater surface, and the process is repeated.In B-C the temperature is further increased and bubbles form at such great rates at such a large number of nucleation sites that they form numerous continuous columns of vapour in the liquid.Transition Boiling: Once the temperature is increased past point C the heat flux decreases because a large portion of the surface is covered by the vapor film, which acts as an insulation due to the LOW thermal conductivity of the vapor relative to the water.Film Boiling: In this region, the heater surface is completely covered by the continuous stable vapor film, point D, where the heat flux reaches a minimum.The heat transfer rate increases with the increasing temperature due to radiation heat transfer between the vapour film and the liquid.