divided into four regimes as illustrated by a typical boiling curve in fig2 the boiling curve is a log log plot of the heat flux per unit area q a against the temperature difference between the wall and the liquid tw in u2212 t l the four regimes are free convective boiling nucleate boiling vapor film boiling and radiation boiling the schematics on top of the curve represent the characteristics of each regime in free convective boiling the heated liquid next to the hot wall rises due to natural convection currents and evaporates at the vapor liquid interface in the nucleate boiling regime vapor bubbles form on the vessel wall and rise to the liquid gas interface without condensing in the bulk liquid as the temperature difference increases the bubbles coalesce on the wall surface and form a vapor film this is referred to as film boiling although the temperature difference is higher in this regime the rate of heat transfer is lower compared to nucleate boiling at still higher temperature difference the wall temperature increases significantly resulting in radiation heat transfer in this regime the heat flux to the liquid again increases with an increase in the temperature difference however this regime is of little practical use for heating due to wall temperatures greater than about 1000 u00b0 c near the melting point of the conventional vessel materials typical values of the heat transfer resistance in different boiling regimes for ammonia are listed in table 1 high heat transfer rates are obtained in free convection and nucleate boiling regimes at significantly low temperature difference tw in u2212 t l due to low resistance to.