Heat Transfer to Boiling Liquids in an Evaporator

Heat Transfer to Boiling Liquids in an Evaporator

The heat required to boil a liquid in an evaporator is usually transferred from a heating fluid, such as steam or hot water, across the wall of a jacket or tube in or around which the liquid boils. A qualitative discussion of the methods used to secure high rates of heat flow can be based on equation (3.9):

 Q = UAΔT

where Q is the rate of heat flow, U is the overall heat transfer coefficient, A is the area over which heat is transferred, and ΔT is the difference in temperature between the fluids.

The overall heat transfer coefficient is derived from a series of individual coefficients that characterize the thermal barriers that oppose heat transfer. Thus, for the heating fluid, the film coefficient for a condensing vapor, such as steam, is high provided permanent gases and condensate are removed by venting and draining. With liquid heating media, the velocity of flow over the heat transfer surface should be as high as is practicable. If the solid barrier consists of a thin metal wall, the resistance to heat flow will be small. Resistance, however, is significantly increased by chemical scale, which may be deposited on either side. The accumulation of scale should be prevented. A glass wall may provide the largest thermal resistance of the system. Neglecting the thermal stability of the boiling liquid, circulation of the liquid should be rapid and, because of its influence on viscosity, the temperature of boiling should be as high as possible. Both factors promote high film coefficients on the product side of the wall.

Other factors described by the above equation are the area of the heat transfer surfaces, which should be as large as possible, and the temperature difference between the heating surface and the boiling liquid. As long as the critical heat flux is not exceeded, the latter should also be large.