The Transfer of Heat Between Fluids

THE TRANSFER OF HEAT BETWEEN FLUIDS

The transfer of heat from one fluid to another across a solid boundary is of great importance in pharmaceutical processing. The system, which frequently varies in nature from one process to another, can be divided into constituent parts, and each part is characterized in its resistance to the transfer of heat. The whole system may be considered in terms of the following equation:

Rate at which heat is transferred  ∝  Total temperature difference/Total thermal resistance

A hot liquid passing through a heavily lagged metal pipe may be con-sidered as an example. The transfer of heat from the liquid to the pipe, con-duction through the pipe wall and across the insulation, and heat loss to the surroundings by natural convection can each be assigned a thermal resistance. A system in which steam is admitted to the outside of a vertical pipe containing a boiling liquid may serve as a second example. This arrangement is common in evaporators, and the evaluation of heat transfer rates demands a study of con-densation, conduction across the wall of the tube and any deposited scale, and the mechanism of boiling.