When saturated vapor flows in a tube that is cooled by an exterior fluid, some of the vapor condenses on the tube wall and forms a liquid film. The main resistance to heat transfer for refrigerants and other low conductivity fluids is the resistance to conduction through the condensated film. There are essentially two different mechanisms of condensation in straight tubes: laminar film condensation and forced-convection condensation.

In this investigation the momentum and heat transfer analogy was applied to an annular flow model using the von Karman universal velocity distribution to describe the liquid film. Since the vapor core is very turbulent, radial temp gradients were neglected, and the temperatures in the vapor core and at the liquid-vapor interface were assumed to be equal to the saturation temp. Axial heat conduction and subcooling of the liquid film were also neglected. An order of magnitude analysis and non-dimensionalization of the heat transfer equations resulted in a simple formulation for the local heat transfer coefficient. The analysis was compared to expemnental data for Rl2 and R22, and the results were used to substantiate a general design equation). for forced-convection condensation.