A model using the population balance concept is developed to predict the drop size distribution for small drops that grow by direct condensation. The resistances to heat transfer due to the drop (conduction through the drop, vapor-liquid interfacial resistance, drop curvature) and due to the promoter layer and the sweeping effect of falling drops are incorporated into the model and are also included in calculating the heat transfer rate through a single drop. The total heat flux is calculated from the drop size distributions and the heat transfer rate through a single drop. Drop size distribution for large drops that grow by coalescence is obtained from the works of Rose and Glicksman. The work in this paper reveals that to adequately calculate the heat flux, all the resistances to heat transfer due to the drop and the promoter layer have to be included. Considering heat conduction through the drop as the only resistance to heat transfer overestimates the heat flux. The amount of overestimation increases as the temperature difference increases. (C) 1997 Elsevier Science Ltd.