Correlations are proposed for the wet surface ratio of a fin-and-tube heat exchanger with plain and wavy fin geometry under dehumidifying conditions. The 'Finite Circular Fin Method' (FCFM) is used for data reduction. It is found that the percentage of wet surface area increases with increasing fin spacing or number of tube rows and decreasing Reynolds number. Despite the addition of tube rows or reduced fin spacing the effective surface area is increased, and its influence on a partially wet surface is apparently the opposite. This is because adding tube rows will provide a more effective temperature drop in air flow than adding fins, Moreover. the heat and mass transfer characteristics of j(h) and j(m) increase with an increase in the area of wet surface. Correlations for prediction of the percentage of wet surface area are proposed. These correlations can describe 83.81% of the area of wet surface to within +/- 10%. (C) 2009 Elsevier Ltd. All rights reserved.