In this study, the dependence of the mass transfer rate on the air velocity of a fin-tube heat exchanger under frosting conditions was investigated via experiments and a mathematical model. An air-side heat transfer coefficient was developed with respect to the air velocity to maximize the mass transfer rate under frosting conditions. The value of the air-side heat transfer coefficient at the maximum mass transfer rate was defined as the critical air-side heat transfer coefficient. The existence of a critical air-side heat transfer coefficient was verified, and equations for determining its value were derived. The frost growth was significantly retarded when the heat exchanger was operated under conditions that avoided the critical air-side heat transfer coefficient. (C) 2014 Elsevier Ltd. All rights reserved.