This study consists of evaluating the effect of the saturated humid air flow rate on the heat and mass transfer in a vertical exchanger. One of the affected parameters is the shear stress along the liquid vapor interface. The second studied point is the effect of flow rate on the diffusion of the vapor in the mixture. A simple mathematical model is built from one-dimensional equations for the mass conservation, enthalpy and momentum. In order to validate the model, an experimental apparatus allows to have access to the local heat transfer passing through the wall. The experimental results are in good agreement with the theoretical predictions. They show that the cocurrent gas/liquid flow allows a better recovery of energy.