The permeation flux of water vapor in direct contact membrane distillation experiments has been studied. Permeation experiments with a PTFE membrane were conducted by using a hat membrane module with channels for hot and cold water created by both separators. We have studied two basic separator configurations : open flow separators and screen separators. The experimental results show as the permeate flux increases, the temperature and the recirculation rate increase. The use of screen separators also increases the flux. These results are explained by the equations for heat and mass transfer. These equations allow to calculate the heat transfer coefficients h in the Liquid films adjacent to the membrane. The h values suggest the appearance of turbulences when screen separators are used. Finally the temperature polarization coefficients are evaluated for different operational conditions.