The concentration of thermo-sensitive solutions can be successfully carried out by low temperature evaporation in membrane contactors. This process of membrane evaporation which is approaching to a sweep gas membrane distillation is based on the concentration of an aqueous solution by its continuous evaporation through a metallic hydrophobic membrane. The driving force of the process is the difference of water vapor pressure between the aqueous feed solution and a dry gas. This work is the first part of a study about membrane evaporation process carried out with flat hydrophobic metallic membranes. In this first part we present a detailed research on membrane characterization, describe the basic principles of the process and develop a complete mass and heat transport model which is validated with some experimental results. Simulations have been carried out in order to study theoretically the influence of different operating parameters and membrane structure on process effectiveness. (C) 2010 Elsevier B.V. All rights reserved.