A new methodology is presented for the thermal and mechanical design of a two-phase flow thermosiphon heat exchanger, using water as the working fluid. The thermal design is made by applying a computer program and taking some experimental input data from exhaust gases of a gas turbine, which are used to heat the inlet air in the turbine. A conceptual mechanical design was produced for the heat exchanger which consisted basically of a rectangular metallic plate to hold up the thermosiphons and to separate the heat exchange zones. A calculation methodology based on the determination of the thickness of the plate, with a fixed number of perforations, was used. It is concluded that the methodology presented, allows adequate thermal and mechanical design of heat exchangers employing two-phase thermosiphon tubes.