CTHE_free is a simple but powerful tool for the thermodynamical design and rating of a concentric tube heat exchanger or a coaxial heat exchanger. It can extimate the capacity of the exchanger and it can evaluate the required surface. CTHE_free allows the use of several common liquids, gases and refrigerants, plus user defined fluid. The fluid database is the best available. The current release is dedicated to the single-phase thermal exchange (liquid or gas/vapor).