This paper describes a new construction of a three flow heat exchanger for liquefiers of small and mean efficiency. It is a highly efficient serpentine heat exchanger consisting of three tubes with different diameters; two of which are soldered on thermal contact and inserted into a third. Thermal characteristic tests of the heat exchanger are carried out and as a result of these, the coefficients of heat conductivity are defined under different conditions of work. On the basis of the experimental data obtained, the corresponding criterion equation of Nusselt is carried out. According to this, a simplified approximate estimate of heat exchange apparatuses of similar type is recommended.