Аннотация:The energy separation of a gas with the Prandtl number unequal to unity in a heat exchanger comprising of two coaxial axisymmetric tubes through which the gas flows with sub- and supersonic speeds is investigated. Supersonic flow is assumed to be laminar, subsonic one is turbulent. Heat exchange between the two gas streams is due to the difference from unity of the recovery factor. The flow is described by one-dimensional gas-dynamics equations for averaged quantities.