Abstract <p>The article is devoted to the justification of the introduction of a flow thermodynamic potential to assess the maximum energy efficiency of gas systems in which there is no mechanical work, but a certain useful effect is realized: energy separation (vortex tubes), mass transfer and bubbling devices (cyclones, ejectors of various types), flow-through chemical reactors, plasmatrons, acoustic gas systems, Leontiev’s pipe, etc. Based on the results of the limiting energy theorem for gas systems, a relation for a thermodynamic potential of a special type is proposed. The thermodynamic flow potential has two equivalent forms: a potential as a function of the mechanical energy of the gas flow and a function of the internal energy of the gas flow.</p>