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Traditional equilibrium thermodynamics can only solve a few equilibrium processes composed of continuous stable equilibrium states. However, the vacuum flash evaporation process is a typical unsteady process. The study of non-equilibrium thermodynamics of the two-phase flow model is helpful to improve our understanding of the basic law of the flash evaporation process. Based on the theory of non-equilibrium thermodynamics, the flash chamber in the vacuum flash ice making system was studied in this paper, and the possibility of non-equilibrium steady state evaporation with superheat was obtained. The chemical potential difference between liquid water and water vapor under non-equilibrium steady state conditions was determined, and the corresponding evaporation entropy was calculated. It is shown that the results obtained by equilibrium thermodynamics are only related to the temperature difference, while the results obtained by non-equilibrium thermodynamics are not only related to the temperature difference, but also the state of the evaporation process. This is because non-equilibrium thermodynamics considers the cooling of liquid water and the evaporation of water vapor as a whole, taking into account the interaction between the two processes. However, the traditional equilibrium thermodynamics theory divides the steady state evaporation process into two independent processes and ignores the influence of each other. [ABSTRACT FROM AUTHOR]