Properties of Saturated and Superheated Steam

When the liquid evaporates in a limited closed space, the liquid molecules enter the upper space through the liquid surface and become vapor molecules. Because the vapor molecules are in turbulent thermal motion, they collide with each other, and collide with the container wall and the liquid surface. When colliding with the liquid surface, some molecules are attracted by the liquid molecules and return to the liquid to become liquid molecules. . At the beginning of evaporation, the number of molecules entering the space is more than the number of molecules returning to the liquid. As the evaporation continues, the density of the space vapor molecules increases continuously, so the number of molecules returning to the liquid also increases. When the number of molecules entering the space per unit time is equal to the number of molecules returning to the liquid, the evaporation and condensation are in a dynamic equilibrium state. At this time, although the evaporation and condensation are still going on, the density of the vapor molecules in the space no longer increases. The state at this time is called the saturation state. The liquid in the saturated state is called saturated liquid, and its corresponding steam is saturated steam, but it is only wet saturated steam at first, and it is dry saturated steam after the moisture in the steam is completely evaporated. The temperature of the steam does not increase from unsaturated to wet saturation and then to dry saturation. If the steam continues to be heated after dry saturation, the temperature of dry saturated steam will rise and become superheated steam.