Chapter 6 Water Vapor As a kind of working medium, water vapor has many advantages, such as proper therme nuclear power plants and many other places. Water vapor is also used as a heat transfer medium in various heat-exchangers. In thermodynamic systems, water vapor is usually not far away from liquid and often experiences phase changes during working processes. Thus, it can not be treated gas. In engineering calculations, the thermodynamic properties of water and water vapor are usually obtained by using water vapor charts and tables It needs to mention that working mediums used in refrigeration engineering, such as ammonia, Freon, etc, and liquefied petroleum gases in gas engineering, such as propane, butane, etc have similar thermodynamic properties with water vapor and follow the same rule of phase changes as water vapor, but with different phase change parameters. If we can grasp the property of water vapor and its phase change characteristics, it may help us to comprehend the properties of other vapors very easily Therefore, we choose water vapor as a representative to discuss. The water vapor generation process at constant pressure is introduced firstly. Then, the water vapor chart and tables are also introduced. In addition, we emphasize how to use them to solve practical problems 6.1 Vaporization and condensation Any kind of substance, including water, may undergo various phase change processes. The process involving a phase change from liquid to vapor is called vaporization. The intensity of vapor increases as the temperature of the liquid increases. There are two ways of vaporization: evaporation and boiling. Evaporation occurs at the liquid-vapor interface when the vapor pressure is less than the saturation pressure of the liquid at a given temperature. Boiling, on the other hand, occurs at the solid-liquid interface when a liquid is brought into contact with a surface maintained at a temperature T sufficiently above the saturation temperature T of the liquid. The boiling process is characterized by the rapid motion of vapor bubbles that form at the solid-liquid interface, detaching from the surface when they reach a certain size, and attempting to rise to the free surface of the liquid. However evaporation involves no bubble formation or bubble motion The process involving a change from the vapor to liquid phase is called condensation. It consumes energy during vaporization process and releases heat during condensation process If the liquid is placed in a closed vacuum vessel, it vaporizes faster than it condenses due to the low vapor concentration in the space of vapor side at the beginning. Gradually, the vapor molecule accumulates and its density increases continuously, and the number of molecules returning to the liquid surface also increases. Thus, the vaporization rate decreases gradually and the condensation rate increases.As these two rates become equal, vaporization and condensation will reach a dynamic equilibrium. This equilibrium state is called the saturated state. At this state, the vapor pressure called saturated pressure and the temperature is called saturated temperature. The liquid and vapor at saturated state is called saturated liquid and saturated vapor, respectively. The rate of vaporization depends on the temperature of the liquid and the rate of condensation is related to the apor molecular density. This molecule density is proportional to vapor pressure. Thus, the rate of condensation depends on the vapor pressure. That is, the temperature at which water starts boiling depends on the pressure; therefore, if the pressure is fixed, so is the boiling temperature. At a gi pressure, the temperature at which a pure substance changes phase is called the saturation temperature T. Likewise, at a given temperature, the pressure at which a pure substance changesChapter 6 Water Vapor As a kind of working medium，water vapor has many advantages, such as proper thermodynamic properties, non-toxic, odorless, cheap, and so on. It is widely used in steam turbines, steam engines, nuclear power plants and many other places. Water vapor is also used as a heat transfer medium in various heat-exchangers. In thermodynamic systems, water vapor is usually not far away from liquid and often experiences phase changes during working processes. Thus, it can not be treated as an ideal gas. In engineering calculations, the thermodynamic properties of water and water vapor are usually obtained by using water vapor charts and tables. It needs to mention that working mediums used in refrigeration engineering, such as ammonia, Freon, etc., and liquefied petroleum gases in gas engineering, such as propane, butane, etc. have similar thermodynamic properties with water vapor and follow the same rule of phase changes as water vapor, but with different phase change parameters. If we can grasp the property of water vapor and its phase change characteristics, it may help us to comprehend the properties of other vapors very easily. Therefore, we choose water vapor as a representative to discuss. The water vapor generation process at constant pressure is introduced firstly. Then, the water vapor chart and tables are also introduced. In addition, we emphasize how to use them to solve practical problems. 6.1 Vaporization and Condensation Any kind of substance, including water, may undergo various phase change processes. The process involving a phase change from liquid to vapor is called vaporization. The intensity of vaporization increases as the temperature of the liquid increases. There are two ways of vaporization: evaporation and boiling. Evaporation occurs at the liquid–vapor interface when the vapor pressure is less than the saturation pressure of the liquid at a given temperature. Boiling, on the other hand, occurs at the solid–liquid interface when a liquid is brought into contact with a surface maintained at a temperature T sufficiently above the saturation temperature Ts of the liquid. The boiling process is characterized by the rapid motion of vapor bubbles that form at the solid–liquid interface, detaching from the surface when they reach a certain size, and attempting to rise to the free surface of the liquid. However, evaporation involves no bubble formation or bubble motion. The process involving a change from the vapor to liquid phase is called condensation. It consumes energy during vaporization process and releases heat during condensation process. If the liquid is placed in a closed vacuum vessel, it vaporizes faster than it condenses due to the low vapor concentration in the space of vapor side at the beginning. Gradually, the vapor molecule accumulates and its density increases continuously, and the number of molecules returning to the liquid surface also increases. Thus, the vaporization rate decreases gradually and the condensation rate increases. As these two rates become equal, vaporization and condensation will reach a dynamic equilibrium. This equilibrium state is called the saturated state. At this state, the vapor pressure is called saturated pressure and the temperature is called saturated temperature. The liquid and vapor at saturated state is called saturated liquid and saturated vapor, respectively. The rate of vaporization depends on the temperature of the liquid and the rate of condensation is related to the vapor molecular density. This molecule density is proportional to vapor pressure. Thus, the rate of condensation depends on the vapor pressure. That is, the temperature at which water starts boiling depends on the pressure; therefore, if the pressure is fixed, so is the boiling temperature. At a given pressure, the temperature at which a pure substance changes phase is called the saturation temperature Ts . Likewise, at a given temperature, the pressure at which a pure substance changes