normally contains some water vapor(or moisture)and is usually referred to as atmospheric air(or moist air). In contrast, air that contains no water vapor is called dry air(air without moisture). It is often convenient to treat atmospheric air as a mixture of water vapor and dry air. Although the amount of water vapor is small, it can not be ignored. The amount of water vapor in the air has significant impact on thermal comfort and many processes such as drying. Therefore, we can not treat moist air simply as dry air for analysis and calculation As dry air is far away from its critical point, it can be regarded as an ideal gas. Atmospheric air contains a very small amount of water vapor and the partial pressure of water vapor is very low. Thus, the water vapor in the atmospheric air can also be treated as ideal gas. That is, atmospheric air can be treated as a binary mixture of ideal gases, dry air and water vapor. However, phase change of water vapor in the atmospheric air often occurs, such as condensing, frosting, snowing, hailing, etc. Thus, it is not exactly the same as an ideal-gas mixture. Therefore, it needs some special state properties in order to describe the characteristics of moist air, such as the phase change of the water vapor and changes of the amount of water vapor 7.1.2 State of Atmospheric Air and its State Properties (1)The pressure of moist air As the Dalton law of partial pressure stated, the pressure of atmospheric air p is the sum of the partial pressure of the dry air pa and the partial pressure of the water vapor p,, that he partial pressure of water vapor reflects the amount of water vapor in moist air. It is referred to as apor pressure. The more moisture the atmospheric air contains, the higher the vapor pressure it has When there is no phase change in water vapor, both the dry air and the water vapor satisfy the ideal-gas relation. Tha Pv=raT Pv=RT where. T is thermodynamic temperature of the atmospheric air K:R,=2870J/(kg. K)and R,=4615J/(kg.K)are the gas constants of the dry air and the water vapor, respectively; v is the specific volume of moist air, m/kg In ventilation and air conditioning or drying processes, atmospheric air is often taken as the working fluid, where the total pressure of the moist air is the local atmospheric pressure. And eq (7-1) can be written as B=p=P+p The local atmospheric pressure varies with altitude, location, season and weather conditions, especially the altitude of a place (2) Saturated air and unsaturated Air When the temperature of the moist air ist, the corresponding saturated vapor pressure is p, (t) fo the moist air. If pr <Ps(t), then water vapor in the moist air is superheated vapor at the state A,as shown in Figure 7-1. The moist air at this point is known as unsaturated air. The unsaturated air is a mixture of dry air and superheated water vapor. Maintaining the temperature t constant, increasing the vapor pressure by adding some water vapor into the air, the state of the moist air changes from A towards C, as shown in Figure 7-1. When it reaches C on the saturated vapor line, P,=P,(),water vapor in the moist air becomes saturated vapor. At this point, the moist air is called saturated108 normally contains some water vapor (or moisture) and is usually referred to as atmospheric air (or moist air). In contrast, air that contains no water vapor is called dry air (air without moisture). It is often convenient to treat atmospheric air as a mixture of water vapor and dry air. Although the amount of water vapor is small, it can not be ignored. The amount of water vapor in the air has significant impact on thermal comfort and many processes such as drying. Therefore, we can not treat moist air simply as dry air for analysis and calculation. As dry air is far away from its critical point, it can be regarded as an ideal gas. Atmospheric air contains a very small amount of water vapor and the partial pressure of water vapor is very low. Thus, the water vapor in the atmospheric air can also be treated as ideal gas. That is, atmospheric air can be treated as a binary mixture of ideal gases, dry air and water vapor. However, phase change of water vapor in the atmospheric air often occurs, such as condensing, frosting, snowing, hailing, etc. Thus, it is not exactly the same as an ideal-gas mixture. Therefore, it needs some special state properties in order to describe the characteristics of moist air, such as the phase change of the water vapor and changes of the amount of water vapor. 7.1.2 State of Atmospheric Air and its State Properties (1) The pressure of moist air As the Dalton law of partial pressure stated, the pressure of atmospheric air p is the sum of the partial pressure of the dry air pa and the partial pressure of the water vapor pv , that is p = pa + pv （7-1） The partial pressure of water vapor reflects the amount of water vapor in moist air. It is referred to as vapor pressure. The more moisture the atmospheric air contains, the higher the vapor pressure it has. When there is no phase change in water vapor, both the dry air and the water vapor satisfy the ideal-gas relation. That is, a a p v R T = v v p v R T = where, T is the thermodynamic temperature of the atmospheric air, K; R a =  287.0 J / kg K ( ) and R v =  461.5 J / kg K ( ) are the gas constants of the dry air and the water vapor, respectively; v is the specific volume of moist air, m3 /kg. In ventilation and air conditioning or drying processes, atmospheric air is often taken as the working fluid, where the total pressure of the moist air is the local atmospheric pressure. And eq. (7-1) can be written as B p p p = = + a v The local atmospheric pressure varies with altitude, location, season and weather conditions, especially the altitude of a place. （2）Saturated air and unsaturated Air When the temperature of the moist air is t , the corresponding saturated vapor pressure is p (t) s for the moist air. If p p (t) v  s , then water vapor in the moist air is superheated vapor at the state A, as shown in Figure 7-1. The moist air at this point is known as unsaturated air. The unsaturated air is a mixture of dry air and superheated water vapor. Maintaining the temperature t constant, increasing the vapor pressure by adding some water vapor into the air, the state of the moist air changes from A towards C, as shown in Figure 7-1. When it reaches C on the saturated vapor line, p p (t) v = s , water vapor in the moist air becomes saturated vapor. At this point, the moist air is called saturated air