(c) (e) Figure 6-1 Phase Change Process of Water at Constant Pressure To analyze this mixture of saturated liquid and saturated vapor properly, we need to know the proportions of the liquid and vapor phases in the mixture. This is done by defining a new property called the quality x as the ratio of the mass of vapor to the total mass of the mixture where Quality has significance for saturated mixtures only. It has no meaning for compressed liquid or superheated vapor. Its value is between 0 and 1. The quality of a system that consists of saturated liquid is O, and the quality of a system consisting of saturated vapor is I (3)Superheat stage: saturated vapor to superheated vapor At the saturated vapor state 4, further transfer of heat results in an increase in both the temperature and the specific volume(Fig 6-1(e). At state 5, the temperature of the vapor is, let us say, 150C; and if ome heat is transferred from the vapor, the temperature may drop by somewhat but no condensation will take place as long as the temperature remains above 100oC (for p=l atm). a vapor that is not about to condense(i.e, not a saturated vapor) is called a superheated vapor Therefore, water at state 5 is a superheated vapor. The temperature difference between superheated vapor and saturated vapor is called the degree of superheat, that is At=t-t 6.2.2 Property Diagrams for Phase-Change Processes phase-change process of water at I atm pressure was described in detail in section 6. 2. 1. However, the variations of propertie during phase-change processes can be more d vapor easily studied and understood with the help of property diagrams. Now we repeat the above wet vapor pressures to develop the p-vand the T-sdiagrams for water Figure 6-2 p-v Diagram of Phase Change As I kg compressed liquid of water is heated at constant pressure, it follows a orizontal line on the p-v diagram, that is, the isobaric line changing from ao, a', a, a"to a, which oresents the state of unsaturated liquid, saturated liquid, saturated liquid and vapor mixture, saturated vapor and superheated vapor, respectively. Under other pressures, the isobaric lines can be obtained bo-b-b-b-b, do-d'-d100 Figure 6-1 Phase Change Process of Water at Constant Pressure To analyze this mixture of saturated liquid and saturated vapor properly, we need to know the proportions of the liquid and vapor phases in the mixture. This is done by defining a new property called the quality x as the ratio of the mass of vapor to the total mass of the mixture: vapor total m x m = where, m m m total liquid vapor = + Quality has significance for saturated mixtures only. It has no meaning for compressed liquid or superheated vapor. Its value is between 0 and 1. The quality of a system that consists of saturated liquid is 0, and the quality of a system consisting of saturated vapor is 1. (3) Superheat stage: saturated vapor to superheated vapor At the saturated vapor state 4, further transfer of heat results in an increase in both the temperature and the specific volume (Fig. 6–1(e)). At state 5, the temperature of the vapor is, let us say, 150°C; and if some heat is transferred from the vapor, the temperature may drop by somewhat but no condensation will take place as long as the temperature remains above 100°C (for p = 1 atm). A vapor that is not about to condense (i.e., not a saturated vapor) is called a superheated vapor. Therefore, water at state 5 is a superheated vapor. The temperature difference between superheated vapor and saturated vapor is called the degree of superheat, that is s t = t −t . 6.2.2 Property Diagrams for Phase-Change Processes The phase-change process of water at 1 atm pressure was described in detail in section 6.2.1. However, the variations of properties during phase-change processes can be more easily studied and understood with the help of property diagrams. Now we repeat the above process at different pressures to develop the p v − and the T s − diagrams for water. As 1 kg compressed liquid of water is heated at constant pressure, it follows a horizontal line on the p v − diagram, that is, the isobaric line changing from 0 to x a a a a a 、   、 、 , which represents the state of unsaturated liquid, saturated liquid, saturated liquid and vapor mixture, saturated vapor and superheated vapor, respectively. Under other pressures, the isobaric lines can be obtained, such as 0 x b b b b b − − − −   , 0 x d d d d d − − − −   .... Connecting saturated liquid state points of Figure 6-2 p v − Diagram of Phase Change Processes