Thermodynamics

T340 | ThermodynamicsP 1 s 1 ≅ sT ƒ@T11}CompressedliquidT 3 sP 3 3}Superheatedvapor12Saturatedliquid–vapor mixture3T 2 s 2 = s ƒ + x 2 sx ƒg2}FIGURE 7–10The entropy of a pure substance isdetermined from the tables (like otherproperties).sThe value of entropy at a specified state is determined just like any otherproperty. In the compressed liquid and superheated vapor regions, it can beobtained directly from the tables at the specified state. In the saturated mixtureregion, it is determined froms s f xs fg 1kJ>kg # K2where x is the quality and s f and s fg values are listed in the saturation tables.In the absence of compressed liquid data, the entropy of the compressed liquidcan be approximated by the entropy of the saturated liquid at the giventemperature:s @ T,P s f @ T 1kJ>kg # K2The entropy change of a specified mass m (a closed system) during aprocess is simply¢S m¢s m 1s 2 s 1 21kJ>K2(7–12)which is the difference between the entropy values at the final and initialstates.When studying the second-law aspects of processes, entropy is commonlyused as a coordinate on diagrams such as the T-s and h-s diagrams. Thegeneral characteristics of the T-s diagram of pure substances are shown inFig. 7–11 using data for water. Notice from this diagram that the constantvolumelines are steeper than the constant-pressure lines and the constantpressurelines are parallel to the constant-temperature lines in the saturatedliquid–vapor mixture region. Also, the constant-pressure lines almost coincidewith the saturated liquid line in the compressed liquid region.T, °C500400300200Saturatedliquid lineCriticalstatev = 0.1 m 3 /kgP = 10 MPaP = 1 MPa100v = 0.5 m 3 /kgSaturatedvapor lineFIGURE 7–11Schematic of the T-s diagram forwater.0 1 2 3 4 5 6 7 8s, kJ/kg • K