Thermodynamics

cen84959_ch03.qxd 4/1/05 12:31 PM Page 128128 | ThermodynamicsT,°CT = 90°CSat. liquidEXAMPLE 3–1Pressure of Saturated Liquid in a TankA rigid tank contains 50 kg of saturated liquid water at 90°C. Determine thepressure in the tank and the volume of the tank.90v fP=70.183 kPaFIGURE 3–31Schematic and T-v diagram forExample 3–1.vSolution A rigid tank contains saturated liquid water. The pressure and volumeof the tank are to be determined.Analysis The state of the saturated liquid water is shown on a T-v diagramin Fig. 3–31. Since saturation conditions exist in the tank, the pressuremust be the saturation pressure at 90°C:P P sat @ 90°C 70.183 kPa1Table A–42The specific volume of the saturated liquid at 90°C isv v f @ 90°C 0.001036 m 3 >kg1Table A–42Then the total volume of the tank becomesV mv 150 kg210.001036 m 3 >kg2 0.0518 m 3P, psia50SaturatedvaporP = 50 psiaV = 2 ft 3T = 280.99FEXAMPLE 3–2Temperature of Saturated Vapor in a CylinderA piston–cylinder device contains 2 ft 3 of saturated water vapor at 50-psiapressure. Determine the temperature and the mass of the vapor inside thecylinder.Solution A cylinder contains saturated water vapor. The temperature andthe mass of vapor are to be determined.Analysis The state of the saturated water vapor is shown on a P-v diagramin Fig. 3–32. Since the cylinder contains saturated vapor at 50 psia, thetemperature inside must be the saturation temperature at this pressure:T T sat @ 50 psia 280.99°F1Table A–5E2FIGURE 3–32v gSchematic and P-V diagram forExample 3–2.vThe specific volume of the saturated vapor at 50 psia isv v g @ 50 psia 8.5175 ft 3 >lbm1Table A–5E2Then the mass of water vapor inside the cylinder becomesm V v 2 ft 3 0.235 lbm8.5175 ft 3 >lbmEXAMPLE 3–3Volume and Energy Change during EvaporationA mass of 200 g of saturated liquid water is completely vaporized at a constantpressure of 100 kPa. Determine (a) the volume change and (b) theamount of energy transferred to the water.Solution Saturated liquid water is vaporized at constant pressure. The volumechange and the energy transferred are to be determined.Analysis (a) The process described is illustrated on a P-v diagram in Fig. 3–33.The volume change per unit mass during a vaporization process is v fg , which