Thermodynamics

cen84959_ch03.qxd 4/1/05 12:31 PM Page 133Compared to saturated vapor, superheated vapor is characterized byLower pressures (P P sat at a given T)Higher tempreatures (T T sat at a given P)Higher specific volumes (v v g at a given P or T)Higher internal energies (u u g at a given P or T)Higher enthalpies (h h g at a given P or T)Chapter 3 | 133EXAMPLE 3–6Internal Energy of Superheated VaporDetermine the internal energy of water at 20 psia and 400°F.Solution The internal energy of water at a specified state is to be determined.Analysis At 20 psia, the saturation temperature is 227.92°F. Since T T sat , the water is in the superheated vapor region. Then the internal energy atthe given temperature and pressure is determined from the superheatedvapor table (Table A–6E) to beu 1145.1 Btu>lbmEXAMPLE 3–7Temperature of Superheated VaporDetermine the temperature of water at a state of P 0.5 MPa and h 2890 kJ/kg.Solution The temperature of water at a specified state is to be determined.Analysis At 0.5 MPa, the enthalpy of saturated water vapor is h g 2748.1kJ/kg. Since h h g , as shown in Fig. 3–41, we again have superheatedvapor. Under 0.5 MPa in Table A–6 we readT, °C h, kJ/kgT0.5 MPa200 2855.8250 2961.0Obviously, the temperature is between 200 and 250°C. By linear interpolationit is determined to beT 216.3°Ch > h gFIGURE 3–41At a specified P, superheated vaporexists at a higher h than the saturatedvapor (Example 3–7).h gh3 Compressed LiquidCompressed liquid tables are not as commonly available, and Table A–7 isthe only compressed liquid table in this text. The format of Table A–7 is verymuch like the format of the superheated vapor tables. One reason for the lackof compressed liquid data is the relative independence of compressed liquidproperties from pressure. Variation of properties of compressed liquid withpressure is very mild. Increasing the pressure 100 times often causes propertiesto change less than 1 percent.