Thermodynamics

cen84959_ch03.qxd 4/1/05 12:31 PM Page 140140 | ThermodynamicsT, °C60017.310.8 5.02.40.50.00.00.050037.120.8 8.84.10.80.10.00.0IDEALGAS400300271.0 17.6152.720 MPa7.4 1.310 MPa49.516.7 2.630 MPa0.10.20.00.00.00.05 MPa25.72001 MPa6.07.60.50.00.0FIGURE 3–49Percentage of error([|v table v ideal |/v table ] 100)involved in assuming steam to be anideal gas, and the region wheresteam can be treated as an ideal gaswith less than 1 percent error.10000.001100 kPa10 kPa0.8 kPa0.010.111.6100.00.10.00.0100 v, m 3 /kgIt can also be expressed asIDEALGASZ = 1REALGASESZ> 1= 1< 1FIGURE 3–50The compressibility factor is unity forideal gases.Z v actualv ideal(3–19)where v ideal RT/P. Obviously, Z 1 for ideal gases. For real gases Z canbe greater than or less than unity (Fig. 3–50). The farther away Z is fromunity, the more the gas deviates from ideal-gas behavior.We have said that gases follow the ideal-gas equation closely at low pressuresand high temperatures. But what exactly constitutes low pressure orhigh temperature? Is 100°C a low temperature? It definitely is for mostsubstances but not for air. Air (or nitrogen) can be treated as an ideal gas atthis temperature and atmospheric pressure with an error under 1 percent.This is because nitrogen is well over its critical temperature (147°C) andaway from the saturation region. At this temperature and pressure, however,most substances would exist in the solid phase. Therefore, the pressure ortemperature of a substance is high or low relative to its critical temperatureor pressure.