Thermodynamics

cen84959_ch03.qxd 4/1/05 12:31 PM Page 119Chapter 3 | 119T, °C373.95Critical pointP = 25 MPaP = 22.06 MPaP = 15 MPaP = 8 MPaP = 1 MPaP = 0.1 MPaP = 0.01 MPaSaturatedliquidSaturatedvapor0.003106v, m 3 /kgFIGURE 3–16T-v diagram of constant-pressure phase-change processes of a pure substance at various pressures (numerical values arefor water).ture (179.9°C) at this pressure. Second, the specific volume of the saturatedliquid is larger and the specific volume of the saturated vapor is smallerthan the corresponding values at 1 atm pressure. That is, the horizontal linethat connects the saturated liquid and saturated vapor states is much shorter.As the pressure is increased further, this saturation line continues toshrink, as shown in Fig. 3–16, and it becomes a point when the pressurereaches 22.06 MPa for the case of water. This point is called the criticalpoint, and it is defined as the point at which the saturated liquid and saturatedvapor states are identical.The temperature, pressure, and specific volume of a substance at the criticalpoint are called, respectively, the critical temperature T cr , critical pressureP cr , and critical specific volume v cr . The critical-point properties ofwater are P cr 22.06 MPa, T cr 373.95°C, and v cr 0.003106 m 3 /kg.For helium, they are 0.23 MPa, 267.85°C, and 0.01444 m 3 /kg. The criticalproperties for various substances are given in Table A–1 in the appendix.At pressures above the critical pressure, there is not a distinct phasechangeprocess (Fig. 3–17). Instead, the specific volume of the substancecontinually increases, and at all times there is only one phase present.Eventually, it resembles a vapor, but we can never tell when the changeTT crLIQUIDv crP > P crP crCriticalpointPhasechangeVAPORP < P crFIGURE 3–17At supercritical pressures (P P cr ),there is no distinct phase-change(boiling) process.v