Critical Point

CHANGE OF PHASE Thermodynamics Point C is the saturated vapor state, and line BC is the constant-temperature process in which the change of phase from liquid to vapor occurs. Line CD represents the process in which the steam is super-heated at constant pressure. Temperature and volume both increase during the process. Now let the process take place at a constant pressure of 100 psia, beginning from an initial temperature of 60°F. Point E represents the initial state, the specific volume being slightly less than 14.7 psia and 60°F. Vaporization now begins at point F, where the temperature is 327.8°F. Point G is the saturated-vapor state, and line GH is the constant-pressure process in which the steam is superheated. In a similar manner, a constant pressure of 1000 psia is represented by line IJKL, the saturation temperature being 544.6°F. Critical Point At a pressure of 3206.2 psia, represented by line MNO, there is no constant-temperature vaporization process. Rather, point N is a point of inflection, with the slope being zero. This point is called the critical point, and at the critical point the saturated-liquid and saturated-vapor states are identical. The temperature, pressure, and specific volume at the critical point are called the critical temperature, critical pressure, and critical volume. A constant pressure process greater than the critical pressure is represented by line PQ. There is no definite change in phase from liquid to vapor and no definite point at which there is a change from the liquid phase to the vapor phase. For pressures greater than the critical pressure, the substance is usually called a liquid when the temperature is less than the critical temperature (705.47°F) and a vapor or gas when the temperature is greater than the critical temperature. In the figure, line NJFB represents the saturated liquid line, and the line NKGC represents the saturated vapor line. Fusion Consider one further experiment with the piston-cylinder arrangement of Figure 4. Suppose the cylinder contained 1 lbm of ice at 0°F, 14.7 psia. When heat is transferred to the ice, the pressure remains constant, the specific volume increases slightly, and the temperature increases until it reaches 32°F, at which point the ice melts while the temperature remains constant. In this state the ice is called a saturated solid. For most substances, the specific volume increases during this melting process, but for water the specific volume of the liquid is less than the specific volume of the solid. This causes ice to float on water. When all the ice is melted, any further heat transfer causes an increase in temperature of the liquid. The process of melting is also referred to as fusion. The heat added to melt ice into a liquid is called the latent heat of fusion. HT-01 Page 36 Rev. 0