Thermodynamics CHANGE OF PHASESublimationIf the initial pressure of the ice at 0°F is 0.0505 psia, heat transfer to the ice first results in anincrease in temperature to 20°F. At this point, however, the ice passes directly from the solidphase to the vapor phase in the process known as sublimation. Sublimation is a special termused for cases in which the transition between the solid phase and the vapor phase occursdirectly, without passing through the liquid phase. Further heat transfer would result insuperheating the vapor.TriplePointFinally, consider an initial pressure of the ice of 0.08854 psia. Again, as a result of heat transfer,the temperature will increase until it reaches 32°F. At this point, however, further heat transfermay result in some of the ice becoming vapor and some becoming liquid because it is possibleto have the three phases in equilibrium. This is called the triple point, defined as the state inwhich all three phases may be present in equilibrium.Figure 8 is a pressure-temperature diagram for water that shows how the solid, liquid, and vaporphases may exist together in equilibrium. Along the sublimation line, the solid and vapor phasesare in equilibrium, along the fusion line, the solid and liquid phases are in equilibrium; and alongthe vaporization line, the liquid and vapor phases are in equilibrium. The only point at whichall three phases may exist in equilibrium is the triple point. The temperature and pressure forthe triple point of water are 32.02°F and 0.08865 psia. The vaporization line ends at the criticalpoint because there is no distinct change from the liquid phase to the vapor phase above thecritical point.Rev. 0 Page 37 HT-01
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