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Critical Point
Condensation

Thermodynamics Heat Transfer and Fluid Flow Volume 1 of 3
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Thermodynamics CHANGE OF PHASE Sublimation If the initial pressure of the ice at 0°F is 0.0505 psia, heat transfer to the ice first results in an increase in temperature to 20°F.   At this point, however, the ice passes directly from the solid phase  to  the  vapor  phase  in  the  process  known  as  sublimation.   Sublimation  is  a  special  term used  for  cases  in  which  the  transition  between  the  solid  phase  and  the  vapor  phase  occurs directly,  without  passing  through  the  liquid  phase.     Further  heat  transfer  would  result  in superheating the vapor. Triple Point Finally, 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 transfer may result in some of the ice becoming vapor and some becoming liquid because it is possible to have the three phases in equilibrium.   This is called the  triple point, defined as the state in which all three phases may be present in equilibrium. Figure 8 is a pressure-temperature diagram for water that shows how the solid, liquid, and vapor phases may exist together in equilibrium.  Along the sublimation line, the solid and vapor phases are in equilibrium, along the fusion line, the solid and liquid phases are in equilibrium; and along the vaporization line, the liquid and vapor phases are in equilibrium.   The only point at which all three phases may exist in equilibrium is the triple point.   The temperature and pressure for the triple point of water are 32.02°F and 0.08865 psia.  The vaporization line ends at the critical point  because  there  is  no  distinct  change  from  the  liquid  phase  to  the  vapor  phase  above  the critical point. Rev. 0 Page 37 HT-01







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