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
