THERMODYNAMICS  THEORY



Tv Diagram

Phase Change Process
Under Constant Pressure
The Process of Creating Tv Diagram


The phasechange process of water under 1
atm described in the previous section can be repeated for different
pressures. Put all the processes in the Tv diagram. A line connected
by all the saturated liquid states is called saturated
liquid line. All the
saturated vapor states are connected to create the saturated vapor line.
When pressure becomes as high as P_{cr }(critical pressure),
the saturated liquid state and the saturated vapor state become a single
point in Tv diagram. This point is called
the critical point. For water, P_{cr} equals 22.09 MPa, T_{cr} (critical
temperature) equals
374.14 ^{o}C. At pressure above the critical pressure, there
will not be a distinct phase.
The saturated liquid line and saturated vapor line divide the region
on the Tv diagram into three regions: subcooled liquid region, saturated
liquidvapor region and superheated vapor region. 



The Construction of Tv Diagram 





Pv Diagram

Pistoncylinder Device for Pv Diagram
The Process of Create Pv Diagram


The water vaporization process can also be described
in Pv diagram. The method of creation the Pv Diagram is much like the
method
for
the Tv diagram. By considering the pistoncylinder device again, the
temperature keeps constant by
heat
transfer. The pressure is changed by removing
the weight. Like the process in previous section, the water will become
saturated liquid,
saturated mixture, saturated vapor and superheated vapor if enough heat
added. The process is repeated under several different temperatures and
plot them in a Pv diagram. By connecting all the saturated
liquid
states
under
different
temperatures, one
can create
the saturated liquid line and connecting
all the saturated vapor states, one can create the saturated vapor line
on the Pv diagram. There are three regions on the Pv diagram: subcooled
liquid region, saturated liquidvapor region, and superheated vapor region.




The Construction of Pv Diagram 





Pv Diagram Including Solid Phase



The above diagram can
be extended to include solid phase as well as the solidliquid and solidvapor regions.
Under some conditions, all three phase can coexist in equilibrium. On
a Pv diagram, it forms the triple line and on a Tv diagram, it forms
only a point called the triple point. 



The Construction of Pv Diagram of a Substance
that
Contracts (left) or Expends (right) on Freezing 





PT Diagram



The PT diagram is called the phase diagram
because three phases are separated from each other by three lines (sublimation,
melting, and vaporization). The three lines meet at the triple point where
three phases
coexist in equilibrium.




The Construction of PT Diagram 


