Pressure-Temperature (P-T) Diagram

A P-T diagram is the most common way to show the phases of a substance. Figure 9 is the P-T diagram for pure water. A P-T diagram can be constructed for any pure substance. The line that separates the solid and vapor phases is called the sublimation line. The line that separates the solid and liquid phases is called the fusion line. The line that separates the liquid and vapor phases is called the vaporization line. The point where the three lines meet is called the triple point. The triple point is the only point at which all three phases can exist in equilibrium. The point where the vaporization line ends is called the critical point. At temperatures and pressures greater than those at the critical point, no substance can exist as a liquid no matter how great a pressure is exerted upon it.

Pressure-Specific Volume (P-n) Diagram

A P-n diagram is another common Figure 10 P-v Diagram for Water type of property diagram. Figure 10 is the P-n diagram for pure water. A P-n diagram can be constructed for any pure substance. A P-n diagram is different from a P-T diagram in one particularly important way. There are regions on a P-n diagram in which two phases exist together. In the liquid-vapor region in Figure 10, water and steam exist together.

For example, at point A, water with a specific volume (nf), given by point B, exists together with steam with a specific volume (ng), given by point C. The dotted lines on Figure 10 are lines of constant temperature. The quality of the mixture at any point in the liquid-vapor region can be found because the specific volumes of water, steam, and the mixture are all known. The quality can be found using the following relationship.

v = xvg + (1 - x)vf

where:
v= specific volume of the mixture (m3/kg)
x = quality of the mixture (no units)
vg= specific volume of the vapor (m3/kg)
vf= specific volume of the liquid (m3/kg)
vfg= specific volume change of vaporization (m3/kg) or vfg = vg - vf