CHANGE OF PHASE
If the temperature of the liquid is lower than the saturation temperature for the existing pressure,
it is called either a subcooled liquid (implying that the temperature is lower than the saturation
temperature for the given pressure) or a compressed liquid (implying that the pressure is greater
than the saturation pressure for the given temperature). Both terms have the same meaning, so
either term may be used.
When a substance exists as part liquid and part vapor at saturation conditions, its quality (x) is
defined as the ratio of the mass of the vapor to the total mass of both vapor and liquid. Thus,
if the mass of vapor is 0.2 lbm and the mass of the liquid is 0.8 lbm, the quality is 0.2 or 20%.
Quality is an intensive property. Quality has meaning when the substance is in a saturated state
only, at saturation pressure and temperature. The area under the bell-shaped curve on figure 6
shows the region in which quality is important.
Figure 6 T-V Diagram Showing the Saturation Region