Property Diagrams

The phases of a substance and the associations among its properties are most generally shown on property diagrams. A large number of various properties have been defined, and there are some dependencies among properties. For illustration, at standard atmospheric pressure and temperature above 212°F, water exists as steam not a liquid; it exists as a liquid at temperatures between the range of 32°F and 212°F; and, it exists as ice at temperatures below 32°F. Additionally, the properties of water, ice, and steam are associated. The saturated steam at 212°F and standard atmospheric pressure has a specific volume of 26.8 ft³/lbm. At any other pressure and temperature saturated steam has a various specific volume. For illustration, at 544°F and 1000 psia pressure, its specific volume is 0.488 ft³/lbm.

There are five fundamental properties of a substance which are generally shown on property diagrams. These are: temperature (T), pressure (P), specific volume (ν), specific enthalpy (h), and specific entropy (s). Whenever a mixture of two phases, like water and steam, is included, a sixth property, quality (x), is also employed.

There are six various kinds of normally encountered property diagrams. These are: Pressure-Temperature (P-T) diagrams, Pressure-Enthalpy (P-h) diagrams, Pressure-Specific Volume (P-ν) diagrams, Enthalpy-Temperature (h-T) diagrams, Temperature-entropy (T-s) diagrams, & Enthalpy-Entropy (h-s) or Mollier diagrams.