ABSTRACT
Engineers have prepared tables of thermodynamic properties for most compounds of practical interest. In these tables, the values of the molar or specific volume, v, enthalpy, h, and entropy, s, are given as a function of temperature and pressure. To use these tables, one must first determine the physical state of the compound at the known values of T and P. The key piece of information is the value of the vapor pressure, P s (also called saturation pressure), at the temperature of interest. This value is normally reported in the first column of the tables, together with the values of v s , h s , and s s for both the liquid and the vapor. If the pressure of interest is higher than the saturation pressure, the compound is in the state of compressed liquid. In this case, as for all practical purposes, liquids are incompressible, the difference in pressure is ignored, and the values of the thermodynamic properties of the liquid compound are considered to be equal to those it has as saturated liquid. Only for work at high pressure may corrections be needed. In the case that the pressure of interest is lower than the saturation pressure, the compound is in the gas phase and its properties are those reported for that pressure. Usually, interpolation between two values of pressure from the tables is required. Sometimes, it is also necessary to interpolate between two values of temperature before proceeding with the interpolation of pressure. In technical language, a compound in the gas phase is referred to as a superheated vapor, meaning that its temperature is higher than the saturation temperature corresponding to its pressure.
