Most thermodynamic textbooks state that the specific volume, specific internal energy, and the entropy of fluids in the compressed liquid region are independent of pressure and vary only with temperatures. Therefore in the compressed liquid region, these properties at given pressures and temperatures are approximated by their saturated liquid properties at the given temperatures. Examination of property values in the compressed liquid region verifies that these assumptions are valid at low temperatures close to the triple point of the fluids. However, the data show that, with increasing temperature, the internal energy and entropy of fluids exhibit higher dependencies on pressure in the compressed liquid region. In a similar fashion, in most applications, it is assumed that the values of constant pressure specific heat (cp) and constant volume specific heat (cv) are approximately the same in the compressed liquid region. Again, examination of these properties in the compressed liquid region validates this assumption for water at low temperatures. However, with increasing temperatures away from the triple point, the deviations between the two specific heat values increase. For fluids other than water the values of cp and cv in the compressed liquid region are not very close, even at temperatures close the triple point. Thermodynamic property behavior of several common fluids in the compressed liquid region is examined in this paper. The paper presents data on the behavior of v, u, h, cp and cv in the compressed liquid region and establishes the range of pressures and temperatures in this region where it is valid to assume that the v, u, h, and s are functions of temperature only and the ranges for which the values of cp and cv can be assumed to be nearly the same.
Approximation of Internal Energy and Enthalpy of Fluids in the Compressed Liquid Region