Fundamental EoS Implementation for {Water+Ammonia} in Modelica

Abstract

The implementation of a library for the calculation of thermodynamic properties for the mixture {water + ammonia} based on a fundamental equation of state (EoS) for the Helmholtz free energy is developed and presented. The model uses the formulation of Tillner-Roth and Friend (1998a) in order to provide the best available single state thermodynamic data. The calculation of the vapour-liquid equilibrium (VLE) using the fundamental equation of state is examined. However due to difficulties found under certain pressure and temperature conditions, another method for calculating the VLE had to be used. The problems found included unreliable results and difficulties setting the initial values. Saturation temperature polynomials by Johnson et al. (2001) have been found to be faster and more reliable and have been implemented instead. It?s possible to calculate thermophysical properties in single and two-phase region at pressures from the melting point up to 40 MPa.