Relationships for thermodynamic evaluation of ammonia-water heat pump systems

Thermodynamic property relations are relevant ingredients for the first and second simulation, analysis and optimization of ammonia-water heat pump systems. Many thermodynamic relations exist, many of which may be considered empirical, semi-empirical or in some case, derived with the use of some experimental data. The availability of experimental data especially the thermodynamic properties of the mixture over the whole operating range normally encountered in ammonia-water heat pump systems have allowed the formulation of unified and conceptually simple equations. This paper presents relevant thermodynamic property relationships for liquid enthalpy, vapour enthalpy, vapour pressure, water concentration in vapour. and liquid entropy, all in terms of concentration and temperature for ammonia- water mixtures which are developed using Chebyshev series surface fits. The application of the Chebyshev polynomials increases the accuracy and enables wider operational range to be covered in a single final property equation. This therefore reduces the need for interpolation of extrapolation during the design or simulation of systems operating on ammonia-water working fluid. These equations are very useful in the computer-aided analysis of ammonia-water heat pump systems.