Charge optimisation study of a reversible water-to-water propane heat pump

This paper presents the results and conclusions regarding the optimisation of the charge obtained throughout a thorough development study performed with a reversible water-to-water heat pump using propane as refrigerant. The paper first describes the influence of the charge on the capacity and COP of the unit at given operating conditions and then analyses the causes of such influence. Then, the theoretical study carried out with a detailed mathematical model to estimate the amount of refrigerant in each part of the unit is discussed. The second part of the paper is devoted to analysing the influence of the design of the evaporator (BPHE with and without integrated orifice distributor) and the operating conditions on the optimal charge. Finally, the effect of employing a POE or a mineral oil for lubrication on both performance and optimal charge is discussed.