Optimization of the intermediate pressure for two-stage cycles with vapor-injection for subcritical and transcritical cycles.

In this paper, the optimum intermediate pressure of two-stage cycles is presented taking into account the fact that the heat sink does not have infinite capacity (constant temperature). The refrigerants R-290 and others (R-152a, R-22, R-32, R-410A, R-134a, and R-1234yf) were considered for the subcritical cycle optimization and the CO2 for the transcritical cycle optimization. In both cases, the intermediate pressure optimization was performed for two-cycle configurations (flash tank and economizer) considering a wide range of operating conditions and secondary fluid conditions. Based on the optimization results, correlations are proposed in order to estimate the optimum intermediate pressure for the two kinds of cycles. In the subcritical cycles, the subcooling is taken into account in the correlation, besides the evaporating and condensing temperatures. In supercritical cycles, the optimal rejection pressure of the gas cooler and the refrigerant outlet temperature of the gas cooler are taken into account in the correlation. Finally, the proposed correlations are used to calculate the optimum intermediate pressure of a sanitary hot water application using an air to water heat pump with R-290 (subcritical) and CO2 as a refrigerant (transcritical).