Entropic optimization of the economizer's pressure in a heat pump cycle integrated with a flash-tank and vapor-injection system

•Defining the Economizer Optimum Pressure (EOP) in a 2 stage heat pump cycle.•Evaluating the effects of condenser and evaporator temperatures and refrigerant on the EOP.•Proposing a general criterion to find EOP. The heat pump cycles with the vapor injection improve the efficiency coefficient by reducing the work of the compressor. Despite the extensive literature on this topic, there are many challenges in determining the optimized condition of the pressure of the economizer. In the current study, a heat pump cycle integrated with a vapor injection system and an economizer was optimized using the entropic analysis. The findings were also compared with optimization based on the energy analysis. A thermodynamic model based on the mass balance, energy, and entropy was offered for various refrigerants, including R125, R134A, R404A, R410A, and R507A. Furthermore, the Economize's Optimum Pressure (EOP) was determined based on the minimum entropy generation and the maximum coefficient of performance in different conditions. Based on the obtained findings, a dimensionless coefficient (α) was finally defined for the economizer's pressure. It was revealed that the optimum value of (α) in a wide range of conditions was in a range from 0.25 to 0.35.