7. Performance Assessment of a Two–stage Heat Pump–Drying System

In this study, energy and exergy analyses of a two–stage heat pump–drying system are conducted, and the performance of the overall system is then evaluated. The system has two cycles: a heat pump and a drying cycle. The working fluid of the heat pump and the drying cycles are R-134 A and air, respectively. The two–stage heat pump consists of two evaporators; one operates at high pressures while the other one operates at lower pressures for additional cooling and dehumidification of the drying air. After the condenser, two sub–coolers are used for additional heating. Also, the exergy destruction rates, energy and exergy efficiencies of each unit, cycle, and the overall system are calculated. Each unit and cycle’s exergy destruction rate is investigated to identify the corresponding unit’s relative irreversibility, and as a result, potential points to improve the system’s exergetic performance. Parametric studies are conducted to understand the effect of ambient temperature on exergy efficiencies and exergy destruction rates of each unit, cycle, and the overall system. The selection and design of these to reduce exergy destruction, and as a result, increase the exergy efficiency of the system by minimizing irreversibilities is discussed in this study. Furthermore, the effect of air mass flow rate on system COP is studied. The initial results with 0.5 kg/s air flow rate at ambient temperature and pressure of 5 °C and 1 atm gives energy and exergy efficiencies of 62 % and 35 %, respectively. The COP of the system is calculated to be about 3.8.