Experimental study on the performance of a quasi-two-stage heat pump sludge drying system

To address the issues of slow temperature rise in the drying chamber and elevated compressor discharge temperatures during the latter stages of drying in closed heat pump drying (CHPD) systems, this paper employs a dual-evaporator design to establish an experimental platform for a closed quasi-two-stage compression heat pump drying (HPD) system. Analyze the impact of the superheat setting value of the electronic expansion valve on system performance and compare it with a single-stage heat pump system. Finally, this system was employed to conduct drying characteristic experiments on the sludge. Results indicate that when the main EEV superheat increases from 1 K to 9 K, the compressor discharge temperature rises by 20.6% while compressor power decreases by 9.8%. The system heating capacity and coefficient of performance (COP) reach their peak values at a main valve superheat of 5 K, measuring 11.93 kW and 4.72, respectively. Set the main EEV superheat to 5 K. When the auxiliary EEV superheat increases from 11 K to 19 K, the compressor discharge temperature rises by 18.2%, while compressor power decreases by 5.9%. The system heating capacity and COP reach their peak at a superheat of 15 K, measuring 12.79 kW and 4.83, respectively. Compared to single-stage compression systems, the overall reduction in compressor discharge temperature for quasi-two-stage compression systems ranges from 9.2% to 11.7%. Increasing the supply air temperature and air velocity, as well as reducing the sludge thickness, all help to shorten the drying time, but the extent of reduction diminishes progressively. When the supply air temperature is 50 °C, the air velocity is 1.6 m/s, and the sludge thickness is 6 mm, the system achieves peak average COP and average SMER values of 4.17 and 1.56 kg/(kW·h), respectively.