Experimental comparative study of an open and completely closed air source heat pump for drying sub-tropical fruits

Abstract

Heat pump drying (HPD) is an energy efficient and environmentally friendly drying method. Heat pumps (HPs) deliver more heat during the drying process than the work input to the compressor. The performance of HP and the dryer is significantly affected by the surrounding conditions. Therefore, the present study investigates the performance of an open and completely closed air duct HPD systems. Also, the influence of refrigerant charge and condenser fan speed was evaluated. The results demonstrated that, the completely closed air duct HPD system had slightly higher coefficient of performance (COP) and heating capacity than the open HPD system because of the heat from the moist exhausted air from the dryer were recovered at the evaporator. However, the drying air temperatures for the closed HPD system were lower than that of open system. This was due to the circulation of cold air from the evaporator. Also, COP increased with the increase of refrigerant charge in both systems. But, at high refrigerant charge the COP values for both systems were decreased because of the decrease in heating capacity which was possible due to the accumulation of refrigerant in the condenser. Similar trend of results were observed at high condenser fan speed, this was due to the increased fan power consumption. Therefore, for the system developed, the optimum refrigerant charge was observed to be 1650 grams and 840 rpm of condenser fan speed (60% of the full speed) and the obtained COP values at this point were 3.85 and 3.75 for the closed and open HPD system, respectively.