Industrial - Scale Heat Pump Drying

Abstract

The prototype of an industrial-scale heat pump dryer (HPD) was designed, constructed and evaluated for performance. Drying chamber contained four trolleys with product capacity of 600-700 kg. In the experiments, papaya glace' was dried in closed system at an average temperature of 55°C, specific air flow rates of 25-32 kg dry air/h-kg dry papaya glace' and the evaporator by-pass air of 81%. The papaya glace' with initial moisture content of 83-86% dry basis was dried to final moisture content of 12-14% dry basis within approximately 32 hours. No significant variation of the average final moisture gradient along horizontal and vertical of dryer was observed (due to uniform air distribution within drying chamber). The results of experiment indicated that drying rate decreased rapidly with time, while total power consumption remained nearly constant. At high initial moisture content, the rate of moisture removal was higher than that of low initial moisture content. (The maximum average drying rate and SMER were 9.34 kg water evap/h and 0.782 kg water evap/kWh, respectively. The maximum average MER is 9.21 kg water cond/h, and the relative lowest average specific energy consumption is 4.92 MJ/kg water evap. The (COP_hp)_sys varied from 4.2 to 4.9 which was higher than (COP_hp)_used obtained by the internal condenser load which varied from 3.3 to 3.8. This attributed to the excess heat rejected to atmosphere through the external condenser.) The quality of dried papaya glace' in terms of color was acceptable due to low drying temperature. In conclusion, HPD achieved high energy efficiency, and it was feasible to be commercialized, particularly in food drying industry.