A series air to air heat pump assisted-dryer has been designed and tested under various operating conditions. Air flow rates of 500 to 1400 m3/hr at supply air temperatures of about 25 to 50 °C are used. This temperature range covers ambient air and warm air heated by either solar collector or waste heat recovery device. R22 is used as the working fluid in the heat pump circuit. The results showed that average refrigerant temperature lift of 33°C and Rankine coefficients of performance (COPR) up to 4.6 are obtained. This (COPR) proved that the heat pump is an efficient device that saves energy and money. Also, exergetic efficiency of the heat pump up to 0.35 and outlet air temperatures of 55°C are achieved. This air temperature level is adequate for drying of many products.
The heat pump assisted-dryer is tested with continuous and intermittent drying techniques. The continuous drying is used to dry onion, carrot, potato and sweet potato in which specific energy consumption lower than the conventional dryers by 18 to 32% are achieved. The intermittent and continuous drying techniques are compared for onion drying. The results showed that the use of intermittent drying saves about 30% of the energy consumed by the continuous drying when the appropriate heating and resting periods are used.