An experimental investigation of a heat pump dryer used for drying of wheat

In this study, a fluidized bed type heat pump dryer was designed and constructed for drying granular wheat particles. The system consisted of two main units: fluidized bed dryer as air circuit and the heat pump unit as refrigerant circuit. In this thesis, the introduction of heat pump to fluidized bed type dryer, and the effects of swirling flow field in the bed, drying medium, on the wheat drying were experimentally investigated. Experiments were performed to investigate the effects of various system operating conditions and parameters on the performance of the system. These investigated operating parameters are the air mass flow rate, temperature of the air flow and swirl number of the flow field in the bed. Also, the various air circuit configurations including recycle of the exhaust air flow discharging from the bed into the ambient or evaporator or again the bed were investigated in order to test their effects on the system performance and efficiency. In order to investigate the system performance, in this experimental study, the measured parameters were: mass flow rate of air passing through the bed; dry and wet bulb air temperatures at inlet and outlet sides of the dryer bed and evaporator of the heat pump; dry and wet bulb temperatures and the absolute pressure of the ambient air; temperatures of the refrigerant working in the heat pump at the inlet and outlet sides of the compressor, condenser, evaporator and expansion valve; the gage iipressures of the refrigerant at the inlet and outlet sides of the compressor and expansion valve; power consumed by the compressor, centrifugal fan and electrical heater; axial and tangential velocity components of air flow in the bed and the static pressures at upstream and downstream sections of the bed. In order to evaluate the system performance, the calculated parameters were: moisture extraction rate, MER; specific moisture extraction rate, SMER; dryer efficiency, DE; coefficient of performance, COP, and swirl number, S. Among the tested configurations, the configuration 6 which has a closed cycle has the highest MER, SMER, DE and the second highest COP. Thus, the configuration 6 is recommended as the best configuration. Higher mass flow rates in the bed result in higher MER, but lower SMER and DE when higher air flow temperature in the bed result in higher MER and DE and lower SMER. The swirling flow which has a swirl number in the range of 0.2-0.85 in the bed increased the MER of the dryer by 5-25 per cent and the SMER by 12-28 per cent and the DE up to 38 per cent. When the cost rate of heat pump is compared with that of the direct electrical energy, it is observed that the heat pump is more economical device at the range of about 110 percent than the direct electrical heating. Keywords: Heat pump dryer, fluidized bed dryer, wheat drying, swirling flow m