Abstract:
A closed cycle heat pump drying system was developed to study the thin layer and deep bed heat
pump drying characteristics of rice and chiliin industrial scale. Two units of dehumidifiers assembled with heat
pumps with a capacity of 0.95 kW were placed in a modified air-tight room (10 m x 3 m x 3 m) for
dehumidifying air. The commodities to dry were tested inside this dehumidified room for the drying rate
variations and the efficiency of the system. A microprocessor-based primary control system was used to vary the
duty cycle of dehumidifiers and thereby control the operational behavior of the dehumidifiers. The secondary
control system was used in controlling the relative humidity in the dehumidified room by using RH sensor and a
microprocessor-based control system. The variation of moisture content of rice and chili was observed in thin
layer and deep bed drying in the dehumidified room. After 65 minutes of operation, the relative humidity and
temperature of the empty store room reached 16% and30⁰C simultaneously. The calculated MER value for heat
pump drying system was 1.2 kg-water/h. The SMER for rice and chili was 0.7 kg-water/kWh and 1.4 kg water/kWh respectively. The optimum final moisture content of rice and chili could be achieved by rice and
chili. Thin layer heat pump drying rates were higher than the deep bed heat pump drying of rice due to the
higher air circulation through thin layers of rice. Chili had similar thin layer and deep bed heat pump drying
rates because of the low density and high porosity of chili layers. In conclusion, thin layer and deep bed heat
pump drying technology could be applied to achieve the optimum moisture content for rice and chili
respectively in industrial scale. Further studies are required to optimize the performance of the heat pump drying
system
