The objective of this research was to study the influence of drying air velocity characteristics on paddy drying. The pneumatic dryer prototype was used in this study. Test drying 15 kg of Hom Pathum paddy at 22 – 25% initial wet basis moisture content was tested drying under the drying air temperature 80OC until its moisture is 14% wet basis with the two drying air velocity characteristics. The first characteristic of drying air velocity is constant at 12.80 m/s and another is reduced velocity every 15 minutes throughout the test process from 12.80 - 8.44 m/s. Drying in a vertical tube with a diameter of 3.81 cm and height 1 m for study the drying behavior drying rate (DR) specific energy consumption (SEC) and thermal efficiency of drying. The results showed that drying with constant air velocity gives a shorter time higher drying rate lower primary SEC consumption and higher thermal efficiency than drying with reducing air velocity.

The purposes of this research were to develop and to test performance of mini heat pump dryer for slice banana. Drying chamber of 0.48×0.47×0.39 m3 was designed and it could contain three trays. The three trays had dimension of 0.33×0.409 m2. A blower with 174.6 W motor of air condition was modify and could adjust flow rates of three levels for air cycle system in dryer. The air system had bypass air at evaporator. The refrigerant of R-134a and four main parts were used in the heat pump system. The four main parts were a 497 W hermetic compressor a 1800 W condenser an expansion valve and a 1300 W evaporator. Experiments for test performance of the mini heat pump dryer used close loop air system an evaporator bypass air of 60 % a temperature of 60 oC and various flow rates of 0.117 0.128 and 0.140 m3/s. Slide bananas of 400-500 g had an initial moisture content of 244-281 %dry basis and were dried until final moisture content lower than 6.24±0.007 %dry basis. It was found that the flow rates of 0.117 0.128 and 0.140 m3/s had coefficients of performance (COPh) of 3.32±0.066 3.88±0.031 and 4.24±0.08 respectively and had specific moisture extraction rates (SMER) of 184.5±11.85 186.2±8.17 and 200.1±21.34 g/kWh respectively. The flow rate at 0.140 m3/s was the best condition because it had high specific moisture extraction rate.

This research developed a new model to determine the moisture diffusion coefficient for the thin-layer paddy drying with the spouted free-fall bed dryer. The design and construction of the dryer was done to experiment with the drying kinetics. The experiment has studied under conditions of hot air temperature of 60-150 °C the initial paddy mass of 5 - 15 kg and hot air velocity of 19 m/s. The results showed that the higher hot air temperature and the lower initial paddy mass resulted in a significant reduction in drying time. The experimental results were used to calibrate 12 of the most popular thin-layer drying models to find a suitable one considering based on the root mean square error (RMSE) chi-squared value (χ2) and the correlation coefficient (r). It was found that the popular thin layer drying model was able to accurately predict the effect of the moisture ratio considering both the influence of hot air temperature and the initial paddy mass in linear models. The highest accuracy was RMSE = 0.0279 χ2 = 0.00084 and r = 0.9938. In addition the experimental results have been used to synthesize the new diffusion moisture coefficient which is a function of initial paddy mass temperature and drying time. This developed model was based on an exponential function with only 7 model constants. The simulated results observed that the new model achieved the average error only 1.8 percent compared with the best model that uses up to 24 model constants.