Maintaining paddy quality by appropriate ventilantion under hot and humid climates

Product quality, drying capacity and energy consumption have to be taken into consideration in drying design Long period in-store drying of paddy may cause the deterioration of product due to successive fungi, yellowing and dry matter loss. Determination of air flow rate and thickness of grain bed is usually a problem due to time consuming during calculation process. The objectives of this paper are thus to demonstrate the development of a design chart for in-store paddy drying under tropical climates and to investigate appropriate design and operating drying parameters. First of all, a near equilibrium drying model was developed and then effects of heat and moisture liberated from respiration process were included to improve accuracy of the model. Comparative results between experiment and simulation showed that they were in good agreement. Effect of heat liberated from respiration process was found to be significant when the initial moisture content of paddy was high and it was negligible if the initial moisture content was low enough. Then simulation results under various conditions were used to construct four quadrant charts. From them, minimum specific air flow rates (cubic m/min-cubic m) which corresponded to the dry matter loss of 0.5 percent could be found. It was assumed that paddy would remain the same grade as it was at the beginning if the dry matter loss during drying was not higher than 0.5 percent. The minimum specific air flow rate increased with initial moisture content but decreased with bed thickness. Other interesting values such as pressure drop, specific energy consumption and drying time could also be obtained from the charts. These charts are expected to be useful for designers to reduce calculation time and to help select appropriate design and operating parameters. It was suggested that for the in-store drying of paddy under hot and humidimates, the specific air flow rate should be around 0.5 to 1.0 cubic m/min-cubic m of paddy, the bed thickness of 3 to 4 and the initial moisture content of 18 to 19 percent wet-basis. In order to minimize energy consumption, ventilation should be done only when inlet air relative humidity was lower than 75 percent.