Design and testing of a combined conduction-convection rotary dryer for rice

A new rotary drum dryer prototype was designed, fabricated and tested to combine conduction and convection heat and mass transfer principles for partial paddy drying. Its main feature was the provision for ambient air forced inside the drum counter-flow to the direction of the cascading grains. Convection drying was combined with conduction heating of grain to increase moisture reduction rates and reduce the high relative humidity inside the drum. Preliminary drying trials of the new prototype confirmed that the airflow inside the drum was used for dryrating the hot grains and blowing away water vapor and humid air inside. Test results of the prototype showed that its partial drying capacity (5 rpm drum speed) doubled that of the benchmark pre-dryer and quadripled at 7 rpm, requiring only a single-pass operation. Fresh ambient air forced inside the drum brought about "dryeration" of the hot grains, resulting in the cooler grain output and increased moisture reduction rates. Results of the partial drying test of the new prototype using freshly harvested BPI-Ri-10 paddy (21 percent initial moisture) showed that partial drying capacity, find moisture content and moisture reduction rate were all significantly affected by counter-flow air velocity. Neither air velocity nor drum surface temperature nor their interaction influenced total milling recovery (TMR) and head rice recovery (HRR). Although this needed further validation, results showed that, in terms of milling yield and quality, it is better to perform single-stage rather than two-page drying for paddy with initial moisture close to 18 percent moisture content. Results of additional drying tests using re-wetted BPI-Ri-10 paddy showed that both drum speed and counter-flow air velocity significantly affected partial drying capacity. In addition to these two factors, drum surface temperature also significantly influenced final grain MC and MC reduction rate. Only drum speed significantly influenced total milling recovery and head rice recovery. The thermal or heat utilization efficiency of the new prototype was 50 percent higher than the highest value obtained with the benchmark rotary pre-dryer in a single-pass operation and 237 percent higher than that of the benchmark pre-dryer tested in this study, which required two passes. With the combined conduction-convection rotary dryer prototype, the objective of coming up with a new and significantly improved design based on the benchmark pre-dryer was realized