Design and testing of a conduction-convection type rotary drum paddy dryer

A new rotary drum dryer prototype was designed to combined conduction and convection heat and mass transfer principles for partially drying of paddy rice. Its main feature was the provision for ambient air to be forced inside the drum wherein the cascading grains are heated by coming in contact with the hot inner drum surface. The premise here was that simultaneous convection drying and cooling would complement conduction grain heating and this would increase heat and mass transfer rates. Moreover, high relative humidity inside the drum limiting moisture evaporation from the grain surface would also be reduced. Preliminary drying trials of the new prototype confirmed that the proper function of airflow inside the drum was not for convection heating but for cooling the conduction-heated grains and blowing away the humidity inside the drum to promote moisture evaporation from the grain. Partial drying experimental results showed that the partial drying capacity of the new prototype was five times higher than that of the IRRI [International Rice Research Inst., Los Banos, Laguna, Philippines] pre-dryer. While it would require two or even three passes (when initial MC is more than 24 percent) through the rotary drum for the latter to partially dry paddy to 18 percent, it took only a single-pass operation for the former. In addition, grains coming out of the new prototype was 12 deg C cooler than the output from the IRRI pre-dryer. This reduced to half the cooling time needed for the grains to attain the partially dried MC level or an even lower level. Fresh ambient air forced inside the drum in a counter-flow direction brought about evaporative cooling of the hot grains as shown by the increase in moisture content reduction rate by as much as 50 percent at 2 m/s air velocity, and the cooler grain output. Partial drying capacity (PDC) as well as grain flow rate (GFR) were very significantly affected by air velocity. On the other hand, as counter-flow air velocity increased, the moisture content reduction rate (MRR) also increased. In terms of effect on total milling recovery (TMR) and head rice recovery (HRR), single-stage drying performed better than two-stage drying for paddy rice with MC already close to the 18 percent skin-dried level. Paddy samples (with initial MC of 21 percent) single-staged dried in a laboratory heated air dryer (LHAD) turned out significantly higher TMR and HRR values than the samples partially dried first in the rotary dryer then completely dried in the LHAD to 13-14 percent MC. The thermal efficiency (TE) of the new prototype was calculated to be 14.3 percent which was within the range of 11.8 to 22 percent efficiency of the IRRI pre-dryer. This low efficiency could be offset by the low operating costs incurred using rice hull as fuel. It is necessary to determine TE during continuous partial drying and compare it to experimental TE values