Performance evaluation of the IRRI [International Rice Research Institute] rotary dryer in partial drying of paddy under field condition [Philippines]

Preliminary test results showed that both grain flow rate and retention time of paddy in the rotary dryer were affected significantly by hopper opening and drum inclination. At a constant drum rotational speed of 7.0 rpm and full hopper opening of 44 mm, the logarithms of grain flow rate and retention time were found to decrease and increase, respectively, as drum inclination increases. During partial drying of wet paddy samples, furnace temp. ranged from 400 to 800 deg C, drum surface temp. from 172 to 231 deg C, while paddy temp. ranged from 60 to 80 deg C. The initial moisture values of 22.2, 26.1, and 28.1% wet basis (or 28.4, 35.2 and 39.0% dry basis) of paddy samples were reduced to the safe moisture level of 18.0% w.b. (22% d.b.) in one pass at average drying times of 19.1, 28.3, and 29.7 minutes, respectively. The effective drying capacity of the rotary dryer was about 45.2 kg/hr or 0.45 tons/day, which may be adequate to relieve peak harvest demands thereby avoiding handling delay problems. Significant differences were obtained in both final moisture content and effective drying rate for different initial moisture contents and drum inclination values. Moreover, a logarithmic relationship was found between effective drying rate and initial moisture content, implying that the higher the initial moisture content, the higher the effective drying rates at low drum inclinations. Gelatinization of the starch molecules of high moisture paddy partially dried might have resulted in higher total milling and head rice recoveries than those of the control samples but this needs to be verified carefully in the laboratory. Statistical analysis showed a highly significant effect of initial moisture content especially on head rice yield for both one and two passes through the rotary dryer. To avoid the destruction of rice seed viability, the rotary dryer should not be used. Drying costs may be reduced by using crop residues as fuel, although the overall thermal efficiencies obtained for partial drying seem low.