Straw can collect in the dryer and block rice flow. Fines reduce air flow and concentrations of foreign material may cause hot spots in storage. A dryer is controlled by setting air temperature and discharge rate with the goals of operating at maximum drying capacity while maintaining milling yield. Milling yield is kept high by limiting the points of moisture removed per pass. Rice is increasingly subject to cracking with increasing difference between moisture in the center versus the outside of the kernel.
Tempering equalizes moisture, so the next pass can safely remove additional moisture.
Generally more points of moisture can be safely removed when rice is wetter. Capacity is greatest when using high air temperature and fast discharge rates. Moisture removal is fastest at the beginning of drying, the first point of moisture is released much faster than the second point in a drying pass. Table 1 shows the value of operating dryers at high discharge rates and high air temperatures. Effect of different column dryer operating conditions on rice quality and dryer capacity.