Ventilation Drying of Grain by Near-Ambient Temperature and Dehumidification Air

Abstract

Ventilation drying test of rice and wheat was performed with a pilot model of a heat pump dehumidifier (H. P. D.) under practical applications. The drying efficiency, the unevenness of drying, the energy consumption and cost, and the percentage of cracked rice were investigated. The results can be summarized as follows.
(1) Drying rate and drying efficiency: For rice harvested during the middle harvesting season, the average drying rate was about 0.17, 0.14, 0.12 (%/hr) when the relative humitidy of drying air was about 54%, 58%, 60%, respectively. For rice harvested during the early harvesting season, it was 0.19 (%/hr) when the relative humidity of drying air was 62%.
For barley, the average drying rate was 0.2 (%/hr) at 47.5% of relative humidity of drying air, and 0.24 (%/hr) at 46.8% of relative humidity for wheat. Air at a flow rate of 0.33-0.39 (m³/s·t), and temperature of 24.5-31.0°C was used in the all drying tests (Table 3, 4).
(2) Coefficient of constant rate of drying: There is a constant rate of drying and decreasing rate of drying for batch type deep bed drying condition. The coefficient of constans rate of drying in this study was 0.605-0.651 for rice and 0.521-0.580 for wheat (Table 5).
(3) Energy consumption and cost: The energy used to remove 1kg of water from grain in this experiment was between 397 and 713 (kcal/kg) at an average of 493kcal/kg. It costed 11.6-20.7 Yen. Compared with that of fire heated drying, it was cheaper. In this case, 38% to 50% of the energy consumption was for the heat pump and the rest was for the blower (Table 9).
(4) Effect of grain mixing: Turning over of the grain during the drying process caused elimination of the unevenness of drying temporarily, but had little effect on the drying rate. It is suggested, therefore, that mixing after drying has been finished would be more effective (Table 7).
(5) Humidity of the drying air when the drying is to be completed: In order to prevent the grain from over drying, it is necessary to provide the drying air which has the humidity corresponding to the required grain equilibrium moisture content. It is desired that the relative humidity of the drying air at the end of drying process is 60% for rice, and 50% for wheat (Table 8).
(6) Low temperature drying and cracked rice: Because the drying temperature was low and there was no over drying, the amount of cracked rice was lower compared to that from fire heated drying and sundrying, especially for the rice harvested during the early harvesting season.