Thermal energy structure of grain hot air drying and analytical method

Abstract: The impact to grain drying involves a number of factors, such as environmental factors, grain physical properties, and flow characteristics, as well as the processing technology and equipment geometry. The change of environmental conditions and physical property characteristics, and differences in processing technology, which makes the system energy loss, was a major difference in the quantity and quality. To essentially illustrate these differences, to improve the comparability of dryer performance test results, and to form objective and fair evaluation standards, the researchers investigated state parameters of grain drying systems and energy transfer by taking moisture migration as a certain amount of energy transfer. Based on the exergy analysis and thermodynamics, the thermal structure of grain drying and its transformation and transfer were analyzed. Using the induced air, the experiment was studied under the conditions of temperature was 26 to 35 ℃, relative humidity was 40% to 55% of ambient air, and the initial temperature and dry basis moisture content of paddy was 36℃ and 38.6%, respectively. The results showed that the paddy temperature reduced 11℃ and the average drying rate was 1.2%/h in drying 2 hours. The paddy temperature picked up to 29℃ when the dry basis moisture content of paddy reduced to 17%. Another experiment was investigated on a 3100kg circulating paddy dryer with hot air, which the air volume was 9556.645 kg/h and the ratio of drying and tempering is 1:3.2. The results showed that the unit gas consumption increased 3.8 times from 113.0 kg/kg to 546.4 kg/kg, the unit heat consumption increased 5.4 times from 2548.9 kJ/kg to 16352.7 kJ/kg, and the exhaust gas heat loss of drying chamber increased 3.9 times from 6.2% to 30.6%, when the dry basis moisture content of paddy decreased from 27.06% to 16.96%. The main reason that the average exhaust gas heat loss was higher, the average heat rate and exergy efficiency of drying chamber was lower, was the poor performance of the energy matching in drying process. The change of energy efficiency in dryer chamber was from -3.9% to 59.9%, which indicated that the energy consumption of grain drying not only had subjective exergy, but also contained objective exergy. This paper pointed out that the evaluation of drying process and dryer energy utilization efficiency could not just stay on the subjective thermal efficiency, and must consider the effect of objective energy. The result provided the reference for reasonably evaluating energy matching of drying system and dryer energy utilization efficiency, and forming a fair evaluation standard.