Design And Experiment Of Direct Gas-heating And Microwave-assisted Corn Dryer

People regard food as their prime want. Grain is the material foundation for human beings to survive and multiply always. China is the biggest country of grain production and consumption in the world. Therefor grain security is not only a key issue for our livelihood but also an important precondition for ensuring a sustained and rapid growth of the national economy and building a harmonious society. Because of backward grain drying technology and low level of mechanization of grain drying in China, 5% of annual grain yield is lost. As one of the most important corn production area in China, Northeastern area is a quite centralized area of high moisture corn. People rely mainly on mechanical dryers to dry the high moisture corn to safe storage moisture.Because there is not enough time for natural-air corn drying due to the low temperature during corn harvest season and early frost. The study was supported by the sub-project"study of online classification and detection and direct gas-heating and microwave-assisted drying system for corn"of national high technology research and development projects(863). Base on the present drying conditions of the high moisture corn in the Northeast area in China, a direct gas-heating and microwave-assisted corn drying system was developed, which is characterized by high efficiency, energy conservation, low carbon, environmental protection and technology integration.According to the analysis of traditional drying technology and microwave drying process, a direct gas-heating and microwave-assisted drying process for corn was proposed. The key technical processes are as follows: the corn is preheated and dried by microwave and then dried by hot air; using natural gas as the hot air source without using the heat exchanger. 5HTW-1 direct gas-heating and microwave-assisted dryer was designed base on the processes, which has the design parameters: the maximum productivity is 1t/h, and the dropping extent was 5% to 7%, and when the production rate of the dryer is 0.5t/h and the dropping extent is about 10%. The dryer is composed of a microwave drying section and three hot air drying sections.The circulating hot air cavity was designed based on the feasibility analysis of the drying medium recycling. Reasonable cycle air flow was selected. When the production rate of the dryer is 0.5t/h and the dropping extent is 10.5%, the calculation results showed that after the recycled drying medium through the circulating hot air section, the natural gas consumption was 7.65m3/h, only 35% of the consumption without cycle utilization of the medium and the theoretical specific thermal energy consumption was 6055.3kJ/kg, a reduction of 58%. The continuously variable transmisson system was designed to meet the actual needs of production. The speed of the conveyor belt was continuously variable between 0~2.93m/min.Many of low-power household magnetrons were used to produce high-power microwave feedback instead of the high-power microwave source. The cooling system which can work at low temperature in winter in Northeast China was designed. The size of the microwave drying cavity was optimized by a computer program using indexes of oscillation modes in the cavity. The results showed that, when a=1.22m, b=0.51m, l=3.509m, the number of the oscillation modes in microwave cavity is up to 201. The maximum electric field intensity of microwave cavity with corn was 17.1kV/m, lower than the limit value of air breakdown strength(30kV/m). There was no high-frequency breakdown. According to the distribution rules of the feed ports, the positions of the feed ports were designed.The microwave leakage proof device, including the suppressors, mierowave oven doors and leakage proof plate, were designed to reduce the microwave leakage, ensureing the safety of operators and reduceing electromagnetic pollution. The performance test results of microwave leakage test showed that the maximum value of leakage through the feed inlet was about 3mW/cm2, which conforms to the China national safety standards.Orthogonal polynomial regression experiment was conducted and the regression model describing the relation between dropping extent, microwave power and natural gas consumption was established when the production rate was 0.5t/h. The model was M = 0. 065P-0.071V~2 +2.04V?3.29.The results showed that when V=9.84m3/h, P=9.1kW, the specific thermal energy consumption was minimum when dropping extent was 10.5%. The replicated tests were conducted for the parameter. The results showed that the dropping extent was 10.43±0.32%, the moisture uniformity 0.43±0.21%, the broken rate increases by 0.33±0.57%, the fissuring rate increases by 16.67±1.53% and the specific thermal energy consumption was 6278.48±101.64kJ/kg, 6.3% lower than that stipulated in national standards when the production rate is 0.5t/h. The specific thermal energy consumption will further decrease if the test is conducted based on the condition stipulated in the national standards.5HTW-1 direct gas-heating and microwave-assisted dryer has certain application value, and has reference value for direct gas-heating and microwave-assisted equipment to reduce thermal energy consumption, improve drying efficiency and quality of drying products.