Optimization of hot-air microwave combined drying control system based on air outlet temperature and humidity monitoring

Microwave drying is one of the most important drying methods in agriculture. The online measurement and control of material dried, as a challenging research issue, is helping to improve the drying quality of the final product and the energy utilization efficiency of microwave dryers. In order to realize the online measurement of temperature in microwave drying, a detection strategy was based on the temperature and humidity monitoring at the air outlet of coupling hot air and microwave drying method. LabVIEW programming software was employed to collect the temperature and humidity signal value at the outlet of the dryer to estimate the drying degree of the material. According to the amount of moisture removal in real-time, the microwave input power was adjusted gradually in the microwave drying process. Taking potato as material dried, the microwave power of microwave and hot air coupling drying is monitored to realize the real-time regulation. The results showed that the total color difference value of the product of 3.09, the rehydration ratio of 2.92, the unit energy consumption of 17 419.35 kJ/kg, and the drying rate of 0.442 g/s were obtained. The minimum comprehensive weighted score of the product was 26.36. Compared with the orthogonal experiment, the optimal drying process was obtained as the total color difference value of the product of 2.84, the rehydration ratio of 3.01, the unit energy consumption of 17 419.35 kJ/kg, and the drying rate of 0.397 g/s. The minimum comprehensive weighted score of the product was 20.67, and the difference was not significant. The control strategy not only solves the non-uniform phenomenon of drying, and makes the microwave drying real-time and continuous, but also improves the drying efficiency and quality.