Study On Temperature And Humidity Controlled Microwave Drying System And Drying Characteristics Of Fruits And Vegetables

This subject mainly studied on the characteristics of temperature and relative humidity, discussed the feasibility of this current technology. Effects of some factors on the drying process were studied, the process was optimized, and the mathematic model of coupled drying process was simulated.Firstly, a new microwave drying system with temperature and relative humidity controlled was developed in this study. The system had five units, a microwave drying unit, a temperature monitoring and control unit, a one-way air flow control unit used to evacuate the gaseous moisture, a humidity measuring unit and a PC-based control unit. The microwave drying unit could change heating power linearly after modified with online mass weighting. As a single-loop control system, the temperature unit selected PID control algorithm to adjust microwave heating power so as to achieve the purpose of controlling the temperature of the products. It could provide accurate temperature and the error was less than ±0.5 ℃. The air flow unit and humidity measuring unit were to measure and control the humidity. Due to the change of the products’ moisture, it’s difficult to make the relative humidity constant. In this passage, air flow and auxiliary water were selected as two intermediate variables to control the humidity. The technology filled the gaps on microwave drying equipment humidity studies. The PC-based control unit could achieve automated control of the entire system by Labview software and a data acquisition card. It could record all the experimental datas, including temperature, humidity, air flow and power to prepare for subsequent experimental study.Using carrots as the research object, this study selected four schemes to change internal temperature of material, including constant temperature, changing the temperature stepwise from low to high and vice versa, changing the temperature linearly from 60℃ to 80℃ and vice versa, drying-rate control strategy. The results showed the drying-rate control strategy was the best, followed by 70 ℃ fixed temperature schemes, stepwise raising temperature schemes and linearly raising temperature schemes. To apply the drying-rate control strategy in production, a variable temperature controller was designed and the results were close to the drying-rate control strategy. However, its operation was simple and could be widely used in industrial production.This is the first to explore the effect of the humidity parameters in microwave drying industry at home and abroad. It was found that the relative humidity of drying container was determined by the relative humidity of air, the air flow rate, the drying rate and complementary water. However the relative humidity of air was constant in all the experiments and the drying rate was uncontrollable. Therefore the study selected changing the air flow rate and adding complementary water to make the relative humidity variable. Four schemes were designed, supplying the air flow with fixed rate levels, changing the air flow rate stepwise from fast to slow and vice versa, Changing the air flow rate linearly from 1 L/min to 6 L/min and vice versa, adding water in the container respectively. The results showed the low rate flow schemes and accelerated schemes could also help to the dried quality. When complementary water was used, the dried quality was better. Especially adding 25 ml water, the dried carrots were best. The results were applied into the actual production, putting fruits and vegetables directly into the microwave drying equipment with different number of openings sealed bag. The results showed drying results with 4 holes are close to 25 ml water, greatly improving the drying process. It could be widely used in the drying industry.Using rehydration rate as the index, through the Box-benhnken central composite experimental method of experimental design, the test determine the optimal process of the microwave drying of carrots. The optimal drying conditions were as follows: the internal temperature of the material 66 ℃, air flow rate of 3.5 L/min, complementary water of 25.33 ml. Under such conditions, rehydration rate of dried carrots was 3.32, validation of experimental and predicted values are in good agreement.