Experiment And Research On The Control System Of Heat Pump Dryer

The drying operation has become an important part of agricultural products processing. The traditional drying method, in which materiel is dried by circular hot air, couldn’t meet the requirement of customers anymore because of the huge energy consumption, time consuming and low products quality. Although there are several new drying technologies such as vacuum freeze-drying, could produce high quality dried good, but the cost restrict the application and popularization.Heat pump drying technology is able to do well in both drying quality and the cost. It has been widely used in wood, agricultural products, medicinal herbs processing. However, most of the existing heat pump dryer, in which dry parameters such as temperature, humidity are still being controlled by human experience, is labor consuming, low precision and unsatisfied control effect.This subject combines the heat pump drying technology with automatically detection technology and SCADA technology, with the aim of establishing a precise control system of heat pump dryer as a platform for the further research about heat pump drying. The system is mainly constituted by heat pump drying equipment, data acquisition system and monitoring system.The heat pump drying equipment, includes condenser, evaporator, compressor, expansion valve, drying chamber, auxiliary heaters, fans composition, is a completely closed heat pump dryer in structure.The data acquisition system, in which PLC analog input modules are taken as the acceptor of temperature, humidity, weight, fluid speed signals from sensors, will convert the digital signals into engineering values after A/D conversion, and save it. Then, OPC server will access data by the communication protocol which is established between PLC and personal computer.Monitoring system is mainly with functions of PLC automatic control and host computer monitoring and controlling. The PLC program, in which PID algorithm is used for temperature and humidity controlling, frequency converter is used to control the flux by fan’s speed adjusting, has data processing capability. Monitoring and controlling platform which is developed from configuration software to the human-machine interface receive data from OPC server as an OPC client with functions of real-time data display, data archiving, drying setting, drying control, data and alarm information query and so on.The research took carrots as material for multiple sets of drying experiments to verify the performance of the heat pump drying control system. In the whole process of drying experiment, heat pump drying device run in normal, temperature and humidity was accurate and stable, the fan’s speed was constant, and fluid speed fluctuated slightly. The experiment showed that the drying quality of this system is excellent, and the drying data proves that the material weight, the moisture content, and the drying rate changed by the drying rule. Heat pump drying control system’s performance achieves the project objective, and it can be used as an experimental platform for in-depth study.By analyzing the archived data, an overlapping rule about drying rate in decline stage under different temperature through the time-drying rate curves has been found and a new research direction has been proposed for further research.