Design And Research On CO₂ Heat Pump Drying System

Drying is an important part of agricultural products processing.The heat pump drying technology which is able to do well in both drying quality and costs is particularly appropriate to agricultural products processing.The commonly used refrigerant of traditional heat pump drying system is Freon,which has low drying temperature,long time-consuming,and can do a lot of harm to the environment.It is essential to apply natural refrigerant CO₂ for ozone depletion.This topic will apply CO₂ trans-critical cycle to heat pump drying technology,combine with automatic detection and control technology to build environmentally friendly heat pump drying device.The device has a small heater,it can carry out the monitoring about drying process in an accurate,real-time and intelligent way,and ultimately achieves the data collection and archiving.The heat pump drying equipment,includes condenser,evaporator,compressor,expansion valve,drying chamber,auxiliary heaters,fans composition,internal heat exchanger,is a completely closed heat pump dryer in structure.The drying characteristics are reflected by the drying curves and drying rate curves,and the system performance of heat pump dryer is evaluated by COP and SMER.The coefficient of performance has been improved by adding the regenerator,which increasing the superheat of compressor suction.The topic analyzed the theoretical of drying medium and refrigerating agent.The topic design heat pump dryer with regenerator basis on the results of CO₂ technology research.The design includes each parameter of cycle point value such as the pressure in gas cooler,the temperature in gas cooler,the temperature in the evaporator.According to the requirements of general design,the crucial parts of the cooling and drying system are designed.Such as the selection of compressor used for CO₂,the design of condenser and evaporator based on current studying on the high pressure heat exchanger,the design of throttling device and auxiliary device,the design of air drying system including draught fan and dryer machine.After finishing these procedures,assembling all structures of carbon dioxide heat pump system.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 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 and so on.In order to adjust the parameters of the refrigeration cycle and the drying cycle,the system use frequency converter to adjust fan speed,the auxiliary electric heater to adjust temperature of drying chamber,the valve to adjust the filling of CO₂,and the knob to adjust the throttle opening.This research has done various experiments under different conditions on the designed CO₂ heat pump dryer.Multiple sets of drying experiments of carrot are made in order to verify the property of drying control system.The influencing factors of moisture content and drying rate are also be found by comparative experiments on the external conditions that influencing the carrot drying characteristics.The influence of temperature on the drying rate is larger than wind speed.When temperature or wind speed increases,the thickness of the material decreases,the drying rate will increase.But the extent of drying rate increase is smaller when temperature or wind speed higher.In the whole process of drying experiments,heat pump drying device operating normally,the 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.The moisture extraction rates is 1kg/h,achieving the object of market dryer standards.And the specific moisture extraction rates is 0.67 kg/kWh,it can be used as an experimental platfonn for further optimization.