Properties And Experimental Analysis Of The Small CO₂ Heat Pump Dryers

The drying method has great influence on the drying results.Present research has more attention on heat pump drying technology,which is able to do well in both drying quality and costs,especially in agricultural products processing.As the early heat drying system mostly use the Freon refrigerants,which has low drying temperature,time consuming and harmful effects to the environment,the heat pump drying that use CO₂ as the working fluid is treated as main research directions.We analyze the working principle and related equipment based on the thermodynamic properties of the original C02 heat pump structure and components to optimize the drying equipment in this paper.Then,the effects of CO₂ filling volume and the expansion valve opening degree on various components of the device,as well as the relationship between performance and equipment are tested by means of testing.Finally,the performance of dry equipment will be investigated by the choice of the material?sweet potato?.Contents of this paper are as follows:?1?According to the design of the existing equipment,structure and thermodynamic properties of the heat pump dryer were analyzed,the pros and cons of different models of the drying cycle were also discussed,which determined that the device has obvious advantages in energy-saving and low pollution.?2?Considering the disadvantages of the existing equipment and the design,production needs of the relevant parts,re-selection and design were conducted.The more powerful compressor with better performance was selected and piping of the heat pump system was redesigned.In order to make better use of energy and protection of the compressor,heat regenerator is added back in the heat pump cycle system,and the length and size of the aperture and outside is on the reasonable choice under this system.To note relevant parameters and operate the device state,PLC data acquisition and monitoring system were optimized,the preparation of the relevant data into the program,more concise record to the experimental data and observations to working conditions were conducted as well.?3?On the basis of the preparation for the study of parameters related to equipment performance a large number of experiments were carried out.Dried working fluid of heat pump dryer is CO₂,which plays critical role of the device,but the studies of its impact on device performance are few.Therefore,the experiment analyzes the influence of the amount of CO₂ filled on cooler,evaporator and other components,and the relevance between the amount of CO₂ filled and equipment performance,the results are shown as follows:The best filling amount of CO₂ for the system is 2.5kg,carrot dry material under the conditions of the system,the COP about 2.693,the MER is 1.168kg/h and the SMER is 1.264kg/kW.The expansion valve interposed between the cooler and the evaporator has a significant impact on the system.So,the system uses an expansion valve manually,By adjusting the opening size of the control CO₂ flow rate and s the volume.Experimental research results show that:The optical expansion valve opening is 3 laps?maximum opening of the expansion valve 6 laps?.?4?In the experimental study of the drying equipment,the selection of sweet potato as a practical drying object properties of the drying equipment.Drying performance of this equipment at different temperatures and wind speeds have been measured,the results are that:temperature has an obvious effect on drying process,it is needed for this material that a suitable limitation should be set in both temperature and wind speed rather than a very high degree,considering the drying quality and energy loss synthetically,the most suitable drying temperature of this device is about 60?,suitable wind speed is 2.0m/s,then the system's COP value about 2.343,MER is 1.178kg/h,and SMER is 1.200kg/kW.This topic analyzed factors affecting heat pump system and heat pump drying equipment,tested drying cycle performance of heat pump drying system,results reflected the main characteristics and performance of the device.Drying experimental data that were measured under certain conditions,may be a little offset for different environments and dry materials experiments,the study of this subject can provide a reliable basis for later improvements and equipment drying experiment.