Study On Dehumidification Performance And Control Technology Of Closed Heat Pump Drying System

Drying is an important process that many industries need.It has a wide range of needs in fields such as agriculture and industry.Heat pump drying technology has the advantages of low energy consumption,high quality of dry products,easy handling,and environmental protection,and is of great significance for alleviating the energy shortage in China and improving the quality of the environment.The heat pump drying system is divided into open,closed,semi-open type,each has advantages and disadvantages and applicability.Closed heat pump drying system is more significant in energy saving and improving drying quality,but the dehumidification of the internal evaporator Regulation technology and unit performance have always been issues that need to be solved in heat pump drying.How to design and adopt reasonable control technology through the system is the key to closed heat pump drying technology.This paper describes the design method of closed heat pump drying system according to process requirements.Discussed,designed and built the corresponding closed heat pump drying system,simulated the airflow organization and temperature distribution of the closed heat pump drying room,and proposed the control logic of the closed heat pump drying system,and designed and produced the corresponding The controller finally used the self-designed closed heat pump drying system to perform the experimental research on the drying of actual materials,and achieved satisfactory results.The design flow and control logic of the closed heat pump drying system were verified and can be widely applied to various materials.Drying.The work and results of the thesis mainly include the following aspects:1.The design process of Closed heat pump drying system.According to the characteristics of the drying object,the form of the heat pump system was put forward.By calculating the heat and moisture load of the drying process,the design of the main unit of the closed heat pump drying system was completed,including the compressor,evaporator,condenser,air supply fan,and electronics.Expansion valves,etc.;the closed heat pump drying system designed can switch four-way valves and expansion valves to provide three modes of temperature increase,cooling and dehumidification.Based on this,several control processes for closed heat pump drying systems are proposed.An intelligent controller was designed and manufactured.The single-chip microcomputer PLC controller was used to realize multi-parameter detection and multi-objective cooperative control to ensure the stable control of temperature in the drying process and to meet the drying process requirements.The drying dehumidification control strategy proposed by the institute was implemented.2.Simulation of indoor air distribution and temperature distribution in closed heat pump drying system.The Fluent software simulates and simulates the air flow in the drying chamber.The purpose is to determine the effect of the air supply method on the temperature in the drying chamber and provide a basis for system design.Simulate the distribution of the temperature and velocity fields of the drying chamber with three types of air supply: top orifice air supply,bottom orifice air supply,and side air delivery.The simulation results show that the temperature in the middle of the drying chamber is lower in the form of the bottom orifice air supply.Evenly,the maximum temperature difference is about 3-4°C.The distribution of temperature field and velocity field is more even than that of the other two methods,and the drying effect is better.Through experimental testing and research,during the actual drying process,the temperature distribution trend of the drying chamber is basically consistent with the simulation results.3.Closed heat pump drying control system and experimental platform design and construction.Based on the design results,a closed heat pump drying control system and experimental bench were designed and constructed.A data monitoring and control platform was designed and manufactured to monitor and collect data on parameters such as dry room temperature humidity and wind speed,and performance status of the heat pump unit.4.Closed heat pump drying system experimental study.Using the designed closed heat pump drying system,the cabbage was selected as the drying material,drying experiments were conducted,and the influence of the return bypass rate on the dehumidification efficiency was studied.The results show that in the same drying medium,the bypass rate increases,and the dehumidification efficiency first increases and then decreases.Under a given operating condition,the system has an optimum bypass rate to maximize the efficiency of dehumidification,and the best bypass rate is mostly concentrated at 50%-80%.By adjusting the bypass rate,the dehumidification efficiency can be greatly increased,up to a maximum of 60% increase;the recirculating wind speed of the drying medium has a certain influence on the water loss rate and the dehumidification efficiency of the material.The higher the circulating wind speed is,the faster the water loss of the material is,and the dehumidification efficiency is decreased,and the circulation wind speed is appropriately increased in the middle and early drying period.The material loses water,and the dehumidification efficiency is improved by reducing the circulating wind speed at a later stage.In the late drying period,the moisture content of the material is low,and the moisture content of the return air is significantly reduced.At this time,it is still difficult to increase the dehumidification efficiency by regulating the bypass rate,and high-temperature drying may be performed first.Increase the moisture content of the return air,and then carry out low-temperature dehumidification,heating and cooling in a cyclic manner to minimize the moisture in the material.The results show that the temperature in the drying room can be fluctuating and dehumidify at 40-50°C.5.According to the research results,a set of general drying control strategies was proposed.According to the analysis of experimental results,in order to improve the efficiency of dehumidification and ensure the drying quality,a set of general drying control strategies was proposed.According to the requirements of different material characteristics and drying processes,and in accordance with the performance of the unit,the control platform was switched to dry mode and the return air was adjusted.The parameters such as pass rate,circulating air speed and return air temperature are used to complete the control of the drying process and achieve efficient,energy-saving and high-quality drying.