Theoretical And Experimental Research Of Water-cycle Heat Pump Dryers

Heat pump drying technology has developed rapidly in these years with its advantages of energy saving, environment friendly, drying in low temperature, etc. In this paper, an indirect heat pump dryer named water-cycle heat pump dryer was systematically studied using both theoretical and experimental methods. Heat pump and dryer were connected through water cycle in a water-cycle heat pump dryer. Compared to the direct heat pump dryer, the water-cycle heat pump dryer didn't discharge heat into environment, was easily controlled, and matched well with drying process, so it's suitable for the moderate or small heat pump dryers.After selecting key variables of water-cycle heat pump dryers, a static mathematic model based on SMER was established. According to this model, energy and variable controlling characters were analyzed under part load operating condition. Dynamic analysis of the water-cycle heat pump dryer was realized through analysis of temperature's change of water and drying materials. The detailed mathematic description of the drying processes was also given when material was heated in both conduction and convection ways. In addition, the variables of each subsystem were sorted into controlling variables and controlled variables, and controlling methods of system were given. Based on the above analysis, an experimental water-cycle heat pump dryer was set up, temperature change of water and materials during boot-strap, and drying characteristic with different heating style were researched.It shows that increasing of temperature and relative humidity of air at dryer's outlet will be helpful for the increasing of SMER. Characteristics of water temperature are closely related to water volumes. Regulating of drying medium's rate is more helpful for heater's heat transfer regulation than regulating the velocity of hot fluid. The COP_h declines by 48% when water temperature increases from 27.5℃to 51.5℃. When the thickness of materials is 20 mm, the drying time with conductive style is less nearly 45% than the one with convective style, and the drying time with convective style is much sensitive to the thickness' increasing of materials.