Thermodynamic analysis and experimental study of a semi-closed heat pump drying system

A type of semi-closed heat pump assisted fluidized bed drying system had been designed based on former grain drying experiments and theoretical researches so as to achieve the vehicle-mounted movable function. A horizontal multi-room fluidized bed was chosen as the drying chamber, and a vapor compression heat pump (R134a as refrigerant) supplied heat for the system. Latent heat of steam discharged from the fluidized bed’s forepart was recovered by the heat pump evaporator, and ambient air was blown into the rear part to recover sensible heat from the post-drying grain. A mathematical model composed of flow in fluidized bed, heat and moisture transfer in grain kernel, coupling of drying process and heat pump operation had been presented according to this system. This model can predict performance parameters of the drying system under various conditions. Model results show that there is a maximum value of drying system’s specific moisture extraction rate (SMER), when air temperature at the drying chamber inlet changes between 60～90℃. A prototype dryer had been installed based on the optimum design under typical weather conditions in northern china’s wheat harvest season. Testing experiments show that the air temperature at the drying chamber inlet fluctuates between 64.4～71.7℃ in the drying process, the fresh air ratio is about 30%, the mean coefficient of performance (COP) is 3.34, and the SMER reaches 1.935 kg/(kW·h). The model calculation is in good agreement with experimental results, and this grain dryer is significantly energy saving.