Structural design and process parameter optimization of heat pump drying system of wheel dehumidification for Pleurotus eryngii

Common methods of drying have three ways: radiation, conduction and convection. Convective drying has been widely used due to its simple equipment and wide application range. The parameters that can be optimized are temperature, wind speed and humidity, but the temperature has an upper limit in each drying stage. Exceeding the upper limit will destroy the quality of agricultural products. It’s not conducive to sufficient heat exchange between wind and material if exceeding the optimum air volume. The humidity is unrestricted for most of the drying period. Low humidity can increase the drying rate and, so humidity is an ideal adjustment parameter. Wheel dehumidification is a common mode in solid dehumidification. However, the traditional wheel dehumidifier has problems such as high energy consumption and unreasonable structure, while heat pump has limited deep dehumidification capacity, but the energy saving effect. In view of the above problems, in this paper, we proposed a model of wheel dehumidifying and for the problems of high energy consumption and unsuitable for drying of agricultural products based on traditional structure of wheel dehumidifying, the optimization design of the dehumidifying structure and the dehumidification system was carried out. Firstly, a conversion mechanism was set up to solve the problem of reasonable conversion between fresh air and circulating air to realize energy-saving drying. Secondly, the surface cooler was replaced by an evaporator and a condenser was set up to recycle the energy. The heat released from the condenser was used to heat dry the inlet air or to regenerate the dehumidification wheel. In order to test and improve the performance of the wheel and heat pump combined dryer, in this paper, we took the sliced Pleurotus eryngii as the research object, and aimed to reduce the color difference of the Pleurotus eryngii, specific power consumption and improve the rehydration, using Box-Benhnken. In the central combined experimental design theory, we carried out three-factor and three-level response surface tests on three factors that affected the drying quality and energy consumption, such as regeneration temperature, dry temperature and conversion point relative humidity. Data analysis was carried out and the response surface mathematical model was established. The four-dimensional renderings was used to analyze the influence mechanism of the above three indicators on the changes of the three test factors. The results showed that the R2 was near to 1and the test factors had a great influence on the drying quality and energy consumption. The order of importance of each factor to rehydration was dry temperature > regeneration temperature > conversion point relative humidity, the order of importance to aberration was regeneration temperature > conversion point relative humidity > drying temperature, the important influence order on SPC was dry temperature > regeneration temperature > conversion point relative humidity. The lower regeneration temperature resulted in the lower drying temperature, the higher rehydration ratio, and the lower relative humidity of the converse. The lower drying temperature, the smaller the relative humidity of the conversion point, and the lower chromatic aberration, and vice versa. The regeneration temperature, the drying temperature and the conversion point relative humidity to SPC showed a trend of low first and then high. When the regeneration temperature was 87 ℃, the drying temperature is 50 ℃, and the relative humidity of the conversion point was 45%. The rehydration ratio of Pleurotus eryngii was 4.028, the color difference was 22.89, SPC was 2633 kJ/kg, and the error between the predicted and absolute value was less than 6 percentage point. This study explored the critical dehumidification mechanism based on enthalpy point and improved the wheel dehumidification structure, and formulated the optimum dehumidification drying process of Pleurotus eryngii. The results can provide the basis for the design of wheel dehumidification and HPD combined dryer and the optimization of drying process.