Heat and moisture transfer properties of four hydrophilic finned-tube evaporators for dehumidification in greenhouse

Efficient dehumidification has been one of the technical problems to restrict the high-quality development of the greenhouse industry in China. It is a high demand to dehumidify and recover the heat energy in the process of dehumidification during winter. Among them, the airside heat and moisture transfer properties of finned-tube evaporators have had direct impacts on the performance of the entire dehumidification system in the greenhouse. In this study, the heat transfer models were established for the evaporators under both low temperatures and high relative humidity. Four hydrophilic finned-tube evaporators were taken with the plain fin, the plain fin with vortex generator, wavy fin, and slit fin. The numerical simulation was verified to compare the experimental data. The heat and moisture transfer properties of four evaporators were assessed for the refrigeration dehumidification systems in the greenhouse using various parameters, including the heat transfer capacity, Nusselt number, friction factor, dehumidification capacity per area, and the enhanced heat-transfer factor. The results showed that average relative errors of less than 3% were achieved in the Nusselt number and friction factor of the models for the grid system. There were the higher Nusselt number, heat transfer capacity, friction factor of the air side in the plain fin with vortex generator, the wavy fin, and the slit fin, compared with the plain fin. The largest heat-transfer capacity and friction factor of the slit fin were achieved under the same inlet air velocity and relative humidity conditions. The friction factor also decreased significantly with the increase of inlet air velocity, with an average decrease of 26.06%, 24.87%, 21.10%, and 29.06%, respectively. By contrast, there was a relatively small effect of the relative humidity on the friction factor. The slit fin exhibited the highest heat transfer capacity and friction factor, whereas, the plain fin displayed the lowest under the same conditions. The other three types of fins improved the convection heat transfer on the air side of the evaporator, due to their unique structure. The dehumidification capacity was augmented with the rise in the inlet air velocity and relative humidity. The wavy fin shared the greatest average increase in the dehumidification capacity per area, with 8.68% and 7.60%, respectively, at inlet air velocity and relative humidity. The wavy fin-tube evaporator presented the highest average enhanced heat transfer factor of 1.084 and 1.041, respectively, under the conditions of inlet air velocity and relative humidity, indicating the most favorable heat and moisture transfer properties. Hence, the wavy fin-tube evaporator was recommended for dehumidification at low temperatures and high relative humidity in the winter of cold regions, when the inlet air velocity exceeded 2 m/s and the relative humidity was over 80% in the greenhouse environment. The findings can provide a strong reference for the design and application of the hydrophilic finned-tube evaporators in the refrigeration dehumidification systems in the greenhouse.