Abstract
In this study, silica gel and sodium polyacrylate desiccants are coated onto a finned tube heat exchanger (Desiccant Coating Heat Exchanger, DCHE), which can absorb the vapor in the process air for dehumidification. In the experiments, the desiccant is coated on fins using the dense coating method, which causes the fixed fin area to be coated with greater amounts of desiccants for a better dehumidification performance. This study discusses the dehumidification performances of a single stage DCHE and two-stage DCHEs in series under different relative humidity conditions of the inlet process air and different regeneration water temperatures. The results show that the thermal coefficient of performance (COPth) of the DCHEs for the two desiccants prepared by the dense coating method is better than that of DCHEs with the general immersing coating method by a factor of 2–2.4. The two-stage DCHEs in series have a lower supply humidity ratio than a single stage DCHE at different inlet humidity levels, and they can be used in the industry when a special low humidity manufacturing process is required. The overall dehumidifying capacities of two-stage series-connected DCHEs at regeneration temperatures of 50 °C and 70 °C are approximately twice as high as those of a single stage DCHE. The COPth value of a single stage or two stages increases with an increase in the inlet humidity of the process air. The COPth values of the sodium polyacrylate single stage and two-stage DCHEs are 1–1.3 times greater than those of the silica gel single stage and two-stage DCHEs at a high inlet air humidity. Finally, the effects of different regeneration water temperatures on the performance of DCHEs are investigated. With an increase in the regeneration water temperature, the COPth value, dehumidifying capacity and regeneration capacity of single stage or two-stage DCHEs increase as well.
Highlights
According to statistical analyses of relevant meteorological data in Taiwan [1], the climate in Taiwan is under high temperature and high humidity climatic conditions throughout the entire year
It can be seen that, during the dehumidification process with an inlet air temperature of 30 ◦C, the cooling water with a temperature of 25 ◦C is introduced into the Desiccant Coating Heat Exchanger (DCHE) and the vapor in the process air is absorbed by the desiccant due to the vapor pressure difference between the process air and the desiccant
At the end of the dehumidification process, the humidity ratio value of the process air behind the DCHE does not return to its initial value, which means that the silica gel desiccant does not attain a saturation state after the dehumidification process period of 30 min
Summary
According to statistical analyses of relevant meteorological data in Taiwan [1], the climate in Taiwan is under high temperature and high humidity climatic conditions throughout the entire year. Chang et al [20] investigated the switching time effect on the dehumidification performance of the DCHEs coated with desiccants of silica gel and sodium polyacrylate, respectively, for process air with different temperatures and humidity levels. To understand the dehumidification effects and the corresponding thermal performance of DCHEs arranged in series, this study conducted several experiments to investigate the vapor sorption amounts, the vapor removal ability per unit mass of desiccant, the vapor removal ability per unit contact area and the corresponding thermal performance of two stages of DCHEs with solid coating desiccants of silica gel and sodium polyacrylate in a series arrangement under different inlet process air humidity conditions and different regeneration water temperatures, compared to those of single stage DCHE. The desiccant coating methods on the performances of DCHEs were investigated
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