Measurement of equilibrium adsorption and adsorption/desorption rate of water vapor to a silica gel thin film coated on a cross fin tube heat exchanger

Adsorption/desorption rate of vapor to adsorbent on an adsorption heat exchanger is one of the important characteristics in order to design and improve the performance of adsorption chillers and dehumidifying/humidifying desiccant systems effectively. In the present study, for a silica gel thin film adsorbent coated on an aluminum plate in a cross fin tube heat exchanger, we measured the equilibrium adsorption of water vapor with TGA and the adsorption/desorption rate with gravimetric method under the different flow velocity of humid air. The equilibrium adsorption was formulated with function of relative humidity. In addition, we obtained overall mass transfer resistance related to both the mass diffusion in the film and the mass transfer in the boundary layer on the film. It was clarified that the influence of the resistance on the reaction rate appeared only at the beginning of reaction, and the resistance decreased with the increase of the velocity, especially in adsorption. Furthermore, the resistance in desorption was less than half in adsorption. Consequently, the resistance in the boundary layer was a negligible quantity for desorption when the flow rate was above 0.1 m/s. Finally, correlations were proposed to predict these resistances.