| Heat exchangers are widely used in the fields of refrigeration, air conditioning, space heating, and chemical engineering, etc. For example, the finned tube heat exchanger which coated hygroscopic coating on the finned surface, can exchange sensible heat and latent heat simultaneously, therefore, it can improve the heat exchange efficiency. Used in air conditioning system, it can significantly improve air-conditioning's coefficient of performance (COP) and reduce energy consumption. This paper is focus on aluminum foil desiccant coating in the finned tube heat exchanger.By means of electrostatic spraying, the hygroscopic coating on the aluminum foil surface was accompolished by spraying and thermosetting the mixture of silica gel powder for dehumidifying and powder coating for bonding. In order to further improve the adsorption performance of desiccant coating, by means of impregnation, the modified hygroscopic coating was finished by impregnating the aluminum foil with hygroscopic coating into the salt solution of LiCl, CaCl2 and MgCl2, The effects of the silica gel mass fraction, the impregnation conditions such as the salt concentration, the impregnation temperature and time on the adsorptive performance of the finned coating were investigated systematically. The composition, surface morphology and pore structure of the modified hygroscopic coating were characterized using scanning electron microscopy, energy dispersive X-ray spectroscopy (SEM-EDS) and porous medium analysis. Thermogravimetry (TG) was used to evaluate the thermal stability of the modified hygroscopic coating.The experimental results showed that the adsorption performance of hygroscopic coating enhanced with the increase of mass fraction of silica gel powder. However, the excessive silica gel resulted in the cracks' appearance on the hygroscopic coating surface, and also influenced the dispersity of silica gel particles on the aluminium foil surface. The adsorption ability of the hygroscopic coating was significantly improved by impregnating hygroscopic salt solution. SEM image showed that desiccant coating can evenly scatter on the aluminium foil surface, EDS analysis revealed the component of the hygroscopic coating. Pore structure analysis showed that, after impregnation, the average pore diameter of the hygroscopic coating slightly increased, while its pore volume and specific surface area slightly reduced. TG analysis showed that the thermal weight loss of the hygroscopic coating mainly concentrated in the lower temperature period (30~200℃) and the higher temperature period (300~600℃). The former corresponded to the desorption of silica gel desiccant and the latter corresponded to the decomposition of epoxy- polyester coating. The result of the static and dynamic state adsorption tests showed that, under lower temperature and humidity, or under higher temperature and humidity, the adsorption performance of modified hygroscopic coating was higher than that of no modified hygroscopic coating.The finned tube heat exchanger with aluminum foil desiccant coating was used in air conditioning system, the test result showed that the novel air conditioning system has a excellent COP. |
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