AN INVESTIGATION OF TEMPERATURE AND HUMIDITY VARIATIONS DURING SILICA GEL-MOISTURE INTERACTIONS

Abstract

Recent successes in heat and moisture recovery from ventilation exhaust air from buildings using energy wheels have caused extensive interest in the HVAC design community. Anew method of heat and moisture recovery using desiccant particles to transfer heat and moisture in an energy recovery system is explored in this thesis. The research of this thesis is the first step of this investigation. The objective is to study temperature and moisture variations during transient flow of humid air passing through a silica gel particle bed. A one-dimensional theoretical/numerical model was developed to simulate the process of heat transfer and moisture adsorption when an initially dry granular silica gel bed is subject to a sudden air flow passing through it at selected temperature and humidity. Eleven governing equations, with initial and boundary conditions were established to describe this complicated problem. The equations reflect the effect of moisture diffusion inside each particle. This diffusion coefficient was found to be very much smaller than the diffusion coefficient for water vapour in air. Theoretical equations were discretized by using the control volume method and coded to simulate the problem. Before the application of the code, some parameters of the silica gel particles, such as permeability, porosity, specific surface area and isothermal moisture adsorption capacity, were measured or calculated. An experimental test facility was set up to measure the temperature, humidity and moisture content in a silica gel particle bed. The initially dry silica gel particles were subject to a sudden air flow passing through them at selected inlet temperature and relative humidities. In total, six experiments were completed for two sizes of silica gel particles. The numerical results were compared with the experimental data of measured outlet temperature, humidity ratio and moisture accumulation in the silica gel bed. Comparisons were made for three conditions of inlet air humidity and two sizes of silica gel particles. A sensitivity study of the numerical model was completed. From the numerical and experimental studies, it was concluded that silica gel particles have very high moisture adsorption capacity. As well, a large amount of heat is released during the moisture adsorption process. This heat release is most evident during the first 1.5 hours of a 10-hour test period. It was also found that, at higher inlet relative humidities and with smaller particle sizes, silica gel particles adsorb more moisture. The values of uncertainty for the bed properties such as specific surface area (S), density of silica gel particles ( Per)' specific heat of silica gel particles (Cper) and moisture-silica gel heat effect (Q) strongly affect the simulation results.