Effects of Physical and Sorption Properties of Desiccant Coating on Performance of Energy Wheels

Desiccant-coated energy wheels are rotary-air-to-air energy exchangers widely used in ventilation systems to reduce the energy consumption required in industrial environments and commercial buildings. In this study, the effects of silica gel microphysical properties, i.e., pore width (P_w), specific surface area (S_A), and particle size (d_p), on the moisture recovery efficiency (latent effectiveness) of energy wheels are investigated. Three silica gel samples with different particle size and pore width (55 μm–77 Å, 150 μm–63 Å, and 160 μm–115 Å) are selected to coat small-scale energy exchangers. The sorption performance of the exchangers is determined from their normalized humidity response to a step increase in the inlet humidity at different flow rates. The results demonstrate that the transient humidity response is mainly specified by the desiccant pore size distribution, specific surface area, and mass of the coating. The transient analytical model is used to calculate the latent effectiveness (ɛ_L) of the exchangers from the transient humidity response. It was found that the exchanger coated with the smallest pore width (63 Å) has the highest available surface area and the highest latent effectiveness. With almost the same particle size (d_p = 150 μm and 160 μm), the latent effectiveness increases by 5% (at wheel speed 20 rpm and Re = 174) as the pore width reduces from 150 Å to 63 Å. Increasing the particle size from 55 μm to 150 μm with almost the identical pore width (P_w = 63 Å and 77 Å) results in a slight enhancement in the latent effectiveness. ɛ_L is also calculated for correlated data (Yoon–Nelson model) where the results agree within experimental uncertainty bounds.