Effectiveness of Desiccant Coated Regenerative Wheels from Transient Response Characteristics and Flow Channel Properties—Part II: Predicting and Comparing the Latent Effectiveness of Dehumidifier and Energy Wheels Using Transient Data and Properties

Abstract

In this paper, a new transient response theory is developed to predict the moisture or water transfer time constant and the fully developed flow moisture effectiveness of dehumidifier wheels and latent energy effectiveness of energy wheels. A new method to experimentally determine the latent energy or water transfer time constant is presented. This analytical model, which uses the new empirical time constant and its theoretical model, is then corrected for entrance length, carryover, sorption temperature changes, and manufacturing variations. The model permits the designer of these wheels and the HVAC applications engineer the opportunity to explore how each flow channel property and wheel operating condition changes the effectiveness. The theoretical latent and sensible effectiveness of energy wheels with molecular sieve and silica gel coatings are determined for two energy wheels, and these results are compared with the effectiveness calculated using experimental data at steady state ARI testing conditions. The comparisons are in agreement within the uncertainty bounds. The analytical model is also used to predict the latent effectiveness for two similar energy wheels where it is assumed that the desiccant coatings are interchanged. This comparison shows that the new hypothetical silica gel coating on the energy wheel would be more effective than the new molecular sieve coating on an energy wheel for the transfer of water vapor. The moisture transfer effectiveness is predicted for two dehumidifier wheels with the same design and desiccants as the tested energy wheels (i.e., the molecular sieve wheel and silica gel wheel). These wheels, which are used for supply air drying, are operated with the same ARI supply air condition and 5 rpm. Considering the comparisons for energy wheels, the molecular sieve and silica gel coatings are predicted to result in similar performance factors but with much lower effectiveness. This reduction in moisture effectiveness for the dehumidifier wheels is caused mostly by the reduction in wheel speed. Further research is required to investigate the performance of dehumidifier wheels at lower wheel speeds.