Run-Around Energy Recovery System with a Porous Solid Desiccant

by Li, Meng

In this thesis, heat and moisture transfer between supply and exhaust air streams are investigated for a run-around system in which the coupling material is a desiccant coated solid that is transported between two exchangers. The finite difference method is used to solve the governing partial differential equations of the cross-flow heat exchangers in the supply and exhaust ducts. The outlet air properties are calculated for several inlet air operating conditions and desiccant properties. The accuracy of the heat transfer model is verified by comparing the simulations with well-known theoretical solutions for a single cross flow heat exchanger and a liquid coupled run-around system. The difference between the analytical predictions and the numerical model for sensible effectiveness for each exchanger and the run-around system were found to be less than 1% over a range of operating conditions. The model is also verified by modifying the boundary conditions to represent a counter flow energy wheel and comparing the calculated sensible, latent, and total effectiveness values with correlations in the literature.

Using the verified model for energy exchangers and the run-around energy recovery system, the sensible, latent and overall effectiveness are investigated in each exchanger and the run-around system during simultaneous heat and moisture transfer. The overall effectiveness of the run-around energy recovery system is dependent on the air flow rate, the solid desiccant flow rate, the desiccant properties, specific surface area, the size of each exchanger, and the inlet air operating conditions. The run-around system can achieve a high overall effectiveness when the flow rates and exchangers properties are properly chosen. Comparisons between the solid desiccant and salt solution run-around system effectiveness (Fan, 2005 and Fan et al, 2006) shows they are in good agreement. In a sensitivity study, the thickness of desiccant on the fibre is investigated in the solid run-around system. It was found that good performance is obtained with very thin desiccant coatings (1 or 2 micron). During the practical use of this system, a desiccant coated fibre could be inserted into very porous balls or cages that protect the desiccant coated fiber from mechanical wear. The performance sensitivity for this kind of run-around system is demonstrated.