Humidity Handling Process Comparison And Exergy Analysis Of Desiccant Wheel And Counter-flow Liquid Desiccant

The research objects of this paper are two air humidity handling methods using the desiccant wheel and counter-flow liquid desiccant respectively. The main contents are the performance as enthalpy recovery device, the influence of module construction and operating parameter on the system composition as dehumidification device, as well as the thought of using exergy to evaluate desiccant wheel system and measure the heat mass transfer ability of components. Based on these ideas, the following studies have been carried out in this paper:At first, the heat and mass transfer numerical models of desiccant wheel and counter-flow solution-air module are introduced, and then validated by experimental data of enthalpy recovery and dehumidification desiccant wheel, and counter-flow packed tower. The impact of rotation speed, enthalpy or humidity difference of fresh air and indoor air, indoor air volume on the efficiency of enthalpy recovery desiccant wheel is tested. The key fact is the rotation speed(Rs), and higher the Rs, higher the efficiency and closer to the maximum.Secondly, based on the mathmatical models, the impact of desiccants mass flow-rate on the states of desiccants and air along the air flow direction is analyzed. It is found that, there is the positive correlation between desiccant wheel enthalpy recovery efficiency and roration speed, while the optimal solution mass flow exists for counter-flow packed tower. Furthermore, the relation between the ideal max enthalpy efficiency and the number of mass transfer units(NTUm) is found according to the heat and mass trasnfer law and energy converstion, and the relation formular is the same for the two devices. Since the specific surface area of structured packings is much smaller than the honeycomb construction of desiccan wheel, the NTUm is the main limiting factor to improve the performance of counter-flow packed tower for the practical application.Thirdly, exergy analysis is conducted about the desiccant wheel dehumidification system. The exergy destruction is classified into two parts, caused by the limited heat/mass transfer ability and caused by the unmatched coefficient. The main reason creating system exergy destruction and the method to improve components can be pointed out clearly.Finally, the performance is compared between desiccant wheel dehumidification system and counter-flow liquid desiccant system. For the former, air is heated to support the regeneration heat into system; hot and cold offset is low, and increases along with wheel size or rotation speed increases. For the latter, solution is heated by heating source to be concentrated; hot and cold offset occurs when dehumidification/ regeneration module NTUm is small or solution circulation flow is high. As a result, heat recovery device is significant for these two systems. When the same NTUm is supplied, the desiccant wheel system needs higher regeneration temperature, while liquid desiccant needs more heat and cooling input.