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Performance improvement in air dehumidifiers: From ideal to actual

LIU XiaoHua;ZHANG Tao;JIANG Yi;Department of Building Science,Tsinghua University;  
Dehumidification is an important component of air-conditioning systems. Improving their energy performance would contribute significantly to the overall energy conservation of buildings. In the present study, the efficiency of ideal air dehumidifiers is analyzed. By accessing various processes in ideal air dehumidifiers using either the condensing or the liquid desiccant method, methods to calculate their energy efficiencies are deduced. Performance discrepancies between these ideal dehumidifiers are then investigated and the associated efficiencies of these dehumidifiers are evaluated in typical environments in cities such as Beijing, Shanghai, Wuhan, and Guangzhou, are compared. Better energy performances are obtained if the liquid desiccant method is adopted. That is, dehumidifiers using liquid desiccants offer the potential to improve energy efficiency. Based on the performance of these ideal humidifiers, the characteristics of actual dehumidifiers using liquid desiccants are then explored. In actual processes, the dissipation between moist air and solutions is the result of limited heat and mass transfer capability, unmatched flow rates, and unmatched inlet parameters. The so-called unmatched coefficient ?m is chosen as the index to evaluate the unmatched dissipation in actual air dehumidifiers using liquid desiccants. As the process between air and solution approaches the iso-concentration line of the liquid desiccant(the air iso-relative humidity line), ?m approaches unity, indicating that the limited heat and mass transfer ability is given full play. Using air dehumidifiers as a typical example, the energy performance and matching properties of key components of two typical air handling processes are investigated. For these two processes with different regeneration approaches, heating the circulating solution for regeneration exhibits a lower ?m values than heating the inlet regeneration air. The energy performance discrepancy correlates well with the discrepancy in the unmatched coefficient, which offers a criterion in developing actual air dehumidifiers using liquid desiccants: specifically lowering the unmatched coefficient helps to lower the unmatched dissipation and improve system performance. The present study focuses on air dehumidifiers and the transfer dissipation method is chosen to investigate its characteristics. Based on the transfer dissipation and transfer resistance analysis, the unmatched coefficient ?m is defined and provides an effective approach for analyzing treatments of air using liquid desiccants.
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