Abstract
Mass extraction/injection and multi-stage system have been proven to be effective approaches to improve the performance of humidification and dehumidification (HDH) desalination system. However, these two optimization schemes cause that the flow ratios of water to air at different positions of humidifier or that of different humidifiers have great difference. A single heating mode is inapplicable to all flow ratios of water to air. Hence, it is necessary to reveal the relationship among mass flow ratio, heating mode and thermal efficiency of humidifier for the selection of appropriate heating mode. A theoretical model of packing tower (typical kind of humidifier) which agrees well with experimental results was established in this paper, and the average relative errors of outlet air temperature, humidity and thermal efficiency are 2.3 %, 4.9 % and 5.8 %, respectively. A comparison among the thermal efficiencies of a counter-flow packing tower with different heating modes (I: solution heated, II: air heated, III: solution-air heated) was conducted. It is found that there exist two values of mass flow ratio (FRI-II and FRII-III) at which the thermal efficiencies of air heated and solution-air heated are equal and that of solution heated and solution-air heated are equal, respectively. However, the thermal efficiency of solution heated is always higher than that of solution-air heated (FRI-III does not exist). It seems clear that there is a relationship between FR and thermal efficiency (η) as follows: FR < FRI-II, ηI > ηIII > ηII; FRI-II < FR < FRII-III, ηI > ηII > ηIII; FR > FRI-II, ηII > ηI > ηIII. Additionally, the effects of inlet parameters on the FRI-II and FRII-III are also evaluated. The results show that the maximum temperature, inlet solution concentration and ambient temperature have positive effects on FRI-II and FRII-III, but ambient relative humidity has negative effect. In the final part, the selection of the heating mode of a counter-flow packing tower is taken as example to introduce the application of FRI-II and FRII-III proposed in this paper.
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