Analysis of hydraulic characteristics and energy efficiency of packed liquid desiccant dehumidifiers

Abstract

In view of the energy efficiency issues of heat and moisture transfer in packed liquid desiccant dehumidifiers (PLDDs) that have not been considered in current researches, the energy efficiency of PLDDs by using the specific moisture extraction rate per unit pressure drop power (SMERP) was analyzed in this paper. The validated models of moisture efficiency and hydraulic performance of PLDDs were used respectively to calculate the dehumidification capacity as well as the power consumption of pressure drop calculated after combining the flow-rate of air. From the perspective of SMERP, the effects of fluid and structural parameters of dehumidifier on dehumidification energy efficiency under three working conditions of parallel, counter, and cross flow are analyzed. The results show that similar laws exhibit for the pressure drop and effective air velocity of the PLDDs, both increasing with decrease in the desiccant temperature, increase in the desiccant concentration, decrease in the void fraction of packing, and increases in the superficial velocities of the air and desiccant. The counter flow is greater than cross flow, which is greater than parallel flow. The effective liquid velocity increases with decrease in the desiccant temperature, with increases in the desiccant concentration, the void fraction of packing and the superficial velocity of the liquid as well as air superficial velocity of parallel condition, while the superficial velocity of air under cross flow condition has almost no effect on the effective liquid velocity. Numerically, the effective liquid velocity presents the maximum in the parallel case and the minimum in the counter case. As the superficial air velocity increases up to 1.7 m/s under the counter flow condition, the device will experience overflow with the pressure drop of 511.7 Pa/m. A more appropriate void fraction is about 0.93. The SMERP decrease from 0.84 kg/kcal to 0.38 kg/kcal and from 2.05 kg/kcal to 0.16 kg/kcal with increase in the superficial velocities of the liquid and the air, respectively, so the superficial velocities of the liquid desiccant and air should be less than 0.01 m/s and 0.9 m/s. With the single-dimensional size changing, the SMERP of the PLDDs under parallel and counter flow conditions decrease with increase in the device height, and that under cross flow condition decreases with increase in the device length. At unchanged volume of the device, the SMERP under parallel and counter flow conditions decrease with increase in the H/V value, while the SMERP under cross flow condition increases. The H/V values of the parallel and counter flows are recommended less than 4 m⁻² with more than 4 m⁻² for the cross flow. The reference value for the engineering application of the PLDDs is provided from another dimension by the research results of this paper.