Mass recovery characteristics of hydrophobic hollow fiber membrane-based refrigerant mass exchangers in vapor absorption refrigeration systems

Abstract

In thermally-driven vapor absorption refrigeration systems, a solution heat exchanger performs the heat recovery to elevate the thermal performance. This study focuses on the use of the hydrophobic hollow fiber membrane (HFM) for mass recovery of water vapor between two streams of lithium bromide (LiBr) solution across the HFM layers. We propose two configurations of the HFM-based refrigerant mass exchangers (HFM-RME): direct-contact and vacuum operating modes. A theoretical analysis explores the characteristics of the mass recovery processes based on simultaneous heat and mass transfer. Because the driving force of the water vapor mass recovery across the HFM layers is due to the difference in vapor pressures between the two streams of the LiBr solution, a larger temperature difference leads to higher mass recovery of the water vapor. In the direct-contact operating mode, the two streams of LiBr solution flow in direct contact across the HFM layers. The conduction heat transfer across the HFM layers reduces the solution temperatures along the length of the HFMs; as a result, it deteriorates the mass recovery of the water vapor. However, in the vacuum operating mode, the two streams of LiBr solution flow through each HFM, and the water vapor occupies the shell side at vacuum condition. The water vapor diffuses through the shell side, as the temperature difference between the LiBr solutions is given. The conduction heat transfer is neglected, and the solution temperature difference is adequately maintained along the HFM length, compared with the direct-contact operating mode. Therefore, the mass transfer of the water vapor, is enhanced by introducing the vacuum operating mode. The present work provides a comprehensive understanding of the heat and mass transfer characteristics in the proposed HFM-RMEs.