DESIGN AND PERFORMANCE TESTING OF A NOVEL 3-FLUID LIQUID-TO-AIR MEMBRANE ENERGY EXCHANGER

Abstract

Liquid-to-air membrane energy exchangers (LAMEEs) allow simultaneous heat and moisture transfer between air and desiccant solution streams that are separated by semi-permeable membranes. Moisture transfer between the air and desiccant solution is accompanied by the release/absorption of phase change energy which increases/decreases the temperature of the desiccant solution as it flows through the exchanger. The resulting change in the desiccant solution temperature decreases the driving potential for heat and moisture transfer (i.e. the differences between the air and desiccant solution temperatures and vapor pressures), which decreases the rates of heat and moisture transfer between the air and desiccant solution inside the exchanger. To overcome this problem, a novel 3-fluid LAMEE prototype is designed, built and tested. The 3-fluid LAMEE is composed of several adjacent parallel air and solution channels separated by semi-permeable membranes with refrigerant tubes within each solution channel. The aim of these refrigerant tubes is to reduce the change in the desiccant solution temperature inside the exchanger to guarantee high differences between the air and desiccant solution temperatures and vapor pressures along the entire length of the exchanger. This thesis has three main objectives. The first objective is to determine the practical nominal air and solution channel widths for flat-plate LAMEEs, and the effects of flow maldistribution caused by membrane deflections on the performance of flat-plate LAMEEs. The results in this thesis show that the practical air and solution channel widths for flat-plate LAMEEs are 5-6 mm and 1-2 mm, respectively. The second objective is to test and compare the rates of heat and moisture transfer between the air and desiccant solution in 3-fluid and 2-fluid LAMEEs under several operating conditions. Results show that the 3-fluid LAMEE can achieve the same heat and moisture transfer rates between the air and desiccant solution as a 2-fluid LAMEE at lower desiccant solution mass flow rates and with smaller membrane surface areas. Therefore the size of LAMEEs can be significantly decreased if refrigerant tubes are installed inside the solution channels. The third objective is to present performance definitions for evaluating the overall performance of 3-fluid LAMEEs. Unlike the traditional energy exchanger effectiveness equations, results show that the overall performance definitions can be used to evaluate the overall sensible and latent effectivenesses of 3-fluid LAMEEs and are less sensitive to the inlet refrigerant temperature.