Experimental detection and reversal of crystallization fouling in a liquid-to-air membrane energy exchanger

Abstract

Liquid-to-air membrane energy exchangers (LAMEEs), which can simultaneously transfer heat and moisture between aqueous salt solutions and air, are designed to control the temperature and humidity in buildings. However, crystallization fouling may occur under some operating conditions where the salt solution becomes saturated and degrade LAMEE performance.

The primary focus of this thesis is to experimentally detect fouling in a LAMEE using non-invasive methods, and to reverse fouling in LAMEEs by changing the solution and air side boundary conditions. The objectives of this thesis work are: (i) to improve the sensitivity of fouling detection in LAMEEs using non-invasive methods and (ii) to reverse fouling in LAMEEs by changing the boundary conditions. Fouling is characterized by the moisture transfer resistance of the LAMEE, scanning electron microscope (SEM) and X-ray photoelectron spectroscopy (XPS).

The fouling detection methods used in this thesis are total uncertainty, slope and random uncertainty methods, which are used to analyze the change in resistance to detect the onset of fouling. The experimental results show that the random uncertainty and slope methods are the most sensitive methods for fouling detection.

The fouling reversal methods involve replacing the salt solution with water and increasing the air humidity, and these are applied on a highly-fouled LAMEE. The resistance graphs show that both methods can effectively remove fouling, and that changing the salt solution to water can remove fouling in less than one hour; whereas, it takes a longer time (several hours) for an increase in the air humidity to remove fouling. SEM imaging confirms that the fouling layer is removed by both methods.