Calibration of indirect methods to detect the onset of fouling in a liquid-to-air membrane energy exchanger

•Digital microscopy is 3–8 times more sensitive than three indirect methods to detect fouling.•The indirect methods are calibrated against digital microscopy to detect the onset of fouling.•The calibrated indirect methods can effectively detect fouling only at their calibration condition. Fouling is a perennial problem that diminishes the performance of heat and membrane exchangers. Therefore, the onset of fouling needs to be timely detected in operating exchangers so that corrective measures can be implemented to mitigate the growth of fouling. This paper presents the sequel to the research that is reported in a previous paper (Olufade and Simonson, 2018). In the previous paper, three indirect methods that are based on the analysis of moisture transfer measurements were applied to detect fouling in a liquid-to-air membrane energy exchanger (LAMEE). In this paper, the indirect methods are calibrated against a benchmark direct method (digital microscopy) to detect the onset of fouling in the LAMEE. Scanning electron microscopy (SEM) is used for autopsy of tested membranes and also serves as a concomitant direct method. The calibrated indirect methods are then applied to detect the onset of fouling at the same condition of their calibration and extrapolated to a different condition. Digital microscopy is found to be three to eight times more sensitive to detect the onset of fouling than the three indirect methods. In addition, there is reasonable agreement between the results obtained using digital microscopy and SEM. The results indicate that the calibrated indirect methods are able to detect the onset of fouling within the uncertainty limits of the digital microscopy method only when they are applied to the same condition of calibration. Care must, therefore, be exercised if the indirect methods are extrapolated to operating conditions that are different from their calibration.