Experimental study on the interaction between particulate fouling and precipitation fouling in the fouling process on heat transfer tubes

This study investigated the interaction between particulate fouling and precipitation fouling on two different enhanced tubes and a plain tube in a shell-and-tube condenser. Three kinds of fouling, including particulate fouling, precipitation fouling, and combined fouling, were measured continuously for more than 720 h in three individual tests, respectively. For the particulate fouling, three tubes suffered a similar fouling process. The asymptotic fouling resistance of the precipitation fouling on enhanced tubes was lower than on the plain tube. However, for the combined fouling, the enhanced tube with a larger start number experienced the greatest fouling. On the same tube, the asymptotic fouling resistances ranked from least to greatest in the following order: particulate fouling, precipitation fouling, combined fouling; and the durations of the induction period ranked as: particulate fouling, precipitation fouling, combined fouling. As for the combined fouling existent in real cooling water systems, the fouling induction period on the plain tube was longer than that of enhanced tubes. Compared with the particulate fouling (SiO2) and precipitation fouling (CaCO3), which had a high bond strength on the tube surface, the combined fouling was soft and easily removed by flow erosion or collision with big particles during the induction period. This indicated that particles mixed in the water could extend the induction period of combined fouling. However, once the combined fouling started to grow, these particles sticking on the heat transfer surface provided nucleation sites for the crystallization process and, thus, increased the fouling thermal resistance significantly. In addition, the results indicated that the variation profile of the accelerated combined fouling after scaling down is similar to the fouling process in a real system. (C) 2019 Elsevier Ltd. All rights reserved.