Scaling in Polymer Tubes Used in Solar Water Heating Systems

An experimental study of the growth of calcium carbonate (commonly termed scaling) on copper, nylon 6,6, semiaromatic high temperature nylon, polypropylene, polybutylene, and Teflon tubes exposed to hard water is presented. The objective of the study is to gain qualitative information on the scaling of polymer tubes in nonisothermal, flowing conditions expected in heat exchangers and solar absorbers. The 89-cm long tubes were placed in tube-in-shell heat exchangers. Water prepared from 10 °C tap water with a total calcium concentration of 4×10⁻³ M, and a pH of 9.0 was pumped through the tubes at 4 cm/s. A 50 percent propylene glycol solution at 60 °C was maintained on the shell-side of the heat exchanger. The experiment was carried out for 540 hours with these conditions. Sections of the tubes were removed periodically to determine the extent of scaling. Results include scanning electron microscope images of the tube surfaces before and after exposure to the flowing water, X-ray diffraction to determine the crystalline phase content of the observed deposits, and chemical analysis to determine the mass of calcium carbonate per unit surface area and to estimate the scaling rate. A model of the scaling process is presented to help interpret the data. The data show conclusively that polymer tubes are prone to scaling. With the exception of nylon 6,6, the scaling rate on the polymers is about the same as that on copper. The nylon 6,6 substrate appears to enhance scaling.