Fouling Characteristics of Water−CaSO4 Solution under Surface Crystallization and Bulk Precipitation

An experimental study has been carried out on the characteristics of the fouling process from Water-CaSO4 solutions to the heat transfer surface with and without bulk precipitation. Effects of rotational bulk solution Reynolds number (Re) and surface temperature (TS) on the fouling mechanism, fouling behaviour and fouling rate, under surface crystallization (SC) and bulk precipitation (BP) conditions, are investigated. It is found that fouling can enhance the overall heat transfer when Re is high enough in the lack of BP. Under SC conditions, the fouling resistance (Rf) and the fouling rate (dRf/dt) decrease with an increase in Re. The effects of TS on Rf and dRf/dt depend on Re, with exponential increases with Re under low Re of 16000, and negligible influence under high Re of 80000. The presence of BP markedly increases dRf/dt by 3.6 times, while decreases the length of the initial period by 4.5 times. The presence of BP alters the overall precipitation fouling mechanisms. A critical Re of 57600 is found separating two distinct fouling mechanisms under the same TS. When Re < 57600, the fouling process is controlled by bulk crystal particulate diffusion. For higher Re, the crystal removal rate becomes the dominating parameter. The current investigation reveals valuable information on fouling characteristics of CaSO4 solutions, which could provide a benchmark for fouling model development and help develop an effective fouling mitigation strategy.