The effects of temperature and hydrodynamics on the crystallization fouling under cross flow conditions

Crystallization fouling experiments were performed in a channel for a cross flow of hot saturated sodium sulphate (Na2SO4) solution over a pipe containing cold water. The heat transfer coefficient and fouling thermal resistance were determined from measurements of local surface temperature and the thickness of the crystalline fouling layer. A systematic assessment of the asymptotic variation of these parameters with time was obtained for a range of hot salt solution Reynolds number based on pipe diameter (RehD) of 165–485, salt solution bulk temperature of 30 °C–50 °C, and cold water (at 5 °C) Reynolds number in the pipe (Rec) of 7500–25000. It was found that the temperature of the salt solution has a large effect on the fouling rate. Increasing the pipe surface temperature decreases the crystallization fouling rate. Increasing the hot solution Reynolds number, via the velocity, decreases the fouling thermal resistance, while increasing the cold water Reynolds number, via the velocity, decreases the fouling thermal resistance, but at the same time it increases the fouling layer thickness. The crystallization process that applies here was found to be under activation control (i.e. chemical kinetic control) so that the mass transport of salt plays only a minor role.

► Increasing hot salt solution Re in the channel leads to decrease the fouling resistance. ► Increasing cold fluid Re in the pipe leads to increase the fouling deposition on the pipe surface. ► The fouling resistance decreases up to 3 times when the temperature increases from 30 °C to 50 °C. ► Corresponding to 80–95% layer thickness. ► The crystallization process is under kinetic control and the mass transport plays minor role.