Transition of convective heat transfer to subcooled flow boiling due to crystallization fouling

Abstract

Crystallization fouling during boiling is more severe to that of convective heat transfer. The former is usually avoided by keeping the surface temperature below the boiling point but there have been reports of accelerated fouling during initially attempted convective heat transfer. This study aimed at examining experimentally whether crystallization fouling during convective heat transfer can turn into boiling. Such occurrence can intensify the detrimental impact of fouling on the performance of heat exchangers and requires different strategies to combat deposition. Fouling experiments were conducted where dissolved CaSO4 solution was used as foulant which was heated in a plain circular tube. The initial surface temperature of the heated tube was adjusted to 80 °C, much below its boiling point and the bulk inlet temperature was 40 °C such that the heat transfer mode at clean conditions was forced convection. It was found that the surface temperature of the heated tube increased as the fouling layer developed such that it increased above the boiling temperature of the solution nearly after the fouling induction period. Streams of bubbles have been seen during the growth of deposit layer. Evidently, the crystallization fouling transformed the single-phase flow to bubbly two-phase flow and the mechanism of heat transfer from convective heat transfer to subcooled boiling. Finally, the operating conditions as well as the deposit characteristics in terms of thermal conductivity, stickiness and morphology would determine the extent of such transition.