Scale formation in tubular heat exchangers—research priorities

Abstract

The common case of CaCO 3 scale formation in pipe flow is considered in this paper. Results of experimental studies, mainly by the author and his collaborators, are reviewed from the standpoint of the main mechanisms involved, in order to identify key issues that require additional research. The discussion is focused on the initial deposition rate as well as on the morphology of growing crystalline deposits as a function of various process parameters. A critical supersaturation ratio ( S c∼7 for CaCO 3) represents a significant transition from one type of scale formation process to another, including the dominant mechanism, the growth rate, and the morphology of scale. As regards generic topics that need clarification, it is suggested that high priority should be placed on (a) the attachment phenomena of nuclei/crystals on the substrate and (b) the effect of particulate/colloidal matter on scale characteristics. Significant gaps also exist in our understanding of the effects of fluid physical and chemical properties, and of fluid mechanics, on elementary processes (e.g., crystal growth and morphology) and on scale characteristics. Clarifying these aspects of scale formation would facilitate the development of fouling mitigation methods. Modelling efforts addressing elementary processes as well as system (global) evolution are considered essential for developing improved predictive tools, and reliable techniques for estimating physical parameter values difficult to obtain by other means.