The Formation and Thermal Effects of Calcium Sulphate Boiler Scale

Abstract

Abstract A fundamental study of the manner in which calcium sulphate scale forms upon an evaporative surface, a redetermination of the solubility values for calcium sulphate in the boiler temperature range, and measurements of the rate of formation and heat conductivity of calcium sulphate scales produced in an experimental boiler, are described in abstract in this paper. Evidence is presented to show that the initial deposition of calcium sulphate scale takes place as the result of the evolution of bubbles, either of dissolved gas or of steam, from an evaporative surface, and that the continued growth of scale is dependent upon the fact that the solubility of calcium sulphate decreases with temperature increase. The work of the authors thus presents a new explanation of the mechanism of initial scale formation, supports Hall’s theory of the growth of scale, negates the colloidal theory of scale formation, and offers a fundamental basis for the study of scale-prevention methods. Photomicrographs of the early stages of deposition and growth of calcium sulphate scale are shown, as well as photomicrographs of scales produced under varying conditions. From the results of their own work and from that of other investigators, the authors present a discussion of the present knowledge concerning calcium sulphate scale. While the effect of scale upon heat utilization in modern boilers is believed to be very slight, calculations based on the heat conductivity of anhydrite scale indicate the importance of preventing overheating of tubes due to the insulating effect of scale deposits within them.