Abstract
The thermal conductivity and density of the alkaline scale formed on the surface of the heat transfer tubes in the distillation-type desalination plant were experimentally examined. The alkaline scale was the mixture of two kinds of salt, calcium carbonates and magnesium hydroxide, and the composition of the scale was affected by its precipitation temperature in the solution.
By the experimental measurements, the densities of the mixtures of two salts were lower than the ones of each single salt. This was thought to mean that the alkaline scale was composed of the porous substances. Therefore, the model of scale structure was assumed as the mixture of the small particles of two salts. The porosity of the scale reached to 40% at the relative proportion of two salts, X=0.55, that corresponded to the porosity for the roughest packing condition of theuniform spheres.
The overall heat transfer coefficients for the test tubes were measured by using a double-tubetype heat exchanger. From the difference of the overall heat transfer coefficients for the tube with and without the scale, the thermal resistance of the scale was calculated. And the thermal conductivities of two salts were experimentally obtained. The thermal resistance of the model scale was given by connecting the resistances of two salts and vacant spaces in series. And the value, 0.63W/m·K, was obtained as the thermal conductivity of the vacant spaces. This value was in good agreement with the one of the water that filled up the vacant spaces in the scale, and thus indicated the appropriateness of the model for the scale structure.
