Local fouling investigations as basis for describing the integral fouling behavior

点击翻译按钮获取中文摘要

Fouling, i.e. the formation of insulating layers on heat and/or mass transfer surfaces, is a phenomenon varying in space and time. However, fouling effects, such as the reduction of heat transfer and/or a pressure drop increase, are typically quantified in a single constant integral value, such as the thermal fouling resistance, for a given equipment. This research proposal builds on the hypothesis that a thorough understanding of local fouling processes allows for a prediction of integral fouling effects. Therefor local fouling processes for the crystallization fouling of the inverse soluble salts calcium sulfate and calcium carbonate in a fluid-heated double-pipe heat exchanger are investigated experimentally as well as theoretically. At two parallel and identically operated heat exchangers complementary and independent investigations on the local distribution and structural characteristics of the fouling layer will be carried out. Local thermal fouling resistances are determined based on local energy balances in combination with heat transfer calculations. A volumetric displacement method reveals the axial distribution of the fouling deposit. One of the two tubes will be cut into ten segments thus allowing for the optical inspection and the determination of segmental pressure drop. The first one allows to distinguish between a compact base layer and a rough growth layer on top. In combination with the segmental pressure drops, constriction and roughness effects may be separated and an equivalent roughness can be extracted. Finally, porosity, density and thermal conductivity of the fouling layer with respect to location and time shall be determined.Based on fluiddynamic as well as heat transfer calculations local velocities, wall shear stresses and temperatures may be calculated. In combination with the solubility of the treated salt solutions loci of supersaturation and thus crystal formation may be identified. Based on a thorough understanding of the interaction of component-related, thermal and fluiddynamic parameters the progress of the fouling process and thus the initiated integral effects may then be predicted.