Thermal and hydrodynamic analysis of the condensation and evaporation processes in horizontal tube desalination plant

Abstract

This paper considers the hydrodynamic and thermal analysis for the case where steam is condensing inside a horizontal tube while simultaneously a thin water film is evaporating on the outside of the tube. An analysis of the collective three-dimensional effects influencing the condensing and evaporating sides are presented at different operating conditions. The operating conditions include the effects of the brine mass feed rate, saturation temperatures of the condensing steam and the falling brine, and vapour flow rates. The analysis is extended to account for further effects produced by the vapour pressure and shear stresses, and also by the temperature differences between both sides of the tube. The model presented in this work consists of three regions on the condensation side and two regions on the evaporation side. The condensation side is categorized by a thin laminar condensing layer (at the top of the tube) , a stratified laminary layer (at the bottom of the tube) , and a laminar or turbulent vapour zone. The evaporation side has a thermally developing laminar liquid film and a thermally fully developed zone. The results of the condensation/evaporation analysis show that the dominant effects are due mainly to the brine mass feed rates, secondly to the saturation temperature differences, and least to the incoming condensing steam rates. Calculations are carried out for vapour flow of Reynolds number between 20 000 and 30 000, inlet saturation temperature of incoming steam between 60 and 70°C, and brine mass feed rate between 0.01 and 0.15 kg m −1 m s −1.