Falling film evaporation of aqueous lithium bromide solution at low pressure

Abstract

Horizontal tube bundles are often used as falling film evaporators in absorption chillers, especially for systems working at low pressure with H2O/LiBr. The heat and mass transfer processes in these apparatuses are affected by several physical effects and the geometrical design of the heat exchanger, for example the material properties of the working fluid or the surface structure of the heat exchanger. Several influencing parameters are summarized and evaluated. Correlations for the prediction of the heat transfer coefficients of falling film evaporators from the literature are discussed concerning their applicability in dependence of the working conditions of the evaporator in an absorption chiller. In this work, experimental investigations are carried out in a falling film evaporator consisting of a horizontal tube bundle with eighty horizontal tubes installed in an absorption chiller because of a lack of consistent data for heat and mass transfer in the literature. The heat and mass transfer mechanisms and the flow pattern in the falling film are analysed and compared with correlations from literature. The deviations of the experimental data from those of the correlations are within a tolerance of 30%. These deviations may be explained by a change of the flow pattern at a lower Reynolds number than compared to the literature.