Absorption of water vapour in the falling film of water–(LiBr + LiI + LiNO 3 + LiCl) in a vertical tube at air-cooling thermal conditions

Abstract

In air-cooled water–LiBr absorption chillers the working conditions in the absorber and condenser are shifted to higher temperatures and concentrations, thereby increasing the risk of crystallisation. To develop this technology, two main problems are to be addressed: the availability of new salt mixtures with wider range of solubility than water–LiBr, and advanced absorber configurations that enable to carry out simultaneously an appropriate absorption process and an effective air-cooling. One way of improving the solubility of LiBr aqueous solutions is to add other salts to create multicomponent salt solutions. The aqueous solution of the quaternary salt system (LiBr + LiI + LiNO 3 + LiCl) presents favourable properties required for air-cooled absorption systems: less corrosive and crystallisation temperature about 35 K lower than that of water–LiBr. This paper presents an experimental study on the absorption of water vapour over a wavy laminar falling film of an aqueous solution of (LiBr + LiI + LiNO 3 + LiCl) on the inner wall of a water-cooled smooth vertical tube. Cooling water temperatures in the range 30–45 °C were selected to simulate air-cooling thermal conditions. The results are compared with those obtained in the same experimental set-up with water–LiBr solutions. The control variables for the experimental study were: absorber pressure, solution Reynolds number, solution concentration and cooling water temperature. The parameters considered to assess the absorber performance were: absorber thermal load, mass absorption flux, degree of subcooling of the solution leaving the absorber, and the falling film heat transfer coefficient. The higher solubility of the multicomponent salt solution makes possible the operation of the absorber at higher salt concentration than with the conventional working fluid water–LiBr. The absorption fluxes achieved with water–(LiBr + LiI + LiNO 3 + LiCl) at a concentration of 64.2 wt% are around 60 % higher than those of water–LiBr at a concentration of 57.9 wt%.