Improved of effective wetting area and film thickness on a concentric helical bank of a generator for an absorption heat transformer

Highlights: • The falling film of lithium bromide - water was improved in the generator of an AHT. • The design of the distributor and the concentric tube helical bank was modified. • Wetting efficiency for different operation conditions was obtained. • Improved operation flow in the heat exchange system was determined. - Abstract: This work was performed in the generator of an absorption heat transformer (AHT) applied for water purification, which has two concentric helical coils. The AHT used LiBr-H₂O to 55%wt for the heat transfer through a heat exchange by falling film. The objective of this study was to define the operating condition of the generator. Different falling film regimes were analyzed: drop mode, liquid column, and jet mode. The effective area of heat transfer of the two helical coils, wetting efficiency, and experimental film thickness were determined for the four operating flows (0.003, 0.008, 0.014, 0.019 kg/s) through digital image processing. The theoretical film thickness was measured and compared with the one calculated by the Nusselt equation. The flow of 0.008 kg/s maintained a drop mode distribution favoring a homogeneous fall along the helical test bank. A wetting efficiency of 99.52% was obtained, so it is proposed as operating flow in the generator. The theoretical film thickness for this flow was 0.289 cm and the one obtained experimentally through digital image processing was 0.293 cm. It was concluded that the distribution in the drop mode was more favorable for a better efficiency in the values of the falling film exchangers.