In the absorber, the LiBr solution is sprayed over horizontal tubes cooled by water flowing inside. It absorbs the water vapor coming from the evaporator continuously and flows in a thin film around the tubes. Then, it is collected in the bottom of the lower shell. To give a description of the heat and mass transport from a horizontal tube covered with a liquid film, the following simplification is made. The film flow along one half of the tube is modeled as that along a vertical cooled wall with a length of half the tube circumference, which is a model suggested by Wassenaar [14,15].A schematic representation of the model is shown in Fig. 2. On one side of the plate, a solution of substance A (LiBr) in substance B (water) flows down as a thin laminar film. At the liquid-vapor interface, the water vapor is absorbed and then transported into the bulk of the film. The heat of absorption is released at the interface and transported through the film and the wall to the cooling medium (water). The cooling water flows on the other side of the plate in a direction perpendicular to the plane of the illustration (cross flow). Therefore, the cooling water temperatures may be assumed constant over the plate height, which is equivalent to assuming a constant circumferential pipe temperature. The absorber model started with the following assumptions [14]: