A heat transfer model of the evaporator in an absorption chiller was developed by modification of Nusselt's film theory. This liquid film model was found to agree with existing experimental correlations in the film Reynolds number region in which the tube is assumed to be fully wetted. Next, two-dimensional numerical computations of vapor flow in the evaporator/absorber were performed with boundary conditions of the evaporator and the absorber respectively derived from each liquid film model. The results showed that the velocity distribution around the evaporator and the velocity field between the evaporator and the absorber are different from those when the vapor evaporates uniformly on the surface of the cylinders in the evaporator.