As the initial flow of a falling film above one or several horizontal tube is known, we propose a model which permits the determination of the local heat transfer coefficient. The model is valid for Reynolds numbers lower than 2000 and for either a constant flux or a constant temperature. The fraction of the film which is evaporated after its passage over the exchange surface is taken into account. The calculated exchange coefficients, the film thickness and the Nusselt numbers are compared with the numerical and experimental results of different authors. The agreement is good, particularly for the case of estimation of the average Nusselt number for Reynolds numbers greater than 1000.