The use of water droplet evaporation in shower towers and passive downdraft evaporative cooling needs the estimation of the time needed to completely evaporate the drops. To solve this problem, a cellular approach is proposed in which the spray is considered as a pile of rigid spheres of equal size; each sphere has multiple layers and contains a drop in its center. The evaporation takes place gradually from the superficial layer towards the internal layers. Parametric studies show the influence of each variable on the evaporation time of the droplets. The model may be used for sizing passive evaporative cooling systems and towers for buildings using the passive evaporative down draught effect. A building equipped with a shower tower has been tested in the framework of the European project PDEC/JOULE in Catania (Italy). The spraying system was successfully sized by using the model proposed in this paper. (c) 2006 Published by Elsevier Ltd.