The waste heat recovery boilers as applied in refuse-waste incinerators are susceptible to fouling. On the tube bundles in the boiler deposits are observed ranging from thin and powdery to thick and sintered with large ridges at the upstream side of the tubes. Both type of layers result in a decrease of the overall heat transfer coefficient of a tube bundles of about 27% and a decrease in efficiency for the boiler of 2%. To optimize the geometry of the bundle such that the formation of layers is minimized a model is developed. The model predicts local deposition rates of particles as a function of the geometry and incorporates both the transport and the sticking of these particles to the surface. First results show the formation of a powdery deposit on the upstream as well as the downstream side of the tube just as observed for the industrial case.