Particle impaction in an in-line super heater tube bundle has been investigated. By using direct numerical simulations for the fluid flow, inertial particles coupled to the fluid through the classical Stokes' drag law have been tracked. Focus has been on the effect of flow velocity, and it is shown that decreasing the flow velocity will drastically decrease the impaction efficiency for some particle radii. Fouling due to the inertial impaction of such particles is consequently very sensitive to the fluid velocity. The results are subsequently combined with particle size distribution measurements from the municipal solid waste incinerator (MSWI) of GKS in Schweinfurt, Germany. Impacting mass fluxes both on the front and back sides of the tubes in the super heater tube bundle are calculated, and it is found that the largest part of the mass impaction stems from particles with diameters in the range from 5 x 10(-5) m to 7 x 10(-4) m. (c) 2012 Elsevier B.V. All rights reserved.