This paper presents a numerical study of dilute gas-particle flows in an in-line tube bank. The physical characteristics of the particle-wall collisions and their contributions to particle phase flow field were investigated employing a Lagrangian particle-tracking model, which includes an algebraic particle-wall collision model and a stochastic wall roughness model. Particles with corresponding diameters of 1 mu m, 15 mu m and 93 mu m were simulated under the gas flow condition of 11.2 m/s. The predicted mean velocities and fluctuations for both gas and 93 mu m particles were validated against experimental data. The numerical predictions revealed that the wall roughness has a considerable effect by altering the rebounding behaviours of the large particles, and consequently affecting the particles motion downstream and shifting particle collision frequency distribution on the tubes. Also, the results demonstrated that the velocity fluctuations for large particles are predominantly determined by the particle-wall collisions. (c) 2006 Elsevier Ltd. All rights reserved.