The impact and erosion on the stainless steel heat exchanger tubes located in the middle row of a 10 x 10 aligned tube bank caused by coal ash particles are investigated numerically by means of direct numerical simulations (DNS) to study the influence of preferential concentration on particle-tube interactions. The immersed boundary technique is applied to account for the coupling effect on the flow by the immersed tubes. A two-way coupled Lagrangian solver is developed for particle tracking. The experimentally validated particle-wall impact and erosion models are used to describe the collision between particles and tubes and the resulting erosion. The results demonstrate that the particles Stokes number has significant effects on particle dispersion patterns as well as collision and erosion characteristics. The global tube erosion of the first tube increases with the increment of the particle size, but the particles with an intermediate Stokes number of 1.6 cause the most erosion to the other downstream tubes with lower collision frequency, due to the preferential concentration. The maximum local tube erosion happens in the front side surface of each tube with certain angle region for all particles where more attention should be paid to prevent from erosion caused by coal ash particles. (C) 2014 Elsevier Ltd. All rights reserved.