The minimum gas speed for a heat exchanger (HE) at which particulate fouling is avoided is investigated. Fouling experiments have been done with particles of different sizes and different materials running under different gas speeds. It is found that the smallest particles in the flow deposit first on the tubes of the HE at areas of minimum flow velocities. Then the large particles deposit and the fouling layer starts to build up. The fouling layer thickness and growth over the HE tube is influenced by the flow speed. As the flow speed in the HE increases, the thickness and the surface area of the fouling layer deposited over the heat exchanger tube are reduced. There is a limiting flow speed above which fouling is avoided. This limiting speed is related to the critical flow velocity required to roll a particle resting on a flat surface. To prevent fouling, the gas speed of a HE should be larger than the critical flow velocity that corresponds to the particle size most likely to stick on the heat exchanger tube. (C) 2004 Elsevier Ltd. All rights reserved.