Gas-solid separator, a key component of a circulating fluidized bed boiler, controls the recirculation of solids around this type of boiler. As the technology matures, the drive is to have smaller size units handling a greater amount of solids. The impact separator is well suited to meet these demands with a low pressure drop from operating at low gas velocities. Complex hydrodynamics are encountered in an impact separator, where a staggered array of collection elements separates solids from an incoming mixture of gas and solid particles. The research was to examine the performance of this separator experimentally in a scale model, built to provide benchmark data. At an inlet gas velocity of 4.0 ms(-1), an overall collection efficiency of 87.5% of particles with a mean particle diameter of 125 pm was realized. The results were compared with those from a similar study. Copyright (C) 2004 John Wiley Sons, Ltd.