in previous works we have shown that by defining various threshold velocities in terms of the modified Reynolds number as a function of the modified Archimedes number a generalized master-curve can be plotted. Then, using this curve, the ratio between various threshold velocities can be easily determined. However, all the threshold velocities were defined for mono-sized particles. The purpose of this paper is to show how the threshold velocities can be used to design of particle-fluid systems for particle size distribution. The analysis provides practical guides for various engineering scenarios, such as selecting the appropriate fluidization velocity for maximum fluidization and minimum entrainment and determining the conveying velocity in pneumatic systems. In addition, the analysis provides a guide to determine whether the deposited layer of particles at the pipe bottom is stationary or moving, for cases where the superficial velocity is smaller than the saltation velocity. (C) 2008 Elsevier Ltd. All rights reserved.