For the reliable design of pneumatic conveying systems, an accurate estimation of the minimum transport boundary is of significant importance. This paper presents results from an effort to establish a unified criterion for scaling-up the unstable boundary for the dense-phase pneumatic conveying of powders, An existing method of representing minimum transport criteria (based on superficial air velocity and solids loading) has been found inadequate for accurately predicting the unstable boundary, especially under diameter scale-up conditions. Using the experimental data from twelve different powders conveyed over a wide range of pipe lengths and diameters, a newly validated improved design procedure has been developed in this study using a Froude number based criterion at the entry to the pipe. The physical significance of Froude number in representing the minimum transport boundary is also discussed. (C) 2009 Elsevier B.V. All rights reserved.