This paper presents the results of an investigation into how the median size of the particles affects pressure drop when bulk solids are conveyed pneumatically in suspension flow along a straight horizontal pipeline. It is apparent that pressure drop increases with increasing particle size; the degree of increase tends to be largest towards the smaller particle sizes, diminishing as size increases and also diminishing with increasing transport velocities. (C) 2000 Elsevier Science S.A. All rights reserved.