This paper presents the results of an ongoing investigation into modelling important design criteria, such as minimum transport condition, straight-pipe pressure drop and solid friction factor for fluidized dense-phase pneumatic conveying of powders. Fly ash (median particle diameter: 19 mu m; particle density: 1950 kg/m(3); loose-poured bulk density: 950 kg/m(3)) was conveyed over a wide range of flow conditions (from fluidized dense- to dilute-phase) under different conditions of pipeline diameters and lengths (viz. 43 mm I.D x 24 m length, 54 mm I.D x 24 m length, 69 mmI.D x 24 m length and 69 mml.D x 70 m length). To define the safe minimum transport boundary, a Froude number based criteria at the pipe inlet has been used (Fr-i = 7). The Froude number based criterion is aimed to address the requirement of different conveying velocities for different pipe diameters. Straight-pipe pneumatic conveying characteristics obtained from two sets of pressure tapings installed at different locations of pipeline have shown that the trends and relative magnitudes of the pressure drops can be significantly different depending on the location of pressure tapping points, thus indicating a change in flow mechanism along the direction of flow. A new approach of modelling solid friction factor using a volumetric loading ratio term has provided better scale-up accuracy when the model predictions were compared with experimental data. This method of modelling solid friction is aimed to address the partial filling of pipe's cross section by the dune of solids, which appears to be a better representation of the flow conditions, especially for the dense-phase pneumatic conveying of fine powders. (C) 2014 Elsevier B.V. All rights reserved.