This paper describes the results of theoretical and experimental studies on the effect of air flow on particle transport through cascading rotary dryers. Two possible models are formulated. In the first, the drag coefficient between the air and the solids is calculated on the assumption that the particles can be regarded as isolated spheres. This approach has been employed widely in the literature to estimate particle residence times in rotary dryers. The results yielded coefficients which were 1–2 orders of magntiude greater than those measured experimentally; this demonstrates the inadequacy of this approach. In the second model, the curtains are treated as flat. two-sided plates and the drag coefficient calculated accordingly. The predictions of this model are in much closer accord with the