Interactions between solids and gas during pneumatic conveying can be utilized for variety of applications including flash drying, solids preheating etc. Experiments on air-solid heat transfer were carried out in a vertical pneumatic conveying heat exchanger of 54 mm inside diameter, using gypsum as the solid material. The effect of solids feed rate (0.6-9.9 g/s), air velocity (4.21-6.47 m/s) and particle size (231-722.5 mu m) on air-solid heat transfer rate, heat transfer area and air-solid heat transfer coefficient has been studied. Empirical correlations have been proposed for the prediction of Nusselt number based on the present experimental data. The proposed correlations predict Nusselt number within an error of +/- 15% for the present data. (c) 2007 Elsevier Ltd. All rights reserved.