Authors report on the hydrodynamic study of the so-called roto-fluidised bed granulator (RFBG) and on results of hydrodynamic experiments carried out using conventional equipment as well as apparatus equipped with mechanical stirring, performed for comparison of bed expansion conditions. The bed shape, relative bed expansion and void fraction as functions of air flow through the apparatus (gas velocity) and rotation speed of the disc are shown on clear diagrams. Based on experimental results and using principles for conventional fluidised systems, relationships have been established for the calculation of expansion (relative volume increase, void fraction) of roto-fluidised beds (RFBs) as a function of gas velocity, and validated by comparisons with experimental data.