A model for predicting the gas velocity stability range is presented for multistage fluidized bed systems with downcomers, when the solid mass flux is very low. The model was successfully applied to a laboratory scale two-stage perspex system, operating in continuous regime, and was capable of correlating the stability limit. This model was used to design a three-stage pilot plant fluidized bed reactor for preparing activated carbons. The stable operation limit for this system was also successfully predicted by the model proposed in this paper.