Cyclones for gas-solid separators are described, which are especially applicable for use in a downflowing third stage separator (TSS) for the removal of dust particles, such as solid catalyst fines, from the flue gas streams exiting the catalyst regenerator in fluid catalytic cracking (FCC) processes. A number of important flow characteristics result from the configuration of the cyclones, particularly with respect to the relative axial lengths over which swirl vanes and central hubs of the cyclones extend. These characteristics include a decrease in growth of turbulent eddies that conventionally result from a sudden expansion, in open cross-sectional area, of gas exiting swirl vanes.