A further increase in air flow causes the particles to become entrained and elutriate the bed into the furnace. If the solids are caught, separated from the air, and returned to the bed, they will circulate around a loop, defined as a circulating fluidized-bed (Fig. 1e). Unlike the bubbling bed, the CFB has no distinct transition between the dense bed in the bottom of the furnace and the dilute zone above. The solids concentration gradually decreases between these two zones.