A review of publication on the expansion of fluidized bed with the fluid velocity. The well-known Richardson-Zaki equation is applied to develop a more accurate relation linking the apparent drag force Fd, the effective gravitational force Fg, and the voidage of fluidized beds under intermediate regime. A correcting const. a in the relation is a function of Archimedes no. which may provide a degree of flexibility for different systems covering a wide range of variables. With addn. of the correlation developed in this work, equivalence correlations between equil. forces and the Richardson-Zaki equation can be found for a whole regime. [on SciFinder (R)]