When a particle size Dp falls through a fluid, the freefall velocity can be estimated by fluid mechanics, by the expression:
Where CD is the drag coefficient determined experimentally. Haider and Levenspiel (1989) found:
For spherical particles, this expression reduces to:
In order to avoid or reduce the particles dragging in a fluidized bed, should be maintained the velocity between Umf and vt. In the calculation of Umf, it is used the average diameter dp of the size distribution present in the bed, while for vt should be used the smallest size of solids present in considerable amounts in the bed.