To avoid or reduce carryover of particles form the fluidized bed, keep the gas velocity between . Recall
Terminal velocity, for low Reynolds number and,
for high Reynolds number
With the expressions for and known for small (viscous–flow) and large (inertial flow) particles or Reynolds number, one can take the ratio of and :
For small
For spherical particles, and assuming
Therefore, a bed that fluidizes at 1cm/s could preferably be operated with velocities < 50 cm/s, with few particles carried out or entrained with the exit gas.
For large
Or,
Therefore, operating safety margin in a bed of coarse particles is smaller and there is a disadvantage for the use of coarse particles in a fluidized bed.
However, make a note that the operating particle size is also decided by the other factors such as grinding cost, pressure-drop, heat and mass-transfer aspects.
. Recall 
for low Reynolds number and, 
for high Reynolds number 
and 
known for small (viscous–flow) and large (inertial flow) particles or Reynolds number, one can take the ratio of 
and assuming 
