The effect of some design parameters on the expansion of particles has been studied in a fluidized bed of 300 mm diameter. Four distributors were examined; three perforated plates, each perforated by holes of 0.8 mm in diameter but different hole densities at 6 mm, 9 mm and 12 mm pitch, and a porous plastic distributor 17 mm thick. Particles of different materials in the Archimedes number range from 100 to 10(5) were fluidized. The inserts were held vertically as arrays. All experimental data for four distributors were correlated within experimental error by the equation: (U-U-mf/U-0-U-mf) = (e-e(mf)/1-e(mf))((1.28+0.00033P3)) where U, U-mf, U-0 are the gas velocity, velocity at minimum fluidization and real or apparent terminal velocity, while e is the bed porosity and e,fis the porosity at the condition of minimum fluidization. P is the hole pitch of perforated plate distributor in millimeters.