When the speed of air flow to pass through the materials bed  increases to a certain value, the air drag force to particles reaches the gravity suffered by the particles. At this point, the bed slightly swells, the particles can turn freely in the original position, but it is unable to move. When the pressure drop of any two sections passing through the materials layer is equal to the sum of the particles and gas on the unit section of the two sections is known as the initial fluidization.