Superficial velocity is also called air tower fluidization velocity, apparent velocity and empty bed velocity . The definition is the nominal speed of the air of bed cross section. At this time, the volume occupied by the solid particles is not considered in the calculation, it is thought that all the cross sections and flowing space from the whole wind plate to the outlet of the hearth all are completely occupied by gas medium. In actual design of fluidized bed boiler, generally the cross section of the upper vertical section of the circulating bed  and total air volume of primary and secondary air are used as a the calculation basis, the calculated  flow velocity of cross section is the designed fluidizing speed of the circulating fluidized bed boiler, namely the superficial velocity of fluidizing velocity of the fluidized bed.