The Gas-Solid Vortex Unit is an advancing fluidization technology with the potential to overcome the limitations of conventional fluidized beds. The conditions for stable fluidization are investigated, that are the upper and lower limit, i.e. minimum and maximum capacity (W-s,W-min and W-s.max) Based on dimensional analysis three non-dimensional groups are identified, governing the fluidization phenomena: the superficial radial particle Reynolds number Re-p,Re- p, the swirl ratio S and the unit loading lambda. Data from different authors is gathered for minimum (42 datasets, 3 geometries) and maximum (251 datasets, 8 geometries) capacity and used in regression analysis. Parameters are estimated for different proposed functional dependencies of the identified dimensionless groups. The model equations describing the minimum and maximum unit loading best, including their 95% confidence intervals, are: lambda(max) = (4.0 +/- 0.4) 10(-3) (Rep,RS(0.454 +/- 0.018))-S-(0.43 +/- 0.011) lambda(min) = (1.15 +/- 0.05) 10(-4) Re-p,Re-R The two equations describe the limits of the operational range of a GSVU for which stable fluidization is possible. The applicability of the model equations is verified against a wide range of data taken from different publications. (C) 2018 Published by Elsevier B.V.