The variable speed fan using a frequency converter will be cheaper to operate, but offers no other major advantage. The fan must be selected so that it does not go into surge at the low flow. System stability is controlled by the fluidizing grid. There must be enough pressure drop across the grid to insure uniform flow at the low flow condition. This pressure drop can be as low as 30 to 40% of the pressure drop of the bed, alone. Exhaust fan with a butterfly is okay as long as gas that leaks in is acceptable. It just needs to be able to provide an adequate vacuum over the bed.