A 24" 10HP vane axial fan will give you 0.42 CFM per bushel in the top ring 9 sqft of vents in roof needed.

A 28" 15HP vane axial fan will give you 0.47 CFM per bushel in the top ring 9 sqft of vents in roof needed.

A 10HP 1750 rpm centrifugal fan will give you 0.55 CFM per bushel in the top ring, 11 sqft of vents needed.

I would go with either 10HP fan. The centrifugal will be more $$