An inlet box allows the air or flue gas to enter the fan from the top or side (perpendicular to the shaft) rather than from an open inlet (parallel to the shaft.) This can help with fans that have flow coming from above. Fans that have a bearing on each side of the rotor need an inlet box to keep the bearings out of the gas stream. Corrosive Gas streams may need an inlet box to keep inlet box damper bearings out of the airstream.