The  impeller driveshaft  can either be top driven or bottom driven. Again there are advantages of each and the choice often varies with scale of operation:

(a) Top driven (laboratory or industrial scale)
 shaft tends to be longer, power development is worse
 shaft seal is easier
 steady bearing required to prevent whipping (required for scales above 100L)

(b) Bottom driven (pilot plant or small industrial scale)
  shaft is shorter, better power development
  more space on top plate for ancillaries
  more difficult seal arrangement

The required  shaft diameter  can be calculated from the following inequality and is largely determined by the length and power transmission requirements: