The function of the impellers is to: (i) reduce bubble size thereby increasing the surface area for oxygen transfer, and (ii) maintain a uniform environment, nutrient supply and heat removal. The choice of impellers depends on: (i) type of fermentation and organism, (ii) impeller design, (iii) power input and (iv) mixing efficiency. Some typical impeller designs are shown below. 

The ratio of the bioreactor tank:impeller diameter (Di/DT) is normally in the range 0.33 -  0.5. For radial flow impellers the diameter tends to be closer to 0.33 of the tank diameter and for axial mixers the diameter tends to be slightly larger. The location of the impellers is determined by geometric and mixing considerations.

First impeller:   0.33 - 0.5 tank diameter from base

Spacing:    1 - 2 impeller diameters (or 1 tank diameter)

Final impeller:    1 - 2 impeller diameters from the aerated liquid surface

Consequently consideration of the liquid level during the process needs to be taken into account, i.e. broth volume will increase with aeration and volume will increase in a fed-batch process.