7
Fluid Flow and Mixing I~O
Figure 7.24 Correlation between power number and Reynolds number for Rushton turbine, paddle and marine propeller
without sparging. (From J.H. Rushton, E.W. Costich and H.J. Everett, 1950, Power characteristics of mixing impellers. Parts I
and II.
Chem. Eng. Prog.
46, 395-404, 467-476.)
7.10 Power Requirements for Mixing
Usually, electrical power is used to drive impellers in stirred ves-
sels. For a given stirrer speed, the power required depends on
the resistance offered by the fluid to rotation of the impeller.
Average power consumption per unit volume for industrial
bioreactors ranges from 10 kW m -3 for small vessels (ca.
0.1 m3), to 1-2 kWm -3 for large vessels (ca. 100 m3).
Friction in the stirrer motor gearbox and seals reduces the ener-
gy transmitted to the fluid; therefore, the electrical power
consumed by stirrer motors is always greater than the mixing
power by an amount depending on the efficiency of the drive.
Energy costs for operation of stirrers in bioreactors are an
important consideration in process economics. General guide-
lines for calculating power requirements are discussed below.
7.10.1 Ungassed Newtonian Fluids
Mixing power for non-aerated fluids depends on the stirrer
speed, the impeller diameter and geometry, and properties of
