Page 350 - APPLIED PROCESS DESIGN FOR CHEMICAL AND PETROCHEMICAL PLANTS, Volume 1, 3rd Edition

318 Applied Process Design for Chemical and Petrochemical Plants

C. n = 0.667; equal mass transfer rate input horsepower of motor and speed of agitator. Select
More directly related to turbulence and motion at the calculated point in the grid, then move to the nearest
the interface. Includes scale-up for rate of dissolving speed and input horsepower (not motor hp). Usually
of solids or mass transfer between liquid phases. adjustments from actual design will be required, but often
Using geometric similarity and equal power per vol- the incremental adjustments will not be great. To aid in
ume results in same n value. these adjustments, equal torque will give equal liquid
D. n = 0.5; equal surface motion motion or solids suspension over a narrow range. To use
Related to vortexes formation, depth of vortex relat- equal torque, set up diagonals from lower left to upper
ed to geometric similarity and equal Froude number. right for the available equipment. Adjust or back-calculate
E. N = O; equal blend time (if necessary) the expected performance and dimensions
Rarely used due to large equipment required to based on the adjustment. Higher speeds require higher
hold speed constant for larger units. horsepower. For equivalent mass transfer, equal power
changes are used.
For turbulent power, (constant power number, P 0): To recheck final design, the diameter of a single,
pitched blade turbine, for turbulent conditions:
p P Ns os (5-49)

Vp 0 3 (5-50) (5-54)

P/V p N 3 D 2 (power per volume) (5-51) OT= impeller diameter, turbulent operation, in.

For P /V constant in two different systems, then Php = horsepower used by system (not motor horsepower
of driver)
(5-52) Sg = specific gravity of fluid
N = agitator impeller speed, rpm
(5-53)

Figure 5-33 provides a selection grid for establishing the Of course economics enters into the solution, so some
industrial standard for the driving equipment, that is, alternate designs may be helpful.

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C.lcullted horlepowers, hp
Figure 5-33. Scaled-up horsepower/speed requirement for an agitator system is readily related to industrial equipment. By permission,
Rautzen, R. R., et al., Chem. Engr., Oct. 25, 1976, p. 119 (32].