TABLE OF CONTENTS David S.Dickey, Ph.D.
MixTech, Inc.
Dayton, OH
For a pitched-blade turbine impeller that is 58 in (1.47 m) in diameter and has four 12-in-wide (0.305-m) blades mounted at a 45 angle, determine the power required to operate the impeller at 84 r/min (1.4 r/s) in a liquid with a specific gravity of 1.15 (1150 kg/m 3) and a viscosity of 12,000 cP (12 Pa s). What size standard electric motor should be used to drive an agitator using this impeller?
1. Determine the turbulent power number for impeller geometry. Power number N P is a dimensionless variable [5] which relates impeller power P to such operating variables as liquid density ?, agitator rotational speed N, and impeller diameter D as follows:
A conversion factor (see below) is used when working with English engineering units; no factor is necessary for SI metric units. For a given impeller geometry, the power number is a constant for conditions of turbulent agitation. Values of turbulent power numbers for some agitator impellers are shown in Fig. 12.1.
The pitched-blade impeller described in this example is similar to the four-blade impeller shown in the figure, except that the blade width-to-diameter ratio W/D is not exactly 1/5. To correct for the effect of a nonstandard W/D on a four-blade impeller, a factor of actual W/D to standard W/D raised to the 1.25 power...
