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Application of the G-Factor - and obsolete design concept.

TABLE 1

 Model

 HP

RPM

Impeller
Diameter

Flow
Ratio

Shear
Ratio

Cost
Ratio

AQG-300

3.0

350

18

0.27

2.54

0.42

AQO1D-300

3.0

45

55

1.00

1.00

1.00

AQO2D-300

3.0

27

80

1.84

1.84

1.48

Each of the following mixers listed in Table 1 has an equivalent G-factor, as each is 3.0 horsepower, each is applied to an equivalent volume of exactly same fluid at a constant temperature (See G-Factor or Mean Velocity Gradient).   

Table 1 says it all.  Torque is inversely proportional to mixer RPM ... and torque costs.  Lower the mixer speed, increase the mixer cost .. it's that simple.  A mixer supplier that designs a flocculator, for example at 20 RPM, and its allowed to incorporate that resulting G-factor within a specification, not only increases the cost (bottom line), it also effectively excludes other mixer manufactures as most economically design flocculators at much higher speeds.  

Rakes' and reels', mixer designs of the 1930's and 50's, still in use today, use even slower mixer output speeds (higher torque).  Applying the resulting G-factors this outdated technology to hydrofoil design would be cost prohibitive.

Designing a mixer for any specific mixing application can result in several mixer solutions, not just one solution.  Stated another way, three mixers may have the exact same torque ratings offering similar process results and each would have different horsepower applied to them and by definition each mixer would result in its own G-factor.  In short, the G-factor is the result of a particular mixer design but it should never be used as a parameter of designing a mixer.  As such, the G-factor should never be specified, or included within an engineering specification, as a constraint, let alone a primary design constraint, of designing any mixer for any application due to its unintended consequences.   

In short, over time, efficiencies of designs make concepts such as the G-factor obsolete.    

04.12.11 

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