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Explain the G-Factor or the Mean Velocity Gradient:


G-FACTOR - or the "Mean Velocity Gradient" is seen frequently in Waste & Water Treatment specifications.  T. R. Camp promoted the "G Factor" many years ago which was based on obsolete flocculators which used rakes and/or reels for impellers.  

Engineers and engineering specifications specify a G-Factor thinking that by doing so that they either are leveling the field of performance between suppliers or equipment or that they are guaranteeing a level of performance.  In fact, specifying the G-Factor guarantees nothing.  provide an advantage to inefficient and/or outdated designs and artificially penalizes advanced technology.  

Flocculator design becomes optimal when using a low-shear-high-flow output device.  Hydrofoil impellers are designed with this specific capability in mind.  The result is that they can be operated at slightly higher speeds than lesser efficient designs such as pitch blade turbines for example.  The higher speeds promotes lower torque, which translates to a smaller gearbox or lower overall initial cost.   

To meet a G-Factor constraint, a concept that was originally based on very low-speed and therefore very high-torque rake designs, your choices are to either utilize a less efficient impeller design of older or outdated technology or be forced into a much larger gearbox torque to meet this artificial torque ceiling.  This can only be achieved by selecting very slow speed agitation using a hydrofoil.  The result is that the G-factor constraint has the effect of adding cost without adding value.  The development of hydrofoil impellers, which produce high flow and low shear has made the "G Factor", as a parameter of design, an obsolete concept for flocculation.

The "G Factor" is defined by the equation:

G = 444[(HP/1000)/m]0.5      sec -1

HP/1000) = horsepower per 1000 gallons of capacity

m = viscosity of water at operating temperature, in centipoise

G = "G FACTOR" in units of inverse seconds

The "G-FACTOR" parameter is still encountered for both RAPID or FLASH MIXERS as well as FLOCCULATORS.  When it is specified, it would be prudent to identify what is meant by its value, what impeller technology the factor is based upon, or its source.  

Engineers are looking for an edge.  If the G-factor is blindly applied as a factor of design, it  has the unintended consequence of driving up the bottom line without adding value.


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