**Explain the G-Factor or the Mean
Velocity Gradient:**

EXPLAIN the "G FACTOR"

**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advantage to inefficient and/or outdated designs and artificially penalizes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either utilize a less efficient impeller design of older or outdated technology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.

04.12.11