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A Discussion of the sources of the G-Factor or Mean Velocity Gradient:

Discussion: Sources of the “G-FACTOR”

Why is the G-Factor an obsolete concept or parameter for modern mixer design?  This is not to say that the use of the G-factor is dead within the engineering community as it still has applicability for obsolete mixer designs.  It also appears frequently in many engineering specifications, but with unexpected consequences.

To understand the answer to this basic question you will need to know the basis of how this concept was applied and to what equipment.  The reason for the irrelevancy of the G-factor, as a primary factor of mixer design, lies in understanding real technological change.  If today's designs were twice as efficient at that used in the 1950's, would you try to design the same energy per unit volume.  The answer of course is of course, "no way", but that's exactly the premise of the G-factor.  

The primary question of each mixer supplier (that is not designing 1950’s equipment) should be, "what is the source of the specified G-factor (that appears in this engineering specification)"?  Understand that the G-factor is an obstructive mixer design constraint.  Oddly enough, this question is almost never asked.  The primary sources of the G-factor include and are based on similar installations, rules of thumb (based on text material), the jar test method, or one supplier is allowed to specifies it.  Although there are other sources, for the purpose of this discussion we will focus on these sources.  The following is a discussion of these methods.

SOURCES of the “G-FACTOR”:

SIMILAR INSTALLATIONS:

Obviously, this is one of the best methods of evaluating the G-factor but it may not be completely reliable.  If the sources are from similar installations, it may also be based on outdated technology.  Inefficient impeller designs and the principles of its application top the list of shortcomings.  For years, the market leader in municipal waste and water treatment designed & specified flocculators at 17 to 20 RPM, as it suited their particular gearbox designs.  Although there is no doubt that these mixers performed, they were not cost effective as torque, being higher than that required for the application, significantly impacts the initial cost.  It is also not clear if these past designs have utilized 1990's efficiencies.  Today's version of the same application would most likely result in output speed range between 20 and 45 RPM.  Even smaller floc tanks have been known to reach up to and beyond 100 RPM as it is now known that fluid shear and impeller type define the process, not just speed alone.

RULES OF THUMB:

This sounds odd, but rules of thumb are still in use every day.  For years, a G-factor of 1,000 seconds-1 was applied blindly to flash mixers.  What is understood today is that above a certain intensity levels, no additional mixing benefit will result.  In other words, additional horsepower adds no mixing benefit, but it may significantly reduce the overall longevity of the mixer design.  A G-factor of 1,000 will also significantly influence the initial cost, in some instances by a factor of 300% .  Further discussion on this topic can be found under the topic "FLASH MIXING”.

JAR TEST METHOD:

Although jar testing is useful in evaluating specific water types and the performance of pin-floc, its resulting floc, its efficiency in separation, among other factors, the G-factors obtained using this method should never be applied to full-scale equipment.  A scale-up from 10 to 10,000 gallons (ratio 1,000:1) is considered to be at the outer extremes of scale up and quite frankly is pushing it.  Typical scale up ratios for small floc tanks from jar testing will typically reach 15,000:1, an order of magnitude beyond what is considered potentially feasible.  It is of fact that pilot scale tests are classified as super pumpers with relatively low shear capability.  Full-scale applications are just the opposite.  In short, you cannot simulate the full-scale fluid shear using the jar test method, one of the primary parameter of design.  This makes no mention of impeller type used to evaluate floc for jar testing.  With these constraints, how can you possibly simulate floc & flash mixing using this method?  Although many have tried to find a correlation, the resulting G-factors using the jar test method have traditionally been higher by a factor from 30 to 50% as compared to the latest technology available today that is used for designing mixing.

ONE SUPPLIER DESIGN:

Another common source of the G-factor is to quantify a number based on only one manufactures design.  If for example, one manufacture designs its flocculators at 20 RPM, the resulting G-factor, which is then added to a specification, applies a severe and significant constraint to designing the mixer.  Lower mixer output speeds, result in higher torque, where there is a direct relationship between torque the bottom line.  

Each mixer manufacturer understands that if they are allowed to specify and exact horsepower & output speed, which ultimately defines a specific G-factor, that manufacturer can effectively lock out a lower cost competitive offering.  In almost every mixer design, there are several solutions to each mixing application, each having their own resulting G-factor.  Simply stated, each manufacturer has specific gearbox rating that avail an efficiency only to themselves and not to all.  The objective should be to leave enough flexibility for each supplier to achieve that suppliers cost optimum understanding that the G-factor is the result of a design, and should never be a primary constraint applied to a design.  

04.12.11 

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