an Engineering & Service Bulletin

Technical Topics

J.H. WRIGHT & ASSOCIATES

IT'S NOT THE HORSEPOWER, IT'S THE TORQUE

Nothing is more dear to the heart of a purchasing agent or engineer than reducing an equipment comparison to "Apples to apples". Yet when it comes to mixing equipment, most persist in equating a 10 HP mixer with a 10 HP mixer and think they've got it all figured out. The trouble is - HORSEPOWER DOESN'T DO THE MIXING, TORQUE DOES !

Since torque is simply :

all one needs to know about a mixer is the horsepower of the motor and the RPM of the propeller to reduce competitive offerings to a numerical comparison.

The problem with the above statements is that it's not readily apparent or easily acceptable intellectually, that torque rather than horsepower does the mixing.

So, I'm forced to prove it to you mathematically and the proof will probably glaze your eyes over, assuming you even follow it all the way through.

We begin with the easily accepted fact that the pumping capacity of the mixer has obviously a direct relationship to its ability to mix. So why not use pumping capacity to compare mixers? Well, one good reason is you have to take the manufacturer's word for it - you can't easily measure pumping capacity. But you can easily calculate the mixer's torque and it can't be fudged.

The following two formulas can be found in any good textbook on mixing:

Where:

Q = GPM (Pumping Capacity)

N = RPM of turbines

D = Turbine diameter

T = Tank diameter

NP = Constant describing any particular turbine design.

Using the above formulas and assuming we wish to achieve 10,000 GPM pumping capacity in a tank 96" in diameter with a series of impeller diameters and speeds, we can construct the following table:

Notice the following:

(A) Torque tracks pumping capacity exactly.

(B) You can achieve 10,000 GPM pumping capacity with anywhere from 6.3 HP TO .4 HP, so how can you use horsepower to compare mixers?

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