Ralph A Heasley, executive director product development
Xanodyne Pharmaceuticals Inc., Newport, Ky.

Try a trip to the bone yard

The easiest change you can make is to increase the temperature of the mixing solution. This should decrease the viscosity. What could work just as well is to change the blades on the unit, or the type of impeller; use one with a smaller diameter which draws less power, but might not do as good a job. One other idea is to find a variable frequency drive and lower the speed; again, this may cause fisheyes (clumps of unmixed solid mixed with air) or increase blending time. A trip to the bone yard for a larger mixer or a one from a used-equipment dealer could be a solution if you can do this in the days allowed.
Dr. Rudy Lisa, senior research associate 
BASF Corp., Wyandotte, Mich.

Reduce the speed

Chances are the high-shear agitator is “over-agitating” anyway. Reduce the current draw by installing a frequency-modulation type speed controller (adaptable to any AC motor with minimal power loss). This will lower the speed of the agitator by 20% or more, until the motor does not exceed its name-plate amperage rating. If, by doing so, the mixing action is still satisfactory for the downstream process, you're off the hook temporarily. You may want to consider replacing the impeller (rotor, stator) or the whole mixer eventually.
Gueric Boucard, president
TEXAROME Inc., Leakey, Texas

Go with slower, larger motor

Clearly there is a problem. Since power draw is proportional to viscosity times speed squared times diameter cubed, reducing the speed by 66% and the blade diameter to 80 % of its original size will require a motor 2.2 times larger. Factoring in the effect of density (power is proportional to density × speed cubed × impeller diameter to the fifth power), the motor needs to be 1.2 × 2.2 = 2.64 times larger, or 5.28-hp. Assuming that the existing  motor is a 1,800 rpm with a 1.0 safety factor, my quick fix solution is to change the motor. Go to a 1,200-rpm 5-hp motor with a 1.15 safety factor premium efficiency. Next, if possible, I would cut the blades down 20%. Obviously, the existing agitator is only 2-hp, which indicates that the application is not a viscous application. Instead of cutting down the blades why not replace the high shear agitator blades with high efficiency blades? Depending on the specific power numbers (Np) of the agitator, this would be a better choice and may eliminate the need to change the motor horsepower or speed. Another alternative would be to select the larger motor and provide it with a variable frequency drive.
Jim Collett, senior project engineer
Emerald Hilton Davis LLC, Cincinnati, Ohio

Experiment

I would try the following ideas:

1. Raise the temperature to lower the viscosity. (Avoid overheating to oxidize any organics in the color concentrate.)
2. Lower the speed of the high shear mixer.
3. Use a smaller blade on the high shear mixer.
4. Use a different type (less aggressive) blade on the high shear mixer.
5. Add a clamp-on mixer that will mix at the higher viscosity.

If this doesn’t work, another solution is to mix in smaller batches in a smaller vessel. For a significantly higher viscosity, a different type of mixer may be needed. I recommend a sigma blade, ribbon, paddle etc.
Gary D. Robertson, engineer
Valley Equipment Co., Jonesborough, Tenn.

Your solution is dilution

Since the viscosity of the color concentrate may be responsible for raising the viscosity of the final mixed solution, why not dilute the concentrate (dye)?Your laboratory could run some experiments to see if this works. The concern would be the water balance. Plodders produce some heat but not enough if a lot of solvent is added.
Scott Shaffer, president
AMS Materials LLC, Fruit Cove, Fla.

Change the motor style

A quick possible solution: Rent a similar mixer, but higher-hp-rated one. Another alternative would be to replace the motor. Is the motor mounted on the outside and to a plate or is it a C frame which has a plate at the front of the motor which mounts directly to the mixer? If it is mounted on the outside to a plate, the 2-hp motor can be de-mounted. An adapter plate can be made and bolted to the existing frame. Then, a larger horsepower motor can be mounted on a stand a few inches above the adapter plate. The new motor can then be made to drive the mixer via a jack shaft similar to the ones go-carts use. The output shaft of the new motor would have a pulley with a belt going to a same-size pulley now mounted on the jack shaft which is coupled to the mixer drive. If the new motor is the same rpm as the old one, using same-sized pulleys will give the same speed. Since I’m not an engineer, the power requirements would have to be confirmed. The parts are generally available from suppliers overnight if needed. The motor adapter can easily be manufactured from sheet steel and simple gussets welded or bolted in to make the assembly rigid enough for the horsepower requirements.
Joe Sprow, industrial specialist
Bureau of Engraving and Printing, Washington, DC

Consider various options

I came up with some short-term and long-term solutions. Short-term improvements include: 1) buy off-the-shelf larger motor — if the shaft and plate modifications are not severe; 2) explore increasing the motor voltage; 3) look at increasing the temperature of the mix tank; 4) replace the impeller (rotor, stator) with one that requires less power or a smaller one. Purchasing a larger motor should be fairly easy but mounting it to the existing agitator might be a challenge. Increasing the temperature in the tank could be done with heat-tracing and insulation — the results might not be enough; check with R&D first. Replacing the impeller might be possible in a short time. Cutting the impeller back may cause an imbalance in the shaft if not done correctly.