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Powder Flow Testing at Elevated Humidity and Temperature

An Interview With Dr. Erik Miller

Do temperature and humidity affect your powder flow behavior in gravity discharge from a bin? You may think so, but how do you know for sure? Is there a way to run tests that can conclusively prove what the outcome will be? Unfortunately, using your process equipment to find out can be disastrous if you have to shut everything down because of erratic flow or blockages.


Brookfield Responds: Q&A

Powder Question Challenge

05/2015 - My Company produces various types of sugar. It seems that the granular sugar discharges from our hopper with relative ease, but the powder sugar is more problematical. Is there a way to quantitatively determine how difficult the powder sugar will be to process?

Brookfield Answers: The shear cell is the only scientific instrument that can quantitatively measure the flow behavior of the sugar particles. The reason is that the shear cell shears the particles against one another to evaluate the inter-particle friction, which is essentially resistance to flow. This data can be used to calculate the dimensional requirements for hopper opening and half angle to ensure continuous flow.

Powder Question Challenge

Customer Challenge: The Flow Function that we measure with the Powder Flow Tester on our product appears to be shifting upwards. Our powder is a special flavoring that we blend for a new brand of low calorie corn chips. The production staff tells us that the powder seems to come out of the hoppers ok, but we hear grumbling that the flow can become erratic at times during the run. We are not sure if the problem is with the ingredients in our blended powder or the PFT. How do we know whether our PFT is measuring correctly?

Brookfield Answers: Checking the PFT for proper calibration has two possibilities. Use specially calibrated powder known as BCR-116 limestone, which is available fromBrookfield(Part Number PFT-BCR-116), and run the Flow Function in accordance with our published test procedure for performing a calibration check. The measured values for Unconfined Failure Strength must lie within established min/max limits for the test to pass.

The second approach is to use a special calibration kit (CalKit) which checks the sensors in your instrument that measure torque, axial load, and position of lid. If the sensors are out of calibration, the CalKit will identify the problem and recalibrate the sensors. You will also receive a new Certificate of Calibration for your PFT.

Concerning your blended powder, consider performing the Flow Function test on the ingredient that is the primary carrier for the mixture. It has the greatest influence on ‘flowability”. This powder may constitute the greatest percentage by volume. The objective of the Flow Function test is to find out whether this material is changing by performing a QC check during incoming inspection.

Powder Question Challenge

Customer Challenge: We use a Flodex Tester to make a quick assessment of our powder’s flow behavior before we start processing operations by opening the discharge on our main hopper. It’s a relatively inexpensive device and easy to operate. However, it only gives a “go” or “no go” indication of whether we are likely to experience problems. And it doesn’t always agree with what actually happens once we start operations. Is there a better way to measure the flow behavior of our powder?

Brookfield Answers: There are several ways to make measurements of powder flow behavior. Some are popular, because they have been around for a long time, like the Flodex Tester. If your material is relatively “free flowing” like granulated sugar or crystalline salt, this may be a satisfactory method.

Powder Question Challenge

03/20/2014 - Our powder sometimes has a sticky quality that increases the flow problems that we experience in processing. We think that it could be related to humidity. But we also have suspicion that one or two ingredients from different suppliers may be contributing to the stickiness. Does the Brookfield shear cell give you the ability to detect this type of characteristic before it becomes a problem out on the production floor?

If your material has a granular free-flowing nature like sugar, but changes due to humidity/temperature, then the Powder Flow Tester is designed to characterize this property and quantify it for you in terms of a reduced flow capability. This is exactly the type of application the PFT is used for; any type of powder that shows cohesive behavior and is being used in gravity feed systems and the like are what the PFT can measure.

Brookfield has a Powder Laboratory where the relative humidity can be elevated from 30% to 60% and the effect on flow behavior of customer powder samples is measured with short turnaround time. The testing is free for a limited number of samples from each customer.

Powder Question Challenge

03/04/2014 - The application on your website called “Solid Dosage – Stacking Sacks of Powder for Processing” is relevant to one of the problems in our company. We didn’t know how to determine the recommended “stacking height vs. time” in order to avoid caking of the powder. Your article gives us a clear way to make this calculation by running the time consolidated flow function test with the Brookfield Powder Flow Tester. The formula for calculating the arching dimension of the powder is:
Arching Dimension = 2 x Critical consolidating stress x 1000
Density of powder x gravity

What do the factors 2 and 1000 represent?

The answer to your question is easier than you may think. The important note is that the Powder Flow Pro software, which runs the Time Consolidated Flow Function test, does the calculation for you.

1000 is for conversion from units of kPa to Pa.

2 is an arch shape factor for a circular outlet in a conical hopper. This is a simplification of the Jenike H(θ) function which is approximately 2.

Note that for a wedge or plane hopper with a slot outlet the arch shape factor is 1, which is again a simiplifcation of Jenike H(θ) function which is approximately 1.

Once you know the arching dimension related to the time period that the sacks will be stacked, you can determine a maximum height to avoid caking.

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