Insufficient Product Yield

This issue marks the introduction of Equipment & Processing Report’s new “Troubleshooting” column. When a process does not produce the expected results, or when a piece of equipment behaves strangely, even experienced personnel sometimes may be at a loss to understand the reason why. This column will be a forum to discuss production problems. Readers will submit anonymous questions, which equipment vendors and industry professionals will attempt to answer. In this way, the column may help promote greater understanding of the industry's products and processes.

Q: Our dry-powdered product is not meeting a critical final-quality specification related to the particle-size distribution. We use a volumetric dry-material feeder and jet mill to create the finished product. When we charge the feeder hopper with a batch of material, the resulting particle-size distribution changes from the beginning of the run to the end of the run. Maybe the load in the mill (i.e., the feed rate through the system) drops off because the material is sensitive to head load and we have no feedback control on the material feed rate. Why doesn’t our particle-size distribution stay consistent throughout the run? How can we meet our final-quality specification?

A: Try performing additional testing in the laboratory using a gravimetric feeder, which has a scale that is monitored by a controller. The controller uses the data from the scale to calculate a feed rate and provide a feedback control loop to maintain a set feed rate.

It is indeed likely that your material’s flow properties are sensitive to head load, and without the feedback control loop, the feed rate drops off as the feed hopper empties. The control loop in the gravimetric feeder should be able to increase the speed of the auger. This increased auger speed could compensate for the change in material flow that results from the change in head load. Also, the increased auger speed could help maintain the feed rate for more than 95% of the hopper capacity. Using a gravimetric feeder that provides a feedback control loop could increase your product quality and yield.

—Chad Lorensen, business-development manager at Schenck AccuRate (Whitewater, WI)

If you have a problem with your equipment or process, an industry expert may have the solution. Please send your question to Erik Greb, editor of Equipment and Processing Report, and we may be able to provide an answer in a future issue. All questions will remain anonymous.