When Considering a Dryer, Remember Material Must Go Somewhere

To select a dryer, we must know something about our material. To select a dryer that will work well we must know a great deal about our material. Such a census of our material’s properties will likely extend farther than we may wish to go but is necessary.

Once the data has been gathered, we are presented with a range of options as to how drying will take place. Many dryers use heat, directly or indirectly, and some medium to achieve drying. There are also dryers that rely on freezing, microwave technology, and radiant heat to accomplish their task. The form and function of dryer technology may vary with the material and process, such as batch or continuous use dryers. Regardless of these details the material exiting the dryer will not be the same as what initially entered.

In light of these many steps and topics at play, we must be mindful of a final piece. The material that leaves the dryer must go somewhere. Material handling often rightly receives much greater scrutiny at the dryer inlet. Going into the dryer the material is wet, often drastically so, and can present challenges in simply trying to coax it into the unit.

At the outlet the material once dried is almost universally a better-behaved substance. This improvement in flowability being a function of the reduction in moisture content of the material. We still must handle this outgoing material, which may not be problem free in storage.

Just because the material has passed through a dryer does not mean that it can now be put into a generic bin only able to receive the freest of free-flowing materials. Three reasons are more prevalent when problems occur with the dryer discharge material.

First, the decrease in moisture content may not be enough to significantly improve material flowability. Moisture content is one of the strongest levers for changes in cohesion and flowability for bulk solids. The change in flowability based on moisture content is often nonlinear. It is possible to reduce the moisture content of a material yet not to a point that flowability has noticeably changed.

Second, moisture content is not the only factor to consider when assessing flowability. For many bulk solids, temperature also drives flowability. For certain bulk solids they may show worse flowability when at the elevated temperature of the dryer outlet.

Third, some materials have poor flowability even when moisture is lacking. Fine powders or fibrous particulates are both prone to flow problems and may not reliably discharge from a bin even after having been dried.

When we consider a dryer, we must remember that the material must go somewhere. That material may or may not present difficulties to the receiving equipment. Those providing the equipment for the dryer’s downstream equipment must be made aware of what state the material will be in and how it will behave.

As a final word of caution here, even the best laid plans seem to go awry. The material leaving our dryer will likely be dried to the design point, but things can go wrong. Varying feedstock, unexpected circumstances, and the seemingly random events of normal plant life all can result in something coming out of our dryer that is a bit more wet than we hoped. There is an inherent vulnerability to our receiving equipment after the dryer, and we would do well to also keep this in mind.

Scott Miller has been providing consulting services for Solids Handling Technologies Inc. since 2016. He works closely with clients to evaluate and help address their bulk handling needs. Prior to his work with Solids Handling, he spent several years as a plant engineer at a waste-coal power plant in PA, experiencing firsthand the challenges of difficult flowing materials. Miller has a BS in Mechanical Engineering from Geneva College in Beaver Falls, PA. For more information, visit www.solidshandlingtech.com.