Industrial Drying Machines: Avoiding Over-Drying and Under-Drying

Key Takeaways:

• Industrial drying may seem simple on paper, but is in fact a complex process that when performed wrong will result in over- or under-drying.

• Over- and under-drying are much more than just thermal issues, and will result in products with unacceptable structural, sensory, moisture, performance or functional properties.

• To avoid over- and under-drying, manufacturers must ensure that process variables and industrial drying machines stay within original specifications, or they will have to perform system modifications. 

On paper, industrial drying machines are simple enough. Start with a high-moisture product, apply heat and friction to release some of that moisture, and out comes a low-moisture end product ready for the next step in the process. 

There seems to be only a few variables involved: feed or batch volume, thermal energy, residency time, and handling turbulence. It should be easy enough to set these parameters once and hit the required product specs every time, right? Not exactly. 

In practice, industrial drying is the epitome of a dynamic control application, which must automatically and instantly account for a huge number of variables ranging from ambient temperature to air density, product tack to equipment cleanliness, and so on. 

For these reasons, dryers don’t operate exactly the same way from batch to batch. While the end product may certainly come out the same, the precise way in which the dryer got there will be unique to the natural variations seen in each run. 

These variations introduce the concept of over- and under-shooting drying specifications. In short, when an industrial drying machine is pushed outside of its design parameters such that the dryer couldn’t successfully pull a given variable back into range, the dryer is said to have over- or under-shot its target.

Symptoms of Over-Drying and Under-Drying in Industrial Drying Machines

Many sources will define over- and under-drying as a thermal issue, and that is often true. However, over- or under-drying can technically present in other ways. To make this point, let’s compare how over- and under-drying impact the most important properties of dried products.

Product / Process Parameter	Over‑Drying	Under‑Drying
Structural Integrity	Materials become brittle, fracture easily and generate excess fines or dust	Materials remain soft, tacky or unstable, and may deform or clump
Chemical Components	Active compounds degrade, desirable volatiles are destroyed and functional performance declines	Unwanted moisture and solvents are retained and the risk of undesirable microbial and chemical activity increases
Sensory Attributes	Discoloration, scorching, off‑flavors and aroma loss occur	Colors are dull, aromas and flavors are inconsistent, and other sensory properties remain “raw”
Physical Handling	Materials become too light and dusty, complicating conveying and packaging	Materials cake, bridge and adhere to equipment surfaces, disrupting flow
Uniformity	Powders shrink or harden excessively, and break down into uneven particle sizes	Moisture, texture, hardness, density and ingredient dispersion present unevenly across the batch
Rehydration Behavior (where applicable)	Rehydration capacity is reduced, and functional benefits are lost	Rehydration becomes inconsistent, equally costing functional benefits
Downstream Process Performance	Materials may break down during milling, blending or compaction	Materials may jam, smear or fail in granulation, coating or packaging
Energy and Equipment Stress	Energy consumption and physical wear on equipment are increased due to extra heat and friction	Energy is wasted due to inefficient drying, and downstream processes are strained under higher moisture loads

Tips for Avoiding Off-Spec Drying  

Industrial drying machines can only avoid over- and under-drying products when three things are true: 

• Input materials and utilities are provided within expected parameters
• The system is operated under expected conditions
• The entire process was originally engineered in accordance with all of these variables 

If any of these items are not provided, a dryer will have no hope of achieving on-spec products. 

Beyond these three general requirements, here are some specific suggestions that manufacturers can follow to avoid over- and under-drying in their own operations:

• Use advanced instrumentation and monitoring: Use modern, advanced digital sensors for critical control points (temperature, moisture, level, etc), ideally in redundant pairs across multiple zones throughout the process (not just one point).
• Manage overall airflow: Go beyond traditional practices of only monitoring incoming air temperature and mass rate. Instead, monitor and control for supply and exhaust air profiles to ensure correct thermal transfer and distribution. 
• Standardize infeed materials: Many off-spec drying problems can be resolved by simply eliminating variations in infeed materials (i.e., particle size, moisture content, density, etc) via enforcing specifications and/or installing pretreatment gear. 
• Calibrate incessantly: Instruments drift over time, and a dryer’s control system can’t accurately control what it cannot correctly measure. Even small sensor deviations can cause major biases in control outputs, all solved by active calibration. 

• Train continuously: The parallel to calibrating instruments is of course training (and re-training) human operators, ensuring that they are skilled in their dryer’s operational nuances and reactions as variables change.

We said above that while industrial drying might seem like a simple application on paper, the reality is that drying can be one of the most complex and sensitive processes in your entire plant. 

Consistency and proactive management are the name of the game with industrial drying machines the only ways to protect against over- and under-drying. Do you have questions about drying processes? Reach out to us for answers.