Rotary Dryer Design 101: Defining Your Materials Part 1 – Percent Moisture

This article was co-authored by:

Shane Le Capitaine
Thermal Processing Expert

Carrie Carlson
Technical Writer

Rotary dryers are used prolifically throughout industrial processing operations to dry bulk solids, helping to prevent downstream equipment from clogging, creating a more flowable feedstock, and producing a finished product ready for market. 

Achieving optimal performance of a rotary dryer is the result of finding the right balance between the characteristics of the material to be processed and how the dryer design works with those characteristics to promote maximum efficiency and product quality. As such, the process of engineering a custom rotary dryer begins with analyzing the material, and that starts with assessing the inlet and outlet moisture content. 

Rotary Dryer Design: Inlet and Outlet Moisture

Establishing the difference between actual (inlet) and desired (outlet) moisture content determines the amount of moisture the dryer will need to remove, which will, in part, influence some of the key design parameters of the system. 

While rotary dryers can be used to remove a significant amount of moisture from a wide range of materials, there are a couple factors to keep in mind in evaluating moisture content: 

Maximum Allowable Inlet Moisture Content of a Rotary Dryer

Each material responds uniquely to drying. In general, however, those with a moisture content higher than 30% can be problematic, leading to sticking issues on the dryer’s interior. Rotary dryers are still an option when this occurs, but some level of preconditioning may be required to bring the moisture content into a more suitable range. 

A number of options are available for preconditioning material to bring down the moisture content. This might include pre-drying, or the incorporation of a back mixing step. Back mixing is a technique in which dried material is blended with raw feedstock to reduce the overall moisture content of the material going into the dryer. This is typically carried out using a pugmill mixer (also known as a paddle mixer). 

Excessive moisture is often found in sludge-like/low-solids materials such as dairy or hog manure, wastewater treatment sludge, and some industrial by-products.

Removing Bound Moisture

It’s important to recognize that there can be two different types of moisture in any given material: free moisture and bound, or crystalline moisture. 

Free moisture is that which is readily removable and not chemically attached, while bound moisture is actually chemically incorporated into the makeup of the material. 

In some cases, the presence of bound moisture may not be readily apparent, presenting a challenge during the drying process when a portion of the moisture (that which is chemically bound) refuses to be removed. For this reason, testing the feasibility of an intended drying process is often essential, as it will identify the presence of bound moisture up front. 

The removal of bound moisture is a chemical reaction, typically requiring much higher temperatures than those at which a typical rotary dryer operates. As such, when bound moisture is present, it may be necessary to process the material at higher temperatures, or in extreme cases, a rotary kiln following the dryer. Rotary kilns operate at significantly higher temperatures in order to cause a chemical reaction and remove crystalline moisture. 

It may seem inefficient to use both a dryer and a kiln for removing moisture, but it’s actually more efficient in some cases; rotary dryers have a comparably high rate of heat transfer, offering the best efficiency for removing free moisture. Although it is physically possible to use a rotary kiln to remove free moisture, the less efficient heat transfer is not practical for removing large amounts of free moisture.

How Moisture Content Influences Rotary Dryer Design

Once the basic parameters around inlet and outlet moisture are established, engineers tailor the dryer design to meet that goal. While the desired end product moisture content is at the heart of every design decision, it’s important to note that no aspect of dryer design can be looked at in isolation. In other words, reaching the intended moisture content cannot be tied back directly to any one parameter; all of the variables are used in collaboration to reach the desired result.   

The major parameters used to target the desired moisture content include:

Retention Time

Retention time, sometimes also referred to as residence time, is the amount of time that the material will need to reside in the dryer to reach the target moisture content. Retention time is primarily controlled through dryer length, diameter, drum slope, and drum rotational speed.

Air Flow

The raw material characteristics and product requirements impact the choice of air flow configuration of the dryer, or the direction of the air flow in relation to the flow of material – co-current (in the same direction) or counter current (in opposing directions). 

Heat sensitive materials require the hottest gases to come in contact with the wettest material. In this case, the co-current air flow configuration would be chosen. It’s important to note, however, that the selection of air flow configuration must take other factors into consideration as well. 

Drum Diameter & Length

As mentioned, the overall dryer size is also influenced by the amount of moisture that needs to be removed, though again, several variables come into play when sizing the drum. In general, a longer drum will promote a longer retention time, and therefore an ability to remove more moisture. The heat transfer properties of the material are also important in this equation, as some materials more readily release their moisture than others (a factor that also influences air flow configuration selection).

Developing a Rotary Dryer Process Through Testing

Developing an efficient drying process that yields the desired product quality is not always a straightforward endeavor. This is particularly true of materials presenting unique challenges, or when developing a process around a novel material. 

Often it is necessary to conduct various tests to gather critical data points and assess how the specific material responds to the drying process. In addition to gathering data for scale-up, testing also de-risks the path to commercialization. 

The FEECO Innovation Center offers a pilot-scale rotary dryer for continuous testing of a wide range of materials. Through testing, experts in the Innovation Center identify key process parameters, work out critical process variables, and develop an efficient approach to drying the representative sample. Gathered data includes:

  • Retention time
  • Dryer temperature
  • Drum speed and slope
  • Air volume
  • Feed rate
  • Feedstock characteristics
  • Gas sampling & analysis

Testing can also be used to identify the necessary preconditioning requirements. 

The Innovation Center is also home to a unique flight simulator which allows various flight designs to be trialed. Flights, or lifters, are a vital component in promoting maximum heat transfer in a rotary dryer. 


There are many factors that work into the engineering process of sizing and designing a rotary dryer for optimal performance. The amount of moisture removal required sets the stage for what the drum will need to accomplish, and ultimately, how it will need to be designed to meet those specifications.

FEECO has been engineering custom rotary dryers around the unique characteristics of the material to be processed since 1951. In addition to our custom rotary dryers, we offer testing in our Innovation Center, as well as a comprehensive rotary dryer parts and service program. For more information on rotary dryer design, contact us today!

About the Authors . . .

Shane Le Capitaine is a Process Sales Engineer and thermal processing and fertilizer production expert.

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Carrie Carlson is a technical writer and visual designer.

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