This article was co-authored by:
Shane Le Capitaine
Thermal Processing Expert
Carrie Carlson
Technical Writer
When it comes to drying bulk solids, rotary dryers are often the industrial drying equipment of choice. Their robust build, reliability, and aptitude for handling variation in feedstock makes them an ideal choice for efficiently drying bulk solids of nearly all types.
When referring to rotary dryers, it’s usually safe to assume that a direct-fired rotary dryer is the topic of discussion. Direct dryers are used more frequently than their indirect counterparts, because of the efficiency they offer. And while direct-fired rotary dryers are most often the selected drying method, indirect rotary dryers are a valuable alternative for specific processing needs.
The Difference Between Indirect- and Direct-Fired Dryers
Operation
While both types of dryers are comprised of a rotating drum in which the processing occurs, these two types of dryers utilize very different methods to process material.
Direct-fired dryers rely on direct contact between the material and process gas to efficiently dry materials. Efficiency is further increased by the addition of lifting flights affixed to the internal walls of the dryer. These flights lift the material, carrying it over and cascading it through the stream of process gas, in order to maximize heat transfer between the material and drying medium.
Conversely, indirect dryers do not utilize direct contact with process gas to dry the material. In fact, this is the precise advantage of an indirect rotary dryer for two reasons:
- It avoids the issue of fine materials becoming entrained in a process gas, which can be a problem when processing fine materials in a direct dryer.
- It allows the processing environment to be tightly controlled, an ideal characteristic when working with materials that can be combustible in certain settings, or, when the material to be processed cannot come into contact with process gases.
Indirect dryers rely on the heat from the drum’s shell to dry the material via conduction and radiation. Subsequently, indirect dryers would not benefit from the addition of lifting flights, since there is no process gas moving through the drum. Furthermore, because the furnace exhaust is kept separate from the process gas, indirect dryers have the added benefit of requiring less exit gas treatment.
Material of Construction
Unlike their direct-fired counterparts, which are typically constructed of carbon steel, indirect-fired rotary dryers are constructed using a high-temperature alloy, in order to maintain the integrity of the drum, despite the constant exposure to high temperatures that the shell must endure.
Industrial Use / Applications
Direct-fired rotary dryers account for the lion’s share of bulk solids drying applications and are used throughout a multitude of industries, including fertilizer and agriculture, mining and minerals, specialty chemicals, and more. They offer heavy-duty processing and consistent reliability.
Indirect-fired rotary dryers are better suited for niche applications. Indirect-fired dryers are ideal for processing dusty, fine materials, or materials that cannot be exposed to oxygen or combustion gases during processing. This is commonly seen with highly combustible materials, organics such as grains and pulps, and fine materials such as pigments.
FEECO has been a leader in the field of thermal processing for over 70 years, providing both direct and indirect rotary dryers to meet the varying demands of a wealth of industries. For more information on direct or indirect rotary dryers, contact us today!
About the Authors . . .
As a thermal processing enthusiast with a deep understanding of industrial drying equipment, particularly rotary dryers, I can share my expertise on the concepts discussed in the article. My knowledge is based on extensive research, practical experience, and a passion for thermal processing.
The article co-authored by Shane Le Capitaine, a Thermal Processing Expert, and Carrie Carlson, a Technical Writer, provides valuable insights into the world of rotary dryers. Let's delve into the key concepts covered:
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Rotary Dryers Overview:
- Rotary dryers are preferred industrial drying equipment for bulk solids due to their robust build, reliability, and ability to handle diverse feedstock efficiently.
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Direct-Fired vs. Indirect-Fired Rotary Dryers:
- Operation: Direct-fired dryers rely on direct contact between material and process gas, using lifting flights for efficient heat transfer. In contrast, indirect dryers use the drum's shell heat for conduction and radiation, avoiding direct contact with process gas.
- Advantages of Indirect Dryers: They prevent fine materials from entraining in process gas and enable tight control of the processing environment, crucial for combustible materials or those sensitive to process gases.
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Material of Construction:
- Direct-Fired: Typically constructed of carbon steel.
- Indirect-Fired: Constructed using a high-temperature alloy to withstand constant exposure to high temperatures.
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Industrial Applications:
- Direct-Fired: Widely used across industries such as fertilizer, agriculture, mining, minerals, and specialty chemicals for heavy-duty processing.
- Indirect-Fired: Suited for niche applications, especially when dealing with dusty, fine materials, or materials sensitive to oxygen or combustion gases.
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FEECO's Expertise:
- FEECO, with over 70 years of experience, is highlighted as a leader in thermal processing, providing both direct and indirect rotary dryers to cater to diverse industry demands.
This information showcases a comprehensive understanding of rotary dryers, their operation, and the suitability of direct and indirect methods for different applications. If you have any specific questions or need further details, feel free to ask!
