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Rotary dryers are often described as the workhorses of the thermal processing industry. These industrial dryers are used to efficiently reduce the moisture content for a variety of materials across many different industries.      A rotary dryer is comprised of a large rotating cylindrical tube (called the drum), through which drying air is passed (referred to as a direct rotary dryer).      Lifting flights are precisely positioned on the interior of the chamber. The flights pick up material from the bed, carry it over, and drop it through the drying air as the drum rotates, maximizing heat transfer between the material and drying air.      Rotary dryers accommodate high capacities and allow for continuous operation; damp material is constantly being fed into the machine at one end, while dried material comes out of the other end. Why Choose a Rotary Dryer? Rotary dryers are extremely versatile, and can be used for a wide variety of applications in the building materials, mineral, fertilizer, and chemical industries. They are commonly used for drying materials such as frac sand, limestone, gypsum, potash, ores, and more.     Rotary dryers in general are an excellent choice, because they are not complex to operate, and will turn out a uniform product. Well-constructed rotary dryers can process hundreds of tons of materials per hour and often last for decades. They operate most efficiently on a continuous basis, because it takes less energy to maintain a certain temperature within the rotary dryer than it does to repeatedly bring it back up to temperature for another shift. Specific Considerations in Choosing a Rotary Dryer There are a number of factors to consider when choosing the best rotary dryer for your specific needs. Some of these include: Direct or Indirect Heat? There are two main designs for rotary dryers when it comes to the application of heat. Direct heat rotary dryers pass a stream of drying air through the drum. The drying air is directed in either a co-current flow (flowing the same direction as the material), or counter-current flow (moving in the opposite direction of the material).     Direct dryers are the most common choice for drying bulk solids, because they are so efficient. However, there may be cases where direct heat is not an option. If the material to be processed is eventually going to be used in creating consumer products, it may not be appropriate for those materials to come into contact with the combustion gases of a directly heated rotary dryer. There may also be cases where the material to be processed cannot come into contact with oxygen, or needs a completely inert environment. In cases such as these, an indirectly heated rotary dryer is the answer. This configuration transfers heat into the chamber via the shell of the rotary dryer itself. This method also avoids the risk of material becoming entrained in the drying air, which can be a challenge when working with fine materials. Operational Surroundings Another factor to consider in choosing a rotary dryer is the location of the operation. Things such as the surrounding atmosphere, air temperature, humidity, and elevation, can all affect the dryer’s performance. For example, an increase in elevation means a decrease in the amount of oxygen in the atmosphere, making the air less dense, and less combustible. At higher elevations, therefore, the dryer’s fan size may need to be increased to compensate.     Rotary dryers are useful in a variety of industrial applications and choosing the right dryer can enhance the efficiency of your entire materials processing system. A Customized Drying Solution It is important to keep in mind that a variety of factors will influence the success of the drying process. In addition to the processing environment, many material characteristics will affect how the material responds to drying, and subsequently, how the dryer will need to be designed in order to work around those characteristics to produce a product with the desired specifications. Characteristics such as bulk density, angle of repose, moisture content, and more, will all need to factor into the design of the rotary dryer itself. When choosing a rotary dryer that will meet your process and product goals, customized equipment will produce the best results.     Dan Baxter is a process sales engineer at Feeco International Inc., Green Bay, WI, a manufacturer of OEM thermal processing, agglomeration, and material handling equipment. For more information, visit feeco.com.

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