In many industrial processes, rotary dryers are relied upon for high capacity, consistent drying of bulk solids — and for good reason. They offer innumerable benefits, both to the process and the product.
One of the most prominent components of a rotary dryer is a large, rotating cylinder called the drum shell. Drying air or process gas flows through the drum in co-current or counter-current modes. In co-current mode, material travels in the same direction as the process gas. In counter-current mode, material travels in the opposite direction of the process gas. Regardless of the direction of travel, the drying air comes in direct contact with the material being dried.
Rotary dryers are used both for post-drying and pre-conditioning. Post-drying is a method of reducing the moisture content of a final product. Pre-conditioning is a method of drying a material prior to subsequent processing.
Rotary dryers offer major benefits when it comes to processing bulk solids. Here are eight reasons why processors choose rotary dryers for their bulk solids drying needs.
1. Custom Processing Capabilities
While standard, one-size-fits-all rotary dryers are available, many rotary dryer designs are customizable and incorporate options that can increase processing efficiency.
For instance, by designing around a material’s unique characteristics and the overall goals of the process, rotary dryers can attain optimum processing efficiency. Various material characteristics affect how the material behaves during the drying process. The characteristics of the material to be dried also influence dryer design to ensure the thermal processing satisfies the production goals. When designing a rotary dryer, consider key material characteristics such as:
- Raw material moisture level.
- Outlet material moisture goal.
- Specific heat of the material to be processed.
- The heat transfer properties of the material to be processed.
- Material limitations (fragility, consistency, end use, particle size, etc.).
- Material surface quality (abrasive, corrosive, etc.).
In addition, site-specific factors can affect how a dryer performs and should be taken into account during the design phase. Typical factors include the humidity level, the elevation and ambient temperatures.
A multitude of components can be customized to suit most processing needs. Factors such as airflow configuration, flight design and pattern, percent fill, retention time and size will affect how the dryer performs to meet its intended goal. Additional customizations such as knocking systems, seal options and materials of construction can be combined to create an efficient drying solution tailored to your unique processing requirements.
2. Creating a Refined Product
When used to dry pellets or granules, rotary dryers offer an advantage over other drying mediums. The tumbling action on the bed of material further rounds and “polishes” the granules, knocking off any loose edges and creating a more refined product.
3. Heavy-Duty Processing
Rotary dryers are robust and offer a heavy-duty design. While they can be used to process fines, they are particularly adept at processing heavier materials such as those found in the mining industry. For this reason, materials commonly processed in a rotary dryer include potash, phosphates, gypsum, limestone, minerals and ores.
A well-designed and constructed rotary dryer also has the potential to last for decades when properly maintained.
4. Consistent Processing
In addition to their heavy-duty processing capabilities, rotary dryers are perhaps most valued for their ability to churn out a consistent-quality product despite inconsistencies in the feedstock or process. Flights within the rotating drum lift and cascade material as it tumbles through the drum, preventing material clumps and promoting an evenly dried final product. They are able to take what is given — lumpy or uniform, wet or dry – and produce a quality product.
Rotary dryers are particularly valuable in settings where variance in the feedstock is a given such as when processing minerals. While a change in moisture or particle size distribution might leave a fluid bed drying operation at a standstill, a rotary dryer will continue to put out a fairly consistent product. This is also true of processing environments where upsets are likely such as power outages or lapses in fuel availability.
5. High Capacity
While rotary dryers do occupy a larger footprint than some other dryer types, this also means they offer significantly higher capacities. Rotary dryers can offer capacities ranging from as little as 1 ton per hour to in excess of 200 tons per hour. Continuous, high capacity processing is a necessity in many processing environments.
6. Energy Savings
Rotary dryers typically require less energy than some other drying alternatives such as a fluid bed dryer. In addition, they offer the opportunity to save on energy costs when running at reduced capacities. For example, fluid bed dryers require a certain amount of energy, even when running at decreased capacities, because it takes a set amount of energy to fluidize a material. With a rotary dryer, however, energy consumption is reduced without process upset when running at lower capacities.
Heat transfer efficiency is increased with a rotary dryer through the use of lifting flights. The flights pick up the material and drop it through the airstream, creating a curtain of material as the drum rotates. Exposing the material to the airflow improves drying and moisture leveling.
7. Ease of Operation
Rotary dryers offer fairly hands-off operation. While a rotary dryer does need to be monitored, there are few things that can cause a process upset with a rotary dryer. There are also minimal variables to be adjusted, allowing for ease of operation.
8. Direct and Indirect Heating Options
Most rotary dryers are of the direct-fired type, meaning the material and drying air are in direct contact with each other. However, when ultra-fine materials must be processed, or the material to be processed must not come into contact with the drying air, an indirect-fired rotary dryer offers an effective processing solution.
With indirectly heated rotary dryers, the drum is heated externally, and the material is dried through contact with the shell of the drum. This avoids any contact between the drying air and material. An indirect dryer is especially beneficial when entrainment of fine material is a concern.
In conclusion, rotary dryers are the backbone of many modern industrial processes, helping to reduce the moisture content of final products and prepare material for additional processing. They offer a number of advantages for both the end product and the process itself. While rotary dryers are available in standardized models, a dryer tailored to the unique characteristics of the material to be processed will produce the most efficient and reliable results.