Rotary Dryers and Coolers Rotary Dryers

The workhorse of industry.

GEA's range of Rotary Dryers are extensively used throughout the process industries and are highly reliable under the most arduous operating conditions. Noted for their process flexibility and robust construction, their design permits the use of the highest possible drying temperatures and in contrast to other dryers, are not sensitive to wide fluctuations in throughput and product size.

Principles of Rotary Drying

The directly heated rotary dryer operates on the principle of lifting and showering the product through a hot gas stream moving either in parallel or counter-flow whereas with the indirect dryer, which is more suited to fine and dusty materials, there is little or no contact between the product and the drying gases since they are heated from the outside, via a stationary jacket fitted with either multiple burners or other external heat source. 

The efficiency of the dryer is largely dependant on the differential between the inlet and exhaust gas temperatures, although the heat transfer rate is also influenced by the relationship between the design of flights and the speed of rotation. However, irrespective of the gas and material temperatures the drying (or residence) time may be critical, as this is governed by the rate of diffusion of water from the core to the surface of the material. 

For greater thermal efficiency and where inertization is required, recycling of exhaust gases can be used. This can be implemented on all our airstream drying systems and retrofitted on customer's existing drying operations.

For special applications, the rotary cascade design is adapted to provide a shortfall, or 'Cruciform', lifter configuration.

Similarly, Rotary Cascade Coolers and Combined Dryer/Coolers are available as required.

 

Features of the Rotary Drying

  • Robust construction with various lifter options
  • Can be co-current or counter-current
  • Permits use of higher drying temperatures
  • Relatively long residence time
  • Drum provides dispersion as material is conveyed along the length
  • Can be designed for open circuit, partial gas recycle and SSD™

Benefits of Rotary Drying

  • Combines flexibility with reliability resulting in a long service life. The rotary is also suitable for operation with a wide range of materials & particle sizes and can more readily accommodate 'upset' up-stream process conditions
  • Co-current is suitable for materials with relatively high moistures and provides rapid evaporation without overheating the product. Counter-current has the greatest thermal gradient and helps achieve low product moistures
  • Reduces equipment size and the associated capital and operating costs
  • Promotes diffusion drying
  • Relatively low fan power and low headroom
  • Enables energy savings and system integration

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Co-Current Rotary Dryers are the most widely used and are particularly suitable for drying materials containing a high moisture content, which are heat sensitive or have a tendency to stick or cake.

The wet material is in contact with the gas at its highest temperature, which rapidly evaporates surface moisture. The initial heat transfer rate is high, causing an immediate and considerable drop in gas temperature, which prevents overheating of the material and the dryer shell. The final product is in contact with the gas at its lowest temperature, enabling the moisture content to be readily controlled, usually by maintaining the dryer exhaust gas temperature at a pre-set value.

Features

  • Wet material is in contact with the gas at its highest temperature, which rapidly evaporates surface moisture
  • The final product is in contact with the gas at its lowest temperature 

Benefits

  • Initial heat transfer rate is high, causing an immediate and considerable drop in gas temperature, which prevents overheating of the material and the dryer shell; co-current dryers are particularly suitable for drying materials containing a high moisture content
  • Enables the moisture content to be readily controlled  

 

Process Flow Diagram

Rotary Dryer Co-Current drawing3

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Counter-Current Rotary Dryers are more suitable for materials that must be dried to very low levels of moisture, where the last traces of moisture are difficult to remove, or where an elevated product temperature is desirable.

They are also used effectively as combined dryer / preheaters. However, since the final product is in contact with the gas at its highest temperature the counter-current dryer is often unsuitable for heat sensitive materials. Although this system can be more efficient, moisture which is to remain in the product, is not so readily controlled.

Features

  • Dry product is in contact with the gas at its highest temperature
  • The final product is in contact with the gas at its lowest temperature

Benefits

  • Suitable for materials that must be dried to very low levels of moisture, where the last traces are of moisture are difficult to remove, or where an elevated product temperature is desired. Often used as a combined dryer/preheater.
  • Enables the moisture content to be readily controlled

 

Process Flow Diagram

Rotary Dryer Co-Current Principle drawing

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