Dryers 101: Types of Dryers

There are many different types and variation of dryers, and selecting the proper Dryer is crucial to achieving the desired results.

Different types of Dryers are required depending on factors such as:

• Capacity
• Product quality
• Product size
• Product consistency
• Hours of operation
• Pounds of water to be evaporated
• Acidity of the product
• Operational environment
• Etc.

One of the more common variations between Dryers is the heat source.  Does the Dryer use waste heat, or is it direct fired, or is it indirect fired?  What sort of fuel is used to generate the heat?  Is it Bio-mass, coal, natural gas, or a bi-product of the plant’s main process?  All of these factors are important to consider when choosing a Dryer.

To begin with let us look at a few of the more commonly used Dryers in industry today.  It is important to note that this list does not include all forms of Dryers.

Flash Dryers
A Flash Dryer uses ductwork, which acts as a container for the uniform transfer of thermal energy from a hot gas stream to a moisture laden product, for the purpose of moisture reduction in the product.  For proper drying the particle size must be uniform and relatively small.  Wet product is introduced in the gas stream where the moisture is flashed off, and then the dried product is removed from the gas stream.

The Ring Dryer is a variation of the flash dryer which grinds and then recycles product to ensure uniform particle size and product moisture content.

Fluid Bed Dryer
A Fluid Bed Dryer uses a container equipped with vertical jets of hot gases, which acts as a container for the uniform transfer of thermal energy from a hot gas stream to a moisture laden product, for the purpose of moisture reduction in the product.  For proper drying the particle must have near spherical shape as well as uniform small size and moisture content.  The wet product must be uniformly introduced into the drying gas stream where the moisture is flashed off.  The dried product is removed from the exiting gas stream.

Rotary Dryers
A rotary dryer uses a rotating cylindrical drum, which acts as a container for the uniform transfer of thermal energy from a hot gas stream to a moisture laden product, for the purpose of moisture reduction in the product.  Wet product is introduced into the inlet of the drum where it is dried as it is conveyed to the drum’s outlet.  The drum is equipped with flighting to disperse the product into the drying gas stream as the drum rotates.  The product can either be conveyed pneumatically or it can be conveyed by gravity if the drum is on a slope.  There are several different types of rotary dryers and they can be used in many different applications.

The two common types of rotary drying systems are single and triple pass; however, there are several other types and hybrids of these designs.

Steam Tube Dryer
A steam tube dryer is a constant temperature drying process.  The drying process always takes place at the steam’s saturation temperature for the steam’s pressure.  Steam Tube Dryers use a rotary drum for the drying vessel and product is handled in a Steam Tube Dryer similar to a Rotary Dryer.  Product can be conveyed with gravity, screw flighting, or pneumatics.

The Steam Disc Dryer is a variation of the Steam Tube Dryer.