Types of Dryer for Wetted Solids

Batch and Rotary Dryers

• Batch dryers process wetted solids in batches; most of them use a recirculatory air system. This type of approach is best for small-scale applications where flexibility is an important consideration. These dryers often rely on a tray design where the solids are loaded onto individual trays, and this can potentially be a rather labor-intensive process. Rotary dryers feature a slowly revolving cylinder tilted to one side. The wetted solid feeds in on one side of the cylinder and the dry product exits on the other.

Rotary, Vacuum and Freeze Dryers

• Heat-sensitive solids like certain biotechnology products may go through freeze-drying, where moisture is removed from the solid at very low pressures that cause the frozen liquid through sublimation to go straight to the gaseous phase. This process is expensive but helps preserve product quality. Vacuum dryers use low pressure to help remove moisture from the product more rapidly; they too are more expensive than room-temperature devices but help to reduce fire risk if the product is flammable.

Pneumatic Dryers

• Pneumatic dryers are especially well suited to handle wetted solids. These also are continuous in nature and very often rely on fluidized bed technology, where the wetted solids are suspended in a stream of hot gas or air. Since the wet solids are brought in continual contact with the hot gas, the rate of drying is greatly accelerated. Infrared dryers are an approach that relies instead on radiant heat; these are reserved primarily for certain niche applications like coated paper.

Band, Pulse and IR Dryers

• In a band dryer, the wetted solids are supported on a band where they are exposed to the dry air. Typically the air flows downward through the material, although in some cases it may be forced upward instead. Pulse dryers use a pulse combustion burner to create a varying, pulsating airflow. The resulting hot air dries the material. Microwave and radio frequency dryers use electromagnetic radiation although from different regions of the spectrum than an infrared dryer; these latter types are also used only for certain very specialized applications.