A rotary dryer is an equipment that allows drying a wet solid, obtaining a dry solid with final moisture usually below 10 %, depending on the operating conditions. The rotary dryer is one of the most widely used equipments for continuous and quick drying of many products at a lost cost when dealing with medium or large flow rates.
The main dryer body consists of a hollow cylinder where the product flows, with the inlet and outlet located on both ends. The cylinder can be over 20 mts. Long and 3.5 mts wide, depending on the operating conditions and flow rate.
The rotary dryer is a direct type dryer, where the hot air current is in direct contact with the product. This air is usually heated in a direct type hot air generator. The hot air can be as hot as 400°C and is contacted with the wettest product to avoid burning it, given that most of the heat is buffered by the evaporating water, cooling the air almost instantaneously. The rotary dryer can have one or more stages, depending on the circulation of the heating air inside it.
The inside of rotary dryers has removing blades for the product which help increase heat transfer coefficients as well as improving product flow. When rotating, the blades lift the product and then let it fall when they are turned upside down, increasing air-particle contact.
The product is discharged into a plenum box ant then transported by screw conveyor, either for recirculation or discharge. Like in a flash dryer a rotary dryer can be equipped with a recirculation system that allows the part of the dry product to be mixed with the inlet in order to achieve the optimum flow rate and water content for each dryer.
These dryers can be designed for residence times ranging from five to sixty minutes and drying capacities ranging from a few hundred kilograms of feed per hour to over 200 tons per hour.