The fan is the most critical component of a dryer. Fans move the drying air through the drying system. A fan that does not perform according to the specifications does not move enough air through the dryer which leads to many problem. Often a dryer is blamed for not working properly when it was in fact a sub-standard fan causing the problems. Depending on the required airflow rate and the needed pressure creation either axial-flow or centrifugal fans are used.
The differences of both fan types are as follows:
Overview on axial flow and centrifugal fans.
Generally it can be said that the cheaper axial-flow fans provide a higher airflow rate at lower pressure creation and are therefore more suitable for shallow bed batch dryers with low resistance to airflow. They are also being used in re-circulating batch dryers where high air volumes are desired to remove water quickly for short drying time. The more expensive centrifugal fans have higher pressure creation and can overcome the resistance of deeper beds but they have lower airflow rates. When a centrifugal fan is the choice backwards curved rotors should be used because of their non-overloading characteristics.
Fan design is an engineering art by itself and many fans sold by Asian manufacturers do not conform to their specifications. In tests conducted in the Philippines fan performance was 30-60% lower than quoted. On the other hand the fan is probably the most important component for getting good performance out of a dryer. When buying a fan it is therefore advisable to request the manufacturer to test the fan on a fan test rig in presence of the customer in order to guarantee that it performs according to the specifications.