here are two basic impeller types, Centrifugal, also called a wheel, and Axial also called a propeller. For centrifugal impellers, air travels into the impellers and exits at a 90 degree angle. With axial impellers, the air travels straight without changing directions. Each of these impeller types has unique characteristics.
Centrifugal impellers are inherently quieter than axial impellers. They are capable of high pressures. The 90 degree turn caused by the centrifugal impeller builds pressure in the housing. This results in air flowing from higher to lower pressure to the fan’s discharge. The centrifugal impellers robust construction allows for general clean air applications as well as higher temperature air and contaminants in the airstream; like restaurants and fume hood exhaust applications. And, because centrifugal impellers turn air 90 degrees they take up more physical space. Centrifugal impellers can be broken into three sub categories: backward -inclined with flat blades, typically referred to as BI; backward –inclined with airfoil blades, typically referred to as AF and forward-curved, typically referred to as FC.
Some of the key differences between the three centrifugal impeller types are: A backward-inclined impeller with flat blades can handle higher pressure than the forward – curved and axial impellers. They can handle clean and high temperature air. They handle contaminated air because the blades are flat and remain cleaner. And they are economical to purchase and operate. The backward-inclined impeller with airfoil blades has uniquely shaped airfoil cross sections, like the wing of an airplane. They can handle higher pressures similar to the BI pressure applications.
Axial impellers are also called propellers. Some of the characteristics are: Axial impellers are designed for high volume applications. They have limited pressure capabilities as compared to centrifugals. They also have higher efficiencies at lower pressures than centrifugal impellers. The combination of higher volume, low pressure, and high efficiencies make axial products an excellent choice for factories and warehouse applications. At high pressures, axial impellers tend to be louder than centrifugal impellers. They are economical to purchase and operate. And they can be designed for reversible airflow. A good example is a factory where fresh cool air is supplied in the morning, and then the fan is reversed to exhaust hot air in the afternoon.