Centrifugal Fans - Basic types and their characteristics

Centrifugal fans, often called as blowers, are special type of industrial fans that are ideal for high pressure applications in several process industries and air pollution systems. Centrifugal fans increase the pressure of an incoming airstream by impellers, a series of blades mounted on a circular hub, which in turn accelerates air radially and alters the direction of the outward flowing air, usually by 90°. As centrifugal fans are constant volume devices that create steadier flow of air than axial fans, they are most widely used in various industrial processes such as in transporting air/gas and material handling.

Centrifugal fans are broadly divided into six basic types based on the blade configuration. Each type of centrifugal fan has its own application range and limited ability to perform in the industrial processes.

Some of the basic characteristics of centrifugal fan types are:

• Forward curved fans

> The forward curved centrifugal fans are also called as 'squirrel cage' fans because the wheel type used in this fan is of squirrel cage wheel. The impeller has forward blades which are relatively smaller, curved in direction of wheel rotation.

> The delicate construction of the impeller, shaft, bearings, and housing of forward curved fans limit its suitability in industrial applications. These fans are ideal for low speed and low pressure applications such as in domestic furnaces, central station units, packaged air conditioning etc.

• Backward inclined fans

> The backward inclined centrifugal fans offer three standard blade types – flat single thickness, curved single thickness and curved airfoil. The blades are generally heavier and larger than forward curved and move away from the direction of wheel rotation. They produce relatively medium to high airflow at static pressures.

> These fans produce highest operating speeds and are found to be more efficient than the other types of centrifugal fans. The backward inclined fans are typically used in low pressure, high volume applications such as heating, ventilation and air conditioning systems.

• Radial blade fans

> Radial blade centrifugal fans also called as paddle wheel fans, consists of an impeller with multiple equally spaced flat blades that are extending perpendicular to the direction of wheel rotation. The wheel is a paddle type wheel which may or may not contain side rims and the blades are comparatively heavier, deeper and narrower than forward and backward inclined blades.

> They produce medium to high pressure and moves relatively a higher mass or volume of fluid. Radial blade types are well suited for material handling and in some moderate pressure industrial applications.

• Airfoil blade fans

> Airfoil blade fans have hollow backward inclined blades that are suitable to handle large volumes of clean air at low to moderate static pressures. These fans produce optimum efficiency and run at greater speed than the backward inclined fans.

> As they can handle large volumes of air, these fans are extensively used in forced and induced draft fans that are suitable for broad operating ranges in industrial applications, HVAC systems, air pollution control, drying & filtration, and other industrial processes.

• Radial tip fans

> The blades of radial tip centrifugal fans have backward curved configuration but cupped in the direction of the wheel rotation. The velocity produced by the Radial tip fan is same as that of backward inclined wheel to move the given mass or volume of air. These fans have compact design than airfoil and backward curved fans and are ideal for high-volume flow rate applications where the pressure requirement is also relatively high.

> Radial tip fans have moderate material handling capabilities but are more efficient than radial blade fans. They are generally used in dust collection systems and general ventilation applications.

• Inline centrifugal fans

> The inline centrifugal fans have rugged construction with backward inclined type wheels enclosed in a tube-axial casing. These are medium duty fans with straight through airflow.

> When compared to radial inclined type fans the inline centrifugal fans are less efficient and are used in low pressure air system requirements like heating and ventilating.

Based on the specific requirements of airflow rate, volume, pressure and efficiency, centrifugal fans of various types are designed to serve the needs of diverse industrial applications.

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