Centrifugal fan

What is a centrifugal fan?

Centrifugal fans accelerate air at a right angle to the axis of rotation of the impeller. This deflection generates high static pressure in heating, ventilation, and air conditioning (HVAC) systems, laboratories, and industrial ventilation systems.

Depending on the blade geometry,
there are two main types

Backward-curved blades:

High efficiency, low noise level, can usually be used without a spiral casing.
The air is diverted in the opposite direction to the impeller's rotation. This geometry allows efficient pressure build-up in the impeller, which often eliminates the need for a spiral casing. The blades are flatter and have larger relative radii. This shape reduces flow losses and enables high hydraulic efficiency with low noise levels. Backward-curved impellers are therefore ideal for applications with high pressure requirements and efficient continuous operation.

Forward-curved blades:

Compact design, high power density, particularly quiet, but require a spiral casing.
The air is accelerated in the direction of rotation of the impeller. This geometry generates high power density in a small space, making it the preferred solution for compact devices such as fan coils, air heaters, or air curtains. The blades are more curved and require a spiral housing for targeted air flow. Although the efficiency is slightly lower than with backward curved variants, forward curved impellers are particularly quiet and are suitable for applications with high volume flow at moderate pressure.

How do centrifugal fans work?

The airflow is accelerated by a rotating impeller and directed centrifugally outwards. This creates static pressure, which is used to transport the air. Efficiency depends heavily on the geometry of the impeller, the air gap to the inlet ring, and the installation situation. Accessories such as jump diffusers or FlowGrid grilles can also significantly improve performance and noise behavior.

The airflow: How air is moved

Centrifugal fans draw in air axially and conduct it radially – i.e., at right angles to the axis of rotation. This creates a higher static pressure than axial panel fans , which convey air parallel to the axis. This feature makes centrifugal fans ideal for applications with complex pipe systems or high counterpressure.

What the Tech:
How is an industrial fan constructed?

How is a centrifugal fan constructed

Impeller:

The impeller of a centrifugal fan can be made of different materials to suit different applications and requirements. Each material has specific advantages and disadvantages in terms of weight, corrosion resistance, and cost.
Metal is often used when high stability and durability are required, as plastic is not rigid enough, especially at high air flow. Metal impellers are also mandatory in special applications, such as cleanrooms.
Plastic, on the other hand, has the advantage that it can be molded into any shape, which allows for particularly aerodynamic designs. Plastic impellers are used in most standard applications because they offer a good combination of flexibility, lightness, and cost efficiency.

Motor:

The motor provides the drive power for the impeller. External rotor motors are often used, in which the rotor directly supports the impeller. This design enables a compact unit and optimal balancing results, as the rotor and impeller are balanced together. Depending on the design, these are AC, EC, or DC motors, with EC motors being efficient and with control characteristics thanks to their electronic commutation.

Housing:

The housing channels the air duct design in a targeted manner and may be optional or mandatory depending on the impeller geometry. Forward-curved impellers always require a scroll housing to efficiently divert the air, while backward-curved variants can also be operated without a housing. Scroll housings are available in single-flow or double-flow designs and are often equipped with connection flanges that allow easy integration into pipe systems.

Inlet and outlet openings:

The inlet and outlet openings control the air duct design and influence the flow characteristics. The inlet ring plays a particularly important role: it ensures an even flow to the impeller and can be equipped with pressure taps to precisely measure the air flow. The outlet opening is usually connected to the scroll housing and directs the air in the desired direction.

What are the advantages of centrifugal fans?

Centrifugal fans offer many advantages, including:

  • Energy efficiency:
    Thanks to their optimized design and special blade geometry, centrifugal fans can minimize energy consumption.
  • Performance:
    Due to their design, centrifugal fans achieve both higher pressures and higher efficiencies than axial fans.
  • Flexibility:
    The curvature of the impellers and the materials used make it possible to customize centrifugal fans to the desired application.

FAQ – frequently asked questions
about centrifugal fans

Backward-curved blades offer higher efficiency and usually do not require a scroll housing. Forward-curved blades are more compact and quieter but require a housing for air guidance.

A scroll housing is mandatory for forward-curved impellers and optional for backward-curved impellers, depending on the application and desired air guidance.

Restricted installation spaces or disturbed airflow can significantly reduce performance and efficiency. An optimal distance from the wall and uniform airflow are crucial.

A FlowGrid is an intake protection grille that reduces turbulence and thus minimizes noise. It is mounted directly at the intake point and can eliminate the need for secondary noise protection measures

Centrifugal fans are available with AC or EC motors. EC technology offers precise control characteristics, higher efficiency, and lower energy consumption.

Technically speaking, the terms centrifugal compact fan and centrifugal panel fan are often used synonymously. In practice, “centrifugal fan” usually refers to the complete unit consisting of the motor, impeller, and, if applicable, the housing, while “centrifugal compact fan” is sometimes understood as a simplified or smaller design. For example, a compact fan can be seen as a component in household appliances or electronic cooling systems. In industrial air technology, however, the term ‘centrifugal fan’ is predominantly used, as it describes the aerodynamics function more precisely.

The noise level depends on the design, speed, and installation situation. Backward-curved impellers are generally quieter, and accessories such as FlowGrid or diffusers can further reduce noise.

Efficiency depends on the blade geometry, the radial air gap, the axial overlap, and the installation situation. Even small changes to these parameters can significantly affect performance.

An installation space that is too narrow or disturbed airflow can significantly reduce air performance and efficiency. Components such as the FlowGrid help to minimize installation losses and noise.