HOW CENTRIFUGAL FANS MOVE AIR – WHAT MAKES THEM DIFFERENT
All fans are rotary-bladed machines designed to maintain a continuous flow of air. They use an impeller to draw in that air – but the exact way they move and expel the air differs, based on fan type.
At a headline level, all centrifugal fans work in the same way too: they move air from the centre of their circular impeller to the outside edge. This differs from axial fans – such as the desktop variety – which move the air in a direction parallel to the axis on which the impeller is mounted.
Centrifugal fans use centrifugal force to throw the air out at an angle of 90 degrees to the impeller’s axis. As the air moves through the powered fan, the impeller blades apply force to the air, causing it to move outwards and away from the centre of the fan.
The curve of the housing directs the air around, increasing its speed and pressure before it exits the fan at the discharge point.
WHAT CAUSES CENTRIFUGAL FORCE? HOW IS IT GENERATED?
Centrifugal force acts on an object that is moving in a circle. The process involves inertia, the natural tendency of an object to remain at rest or in motion in a straight line.
When an object moves in a circle, it constantly changes direction. The object’s inertia wants it to continue moving in a straight line, but the force causing it to move in a circle is also pushing it outwards, away from the centre of rotation. This is known as centrifugal force.
The magnitude of the centrifugal force depends on the:
• mass of the object (in this instance, air)
• radius of the circle in which the air is moving – decreasing the radius increases the force
• speed of rotation – the faster the speed, the greater the force.
WHY USE A CENTRIFUGAL FAN, RATHER THAN AN AXIAL FAN?
Centrifugal fans and axial fans suit different air-movement applications. The choice between them will depend on the specific requirements of the application.
Centrifugal fans are typically better than axial fans for applications such as drying, dust collection, fume exhaust systems, or blowing away debris or excess coatings. This is because centrifugal fans are better able to overcome resistance, thanks to their impeller design.
Find out more here about air delivery solutions for different applications.
Axial fans are better suited to applications that require large-volume air movement such as ventilation, exhaust systems and cooling electronic equipment. They are good at moving large volumes of air because of the way their blades are designed. They push the air in a straight line, allowing them to move larger volumes of air at relatively low pressures.
BENEFITS OF CENTRIFUGAL FANS
Some of the main advantages of centrifugal fans include:
- High airflow rates: Centrifugal fans are designed to handle large volumes of air (though axial fans are better for high-volume, low-pressure applications).
- Increased pressure: Centrifugal fans can overcome higher system resistance.
- Energy efficiency: Centrifugal fans are relatively efficient, as the energy required to move the air increases with the cube of the air velocity, whereas the power required to drive a centrifugal fan increases only with the square of the air velocity. Find out more here – learn about the three Fan Laws.
- Low noise level: Centrifugal fans are generally quieter than other fan types.
- Versatility: Centrifugal fans can be used in a wide range of industrial and commercial applications.
- THE ROLE OF THE IMPELLER IN A CENTRIFUGAL FAN
- The rotating circular impeller at the centre of the fan features a series of angled blades that draw in air and then push it away from the central axis.
- These blades cause the air to move in a circular motion as it is pulled into the centre of the fan. As the air moves through the impeller, the blades apply force to the air, causing it to move outwards and away from the centre of the fan.
- This movement of air creates a pressure difference between the air inside and outside the fan. This causes air to flow into the fan and then be expelled via the discharge port.
- The impeller is responsible for creating the pressure and flow of air that is necessary for the fan to function. The impeller’s rotation speed and blade design are key factors that determine the performance of the fan and its ability to move air effectively and meet the requirements of the application.
- HOW THE FAN’S MOTOR AFFECTS PERFORMANCE
- Virtually all fans supplied by ACI are powered by electric motors. We have worked on some projects involving hydraulically powered fans (which utilise force created by a pump) but these are rare.
- Most centrifugal fans are direct drive: the impeller is mounted on the shaft extension of an electric motor. However, some are belt driven – the motor is used to rotate a belt which then in turn moves the impeller. This creates a gearing effect which increases the rotational speed of the impeller – improving output but reducing energy efficiency.
- Various factors will affect electric motor performance – and therefore the rotational speed of the impeller and the efficiency of the fan as a whole. These factors include the number of poles in the motor and the frequency of the AC mains electricity supply.
