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.
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?
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 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.
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
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.
Impellers can look and perform very differently, depending on their type:
How The Fan Housing Affects Pressure And Airflow
The housing is designed to guide the air as it flows through the fan and push it out – via the discharge point – in a specific direction. This helps to increase the pressure and velocity of the air as it exits the fan. The housing prevents air leaks and protects the impeller and other internal components from external elements.
The shape and size of the housing are also important factors that determine the performance of the fan and its ability to move air effectively.
The typical clearance between blade tips and housing is 0.25% of the impeller’s diameter. Reducing this clearance will increase peak fan pressure (and vice versa). Housings can also include features such as inlet and outlet cones, vanes and scroll sections to further shape and guide the airflow.
A centrifugal fan housing typically looks like an ammonite shell without the spiral. This is called a volute casing; it is a curved funnel that increases in area as it approaches the discharge port.
The ‘ammonite’-style shape of a volute casing uses an outlet flow design. The air leaving the impeller has kinetic energy proportional to its velocity pressure – resulting in greater efficiency.
This is because the kinetic energy of the air helps to maintain its momentum as it flows through the fan, reducing losses due to friction and turbulence. Additionally, the ammonite-style shape of the volute casing helps to smooth out the flow of the air and further reduce these losses – resulting in a more efficient fan overall.
How The Fan’s Motor Affects Performance
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.
How The Number Of Fan Motor Poles Affects Impeller Speed
A two-pole motor has two north and two south poles, while a four-pole motor has four north and four south poles. The more poles a motor has, the slower it will rotate for a given frequency of the alternating current (AC) power supply.
The relation between the number of poles, the power supply frequency and the motor’s speed is described by this formula:
The more poles a motor has, the slower it will rotate for a given power supply frequency.
Example: a four-pole motor running on a 60Hz power supply will rotate at 1,800 RPM, while a two-pole motor running on the same power supply will rotate at 3,600 RPM.
How AC Frequency Affects Fan Motor Performance
In the case of the power grid, the frequency is determined by the number of times the alternating current changes direction in one second. The most common frequency for AC power used in homes and buildings is:
- UK and most other countries – 50Hz
- North America – 60Hz
- Brazil and Japan – 50Hz in some parts of the country, 60Hz in others.
The AC power supply’s frequency affects a fan’s rotational speed because it is closely related to the number of poles in the fan’s electric motor. As the frequency of the power supply increases, the speed of the fan will also increase (assuming that the number of poles in the motor remains the same).
Using that formula again: Speed (RPM) = (120 x Frequency)/number of poles.
Example: A four-pole fan motor running on a 60Hz power supply will rotate at 1,800 RPM, while the same fan motor running on a 50Hz power supply will rotate at 1,500 RPM.
Limitations Affecting Centrifugal Fans
The first relates to the difference between static and dynamic pressure:
- Static pressure (Pa or Ps) – the resistance pressure that the fan must blow against to move air in the desired direction. If the fan is blowing against high pressure, it requires more power and delivers less air.
- Dynamic pressure (Pd) – this is the pressure created by the movement of air. It rises as velocity increases and is always positive.
If you under-specify and/or under-engineer a centrifugal fan, if you ask too much of it, then the static pressure may exceed the dynamic pressure.
At this point, the air being moved will stop moving in the desired direction and back up down the inlet (unlike in a positive displacement compressor which creates a ‘one-way door’ so the air can move in only one direction – it cannot travel back down the inlet).
If you miswire an axial fan – the impeller will rotate in the opposite direction. It will suck instead of blow, so the error is easy to spot.
If you miswire a centrifugal fan during installation – it will still blow air in the right direction – just not as powerfully. This makes underperformance issues harder to diagnose if you are not an experienced fan engineer.
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