Guide · Fan selection
Choosing a fan: airflow, static pressure and noise
A fan is selected on two numbers, not one: how much air it must move and how much resistance it must push against. Get the second wrong and the first is never delivered. This guide explains how to work out both and which fan type suits the result.
1. The two numbers
- Airflow — the volume of air to move, in m³/h, m³/s or litres per second.
- Total static pressure — the resistance the fan works against, in pascals: ductwork, bends, grilles, louvres, filters and dampers added together.
The pair is called the duty point. Every fan has a curve showing the airflow it delivers at each pressure; the duty point must sit on that curve, in the part of it where the fan runs comfortably. A fan that is right at free-air flow can deliver a fraction of it once it is connected to a duct.
2. Working out the airflow
For general ventilation the airflow comes from the room volume and the number of air changes per hour the application needs: airflow = room volume × air changes per hour. For a workshop 20 m long, 10 m wide and 5 m high, the volume is 1,000 m³; at 10 air changes per hour the fan must move 10,000 m³/h. The number of air changes comes from the specification, the standard that applies to the building or the process — not from the fan supplier. Extract from a specific source, such as a kitchen hood or a machine, is sized on the capture airflow that source needs instead.
3. Working out the static pressure
Add up the resistance of everything the air passes through, at the design airflow. If the system has not been designed yet, tell us the duct run, the number of bends, the grilles and any filters; the more exact the resistance, the closer the selection. Axial performance falls away quickly as static pressure rises, so an underestimated resistance is the most common reason an installed fan disappoints.
4. Which fan type suits the duty
| Type | Suits | Watch for |
|---|
| Axial and duct fans | Moving air along a duct or through a wall against modest resistance; general ventilation, kitchen and toilet extract, warehouse and workshop, panel cooling | Performance falls away quickly as static pressure rises |
| Mixed-flow duct fans | Higher pressure than a pure axial in the same envelope; quieter than a centrifugal | Check the duty point against the curve, as for any fan |
| Diagonal and booster fans | In-line boosting of long runs, and retrofit where the static pressure was underestimated | A booster corrects a shortfall; it is better to select for the true resistance from the start |
| Centrifugal fans | Systems where the resistance is too high for an axial: forward-curved for compact air-handling duty, backward-curved and plug fans for higher pressure and better efficiency | Forward-curved fans can show a rising pressure characteristic at high flow; that is normal for the type, but the duty point matters more |
5. Noise in occupied spaces
Fan noise is set by the fan type, its speed and how far along the curve it runs. For occupied spaces, super-silent duct fan constructions are available, and a mixed-flow fan is quieter than a centrifugal in the same envelope. State the noise limit for the space when you send the duty, so it is part of the selection rather than a complaint after installation.
6. Speed control: why EC drive matters
Axial and duct fans are available in AC, DC and EC drive. Fan speed has a steep effect on power: airflow varies in proportion to speed, pressure with the square of speed, and power with the cube. Running a fan at 80 % of its speed therefore delivers 80 % of the airflow for about half the power (0.8³ = 0.51). EC drive offers speed control and materially lower running cost where the fan runs continuously; speed regulators and controllers are supplied for AC and EC fans.
7. When the application sets the specification
Some duties are governed by the area rather than the airflow: car park smoke clearance, classified hazardous areas, rail tunnels, livestock housing. Those fans are in the industrial and specialist range; for explosion-proof fans, confirm the zone, gas group and temperature class when you enquire. Smoke clearance and hazardous-area duties are governed by the authority having jurisdiction and by the system designer. We supply to the specification issued — we do not design or certify the system.
What to send us
- The airflow required, and how it was worked out.
- The total static pressure, or the duct run, bends, grilles and filters if it has not been calculated.
- Where it is used: general ventilation, kitchen extract, workshop, panel cooling, an air handling unit.
- The noise limit for the space, and whether the fan runs continuously.
- The power supply and the drive you prefer: AC, DC or EC.
We will select against the duty and include the technical documentation with our quotation.