Mechanics and drives · ventilationSubscription
Exhaust fan and blower power: motor, air density and affinity laws
Calculate the shaft power of a fan or exhaust fan, the motor power with drive, efficiency and margin, the suggested IEC motor and the air density correction. The affinity laws show the new operating point when the speed changes.
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Where it helps at work
- Check whether an exhaust fan motor is well sized for the operating point.
- Estimate the savings of a variable frequency drive before asking for a quote.
- Correct the catalogue pressure (standard air) for hot air or a plant at altitude.
- Predict the new flow and power after changing the fan pulley.
What you enter
What you get
Worked example: exhaust fan motor and speed reduction
An exhaust fan must move 10000 m³/h of air at 20 °C, at sea level, against 1000 Pa total pressure. The maker’s curve gives 70 % efficiency at the point; the belt drive has 95 % and the motor 92 %. Which motor should be chosen with a 15 % margin, and what happens if the speed drops from 1750 to 1450 rpm?
| Air flow | 10000 m³/h |
|---|---|
| Fan total pressure | 1000 Pa |
| Operating air density | 1.204 kg/m³ |
| Efficiencies: fan / drive / motor | 70 / 95 / 92 % |
| Motor margin | 15 % |
| Current → new speed | 1750 → 1450 rpm |
| Fan shaft power | 3.968 kW |
|---|---|
| With the margin | 4.804 kW |
| Suggested IEC motor | 5.5 kW |
| At the new speed: shaft power | 2.257 kW |
How to read it
The air receives 2.778 kW, but the fan shaft needs 3.968 kW and the motor output 4.177 kW. With the 15 % margin that becomes 4.804 kW, so the suggested IEC motor is 5.5 kW. From the supply, the motor draws 4.54 kW. Reducing the speed to 1450 rpm, by pulley or drive, the flow falls to 8286 m³/h, the pressure to 686.5 Pa and the shaft power to 2.257 kW: power varies with the cube of the speed, which is why slowing down saves so much.
Limits and cautions
- It does not select the fan: efficiency and operating point come from the maker’s curve.
- The affinity laws hold for the same fan in a system with a quadratic curve; with fixed static pressure (filters, dampers) the real point differs.
- Surge, noise, air humidity and motor service factor are not assessed.
- At a cold start the air is denser and may demand more from the motor; check at the lowest operating temperature.
Technical basis
Power from flow, total pressure and efficiencies, air density from the ISO 2533 atmosphere, fan affinity laws and IEC 60072-1 motor ratings.
- ISO 5801 — ventiladores: ensaio de desempenho
- ISO 2533 — atmosfera padrão
- IEC 60072-1 — potências nominais de motores
In the full version, you can
- Calculate the powers and suggested motor with your data and units.
- Correct the catalogue pressure for the plant temperature and altitude.
- Simulate another speed and see the affinity law chart and table.
- Export an unbranded PDF report and keep the work on your device, with a backup file.
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Log in to my accountFrequently asked questions
Should I use static or total pressure?
Total, which is static plus dynamic at the operating point. If the catalogue only gives static pressure, the calculated power is lower than the real one.
Why does power drop so much with speed?
Because flow falls in proportion to speed and pressure with its square; power, the product of both, falls with the cube. Cutting speed by 20 % cuts about 49 % of the power.
When should I correct for density?
When the air is hot, at altitude or otherwise different from catalogue standard air (1.2 kg/m³). At the same speed the volume flow stays, and pressure and power follow the density.