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FAN SELECTION GUIDE
How Are Airflow, Pressure and Motor Power Determined in Fan Selection?
Correct fan selection matches the required airflow and system pressure loss at one duty point and sustains that point safely with a suitably rated motor.
Direct answer
First calculate the required airflow and total system pressure loss. Both values must be met at the same point on the fan curve, and motor power must then be verified for fan efficiency, air density, starting and operating range.
Technical review: MDES Engineering Technical TeamUpdated: 2026-08-11
How is the required airflow determined?
Airflow is calculated from the room function, required air-change rate, occupancy or process load, heat load, contaminant source and applicable design criteria. Capture velocity at the hood or extraction point is also important in local exhaust systems. Branch airflows and simultaneous operation must be assessed in multi-zone systems.
What is the difference between static and total pressure?
The fan must overcome losses from duct friction, bends, dampers, filters, silencers, terminals and other components. Static pressure relates to system resistance, while total pressure includes both static and velocity-pressure components. The pressure definition must be stated clearly when requesting quotations or comparing performance.
Why is the duty point checked on a fan curve?
System resistance increases as airflow rises. The intersection of the fan curve and system curve is the actual duty point. A point near an unstable region, a low-efficiency zone or the motor load limit may cause noise, vibration, excess energy use or inadequate performance. Selection should therefore use approved manufacturer performance data.
How are motor power and speed control selected?
Shaft power is assessed from airflow, fan pressure and fan efficiency. The motor is selected for the duty point, air density, starting conditions, control method and a reasonable margin; unnecessary oversizing should be avoided. A variable-frequency drive can reduce energy use where airflow demand varies, provided the permitted motor and fan speed range is maintained.
How do density, temperature and altitude affect selection?
Fan data may be stated at a reference air density. High temperature or altitude changes density and therefore pressure and power. Moisture, particles, abrasion or corrosive constituents also affect material, coating, sealing and motor protection. Final selection must be corrected to the actual operating conditions.
Key points
- Airflow is calculated from room and process requirements.
- The fan must overcome all duct and component pressure losses.
- The true duty point is where the fan and system curves intersect.
- Motor power is checked against fan efficiency, density, starting and operating range.
- Temperature, altitude and air composition may require performance corrections.
Frequently asked technical questions
Which units are used for fan airflow?
Airflow is commonly stated in m³/h, m³/s or L/s. Units and reference air conditions must match when comparing data.
Why does fan pressure depend on the duct calculation?
Duct length and size, bends, filters, dampers, silencers and terminals create the total resistance that the fan must overcome at design airflow.
Is a larger motor always safer?
No. The motor should cover the duty point, starting condition and a reasonable margin; unnecessary oversizing may increase cost and reduce operating efficiency.
Technical references
- Project airflow and duct pressure-loss calculations
- Manufacturer fan curves, motor power tables and operating limits
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