Most quotation errors on fans are unit errors. CFM against m³/h, static against total pressure, inches water gauge against pascals — each pairing has produced orders for equipment that could not do the job, and every one of them was avoidable in thirty seconds.
Airflow
Volumetric flow, expressed as m³/h, m³/s, CFM or L/s.
Conversions worth committing to memory: 1 m³/h = 0.589 CFM, so 1 CFM = 1.699 m³/h. 1 m³/s = 3,600 m³/h = 2,119 CFM. 1 L/s = 3.6 m³/h.
The trap: volumetric flow is not mass flow. A fan moving 5,000 m³/h of 20 °C air moves considerably more mass than the same fan moving 5,000 m³/h of 200 °C air, because hot air is less dense. Where the process cares about mass — combustion, drying, any application with a chemical or thermal balance — specify the temperature at which the volumetric figure applies. A figure without a temperature is ambiguous.
Pressure, and the distinction that matters
Three values appear on datasheets and confusing them is the single most common specification error.
Static pressure is the pressure the fan develops against system resistance. This is what your ductwork consumes.
Velocity pressure is the kinetic energy of the moving air, proportional to the square of velocity.
Total pressure is the sum of the two.
Fans in ducted systems are almost always selected on static pressure. A supplier quoting total pressure will show a higher number for the same fan — sometimes 10–20% higher in high-velocity systems — and the fan will look better than a competitor quoting static. Always confirm which basis is quoted before comparing.
Units: 1 inch water gauge = 249 Pa. 1 mmH₂O = 9.81 Pa. 1 mbar = 100 Pa. A typical HVAC system runs 200–1,500 Pa; industrial process and dust extraction commonly 1,500–5,000 Pa; high-pressure blowers well beyond.
Power and efficiency
Shaft power (absorbed power) is what the fan actually requires at the operating point. Motor rated power is the motor's capability, always higher. Compare quotations on shaft power at your duty point — motor size tells you what was fitted, not what the fan needs.
Fan total efficiency is useful output divided by shaft power. Realistic figures: backward-curved centrifugal 75–85%, forward-curved 55–70%, axial 65–80% at a good operating point. Anything claimed above 90% deserves a question about the basis.
An approximate check: shaft power in kW ≈ (flow in m³/h × pressure in Pa) ÷ (3,600,000 × efficiency). For 10,000 m³/h at 1,000 Pa at 75% efficiency: (10,000 × 1,000) ÷ (3,600,000 × 0.75) ≈ 3.7 kW. If a quotation shows shaft power far below this, the stated efficiency or the stated duty is wrong.
The fan laws
For the same fan at a different speed, with density unchanged:
- Flow varies directly with speed — 10% more rpm gives 10% more flow
- Pressure varies with the square of speed — 10% more rpm gives 21% more pressure
- Power varies with the cube of speed — 10% more rpm gives 33% more power
The cube relationship is the entire case for variable speed control. Reducing speed to 80% cuts flow to 80% and power to about 51%. Achieving the same flow reduction by closing a damper leaves the fan drawing nearly full power and wastes the difference as heat and noise.
The laws hold for speed changes on a fixed fan in a fixed system. They do not scale between different fan sizes, and they assume the system curve is unchanged.
Noise
Sound power level, in dB, is a property of the fan itself. Sound pressure level, in dB(A), is what a person hears at a stated distance and depends on the room. A specification without a distance is meaningless.
Decibels are logarithmic: +3 dB is double the acoustic energy, +10 dB is subjectively about twice as loud. Two identical fans running together produce +3 dB, not double the number.
Noise rises steeply with speed — roughly 50 log(speed ratio), so a 10% speed increase adds about 2 dB. Selecting a larger fan at lower speed for the same duty is the most reliable way to reduce noise at source, and it usually improves efficiency at the same time.
Reading a datasheet critically
Check the reference density and whether it matches your conditions. Check whether pressure is static or total. Check whether flow is at inlet or discharge conditions where temperature differs. Check whether the quoted efficiency is fan total efficiency or includes motor and drive.
Then find your duty point on the curve and read the efficiency there — not the peak efficiency printed on the sheet, which is at a different flow. A fan with 82% peak efficiency running at 55% at your duty point is worse than a fan with 76% peak running at 74%.
Send us your duty conditions and we will return a curve with the operating point, absorbed power and sound data at that point, stated on a clearly identified basis.