Fan efficiency tells you how effectively a fan converts shaft power from the motor into useful air power in the system. High efficiency is important in industrial plants and mines because ventilation fans often run many hours and consume significant energy. Calculating fan efficiency requires both airflow and pressure data plus the input power to the fan shaft.
The basic concept is:
Fan efficiency = (Air power / Shaft power) × 100%
Air power is the useful power added to the air by the fan. For a fan handling air at roughly standard density, you can express air power in kilowatts using:
Pair = (Q × ΔP) / (102 × 103) (approximate metric form)
where Q is airflow in m3/h and ΔP is fan total pressure rise in Pa. A more exact formula depends on the unit system, but the idea is always volume flow multiplied by pressure, divided by appropriate constants to get kW or horsepower.
Shaft power is the mechanical power delivered to the fan shaft. You can estimate this from the motor electrical input, corrected for motor efficiency and any drive losses. For example, if the motor input is 100 kW and motor efficiency is 95%, the shaft power to the fan is about 95 kW. If there are belts or gearboxes, their efficiency must also be considered.
As a simple example, suppose a mine ventilation fan delivers 120,000 m3/h at a total pressure rise of 2000 Pa, and the shaft power is 80 kW. The air power is approximately (120,000 × 2000) / (102 × 103) ≈ 2.35 kW. That would produce a very low efficiency, which signals that the assumed formula or numbers in this quick example are not realistic. In real calculations, you would use the correct constants (including air density) and verified test data to obtain a meaningful efficiency, often in the range of 70–85% for large, well designed industrial and mining fans.
In practice, engineers often rely on manufacturer test data for fan efficiency at different points on the curve. However, understanding the air power and shaft power relationship helps you verify whether a fan is operating near its best efficiency point or far away from it, which affects energy consumption.
In summary, you calculate fan efficiency by dividing air power (airflow times pressure rise, adjusted for units and density) by shaft power, then multiplying by 100%. For industrial and mining fans, doing this correctly helps ensure that large ventilation systems operate efficiently and cost effectively.
