Axial fan efficiency tells you how well the fan converts mechanical input power into useful airflow and pressure. For large axial fans in industrial plants and mines, even a few percentage points of efficiency can mean large energy savings over the life of the equipment.
The basic definition is the same as for other fans:
Axial fan efficiency = (Air power / Shaft power) × 100%
Air power is the useful power imparted to the air. In metric units, one practical way to estimate air power for an axial fan is:
Pair = (Q × ΔP) / (ηunit)
where Q is volume flow (m3/s), ΔP is fan total pressure rise (Pa), and the denominator contains the appropriate constant to convert to kilowatts while accounting for air density. In many engineering handbooks, this is simplified to a direct formula such as:
Pair (kW) ≈ (Q × ΔP) / 1000 (with corrections for density and units as required)
The key principle is that air power is proportional to flow multiplied by pressure.
Shaft power is the mechanical power delivered to the fan shaft. It is usually derived from the electrical input to the motor, corrected for motor efficiency and any belt or coupling losses. For example, if the motor electrical input is 90 kW, motor efficiency is 95% and drive efficiency is 97%, then shaft power to the axial fan is roughly 90 × 0.95 × 0.97 ≈ 83 kW.
Once air power and shaft power are known for a specific operating point (particular airflow and pressure), axial fan efficiency is calculated by dividing air power by shaft power and multiplying by 100%. Large, well-designed axial fans for mining and industrial ventilation often achieve peak efficiencies in the 75–85% range when operating near their best efficiency point.
In practical design work, engineers rarely calculate efficiency from scratch for every point. Instead, they rely on manufacturer test data that provide airflow, pressure, power and efficiency across the fan curve for various speeds and blade angles. However, understanding the calculation helps verify that the fan is applied correctly and is running close to its optimal region, rather than wasting energy at a poor operating point.
In summary, to calculate axial fan efficiency you determine air power from measured or specified flow and pressure, determine shaft power from motor input and efficiencies, and then divide air power by shaft power to express efficiency as a percentage.
