The importance of fan efficiency

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WITH the growing global emphasis on saving power, fan efficiency is even more important when it comes to product design.

So, what is fan efficiency? Fan efficiency is the energy needed to drive a fan as a comparison of the energy imparted to the air. It is usually expressed as a percentage and traditionally based on the motor output energy. The formula to calculate efficiency is:

Effy% = qv x (PtF / 10Pr)

Where:
qv = volume flow m³/s
PtF = fan total pressure Pa. That is, PtF = PsF (static pressure) + PdF (fan dynamic pressure) Pr = Power absorbed by fan kW.

For example, from page C30 of the Fans By Fantech catalogue, showing “Axial Flow Fans Performance Data”, duty 4m³/s at 200 Pa, AP0634/10/30º ; Absorbed power 1.9kW; PdF = 100 Pa ; Effy% = 4x 200 +100/1.9x10 = 63.16%

A typical pre-MEPS2 motor has an efficiency of 90% and a MEPS2 motor typically has an efficiency of 95%. The actual fan efficiency based on power input will be the fan efficiency x motor efficiency. therefore, for the pre-MEPS2 motor, 63.16% x 0.90 = 56.8% and for the MEPS2 motor, 63.16% x 0.95 = 60% The term “static efficiency” is sometimes used and is calculated Effy% = qv x (PsF / 10Pr)

However, static efficiency is zero at zero pressure and at zero volume flow. As static efficiency does not take account of all the work done by the fan, but only the static pressure, it has declined in use to the extent that it can be considered obsolete.