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Fans - Efficiency and Power Consumption

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Fan Power Consumption

The ideal power consumption for a fan (without losses) can be expressed as

Pi = dp q                                          (1)


Pi = ideal power consumption (W)

dp = total pressure increase in the fan (Pa, N/m2)

q = air volume flow delivered by the fan (m3/s)

Power consumption at different air volumes and pressure increases are indicated below:

Note! For detailed engineering - use manufacturers specifications for actual fans.

Fan Efficiency

The fan efficiency is the ratio between power transferred to airflow and the power used by the fan. The fan efficiency is in general independent of the air density and can be expressed as:

μf = dp q / P                                     (2)


μf = fan efficiency (values between 0 - 1)

dp = total pressure (Pa)

q = air volume delivered by the fan (m3/s)

P = power used by the fan (W, Nm/s)

The power used by the fan can be expressed as:

P = dp q / μf                                     (3)

The power used by the fan can also be expressed as:

P = dp q / (μf  μb μm)                                 (4)


μb = belt efficiency

μm = motor efficiency

Typical motor and belt efficiencies:

  • Motor 1kW - 0.4
  • Motor 10 kW - 0.87
  • Motor 100 kW - 0.92
  • Belt 1 kW - 0.78
  • Belt 10 kW - 0.88
  • Belt 100 kW - 0.93

Power Consumption - Imperial Units

Fan energy use can also be expressed as

Pcfm = 0.1175 qcfm dpin / f  μb μm)                                (4b)


Pcfm = power consumption (W)

qcfm =  volume flow (cfm)

dpin = pressure increase (in. WG)

Fan and Installation Loss (System Loss)

The installation of a fan will influence on the overall system efficiency

dpsy = xsy pd                              (5)


dpsy = installation loss (Pa)

xsy = installation loss coefficient

pd = dynamic pressure in the nominal intake and outlet of the fan (Pa)

Fan and Temperature Increase

Near all of the energy lost in a fan will heat up the air flow and the temperature increase can be expressed like

dt = dp / 1000                              (6)


dt = temperature increase (K)

dp = increased pressure head (Pa)

Standards for Fan Efficiency

  • ISO 12759 "Fans – Efficiency classification for fans"
  • AMCA 205 "Energy Efficiency Classification for fans"
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