Fans - Efficiency and Power Consumption

Fans - Efficiency and Power Consumption

Power consumption and typical efficiencies of fans


Fan Power Consumption

The ideal power consumption for a fan (without losses) can be expressed as
Pi = dp q                                          (1)
where
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:
fan power consumption
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)
where
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)
where
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 / (μf  μb μm)                                (4b)
where
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)
where
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)
where
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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