Fans - Efficiency and Power Consumption

Power consumption and typical efficiencies of fans

Fan Power Consumption

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

P_i = dp q (1)

where

P_i = ideal power consumption (W)

dp = total pressure increase in the fan (Pa)

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

The ideal power consumptions for fans at different air volumes and pressure increases are expressed in the chart below:

Note! For detailed engineering the manufactures specifications must always be used.

Fan Efficiency

The fan efficiency is the ratio between power transferred to the the airflow and the power used by the fan.

The fan efficiency is in general independent of air density and can be expressed as:

h_f = dp q / P (2)

where

h_f = 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 / h_f (3)

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

P = dp q / (h_f h_b h_m) (4)

where

h_b = belt efficiency

h_m = 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

Fan and Installation Loss (System Loss)

Installation of a fan will influence on the overall system efficiency:

dp_sy = x_sy p_d (5)

where

dp_sy = installation loss (Pa)

x_sy = installation loss coefficient

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

Fan and Temperature Increase

Near all energy lost in a fan heats up the air flow. The temperature increase can be expressed as:

dt = dp / 1000 (6)

where

dt = temperature increase (K)

dp = increased pressure head (Pa)

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