Fans - Efficiency and Power Consumption

Fan Power Consumption

The ideal power consumption for 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, 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:

μ_f = dp q / P                                     (2)

where

μ_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 / μ_f                                     (3)

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

P = dp q / (μ_f  μ_b μ_m)                                 (4)

where

μ_b = belt efficiency

μ_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

Power Consumption - Imperial Units

Fan energy use can also be expressed as

P_cfm = 0.1175 q_cfm dp_in / (μ_f  μ_b μ_m)                                (4b)

where

P_cfm = power consumption (W)

q_cfm =  volume flow (cfm)

dp_in = pressure increase (in. WG)

Fan and Installation Loss (System Loss)

The 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 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"

Temperature ^oC
K
^oF

Length m
km
in
ft
yards
miles
naut miles

Area m²
km²
in²
ft²
miles²
acres

Volume m³
liters
in³
ft³
us gal

Weight kg_f
N
lbf

Velocity m/s
km/h
ft/min
ft/s
mph
knots

Pressure Pa
bar
mm H₂O
kg/cm²
psi
inches H₂O

Flow m³/s
m³/h
US gpm
cfm