Fan Capacity Diagrams

Pressure (head) related to air flow volume - is in general documented in capacity diagrams

The head as a function of air flow volume is in general presented in diagrams as shown below:

• The red lines are the pressure head as a function of the air flow volume and the speed of the fan.
• The black lines marked "l" are the throttle lines.
• The blue lines are the power consumption of the fan

Pressure and Head

The operation of a fan is often expressed in the terms of pressure

p_t = p_s + p_d = constant (1)

where

p_t = total pressure (Pa, N/m²)

p_s = static pressure (Pa, N/m²)

p_d = dynamic pressure (Pa, N/m²)

The dynamic pressure can be expressed as:

p_d = ρ v² / 2 (2)

where

ρ = density of air (kg/m³)

v = flow velocity(m/s)

Head developed can be expressed as

dp = p_s2 - p_s1 + (v₂² - v₁²) ρ / 2 (3)

where

subscription 1 is the inlet of the fan and subscription 2 is the outlet

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