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Fan Capacity Diagrams

Pressure, head, air flow volume and fan capacity diagrams.

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Pressure head as function of air flow volume may be expressed in a diagram like the one below:

fan capacity diagram

  • red lines are pressure heads as function of air volume flow and speed of the fan
  • green lines marked - l - are throttle lines
  • blue lines are power consumption

Pressure and Head

Operation of fans are often expressed in terms of pressure

pt = ps + pd = constant                                          (1)


pt = total pressure (Pa, N/m2)

ps = static pressure (Pa, N/m2)

pd = dynamic pressure (Pa, N/m2)

Dynamic pressure can be expressed as

pd = ρ v2 / 2                                                  (2)


ρ = density of air (kg/m3)

v = flow velocity (m/s)

fan pressure head

Head developed can be expressed as

dp = ps2 - ps1 + (v22 - v12) ρ / 2                                                (3)


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

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