Fan Capacity Diagrams

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

• 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

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)

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

subscription₁ is the inlet of the fan and subscription₂ is the outlet