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
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)
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
Systems for ventilation and air handling - air change rates, ducts and pressure drops, charts and diagrams and more.
The affinity laws can be used to calculate resulting volume capacity, head or power consumption when speed or wheel diameters are changed.
High suction pressure at a fan inlet decreases the density of air - and should be corrected for proper selection of the fan.
How to modulate fans and their capacities.
Power consumption and typical efficiencies of fans
The temperature and density of the air influences on the volume flow, pressure head and power consumption in a fan.
Axial and propeller fans, centrifugal (radial) fans, mixed flow fans and cross flow fans.
Centrifugal, axial and propeller fans and their capacity ranges.