When selecting a fan, the engineer will need several key pieces of information to establish the requirements of the application and provide a solution accordingly. One such characteristic is the duty point. For example, what is the air volume flow rate required and what is the pressure loss of the system?
Units of Measurement
The volume of air required is typically measure in meters cubed per hour (m³/hr), sometimes this could be swapped for litres per second (l/s) when discussing a small fan unit.
Air pressure is usually measured in Pascals (Pa).
Air Pressure Considerations
When considering air pressure within a fan system it is normally with consideration to both the static pressure (Ps) and velocity pressure (Pv), to provide total pressure (Pt).
Pt = Ps + Pv
Ps or Static pressure can be said, when considering fan engineering, to be the difference between the atmospheric pressure and the absolute pressure at the point under review.
Or
Ps = Pa – Po
Where Po is the barometric pressure, in the order of 100,000Pa.
And Pa is the absolute pressure, both can be measured using an instrument e.g. a barometer.
When calculating static pressure, it is normal to consider air as incompressible fluid, because the pressure under consideration are relatively low, circa sub 2000Pa. When static pressures are above 5000Pa the errors due to compressibility of air become significant.
Pv or Velocity pressure is related to the density of the air and its velocity as is expressed as follows:
Pv = ½ pv ²
Definition of Air Volume & Air Pressure
Air Volume = The amount of space that the air occupies.
Air Pressure = The amount of force exerted by the air or weight of the air molecules.
Air Volume Flow Rate = The volume of air that passes through the fan, duct, or system over a unit of time – the speed of the air.
System Pressure Loss = The decrease in air pressure within the air movement system that is relative to an increase in the air flow rate.
