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Understanding Air Volume & Pressure

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.

Defining Air Volume & Air Pressure

Air Volumethe amount of space that the air occupies
Air Volume Flow Ratethe volume of air that passes through a fan, duct or system over a unit of time - the speed of the air
Air Pressurethe amount of force exerted by the air or weight of the air molecules
System Pressure Lossthe decrease in air pressure within the air movement system that is relative to an increase in the air volume flow rate

Units of measurement

The volume of air required is typically measure in meters cubed per hour (m³/hr), sometime this could be swapped for liters 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.


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


For a more in depth explanation of pressure, go to our article on Understanding Static, Dynamic and Total Pressure in Air Movement Systems.

Axair’s technical engineers are available to assist with fan selection or any other product enquiries. If you would like more information, contact us, call 01782 349 430 or email sales@axair-fans.co.uk

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