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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

CharacteristicDefinition
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 measured in metres 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 is 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

## Controlling Air Volume and Pressure with Accurate Fan Selection

As mentioned at the start of this article, accurate fan selection ensures that the duty point of the system is achievable.

When the aim of the fan selection is to achieve the most aerodynamic airflow with reduced noise, the design of the fan and its blade configuration are important. The size of the fan, the number and pitch of the blades can significantly alter the airflow and pressure characteristics. Different applications will have specific airflow requirements; therefore, it is advisable to speak to an experienced engineer who can support you with accurate fan selections.

Airflow disturbance, due to incorrectly sized fans, can create turbulence and mechanical vibration. This leads to aerodynamic noise which is a nuisance and can lead to equipment failure. Oversized fans are more likely to operate in stall conditions and cause elevated levels of noise due to creating unnecessary elevated pressure to achieve the same volume of airflow a correctly sized fan would produce.

It is important to calculate any system resistance before selecting a fan to ensure any pressure drops are accounted for, system design can have a significant impact on airflow and overall efficiency. To achieve the highest efficiency, it is important that components match specification requirements, with accurate air volume and pressure data identified at its peak efficiency point.

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 on 01782 349 430 or email [email protected].