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The Principles of Air Flow
Air Flow: A Breeze to Understand with our Essential Guide
Understanding the basic principles of air flow and how to measure pressures within duct systems is essential for maintaining effective ventilation. By applying formulas, you can optimise air flow, ensuring that your systems run efficiency and safely.
In simple terms, air moves from point A to point B when there’s a difference in pressure, with air flowing from the higher-pressure area to the lower pressure area. The flow rate is dictated by the pressure difference between the two points.
Velocity, Static and Total Pressure
Air pressure plays an integral role in ductwork, and there are three main types to consider:
Velocity Pressure
This is the pressure created by air moving at a certain speed. It’s always positive and moves in the direction of the air flow. Typically measured in millimetres of water gauge, velocity pressure is crucial to system efficiency.
Static Pressure
Static pressure refers to the force exerted by air contained within a space, regardless of whether the air is moving. A simple example would be the air pressure inside a sealed tyre.
Total Pressure
Total pressure is the combination of both velocity and static pressure, giving a complete picture of what’s happening within the duct.
Formulas to Keep Your Systems Flowing
Formula for Air Flow Volume
To calculate the volume of air moving through a duct, use this basic formula to ensure that the system is maintaining the correct air flow to meet the needs of the space:
Volumetric Flow Rate = Velocity × Area
Where:
- Volume refers to the amount of air moving through the duct, often measured in cubic meters per hour.
- Velocity is the speed at which the air travels, measured in metres per second.
- Area is the cross-sectional size of the duct in square meters.
Formulas for Velocity and Pressure Calculations
To calculate velocity from velocity pressure:
Take the square root of the velocity pressure.
Multiply by 4.
To calculate velocity pressure from velocity, reverse the process:
Velocity Pressure = (Velocity ÷ 4)²
Applying the Formulas to Real-World Applications
To help better understand how air pressure behaves in real-world applications and to further understand how the formulas can be applied to duct systems, check out the video version of this blog on our YouTube channel.
In the video, Pete breaks down how velocity, static, and total pressures work in different scenarios, such as a sealed duct with trapped pressure, air blowing through a ducting system, and air being sucked through a pipe -
