Air flow rate through piping is the volumetric quantity of air that passes through a pipe or duct per unit of time. It's a crucial parameter in various applications, including HVAC systems, industrial processes, pneumatic systems, and more. It describes how much air is being transported through the piping system. The air flow rate depends on factors such as the pipe's internal diameter, air velocity, pressure, temperature, and the pressure losses caused by pipe friction and fittings.
Air Flow Rate through Piping Formula |
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\( Q_a \;=\; 60 \cdot \pi \cdot v_a \cdot \left( \dfrac{d }{ 24} \right)^2 \) (Air Flow Rate through Piping) \( v_a \;=\; \dfrac{ Q_a }{ 60 \cdot \pi \cdot \left( \dfrac{ d }{ 24 } \right) ^2 } \) \( d \;=\; 24 \cdot \sqrt{ \dfrac{ Qa }{ 60 \cdot \pi \cdot v_a } } \) |
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| Symbol | English | Metric |
| \( Q_a \) = Air Flow Rate | \(CFM\) | - |
| \( v_a \) = Air Velocity | \(ft \;/\; sec\) | - |
| \( \pi \) = Pi | \( 3.141 592 653 ...\) | - |
| \( d \) = Pipe Inside Diameter | \( in \) | - |
For gases such as air, it is important to distinguish between actual volumetric flow rate and standard volumetric flow rate. Because air is compressible, its density and volume change with pressure and temperature. Consequently, a flow rate reported in SCFM (standard cubic feet per minute) can differ substantially from the actual cubic feet per minute flowing through the pipe at operating conditions.
In piping-system design, air flow rate is used to determine pipe diameter, air velocity, pressure drop, compressor capacity, and system performance. An adequately sized pipe should transport the required air flow while maintaining an acceptable velocity and pressure loss. Excessively small piping produces higher velocities and friction losses, while excessively large piping increases material and installation costs.
