Pipe Air Flow Calculator
How to Calculate Air Flow Rate in a Pipe
Understanding the air flow rate within a pipe or duct is critical for HVAC systems, pneumatic conveying, and industrial ventilation. The efficiency of a system depends heavily on ensuring the correct volume of air (measured in CFM) moves through the piping at an appropriate velocity.
This calculator determines the Volumetric Flow Rate based on the physical dimensions of the pipe and the speed at which the air is traveling.
The Air Flow Formula
The calculation of air flow rate ($Q$) is derived from the continuity equation for incompressible fluids. The fundamental formula is:
Q = V × A
Where:
- Q = Volumetric Flow Rate (Cubic Feet per Minute – CFM)
- V = Air Velocity (Feet per Minute – FPM)
- A = Cross-Sectional Area of the pipe (Square Feet)
Step-by-Step Calculation Guide
If you are measuring a round pipe manually, follow these steps to calculate the flow rate:
- Measure the Diameter: Measure the internal diameter of the pipe in inches. For example, let's say the pipe is 10 inches wide.
- Calculate the Radius: Divide the diameter by 2. (10 / 2 = 5 inches).
- Calculate Area in Square Inches: Use the formula $\pi \times r^2$. ($3.14159 \times 5^2 = 78.54$ sq inches).
- Convert Area to Square Feet: Divide square inches by 144. ($78.54 / 144 \approx 0.545$ sq ft).
- Multiply by Velocity: If your anemometer reads an air speed of 2,000 FPM, multiply this by the area. ($2,000 \times 0.545 = 1,090$ CFM).
Why Air Velocity Matters
Maintaining the correct air velocity is just as important as the total flow rate. If the velocity is too low in a pneumatic conveying system, materials may settle and clog the pipe (saltation velocity). Conversely, in HVAC systems, if the velocity is too high, it creates excessive noise and friction loss, leading to higher energy costs.
Standard Pipe Sizes and Air Flow
For general ventilation, typical duct velocities range from 1,000 to 2,000 FPM depending on the application. Industrial dust collection often requires higher velocities (3,500 to 4,500 FPM) to keep heavy particulates suspended in the air stream.
Using this calculator ensures you can quickly verify if your pipe diameter matches the required CFM capacity for your specific application.