How to Calculate CFM From Line Size

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Industrial compressed air systems often “use more electricity than any other type of equipment,” according to a 2003 advisory publication from the U.S. Department of Energy. Manufacturers in many sectors, including automotive, petrochemical, food and paper, rely extensively on compressed air to operate machinery and hand tools. Sizing compressors and air lines requires careful consideration of flow rates through pipe work and expected pressure drops throughout a system. Measuring pressure drop and pipeline size allows engineers to calculate air flow rate in cubic feet per minute, or CFM.

Connect the pressure gauge in accordance with its operating instructions to a pressure measurement point at one end of the air pipeline. Read the air pressure in pounds per square inch and make a note of this figure. Disconnect the gauge.

Connect the pressure gauge to a measurement point at the other end of the air line and repeat the procedure. Write down the air pressure in pounds per square inch at this point.

Subtract the smaller pressure measurement from the larger to work out the pressure drop along the line in pounds per square inch. Make a note of your answer.

Measure the length of pipe in feet between the two measurement points. Write down this figure.

Check the data sheet for the pipe that is used in the section of air line you measured. Note the internal diameter of the pipe in inches and divide it by 2 to work out the radius.

Calculate the square of the radius. Calculate the square of your answer. Make a note of this figure, which is the fourth power of the radius. For example, if the radius is 2 inches, the square of the radius is 4, and the fourth power is 16.

Multiply the fourth power of the radius by 205.33 and divide by the length of the pipe in feet. Multiply your answer by the pressure drop in pounds per square inch, and then multiply by 2,119. For example, if the fourth power is 16, the length of the pipe is 300 feet and the pressure drop is 0.2 pounds per square inch, the answer is 4,641. Make a note of the result.

Check each step of your work. Record the result, which is the air flow rate in the pipeline, expressed in cubic feet per minute, or CFM.

Tips

  • If you do not have the correct pipe data sheet, turn off the air system, disconnect a section of pipe and measure the internal diameter. Reconnect the system and check for leaks before turning the air on again. Ensure that this work is carried out by a suitably qualified person.

    If the air line pipe does not have dedicated measurement points, you will need to fit tee-joints in the line before you can measure the pressure. Ensure that this work is carried out by a suitably qualified person.

    You can use an online calculator to work out the air flow rate once you know the pressure drop, the length of pipeline and the radius of the pipe.

Warnings

  • Working with compressed air can be dangerous. Do not attempt to carry out any work on a compressed air system unless you are qualified to do so.

    This calculation holds good for laminar air flow. If the flow rate is so high that turbulent flow occurs, your results will be inaccurate.

References

Resources

About the Author

Based in Reading, England, Mike Bailey has been writing since 2008. He covers topics such as business, travel and technology for numerous online publications. Bailey holds a Bachelor of Arts in engineering from the University of Cambridge.

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