How to size compressed air branch lines

Finding the right balance in your compressed air piping system can feel like a game of inches—literally. It is commonly understood that a looped main header improves performance by shortening the distance that the air must travel. Further, cross branches are often installed inside the main loop to bring air to users that are not along the perimeter of the plant floor. A common question we receive from system designers is whether these interior cross branches should be the same diameter as the main loop or can they be smaller to save on material and installation costs.

Let’s dive into the physics of air movement and pressure drop to see if "upsizing" actually pays off.

The Goal: Minimizing Pressure Drop

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In any compressed air network, the enemy is pressure drop (∆p). When air travels through a pipe, friction against the pipe walls causes a loss in pressure. To compensate for high pressure drop, you have to run your compressors at higher setpoints, which directly increases your energy bill. Choosing piping with smooth walls definitely helps, but the primary way to combat this is by increasing the pipe diameter. But where does it matter most?

The "Loop" Logic

A loop system is designed so that air can flow in multiple directions to reach a point of use. If a high-demand starts up, air can come from both sides of the loop to feed it.

  • The Argument for Larger Outer Loops: Sizing the loop for the full flow of the compressed air supply decreases the resistance (pressure drop) through the entire network. This allows the system to supply air from both sides of the loop in greater quantities, effectively using the entire network as a more efficient delivery vehicle.
  • The Reality of Small Branches: Smaller diameter branch lines can have an exponentially higher pressure drop compared to larger headers if the branch runs are long. For example, a 1.5" pipe at 500 cfm will create a similar pressure drop in just 100 feet as a 4” pipe would over 13,000 feet. So it makes sense to size branch lines for the users they serve.

Is Larger Always Better?

Not necessarily. While a larger branch line provides a bit of "extra storage" and can reduce pressure drop for very short periods, it isn't a substitute for a dedicated receiver tank for intermittent, high volume users. Also, oversizing the pipe network beyond what's really needed can add an extra upfront cost.

We generally recommend: 

  • Uniformity: Running the entire outer network (main loop) at the same size is the most effective approach for balanced flow.
  • Strategic Upsizing: If you have a specific user that requires a consistent high volume of air, size the branch line to that specific user larger to prevent a localized pressure bottleneck. Here again, point-of-use tanks can smooth out flow and reduce disruption to other users.
  • Path of Least Resistance: Remember that air is lazy—it will always flow more in the path of least resistance. If one path is significantly longer and smaller, it won't contribute much to the supply during steady use. For longer runs of pipe, consider upsizing to combat increased resistance in the air flow.

The Bottom Line

The main header should always be sized for the entire compressor flow. Interior cross-branches can often be downsized to match the actual usage flow of the tools they serve, and they don’t have to be the same size as each other. But don’t be stingy. Be forward thinking and aim to make the installation a one-time expense. High pressure drop creates a perennial energy penalty and will likely disrupt production at some point. Replacing undersized piping is very expensive. These costs far outweigh any incremental capital costs incurred by being generous on pipe size.

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