Appropriate sizing is essential for all air compressor system components, including hoses, and air hose size will affect your compressor's airflow. If you invest in an air compressor system, restricting the flow anywhere in your system can make it significantly underperform or require unnecessary energy costs to run that compressor over its lifetime.
Air travels from the compressor head to your tool through components such as hoses, fittings, valves, and tanks. Each component restricts the flow of air in some way, depending on the geometry of the component and the magnitude of the flow passing through it.
For example, a 100-foot, 1” hose delivering 100 CFM at 90 psi will result in a 3.35 psi pressure drop. If that same hose is tripled to 300 feet, the pressure drop is 10.1 psi, which means the air is now pressurized to only 80 psi.
As a result, your compressor is working harder and using more power than it should to keep up with your air demand, or—if it can’t keep up—your tool performance will suffer. In some cases, where power at the tool is essential, you may be unable to complete your work.
90° fittings like this may restrict airflow
How Fittings Cause Pressure Drops
Pressure drop is due to the restriction created by the pipe or fitting. Anyone who has tried to breathe through a drinking straw can tell you that forcing a large flow of air through a small hole can be difficult. Why? The smaller the diameter, the higher the velocity is required for the air to travel through the hole.
Higher velocities create more friction due to boundary layers at the pipe walls or fitting, creating more losses. With pipes and hoses, the loss is by the length of the pipe. Consider the reduced diameter of the pipe with the fitting when doing pressure drop calculations.
It can be surprising how small the flow diameter is in some fittings, and a quick-connect fitting is one of the worst culprits. Next time you consider a quick-connect fitting, look inside to see how small the actual flow area is, and double-check the specs.
Calculating Pressure Drop of Fittings
You can use quick calculators or charts like those found at Engineering Toolbox, to calculate the pressure drop in your pipes or components.
Note that this calculator is for hard pipe, which is a well-defined shape. Flexible hose typically contains many bends and loops, so creating an entirely accurate calculator is impossible. Although flexible hose will have more losses than a pipe with an identical inside diameter, we can still use the pipe loss calculation to get a decent estimate and see the influence diameter has on pressure loss.
When sizing your air compressor, consider each of the following components that can cause pressure loss:
- Hose reels
- System piping/tubing
- Filters
- Regulators
- Lubricators
- Fittings
Components such as filters will often have pressure, so check the documentation and specifications carefully to match components to the system.
When considering fittings and quick connects, work with your suppliers to ensure they are rated for the maximum pressure your compressor system is rated for and will not cause excessive pressure drop at the required flow rates.
Pressure Drop Sizing Examples
To illustrate the dramatic difference that pipe or hose diameter makes on pressure loss, we used the pressure drop calculator tool to compare 100 ft long pipes with internal diameters of ½”, ¾”, and 1”.
In this example, we have 70 CFM free air delivery (FAD) of compressed air, delivered at 100 psi gauge pressure (equivalent to 114.7 psi absolute pressure*) at the upstream hose entrance.
The approximate pressure loss from end to end for the three pipe sizes is:
- 1″ x 100’ = ~1.4 PSI pressure loss
- 3/4″ x 100’ = ~5.7 PSI pressure loss
- 1/2″ x 100’ = ~44 PSI pressure loss
Your compressor would have to operate at a constant ~134 psi gauge pressure† to maintain 100 psi at the tool with the ½” pipe.
Increasing Pressure vs. Increasing Supply Line Size
Increasing the pressure of your compressor to compensate for flow losses can dramatically impact how hard your air compressor system needs to work. Meanwhile, increasing the size of the supply lines can provide the following benefits:
- Less fuel/energy used by your compressor
- Less heat generated by the compressor
- Longer oil life and service interval
- Lower noise output from the compressor
- Improved safety due to lower operating pressures and lower temperatures
- Lower load on drive system components
- Less wear and longer life of your compressor
Each restrictive fitting, hose, accessory, and bend added to your system results in cumulative pressure drop and can negatively affect the performance of your tool or equipment. Recognizing this and planning and sourcing the right-sized components will enable your air system to perform better.
Keep in mind that formulas and calculators like the one used above are just a guide. Real-life scenarios depend on many factors, each affecting your results. Using a larger diameter hose may cost more, but it can result in long-term savings and may even allow for a lower pressure or output compressor, saving money upfront.
*Absolute pressure is the pressure relative to a perfect vacuum. Gauge pressure is relative to atmospheric pressure. Standard atmospheric pressure is 14.7 PSI so for a calculation such as this one, which requires absolute pressure, atmospheric pressure (14.7 psi) needs to be added to your gauge pressure.
† Note that it may seem that the entrance pressure would need to increase by the same pressure as the loss in the pipe from the first calculation. However, the increase is slightly reduced. As the pressure increases, the volume flow and flow velocity decreases (for the same air mass), and therefore the pressure drop decreases as well.
