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Why does hose size affect my compressor airflow?

It is important to consider appropriate sizing of all components of your air system.  If you are investing in an air compressor system, restricting the flow anywhere in your system could make it significantly underperform or cost you a lot more in energy costs to run that compressor over its lifetime.

As air travels from the compressor head to your tool it travels through components such as hoses, fittings, valves, and tanks. Each of these will restrict the flow of air in some way depending on the geometry of each component and the magnitude of the flow passing through it. For example, a long small hose feeding a high air demand tool can result in a high pressure drop.  The result of this will mean either your compressor is working harder and using more power to keep up with your air demands, or if it can’t keep up, your tool performance will be reduced.  In some cases, where torque or power at the tool is important, you may not be able to complete your work.

Quick calculators or charts can be referred to for calculating the pressure drop in any of your components. The following components should be considered for proper sizing.

  • Hose reels
  • System piping/tubing
  • Filters
  • Regulators
  • Lubricators
  • Quick connect fittings
  • Fittings

Components such as filters will have pressure drop ratings at different flow rates. Be sure to check the documentation and specifications to match them to the system you are installing the components on.  When considering fittings and quick connects, work with your suppliers to make sure 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.

90° fittings like this may restrict airflow

90° fittings like this may restrict airflow

Why does this happen? 

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 trying to force a large flow of air through a small hole can be difficult.  This is because the smaller the diameter, the higher the velocity is required for the air to travel through the hole.  Higher velocities result in more friction created due to boundary layers at the walls of the pipe or fitting creating more losses. With pipes and hoses, the loss is multiplied by the length of the pipe.

It can be surprising how small the flow diameter is in some fittings.  A quick connect fitting is one of the worst culprits.  Next time you are looking at a quick connect, look inside to see how small the actual flow area is.

Pressure drop can be calculated for some components and is made even easier by online tools such as: http://www.engineeringtoolbox.com/pressure-drop-compressed-air-pipes-d_852.html.  Note that this calculator is for hard pipe as it is a well-defined shape. Flexible hose in actual use typically contains many bends and loops and as such it is not possible to create an accurate generic calculator. While flexible hose will have more losses than a pipe with an identical inside diameter we can still use the pipe loss calculation to get an estimate and see the influence diameter has on pressure loss.

Let’s look at some examples.

To illustrate the dramatic difference pipe/hose diameter makes on pressure loss, let’s use this tool to compare 100ft long pipes with internal diameters of ½”, ¾”, and 1” for 70CFM FAD (free air delivery) 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:

1/2″ x 100’: ~44 PSI pressure loss

3/4″ x 100’: ~5.7 PSI pressure loss

1″ x 100’: ~1.4 PSI pressure loss

Your compressor would actually have to be operating at ~134 psi gauge pressure† to maintain 100 psi at the tool with the ½” pipe.

Increasing the pressure of your compressor to compensate for flow losses can have a dramatic impact on the amount of work your compressor system is doing. Conversely, 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, bend etc. you add to your system results in cumulative pressure drop and can have negative results on 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. As always, keep in mind that formulas and calculators like the one used above are just a guide.  Real life scenarios depend on many factors and each will affect your individual results.  Using 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 up front.

If you have any questions about this article or anything mobile compressor related, please contact us.

* Absolute pressure is the pressure relative 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, that requires the use of absolute pressure, atmospheric pressure (14.7 psi) needs to be added to your gauge pressure.

† Note that it may seem at first glance that the entrance pressure would need to go up by the same pressure as the loss in the pipe from the first calculation but it’s actually slightly less. As the pressure goes up the volume flow and flow velocity goes down for the same mass flow of gas and therefore so does the pressure drop.