The Difference Between a Hose and a Tube and Their Effect on Pressure Drop

EXAIR has been manufacturing Intelligent Compressed Air Products since 1983. They are engineered with the highest of quality, efficiency, safety, and effectiveness in mind. Since compressed air is the source for operation, the performance limitations can be defined by its supply. With EXAIR products and pneumatic equipment, you will need a way to transfer the compressed air from the source to the point-of-use. There are three main ways; pipes, hoses and tubes. In this blog, I will compare the difference between compressed air hoses and compressed air tubes.

The basic difference between a compressed air hose and a compressed air tube is the way the diameter is defined.   A hose is measured by the inner diameter while a tube is measured by the outer diameter. As an example, a 3/8” compressed air hose has an inner diameter of 3/8”. While a 3/8” compressed air tube has an outer diameter that measures 3/8”. Thus, the inner diameter of the tube will be smaller than the hose.

Why do I bring this up? Pressure drop… Pressure Drop is a waste of energy, and it reduces the ability of your compressed air system to do work. To cut waste, we need to reduce pressure drop.  If we look at the equation for pressure drop, we can find the factors that play an important role. Equation 1 shows an equation for pressure drop.

Equation 1:

From Equation 1, differential pressure is controlled by the flow of compressed air, the length of the pipe, the diameter of the pipe, and the inlet pressure. As you can see, the pressure drop is inversely affected by the inner diameter to the fifth power. So, if the inner diameter of the pipe is twice as small, the pressure drop will increase by 25, or 32 times.

As an example, we have a 1/2″ black schedule 40 pipe which has an I.D. of 0.622″.  We use this pipe to flow 40 SCFM of compressed air at 100 PSIG through 100 feet.  What would be the pressure drop?  With Equation 1, imperial units, we get a pressure drop of 1.28 * (40 SCFM/60) ^1.85 * 100 feet / ((0.622″)^5 * 100 PSIG) = 6.5 PSID.  Thus, you started with 100 PSIG, and at the end of the pipe, you will only have (100 PSI – 6.5 PSI) = 93.5 PSIG to use.  Sizing pipe is very important when supplying compressed air to your system as pressure drop is a waste of energy.

Let’s revisit the 3/8” hose and 3/8” tube. The 3/8” hose has an inner diameter of 0.375”, and the 3/8” tube has an inner diameter of 0.25”. In keeping the same variables except for the diameter, we can make a pressure drop comparison in Equation 2.

Equation 2:

As you can see, by using a 3/8” tube in the process instead of the 3/8” hose, the pressure drop will be 7.6 times higher.  As an example, if the pressure drop through a 3/8″ hose is 1 PSID, and you decide to switch out to a 3/8″ tube.  The pressure drop will then be 7.6 PSID, and a big loss of pressure.

Diameters: 3/8″ Pipe vs. 3/8″ tube

At EXAIR, we want to make sure that our customers are able to get the most from our products. To do this, we need to properly size the compressed air lines. Within our installation sheets for our Super Air Knives, we recommend the infeed pipe sizes for each air knife at different lengths. (You will have to sign into the website to download).  We also have an excerpt about replacing schedule 40 pipe with a compressed air hose. We state; “If compressed air hose is used, always go one size larger than the recommended pipe size due to the smaller I.D. of hose”. Here is the reason. The 1/4” NPT Schedule 40 pipe has an inner diameter of 0.364” (9.2mm). Since the 3/8” compressed air hose has an inner diameter of 0.375” (9.5mm), the diameter will not create any additional pressure drop. Some industrial facilities like to use compressed air tubing instead of hoses. This is fine as long as the inner diameters match appropriately with the recommended pipe in the installation sheets. Then you can reduce waste from pressure drop and get the most from your EXAIR products.

With the diameter being such a significant role in creating pressure drop, it is very important to understand the type of connections to your pneumatic devices; i.e. hoses, pipes, or tubes. In most cases, this could be the reason for under performance of your pneumatic products, as well as wasting money within your compressed air system. If you would like to discuss further the ways to save energy and reduce pressure drops, an Application Engineer at EXAIR will be happy to help you.

John Ball
Application Engineer
Email: johnball@exair.com
Twitter: @EXAIR_jb

Photo: Manometers by WebLab24_Siti_Web . Pixabay License

Cleaning Out Pipes, Extrusions and Blind Holes

Look at the picture below of a band saw cutting a piece of tubing.  The amount of debris and coolant that is coating the pipe on the inside diameter and outer diameter is substantial.  Cleaning off the outside of a pipe is fairly easy and straight forward.  Cleaning the ID can be difficult.  This is a single instance where one of the EXAIR Back Blow Air Nozzles can turn a cumbersome job into a quick and easy step in the process.

1 – Metal Cutting Bandsaw

The tubing in the photo appears to be around a 3 or 4″ ID tubing which makes it ideal to be cleaned out internally by the model 1006SS – 1/4″ Back Blow Air Nozzle.  This nozzle is ideal for passage ways ranging from a 7/8″ diameter up to a 4″ diameter.

1006SS – 1/4″ Back Blow Air Nozzle

While cleaning out the short section may be able to be obtained with a forward blowing Safety Air Gun, if this was being cut from a 20′ length of tube it would be difficult to remove the debris from the remnant section of tubing.  The advantage being the debris from the saw cut no longer has to be blown out or left in the longer lengths of the tube.   The Back Blow Air Nozzle can easily be inserted and remove debris back from the saw cutting end.  Lee Evans demonstrates this in a video below.

If you would like to discuss any compressed air application, please feel free to contact an Application Engineer.  We will gladly help you determine which EXAIR product may be right for your application.

Brian Farno
Application Engineer
BrianFarno@EXAIR.com
@EXAIR_BF

1 – Metal Cutting Bandsaw image – S.J. de Waard, Creative Commons License [CC BY 2.5 (https://creativecommons.org/licenses/by/2.5)] – https://commons.wikimedia.org/wiki/File:Metal_cutting_bandsaw_(02).JPG