pipe 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 utility 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.

One of the largest systematic problems with compressed air systems is pressure drop. If too large a pressure loss occurs, pneumatic equipment will not have enough power to operate effectively and efficiently. The amount of pressure drop is based on restrictions, obstructions, and piping. When air is forced into small areas, it will cause a high velocity. The high velocity will create turbulent air flow which increases the pressure loss. A restrictive type of pressure drop can be found in different forms, like small diameter pipes or tubing; or restrictive fittings like quick disconnects and needle valves, and undersized filters, regulators and valves.

Why did 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.

It is very important to know the inner diameter of the supply lines to your pneumatic devices. As an example, a model 110006 6” Super Air Knife will need a 3/8″ black, schedule 40 pipe that has an I.D. of 0.493″ (12.5 mm). We use this pipe to flow 21 SCFM of compressed air at 100 PSIG through 50 feet of pipe. What would be the pressure drop? With Equation 1, we get a pressure drop of 1.28 * (21 SCFM/60) ^1.85 * 50 feet / ((0.493″)^5 * 100 PSIG) = 3.15 PSID. Thus, you started with 100 PSIG, and at the end of the 50 ft. pipe, you will only have (100 PSI – 3.15 PSI) = 96.85 PSIG to use.

Let’s look at a 3/8” hose and a 3/8” tube. The 3/8” hose has an inner diameter of 0.375” (9.5 mm), and the 3/8” tube has an inner diameter of 0.25” (6.4 mm). In keeping the same variables except for the diameter, we can calculate the pressure drop with the above equation. 3/8″ hose = 1.28 * (21 SCFM/60) ^1.85 * 50 feet / ((0.0.375″)^5 * 100 PSIG) = 12.4 PSID. 3/8″ tube = 1.28 * (21 SCFM/60) ^1.85 * 50 feet / ((0.25″)^5 * 100 PSIG) = 94 PSID.

As you can see, the 3/8” hose has a pressure drop 3.94 times higher than the 3/8″ NPT pipe. Also, the 3/8″ tube has a pressure drop 7.6 times higher than the hose.

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 in-feed pipe sizes for each air knife at different lengths. (You will have to sign in to 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 drops and get the most from your EXAIR as well as all other pneumatic products.

With the diameter playing such a significant role in creating or mitigating 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 the under performance of your pneumatic products, as well as wasting money through operation of 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

An important component of your compressed air system is the supply piping. The piping will be the middle man that connects your entire facility to the compressor. Before installing pipe, it is important to consider how the compressed air will be consumed at the point of use. You’ll also need to consider the types of fittings you’ll use, the size of the distribution piping, and whether you plan to add additional equipment in the next few years. If so, it is important that the system is designed to accommodate any potential expansion. This also helps to compensate for potential scale build-up (depending on the material of construction) that will restrict airflow through the pipe.

The first thing you’ll need to do is determine your air compressor’s maximum CFM and the necessary operating pressure for your point of use products. Keep in mind, operating at a lower pressure can dramatically reduce overall operating costs. Depending on a variety of factors (elevation, temperature, relative humidity) this can be different than what is listed on directly on the compressor. (For a discussion of how this impacts the capacity of your compressor, check out one of our previous blogs – Intelligent Compressed Air: SCFM, ACFM, ICFM, CFM – What do these terms mean?)

Once you’ve determined your compressor’s maximum CFM, draw a schematic of the necessary piping and list out the length of each straight pipe run. Determine the total length of pipe needed for the system. Using a graph or chart, such as this one from Engineering Toolbox. Locate your compressor’s capacity on the y-axis and the required operating pressure along the x-axis. The point at which these values meet will be the recommended MINIMUM pipe size. If you plan on future expansion, now is a good time to move up to the next pipe size to avoid any potential headache.

After determining the appropriate pipe size, you’ll need to consider how everything will begin to fit together. According to the “Best Practices for Compressed Air Systems” from the Compressed Air Challenge, the air should enter the compressed air header at a 45° angle, in the direction of flow and always through wide-radius elbows. A sharp angle anywhere in the piping system will result in an unnecessary pressure drop. When the air must make a sharp turn, it is forced to slow down. This causes turbulence within the pipe as the air slams into the insides of the pipe and wastes energy. A 90° bend can cause as much as 3-5 psi of pressure loss. Replacing 90° bends with 45° bends instead eliminates unnecessary pressure loss across the system.

Pressure drop through the pipe is caused by the friction of the air mass making contact with the inside walls of the pipe. This is a function of the volume of flow through the pipe. Larger diameter pipes will result in a lower pressure drop, and vice versa for smaller diameter pipes. The chart below from the “Compressed Air and Gas Institute Handbook” provides the pressure drop that can be expected at varying CFM for 2”, 3”, and 4” ID pipe.

To discuss your application and how an EXAIR Intelligent Compressed Air Product can help your process, feel free to contact EXAIR and myself or one of our Application Engineers can help you determine the best solution.

Jordan Shouse
Application Engineer
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Images Courtesy of the Compressed Air Challenge and thomasjackson1345 Creative Commons.

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