Designing a Compressed Air Distribution System

Compressed air is used to operate pneumatic systems in a facility, and it can be segregated into three sections; the supply side, the demand side, and the distribution system.  The supply side is the air compressor, after-cooler, dryer, and receiver tank that produce and treat the compressed air.  They are generally located in a compressor room somewhere in the corner of the plant.  The demand side are the collection of end-use devices that will use the compressed air to do “work”.  These pneumatic components are generally scattered throughout the facility.  To connect the supply side to the demand side, a compressed air distribution system is required.  Distribution systems are pipes which carry the compressed air from the compressor to the pneumatic devices.  For a sound compressed air system, the three sections have to work together to make an effective and efficient system.

An analogy, I like to compare to the compressed air system, is an electrical system.  The air compressor will be considered the voltage source, and the pneumatic devices will be marked as light bulbs.  To connect the light bulbs to the voltage source, electrical wires are needed.  The distribution system will represent the electrical wires.  If the wire gauge is too small to supply the light bulbs, the wire will heat up and the voltage will drop.  This heat is given off as wasted energy, and the light bulbs will dim.

The same thing happens within a compressed air system.  If the piping size is too small, a pressure drop will occur.  This is also wasted energy.   In both types of systems, wasted energy is wasted money.  One of the largest systematic problems with compressed air systems is pressure drop.  If too large of a pressure loss occurs, the pneumatic equipment will not have enough power to operate effectively.  As shown in the illustration below, you can see how the pressure decreases from the supply side to the demand side.  With a properly designed distribution system, energy can be saved, and in reference to my analogy, it will keep the lights on.

Source: Compressed Air Challenge Organization

To optimize the compressed air system, we need to reduce the amount of wasted energy; pressure drop.   Pressure drop is based on restrictions, obstructions, and piping surface.  If we evaluate each one, a properly designed distribution system can limit the unnecessary problems that can rob the “power” from your pneumatic equipment.

  1. Restriction: This is the most common type of pressure drop. The air flow is forced into small areas, causing high velocities.  The high velocity creates turbulent flow which increases the losses in air pressure.  Flow within the pipe is directly related to the velocity times the square of the diameter.  So, if you cut the I.D. of the pipe by one-half, the flow rating will be reduced to 25% of the original rating; or the velocity will increase by four times.  Restriction can come in different forms like small diameter pipes or tubing; restrictive fittings like quick disconnects and needle valves, and undersized filters and regulators.
  2. Obstruction: This is generally caused by the type of fittings that are used.  To help reduce additional pressure drops use sweeping elbows and 45-degree fittings instead of 90 deg. elbows.  Another option is to use full flow ball valves and butterfly valves instead of seated valves and needle valves.  If a blocking valve or cap is used for future expansion, try and extend the pipe an additional 10 times the diameter of the pipe to help remove any turbulence caused from air flow disruptions.  Removing sharp turns and abrupt stops will keep the velocity in a more laminar state.
  3. Roughness: With long runs of pipe, the piping surface can affect the compressed air stream. As an example, carbon steel piping has a relative rough texture.  But, over time, the surface will start to rust creating even a rougher surface.  This roughness will restrain the flow, creating the pressure to drop.  Aluminum and stainless steel tubing have much smoother surfaces and are not as susceptible to pressure drops caused by roughness or corrosion.

As a rule, air velocities will determine the correct pipe size.  It is beneficial to oversize the pipe to accommodate for any expansions in the future.  For header pipes, the velocities should not be more than 20 feet/min (6 meter/min).  For the distribution lines, the velocities should not exceed 30 feet/min (9 meter/min).  In following these simple rules, the distribution system can effectively supply the necessary compressed air from the supply side to the demand side.

To have a properly designed distribution system, the pressure drop should be less than 10% from the reservoir tank to the point-of-use.  By following the tips above, you can reach that goal and have the supply side, demand side, and distribution system working at peak efficiency.  If you would like to reduce waste even more, EXAIR offers a variety of efficient, safe, and effective compressed air products to fit within the demand side.  This would be the pneumatic equivalent of changing those light bulbs at the point-of-use into LEDs.

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

 

Photo: Light Bulb by qimonoCreative Commons CC0

 

Intelligent Compressed Air: Distribution Piping

air compressor

An important component of your compressed air system is the distribution piping. The piping will be the “veins” that connect 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. Some end use devices must have adequate ventilation. For example, a paint booth will need to be installed near an outside wall to exhaust fumes. Depending on the layout of your facility, this may require long piping runs.  You’ll 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 my 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.

Once you’ve determined 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.

pressure drop in pipe

You’ll then need to consider the different materials that are available. Some different materials that you’ll find are: steel piping (Schedule 40) both with or without galvanizing, stainless steel, copper, aluminum, and even some plastic piping systems are available.

While some companies do make plastic piping systems, plastic piping is not recommended to be used for compressed air. Some lubricants that are present in the air can act as a solvent and degrade the pipe over time. PVC should NEVER be used as a compressed air distribution pipe. While PVC piping is inexpensive and versatile, serious risk can occur when using with compressed air. PVC can become brittle with age and will eventually rupture due to the stress. Take a look at this inspection report –  an automotive supply store received fines totaling $13,200 as a result of an injury caused by shrapnel from a PVC pipe bursting.

Steel pipe is a traditional material used in many compressed air distribution systems.  It has a relatively low price compared to other materials and due to its familiarity is easy to install. It’s strong and durable on the outside. Its strength comes at a price, steel pipe is very heavy and requires anchors to properly suspend it. Steel pipe (not galvanized) is also susceptible to corrosion. This corrosion ends up in your supply air and can wreak havoc on your point-of-use products and can even contaminate your product. While galvanized steel pipe does reduce the potential for corrosion, this galvanizing coating can flake off over time and result in the exact same potential issues. Stainless Steel pipe eliminates the corrosion and rusting concerns while still maintaining the strength and durability of steel pipe. They can be more difficult to install as stainless steel pipe threads can be difficult to work with.

Copper piping is another potential option. Copper pipe is corrosion-free, easy to cut, and lightweight making it easy to suspend. These factors come at a significant increase in costs, however, which can prevent it from being a suitable solution for longer runs or larger ID pipe installations. Soldering of the connecting joints can be time consuming and does require a skilled laborer to do so, making copper piping a mid-level solution for your compressed air system.

Another lightweight material that is becoming increasingly more common in industry is aluminum piping. Like copper, aluminum is lightweight and anti-corrosion. They’re easy to connect with push-to-lock connectors and are ideal for clean air applications. Aluminum pipe remains leak-free over time and can dramatically reduce compressed air costs. While the initial cost can be high, eliminating potential leaks can help to recoup some of the initial investment. Aluminum pipe is also coated on the inside to prevent corrosion. While an aluminum piping system may be the most expensive, its easy installation and adaptability make it an excellent choice.

It can be easy to become overwhelmed with the variety of options at your disposal. Your facility layout, overall budget, and compressed air requirements will allow you to make the best choice. Once you’ve selected and installed your distribution piping, look to the EXAIR website for all of your point-of-use compressed air needs!

Tyler Daniel
Application Engineer
E-mail: TylerDaniel@exair.com
Twitter: @EXAIR_TD

Proper Supply Line Size And Fittings Provide Peak Performance

Many times when we provide the air consumption of an EXAIR product, we get a response like…. “I’ve got plenty of pressure, we run at around 100 PSIG”. While having the correct pressure available is important, it doesn’t make up for the volume requirement or SCFM (Standard Cubic Feet per Minute) needed to maintain that pressure. We commonly reference trying to supply water to a fire hose with a garden hose, it is the same principle, in regards to compressed air.

When looking to maintain an efficient compressed air system, it’s important that you use properly sized supply lines and fittings to  support the air demand (SCFM) of the point-of-use device. The smaller the ID and the longer the length of run, it becomes more difficult for the air to travel through the system. Undersized supply lines or piping can sometimes be the biggest culprit in a compressed air system as they can lead to severe pressure drops or the loss of pressure from the compressor to the end use product.

Take for example our 18″ Super Air Knife. A 18″ Super Air Knife will consume 52.2 SCFM at 80 PSIG. We recommend using 1/2″ Schedule 40 pipe up to 10′ or 3/4″ pipe up to 50′. The reason you need to increase the pipe size after 10′ of run is that 1/2″ pipe can flow close to 100 SCFM up to 10′ but for a 50′ length it can only flow 42 SCFM. On the other hand, 3/4″ pipe is able to flow 100 SCFM up to 50′ so this will allow you to carry the volume needed to the inlet of the knife, without losing pressure through the line.

Pipe size chart for the Super Air Knife

We also explain how performance can be negatively affected by improper plumbing in the following short video:

 

Another problem area is using restrictive fittings, like quick disconnects. While this may be useful with common everyday pneumatic tools, like an impact wrench or nail gun, they can severely limit the volumetric flow to a device requiring more air , like a longer length air knife.

1/4″ Quick Connect

For example, looking at the above 1/4″ quick disconnect, the ID of the fitting is much smaller than the NPT connection size. In this case, it is measuring close to .192″. If you were using a device like our Super Air Knife that features 1/4″ FNPT inlets, even though you are providing the correct thread size, the small inside diameter of the quick disconnect causes too much of a restriction for the volume (SCFM) required to properly support the knife, resulting in a pressure drop through the line, reducing the overall performance.

If you have any questions about compressed air applications or supply lines, please contact one of our application engineers for assistance.

Justin Nicholl
Application Engineer
justinnicholl@exair.com
@EXAIR_JN

When Sizing Long Pipe Runs, Make Sure to Add in the Pipe Fittings

IM on Compressed Air Line Sizes for Cabinet Cooler
Installation and Maintenance information on Compressed Air Line Sizes for Cabinet Cooler

 

EXAIR uses this statement in their installation manuals to help determine the correct size pipe for our products. The above statement came from our large NEMA 4-4X Cabinet Cooler installation manual.  There are some important factors to consider when using this guideline to ensure proper air flow.

A customer installed a model 4840 EXAIR NEMA 4 Cabinet Cooler, and he was not getting the proper cooling. In diagnosing compressed air issues, one of the first things that we ask our customers is “What is the air pressure at the device?”  He attached a pressure gauge at the Cabinet Cooler, and he was reading 45 psig; much too low for proper cooling.  He sent me a photo of the setup and some details of the compressed air system supplying the Cabinet Cooler.  We needed to find the restriction to properly supply enough compressed air to the unit.

Westinghouse Cabinet Cooler

In the details that he sent, they ran 43 feet of 1/2” copper compressed air tubing from the header to the Cabinet Cooler. He mentioned that they had one angled Safety Valve at the beginning and twelve elbows in that run.  (Apparently they had to get around and through things to reach the location of the Cabinet Cooler).  They did have a pressure gauge in the header that read 105 psig.

The first thing that I noticed was that they were using compressed air tubing instead of compressed air pipe or hose. Tubing is measured by the outer diameter while the compressed air hoses are measured by the inner diameter.  So, in the statement above when it references ½” I.D. hose, ½” tubing will have a much smaller I.D., and in this case, it had a 3/8” I.D.  With this smaller flow area, this will increase the restriction.  In calculating the pressure drop in 43 feet of ½” tubing, it would be roughly a 27 psi drop at 40 SCFM.  If they have 105 psig at the header, they should be reading 78 psig at the Cabinet Cooler.  Being that they were only reading 45 psig, where is the rest of the restriction?

The answer to that question is in the fittings. When you have pipe fittings like elbows, tees, reducers, etc., they will add pressure drop to your system as the compressed air travels through them.  There is a method to calculate compressed air runs with pipe fittings in terms of Effective Length.  Effective length is a way to estimate the same pressure drop through a similar length of pipe to a pipe fitting.  This can be very important when running compressed air lines for EXAIR products.  Once we have the effective length of a pipe, then we can use the requirements in the installation manual for sizing compressed air lines properly.  The chart below shows the equivalent lengths by fitting category.

Equivalent Length

In the application above, the customer used 43 feet of 3/8” I.D. line, 12 pcs. of 3/8” regular 90 deg. elbows, and one 3/8” angled valve. The equivalent length of pipe can be calculated as 43 feet + 12 * 3.1 feet + 1 * 15 feet = 95.2 feet.  As you can see, with all the fittings, the equivalent length of pipe extended from 43 feet to 95.2 feet.  If we recalculate the pressure loss for 93.2 feet of ½” tubing, then we get a pressure loss of 58 psi at 40 SCFM.  From the header, this will equate to a pressure of 47 psig at the EXAIR Cabinet Cooler.  This is very close to the reading that he measured.  He asked me to recommend the proper size pipe, and by using the equivalent length and the installation manual, I suggest that he should use either ½” NPT pipe or 5/8” O.D. copper tubing for a 95 feet run.  This would only create a 5 psi pressure drop which would properly supply the model 4840 Cabinet Cooler with 40 SCFM.

If you are wanting to use tubing in your compressed air lines, you will need to use the inner diameter for sizing. Also, if you have many fittings, you can add them to your pipe lengths to get an equivalent overall length.  With the above methods to correctly size the compressed air lines, your EXAIR products will be able to work effectively and properly.

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

 

3 Common Mistakes in Your Compressed Air System

Every day I speak with engineers who are having trouble using compressed air products. A common problem they have is not providing an adequate air supply to their unit. I go through a basic troubleshooting technique to ensure that their pressure and flow rate is adequate. I ask them to install tee on the inlet to the compressed air product in order to install a pressure gauge right at the inlet to the pipe. This allows us to know exactly what pressure we are supplying to the product. Customers are always surprised how the gauge on the compressor or the regulator may read 120 PSIG, but the gage on the inlet to the compressed air product is significantly less.

Last year, my colleague, Russell Bowman, made an excellent video showing how the inlet pressure at the knife will have a significant impact on the performance of the Super Air Knife.  In the video, he changes the length and ID of the compressed air supply to illustrate the difference a proper supply line will have on the performance of a compressed air products.

Not providing adequate air supply is commonly caused by these three mistakes, when plumbing compressed air systems.

1. Incorrectly Sized Piping – This can be the single biggest problem. A lack of planning before installing a compressed air product. Not all compressed air systems are created equal. Though a 1/4″ shop air hose may work for a number our products, some of our products require a larger air line because they require more volume of air to be effective. We often speak with customers an illustrate this problem by stating small air lines are like trying to feed a fire hose with a garden hose – there simply is not enough volume to create the pressure necessary to reach the fire, or solve the application in our scenarios. We publish the flow rates for all of our products and make inlet pipe size recommendation in the installation and maintenance guide furnish with the products so you may avoid this common problem. We also have air data tables in our Knowledge Base or  you may consult an application engineer who will be happy to make the proper recommendation.

2. Quick Disconnects – These handy connectors are great when operating a brad nailer, or a small blow gun, but the small through diameter can severely limit the flow rate into a long air knife, large diameter air operated conveyor, or big vortex tubes.  Due to this fact it is strongly advised to use threaded fittings or over-sized quick disconnects.

3. Adding extra hose or pipe – Extra hose is never a bad thing, right? No, an extra 30 feet of air hose can significantly drop the pressure of a compressed air system. 20 feet of ½ Pipe can flow 70 CFM with a 5 PSI pressure drop.  50 feet of ½” pipe will only flow 42 SCFM with the same 5 PSIG pressure drop. Keep your hose or pipe lengths to a minimum to improve the volume of air you can deliver to a compressed air product.

Dave Woerner
Application Engineer
DaveWoerner@EXAIR.com
@EXAIR_DW

Compressed Air Calculations, Optimization, and Tips

EXAIR uses our blog platform to communicate everything from new product announcements to personal interests to safe and efficient use of compressed air. We have recently passed our 5 year anniversary of posting blogs (hard for us to believe) and I thought it appropriate to share a few of the entries which explain some more of the technical aspects of compressed air.

Here is a good blog explaining EXAIR’s 6 steps to optimization, a useful process for improving your compressed air efficiency:


One of the Above 6 steps is to provide secondary storage, a receiver tank, to eliminate pressure drops from high use intermittent applications. This blog entry addresses how to size a receiver tank properly:

Here are 5 things everyone should know about compressed air, including how to calculate the cost of compressed air:

These next few entries address a common issue we regularly assist customers with, compressed air plumbing:

In a recent blog post we discuss how to improve the efficiency of your point of use applications:

Thanks for supporting our blog over the past 5 years, we appreciate it. If you need any support with your sustainability or safety initiatives, or with your compressed air applications please contact us.  

Have a great day,
Kirk Edwards
@EXAIR_KE

How to Size Pipes for Your Compressed Air System

Most facility’s compressed air systems have evolved over time. A spur added here a spur added there. Eventually pressure drop issues develop. Common practice is to increase the air pressure at the compressor. While it may address the symptom it does not address the problem and is very costly. For every 2 PSI increase in pressure requires 1% more energy.

A properly designed system will be a loop with spurs. This will ensure all airsystem

drops will share the air equally. The header loop should be able to carry all the air the compressor is capable of producing.  Best practices suggest the distribution header should be sized to allow an air velocity not to exceed 30 ft/second. The formula to calculate this is:

A =    144 * Q * Pa
       V *60 x (Pd +Pa)

Pipe Diameter = √ (A*4/3.14)

Where:

A = cross sectional area if the pipe bore in square inches or ∏ x diameter squared / 4
Q = Flow rate SCFM
Pa = Prevailing absolute pressure. Sea level is 14.7
Pd = compressor gauge pressure minus prevailing absolute pressure
V = Design pipe velocity ft/sec

Example: Size a header for 500 SCFM at 100 PSI at an elevation at sea level

A = 144 x 500 x 14.7 / 30 x 60 (100 + 14.7) = 5.13  square inches

Pipe diameter then is square root of  (5.13 * 4) / 3.14 = 2.56″

So an 2.56″  internal diameter pipe would be the proper size header.

The same formula can be used to calculate the sizes of the drops. In this case you would use the demand flow rate for Q.

Joe Panfalone
Application Engineer
Phone (513) 671-3322
Fax (513) 671-3363
Web: http://www.exair.com
Twitter: http://www.twitter.com/exair_jp
Facebook: http://www.facebook.com/exair