Confused about compressed air pressure drop? You’re not alone. Pressure drop—the loss of air pressure between the air compressor and the endpoint where the air is used—is a common problem with compressed air systems. In addition to causing problems with pneumatic tools and equipment, pressure drop also drives up your energy bills. Understanding the causes of pressure drop in your compressed air system and how to address them could save you hundreds or thousands on your energy bill annually.
What is Pressure Drop?
Put simply, pressure drop in a compressed air system is the difference between the pressure discharged by the air compressor and the pressure delivered for compressed air applications. As compressed air moves through the distribution system, small losses along the way can add up to a large loss of pressure by the time air reaches the end of the line. For instance, you may have your compressed air system set at 100 PSI but only see 90 PSI at the point of use. That 10 PSI difference is what we call pressure drop.
While all compressed air systems lose a bit of air pressure somewhere along the distribution route, it becomes a problem when pressure drop is excessive. An efficient compressed air system should lose less (ideally much less) than 10% of pressure between the air receiver tank and the point-of-use. If you are losing too much pressure in your compressed air system, tools and equipment may not receive enough pressure to operate correctly.
Aire Tip: Pressure drop between the air receiver tank and the end-use should not exceed ~3 PSI.
How Does Compressed Air Pressure Drop Impact Energy Bills?
If you are losing a lot of pressure in your delivery system, you will have to raise the overall pressure of your compressed air system to compensate. For example, you may have tools that require 90 PSI to operate. If you are experiencing a 30 PSI pressure drop across your system, you will have to run your air compressor at 120 PSI to compensate.
As a rule of thumb, every 2 PSI in over-pressurization adds about 1% to the energy costs of your air compressor. So, if you are over-pressurizing by 30 PSI, you are increasing your compressed air energy bill by 15%. This adds up fast: energy costs for a 100 HP air compressor operating in a 24/7 environment can easily run to $78,000 per year (at $0.10/KWh). A 15% increase in energy use would add another $12,000 to the energy bill.
Aire Tip: Many plants are operating at a higher PSI than they need. Read more: Reduce Plant Pressure to Save Money and Energy
What Causes Pressure Drop in Compressed Air Systems?
Pressure drop can happen at numerous points between the air tank and point-of-use. Anything that causes an obstruction in airflow or loss of air will result in a loss of pressure for the system. Causes of pressure drop in the compressed air system can be broadly separated into two categories: the air distribution system and compressed air system components.
- The distribution system consists of all of the piping, tubes, and hoses that carry compressed air from the air receiver tank to the point of use. A poorly designed or maintained air distribution system can lose pressure through excessive friction, obstructions, or leaks.
- Compressed air system components that can cause pressure drop include air dryers, filters, moisture separators, after-coolers, and oil separators.
If the piping is not sized properly for the airflow, you will see the pressure drop in the system. Airflow is measured in cubic feet per minute, or CFM. As air moves through the distribution system, it encounters friction with the pipe walls. The more friction it experiences, the more the air slows down. Undersized piping results in excessive friction, putting a significant drag on air velocities in the distribution system.
Poorly Designed Distribution System
The design of the distribution system also impacts friction. The longer the run between the air compressor and the endpoint, the more friction it will encounter along the way. Air also slows down when it encounters rough patches in the piping (such as a join) and when it is required to take a turn. A compressed air distribution system with lots of joins, tees, and elbows will result in more pressure loss than a straight, smooth pipe of equal length. Dead ends in the distribution system also result in pressure drop.
Corrosion in the Distribution System
Corrosion in compressed air pipes can significantly impact airflow and performance. Corrosion roughens the interior of the pipe, generating more friction as air passes through. Over time, it can even cause blockages as material flakes off and accumulates in bends or low points in the system. Excessive moisture in compressed air pipes can lead to corrosion in susceptible materials.
Leaks in the Distribution System
Leaks in the compressed air system reduce air pressure in two ways. First, any loss of air will cause a drop in the pressure of the system, making the air compressor work harder to maintain the proper PSI. Second, a leak changes the airflow pattern from a smooth laminar flow through the pipes to a turbulent flow, increasing friction and reducing airspeed. Leaks are most commonly found in couplings, hoses, joints, and fittings. Quick couplers used to connect equipment to the distribution system are a common culprit.
Blocked inline filters are another cause of pressure drop in a compressed air system. Inline filters are positioned after the compressor to filter particulate and oil carryover out of the compressed air stream. They may be placed directly after the air compressor, after the dryers or immediately before the end application. As these filters become loaded with particulate and oil, it takes more energy to move air through the filter media. This causes a drop of pressure between the dirty and clean sides of the filter. Undersized filters, just like undersized pipes, will also cause obstructions in the system.
Undersized Air Dryers
The compressed air dryer is positioned after the air compressor to remove moisture from compressed air. If the dryer is not sized properly for the CFM and PSI of the system, it can become a choke point for airflow. The air dryer will be rated for a maximum CFM, but performance will also be impacted by temperature and pressure.
Other Air Compressor System Components
Any other component that air flows through between the air receiver tank and the endpoint can become a potential source of pressure drop. That includes aftercoolers and water separators. If these components are not sized for the CFM of your system, they generate excessive friction and slow air down.
How is Pressure Drop Measured?
The easiest way to determine the pressure drop for a compressed air system is to simply measure it. A compressed air flow meter measures airflow (CFM) and pressure (PSI). Some will also measure temperature and total flow. A flow meter may be installed permanently in line with the compressed air distribution system or temporarily inserted during a system audit.
To find the source of pressure drop in your system, it is best to measure PSI at several points.
- Immediately after the air receiver tank.
- Before and after the air dryer.
- Before and after any inline filters, water separators, or after-coolers.
- At each of the drops in the distribution system, both before and after hoses and couplers.
By doing this, you can create a “pressure map” of your compressed air distribution system that will help you identify where you are seeing the biggest losses in pressure. This will tell you where you need to look for opportunities for improvement. For example, you may discover that you are seeing a large pressure drop before and after an air dryer or after-cooler, indicating that the component may not be sized properly for your CFM. Large drops in pressure at the end of long distribution runs may indicate a problem with the design of the distribution system or excessive leaks in the piping system.
Pressure Drop Calculator for Compressed Air
It is also possible to calculate the expected pressure drop for your distribution system. Remember, all compressed air systems will experience some pressure drop due to friction in the distribution system. You can minimize this with an efficient compressed air distribution system design. This online pressure drop calculator will help you determine the expected pressure drop based on the length and design of your distribution system.
How to Minimize Pressure Drop in Compressed Air Systems
You can take steps to minimize pressure drops in your compressed air system. Ensuring that all compressed air system components are adequately sized for your CFM, improving the efficiency of your distribution system, fixing leaks, and properly maintaining filters and system components will keep pressure drop to a minimum.
The fastest and most cost-effective way to improve pressure drop in your compressed air system is to make sure filters are clean. Change inline filters at least once a year or after 8,000 hours of operation. If you have a lot of oil carryover or dry particulate in the compressed air stream, filters will need to be changed more often. Also, make sure your inline filters are sized appropriately for your airflow.
The next step in reducing pressure drop in your compressed air system is to find and fix any leaks in the distribution system. Pay special attention to hoses and quick couplers in the “dirty thirty”—the last 30 feet of connectors between the main distribution piping and the equipment using compressed air. Fixing leaks is almost always a good investment to improve the performance of your compressed air system and reduce energy costs. You may even be able to get compressed air leaks fixed for free through a program with your energy provider.
Read more: How to Detect and Fix Compressed Air Leaks
Replace Corroded Air Compressor Piping
If you suspect that you may have corrosion in your compressed air piping, the problem will need to be found and fixed. Bare steel and iron pipes are especially prone to corrosion. If your piping system is old and has not been maintained, you should remove and inspect a few sections of pipe to look for signs of corrosion on the inner surface. If corrosion is present, you may also notice flakes of rust in the compressed air stream or embedded in inline filters. The affected portions of the compressed air piping system should be replaced. Consider replacing the piping with a material that is not prone to corrosion, such as aluminum piping. Also, look at upgrading your air drying system to remove corrosion-causing moisture from compressed air.
Streamline Your Compressed Air Distribution System
If your distribution piping is a mess of dead ends, elbows, and tees, it may be time to consider replacing it entirely. An efficiently designed distribution system will save considerable energy in the long run. A loop-style distribution system is generally the most efficient configuration. Make sure pipes are properly sized for your CFM and try to minimize the length of runs and the number of tees and elbows. Also, minimize the use of inefficient hoses and quick couplers.
Upgrade Undersized Air Dryers, After-coolers and Oil/Water Separators
If you are experiencing excessive pressure drops before and after the air dryer or other system components, you may need to consider a replacement. Make sure air dryers, aftercoolers, water separators, and other compressed air accessories are properly sized for the application and well maintained.
Need Help Diagnosing Compressed Air Pressure Drop?
Sometimes, the causes of pressure drop in a compressed air system are not obvious. If you need help measuring pressure drop in your compressed air system, diagnosing the causes of pressure drop, or mitigating a pressure drop problem, contact us. We will be happy to help you find and fix the source of your pressure drop problem.