CFM vs. PSI Air Pressure: What You Need To Know
Compressed air pressure and flow seem simple enough, right? You turn the compressor on and compressed air comes out. Well, not exactly. Quite a bit of science like thermodynamics, goes into that air compressor.
No matter your mobile air compressor application, big or small, it’s helpful to understand at least some of the science behind an air compressor system. That’s because every application is different, and every application demands a compressor that meets the specific needs of that application.
So, what do you need to know? For starters, it helps to know the difference between pressure and flow.
Air pressure is measured in pounds per square inch or PSI, while air flow (also referred to as volume) is measured in cubic feet per minute or CFM. How psi and cfm work: PSI and CFM are the two operational standards for measuring pressure within an air compressor, and the key measurements that operators must consider when sizing a compressor to a specific application. Sizing a towable air compressor to an application correctly optimizes its use of energy, making its operation as efficient and cost-effective as possible.
What is the difference between the two and why are they important?
PSI measures the amount of pressure placed on a single square inch of space. In compressor terms, PSI is the amount of force that an air compressor can deliver 100 PSI, for example, equates to 100 pounds of force exerted per square inch. PSI is the measure of force that’s applied to a defined area, and determines the compressor’s ability to perform a specified amount of work at a given point in time.
Each pneumatically powered device has a stated pressure range in which it works reliably. In order for that device to work reliably, the portable air compressor must provide the right amount of pressure, or force. Too little pressure means the job won’t get done, whereas too much pressure can damage the equipment and lead to unanticipated malfunctions.
Trying to use 75 PSI to operate a device that demands 100, simply will not work and the compressor will behave erratically. Conversely, using 200 PSI for a 100 PSI application, such as powering a pneumatic wrench, will not only cause the compressor to act erratically, but also lead to equipment damage. This is the last thing you want on a job site.
As we mentioned earlier, flow is measured in CFM, and indicates an air compressor’s ability to continuously perform a task per minute of operation. This is the compressor’s “flow rate,” the amount of air that it can produce in one minute at a given pressure level. Compressors that have high flow rate, such as 200 CFM, are ideal for larger applications such as HVAC systems while small, mobile compressors that deliver around 2 CFM are better suited for less demanding applications, such as pneumatic power tools.
The compressor’s CFM correlates directly to its horsepower. When comparing a small air compressor sold at home improvement stores to an towable compressor located at a large manufacturing facility, it’s easy to see why the industrial compressor can achieve a much higher CFM than the smaller, at-home compressor. A 60-horsepower machine will generate a CFM of approximately 150, while a 150-horsepower machine will generate roughly 2000 CFM.
At this point, when talking about the relationship between CFM and PSI, it would be helpful if you could recall that high school chemistry class where your instructor explained Boyle’s Law. Back in the mid-17th Century, Robert Boyle came up with a simple equation that defines the relationship between pressure and volume in gases: P1 X V1 = P2 X V2, (where P1 is the initial pressure, V1 is the initial volume, P2 and V2 are final pressure and final volume, respectively). In plain English, the volume of a constant mass of ideal gas at a constant temperature is inversely proportional to the pressure applied on it.
Let’s say you have a compressor that doesn’t have the CFM you know you need for your application and want to figure out how much more horsepower you’ll need to generate that needed volume. Simply plug in the numbers that represent your compressor’s PSI and CFM, along with the desired CFM for your application, and the equation will tell you just how much horsepower is needed to make up the volume difference.
Overpressurizing your compressed air system is not good. What is overpressurization? It’s what happens when an operator increases a compressor system’s PSI in order to make sure that optimal pressure is achieved. For example, an industrial application may call for 75 PSI, yet an operator may run the compressor system at 100 to 125 PSI. This is also called artificial demand and can result in significant energy loss with high energy cost.
When operating your compressor system, leave nothing to chance. It’s important to ensure that your system’s pressure (PSI) and flow (CFM) rate are in line with the needs of your specific application. Not aligning compressor specs with application demands can lead to high costs and energy usage, as well as a negative impact on the service life of your compressed air system.
If you have any questions about the pressure or flow rate required for a given application, make sure to consult with an Atlas Copco air compressor professional for expert guidance.
Clayton started his compressor career early in the Midwest doing compressor service, applications and has been running compressors for over 15 years. He is the Product Marketing Manger for Mobile Compressed Air with Atlas Copco.