So many times, when reviewing the air requirements for our products, I am met with psi. “I have 100 psi running to this Super Air Knife, but it has almost no air flow”. PSIG and SCFM work hand in hand, and both are critical in optimal performance of our products. These two measurements are related, but they are not immediate family. PSI is more about power, whereas SCFM is more about flow and consistency. When they both combine, force is the outcome. We need both PSI and SCFM to produce force and for products to perform. It seems that most have a solid grasp on PSIG, but the SCFM tends to cause some confusion. Mostly the “S” in the SCFM.
Once we redirect the conversation to SCFM, we almost always find the heart of the issue. But SCFM can be confusing and many times is mistakenly interchanged with CFM. CFM is the easiest to explain, so let’s start there.
CFM is Cubic Feet per Minute of airflow. This is exactly as it sounds, 1 CFM = 1 12″x12″x12″ box full of air moving through the product (Air Knife, Nozzle, Vortex Tube, Air Wipe and so on) per minute. To visualize this, take a look at your office (or visualize one). Let’s say it is 12x12x8 feet, like the sample to the right. That is 1152 cubic feet of air inside that office. If we used a 48″ Super Air Knife, running at 80 psig, it will consume 139.2 standard cubic feet of air per minute, or in other words, it will use all of the air in that office in 8.27 minutes. Assuming that air was not replaced, the Super Air Knife would starve and not have any airflow even though your gauge may still show 80 PSIG. So, when we break it down to this simple form and example, CFM is pretty easy to comprehend.
Now the confusing part: what is the S in SCFM? The S simply stands for Standard. I know, this is shocking, but what is “Standard”? Standard represents values that are a baseline of measurement, even though few of us reside or work in these “ideal” surroundings. Air volume can be altered by several things, the most common are atmospheric pressure, temperature, and relative humidity.
Some genius somewhere decided that the best place to measure for a Standard CFM at sea level. I imagine they chose Key West Florida, on the beach, which sits around 14.7 psia atmospheric pressure. Then, to be precise they measured this when the thermostat read 68°F, and the hygrometer showed 0% relative humidity. Outside of creating a lab with these settings, that beach, on that day, at that precise time, is where you can get a true CFM of air equal to the Standard.
To summarize the “S” or Standard conditions are 14.7 psi atmospheric pressure, 68°F, and 0% relative humidity.
Since most of us will never work in that exact environment, SCFM is “mostly” accurate. This is the reference point that we all can use, and has become the “Standard” across the globe. As you can imagine, it would be impossible for companies such as mine to publish or calculate every Actual CFM (ACFM). But we can do this…
ACFM is just what is says – the exact CFM that you will use based upon your elevation (atmospheric Pressure), your temp, and your relative humidity. I have a peer that wrote a recent blog that goes into depth on this topic as well as ACFM and ICFM (Inlet) and how to calculate each of these. You can see that blog here.
Thank you again for stopping by to find out more about the S in SCFM. If you have any questions about this, or anthing else related to our products, please do not hesitate to reach out.
Thank you for stopping by,
Brian Wages
Application Engineer
EXAIR Corporation
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Cover photo by PIRO4D, and licensed by Pixabay
