On the surface, the question seems simple: How much air does my compressor put out?
But online search engines and even that compressor guy up the street start throwing terms around like ACFM and SCFM and they frankly sound about the same; some sites or people may even use them interchangeably.
So, what’s the deal? Are ACFM and SCFM the same thing? And if they aren’t, then what is the difference? How much does it matter?
Air basics
First, let’s address a basic principle that is sometimes left out of compressed air discussions or overlooked by someone new to compressed air systems.
A screw style air compressor, either oil free or oil flooded, is positive displacement. You turn the rotors at a given speed, and they will give you a specific air volume in return.
However, an air compressor pulls air from its surrounding environment. That environment looks different depending on where you are; a cubic foot of space at sea level on a 60-degree day holds more air than a cubic foot of space in Colorado on a 90-degree day.
So, temperature and altitude drastically affect the amount of air in a given compressed air environment.
Actual Cubic Feet per Minute
To make rating simple, compressor manufacturers typically use Actual Cubic Feet per Minute (ACFM) when labeling their compressor capability.
ACFM does not change based on a compressor’s surrounding environment. Therefore, ACFM always represents how much volume of air a compressor can move in one minute.
Picture it like this: if a compressor is listed at 40 ACFM, then it can empty 40 1’x1’x1’ boxes of air in one minute . . . regardless of temperature, altitude, humidity, etc.
Manufacturers use ACFM to rate a compressor because it enables them (and you) to easily compare compressors against one another. You can easily decipher that an 80 ACFM rated compressor can move more volume per minute than a 40 ACFM rated compressor.
Standard Cubic Feet per Minute
Standard Cubic Feet per Minute (SCFM) is different than AFCM in a significant way.
While ACFM tells you how many boxes your air compressor can empty, SCFM is a standard rating that we apply to know how much air is in the boxes.
Remember that environment lesson above? Well, this is where it comes into play.
Temperature, altitude, etc. affect how much air is in each cubic foot of space; which means that your compressor is moving different volumes of air depending on those factors. It’s important to identify them because they influence how much air is going downstream to the equipment you have on site that requires it.
SCFM, then, is a specific calculation to your environment. SCFM can range anywhere from matching up with the ACFM to being 15% less than ACFM in some areas. During summer in the southeast states, you will see a 7%-10% reduction in SCFM from ACFM on hot days.
A good way to remember this to equate SCFM with “SITE” conditions.
A European take
While most of our business is in is the Southeastern US, we want to take note that European countries typically apply the term ACFM to identify how much air you have at pressure. This is different than the US where we apply the term to free air, before pressurization.
Always identify whether you are discussing ACFM, SCFM, or using European terminology. CFM alone is general and misleading, so ask for clarification on this one too. The end result could be an air compressor that is a total misfit for your application otherwise.
If you want more exact information on these terms or would like specific calculations, contact our team!