This page is providing information about SCFM and the measuring of compressed air flow.
Since SCFM is the measurement of free air flow, confusion with how this applies to compressed air flow, and even above questions about compressors themselves, is an issue that draws a steady stream of visitors to this air compressor information website.
I hope the information on this page helps clear any confusion about SCFM that you may have.
Given the various concepts and discussions about this topic I am not too surprised at the confusion. Even though I think I understand the difference between SCFM, CFM, ACFM and other acronyms that have to do with the measurement of compressed air flow, my definition may not be the one that any specific compressor vendor is using to provide flow characteristics for their compressor. It also might vary from the definitions of the air cylinder, or air valve manufacturers engineering specs too!
The definition of SCFM is suspect due to the changing parameters from different sources. And it is how that definition applies to sizing a compressor that complicates the issue with many folks.
So, press on, and lets see if we can figure it out together.
Compressor discharge flow measurement is confusing and fraught with error for the person that is not dealing with these issues regularly. Heck, even if you are involved with fluid power, applying SCFM in a meaningful way to compressed air flow is confusing.
I have canvassed a variety of sources and have found that even those that are considered most reputable have differing definitions of SCFM, and how that definition is applied for us compressed air users.
SCFM Measure Has Rigid Parameters
In order for specific air flow to be measured in SCFM, most experts agree that the air flow must meet the following set of rigid environmental circumstances;
- The air temperature must be 68 degrees Fahrenheit
- The relative humidity of the air must be 36%
- The air itself must be measured at sea level
Having said that, some other sources indicate that the relative humidity of the air must be zero in order for the air flow to be measured in SCFM. Another source says that the R.H. must be at 50%, for the flow to be rated as SCFM.
Confusing? You bet!
Consider this! Few of us have plant locations exactly at sea level where the ambient air pressure is 14.7 PSI. We have little control over the exact air temperature or the relative humidity in our plant. As air exits the compressor discharge it will always be a lot hotter than 68 degrees F. That compressed air will have a much higher humidity level than the 36% (or the zero, or the 50%) that is specified for an SCFM rating.
Simple Definition Of SCFM
The acronym SCFM stands for Standard Cubic Feet of air per Minute.
If you need more than a simple definition, if you are trying to understand how that term applies to measuring compressed air, then please read on.
An Airflow Yardstick
Some compressor manufacturers use SCFM as their yardstick to measure free air flow into the intake filter on an air compressor, not the flow of compressed air from the discharge of their compressor.
Some compressor manufacturers will use the term CFM for the flow of air out of their compressor discharge. Others will use the acronym ACFM (Actual CFM) as the measure for air from their compressors. Another might use ACFM as “after-compressor feet per minute”.
And some manufacturers will use the acronym SCFM as their measurement of air from the discharge port on their compressors.
For us then, SCFM must be considered as a theoretical measure of flow that does not have a lot of impact on us when it comes to determining the compressed air flow needed to do work.
Why not just ignore SCFM? What you do need to know how much flow and pressure a specific compressor will generate for you.
You will have to know how much air and at what air pressure a specific air tool or compressed air using application will need. This to help you ensure that all your applications have enough compressed air flow and pressure from the air compressor to do their work! In most cases, that is all you care about.
CFM & Pressure
When you first start to source an air compressor then you will first need to know how much compressed air your equipment will use in CFM and at what air pressure that CFM must be.
Your air-driven equipment will often have a tag that identifies the flow requirements, or, you can check the manufacturers web site for that information, or to obtain a contact from whom you can obtain that data.
Remembering that in compressors over 10 HP in size, normally each HP of motor capacity will generate 3-4 CFM of compressed air at 90 PSI, and under 10 HP you should get 2-3 CFM of compressed air at 90 PSI for each HP of electric motor size.
Knowing this, and having researched to find out the number of CFM and pressure required to run your air equipment is, you can now source a compressor of the correct size.
When you find the compressor that you think suits your needs, if it were me doing so, I would provide the manufacturer with my plant of workshop requirements in CFM at the pressure you need. Let the compressor manufacturer guarantee to you that their recommended air compressor will provide sufficient flow and pressure, regardless of what term they use to describe the compressed air flow from their own brand of compressor. That’s their job, after all.
Higher Discharge At Lower Pressures
Be a bit careful about compressor flow claims. An air compressor will show a higher discharge flow rate at a lower pressure level than it will at a higher pressure level. Compressor manufacturers will often highlight a high flow rate and lowlight the CFM. It will likely be air flow at the higher pressure that you will require for your applications!
Make sure, when you are discussing your compressor size with your vendor, whether they measure the outflow flow from their compressor in SCFM or CFM or ACFM or whatever, insist that they assure you that you will get the CFM you need at the pressure you need for the compressed air equipment to run.
An Engineering Perspective On SCFM
The following is a contribution from a visitor… a consideration of SCFM from an engineers perspective. I appreciate the information and am happy to share this with visitors.
“Using scfm provides a more precise measurement of air flow.
Air gets denser as you compress it, less dense as you heat it. In order to provide accurate flow measurement to an end user or to provide an “apples to apples” comparison of flow, that variability has to be removed.
So a “standard” cubic foot was created – based as you’ve said – rather loosely on a standard pressure and temperature.
Typically being 1 atmosphere and temperature of either 60F, 68F or 15C. In other words, 1 scf is the space that 1 cubic foot of air occupies at atmospheric pressure and a standard temperature. At 90 psig that same cubic foot takes up a lot less space than it did at atmospheric pressure so 1 cfm @ 90 psi contains 7 scf.(standard cubic feet)
In real life, cfm is the useful output rate and it is a measure of exactly how many cubic feet leave the compressor at the rated pressure in a minute. There is no regard for the density of that cubic foot or whether or not my compressors cubic foot contains as much air as your compressor. That’s where scfm comes in – it equalizes the playing field, providing standardized correction to capacity claims at 110 psig versus those at 90 or 150 psig for instance.
In reality no one using compressed air to drive a tool really cares about scfm – we’re all concerned about how many cfm at my required pressure can i get so i know i can drive this air wrench etc. As long as i get 30 cfm of 90 psi air and my wrench works i don’t care how many scf that air contains.
However if you deal with the accounting side of a large firm you’ll know they bill or pay based on scfm used so that every cubic foot used is corrected for pressure and temperature, preventing over or under charging. It provides consistency day to day and season to season – same goes for the gas company, your bill will show use in scf (perhaps 1000’s of scf).”