The terminology and jargon used on an air compressor can be a bit confusing. The first problem most people encounter when comparing different air compressors is SCFM (standard cubic feet per minute).
What is SCFM on an air compressor, anyway? And adding to the confusion, tools are generally rated in CFM (cubic feet per minute) and PSI (pounds per square inch). So, what’s the difference between these two technical units for measuring gas?
Let’s break all of these critical measurements for air compressors down to better understand what you need and what your compressor can do.
Table of Contents
- What is SCFM?
- What is CFM?
- What does 2.6 scfm 90 psi mean?
- What is the difference between SCFM and CFM?
- SCFM vs. CFM in Evaluating an Air Compressor
- How Much SCFM Do I Need to Run Air Tools?
- What Does the Volumetric Flow Rate for Air Compressors Mean?
- What Does the Pressure for an Air Compressor Mean?
- Summary Table
- An Engineering Perspective On SCFM
- Further Reading on SCFM and CFM
What is SCFM?
Firstly, let’s understand what SCFM is… If you’ve ever shopped for or researched air compressors, you already know that SCFM is one of the essential terms to understand. Air compressors are rated for a specific SCFM, and knowing what SCFM you need is vital to getting an air compressor suitable for your required applications.
What Does SCFM Stand For?
SCFM is the standard cubic feet per minute of air that an air compressor can process.
However, more important than simply defining what the letters mean is understanding why it’s so important. The SCFM is a vital measurement to understanding what your air compressor is capable of. Using the SCFM, you can figure out what sort of tools you can run and how many of them.
What is SCFM on an Air Compressor?
The “standard” in SCFM refers to the standards against which the compressor is compared. You can’t compare two units to one another unless the air they are pumping is identical. The Compressed Air & Gas Institute (CAGI) and the American Society of Mechanical Engineers (ASME) have established “standard conditions” as 14.7 PSI (atmospheric pressure), 68 degrees Fahrenheit, and 36 percent relative humidity at sea level.
As SCFM is measured at these standard conditions, it allows the measurement of mass flow rate. Mass flow rate is the amount of substance or particles that pass through a surface at a specific unit of time.
For air compressors, SCFM is the agreed-upon measure of the overall power of a compressor. As described above, this standard helps you compare one compressor to another. As SCFM is a measure of air volume. The bigger and more powerful your air compressor is, the higher the SCFM rating is.
What is CFM?
CFM is the usual unit of measure for the volume of discharge air from a compressor over time. As you may have guessed, it’s units are based on; volume / time. CFM is the acronym for cubic feet per minute. A compressor has a specific number of cubic feet of compressed air per minute (CFM) of flow from its discharge port.
For more on what is CFM and what does CFM mean on an air compressor please visit our article.
What does 2.6 scfm 90 psi mean?
Air compressors are typically rated with an SCFM at a specific pressure. So when a compressor specification says 3.8 scfm at 90p psi, this means that the air compressor PUMP has the ability to deliver 3.8 scfm at a regulated pressure of 90 psi.
Some air compressors are rated with 2 different scfm ratings i.e.
- 3.1 scfm at 90 psi
- 4.2 scfm at 40psi
This basically means that the COMPRESSOR PUMP can produce 3.1 SCFM at 90 psi, but it can increase slightly more SCFM (4.2) at 40 psi.
This is a cause of MUCH confusion. Once again, the compressor manufacturers are referring to the rating of the COMPRESSOR PUMP. The SCFM and pressure you get at the outlet of the air tank is only related in terms of the ability of the compressor pump to replenish the air used. i.e. if you use 8 SCFM at 90 psi from a compressor that’s rated at 3.1 scfm at 90 psi, your air tool will work until you drain the air tank and the compressor pump has a chance to catch up.
See my detailed guide on how to calculate SCFM of an air compressor to understand how the relationship between SCFM and psi on your compressor is completely different if talking about the compressor pump vs the air tank outlet.
What is the difference between SCFM and CFM?
So now let us put SCFM vs. CFM, what is the difference? Well, the simplest way to think about it is to divide the two terms up. The SCFM is more indicative of how the air which is pressurized to a standard value will perform when delivered to the air tool from the compressor. The air compressor CFM is more relevant to the internal capacity of an air compressor.
Since SCFM is measured at these hypothetical standard conditions mentioned above, chances are your real-world CFM is different. If you measure what the machine is actually doing, the measurement is known instead as ACFM or, actual cubic feet per minute.
SCFM vs. CFM in Evaluating an Air Compressor
While the air compressor is measured in how much air it can compress in a given minute, tools are rated by how much air they need to operate. Tools will be rated in both PSI (pounds per square inch of air pressure) and in SCFM.
Manufacturers provide the CFM and SCFM ratings of their air compressors or tools measured at sea level. SCFM gives us an air compressors volumetric flow rate at a standardised condition while there are no real standards for CFM.
Is SCFM higher than CFM?
Yes, CFM ratings are commonly lower than SCFM ratings… why you ask? The air compressor CFM rating comes after the air is pressurized, typically at 90 PSI, therefore this results in a smaller air volume.
On the flip side, as you know now, the air compressors manufacturers expand the air to a standard condition while taking into account the variables mentioned. This standard condition has a significantly lower pressure (14.7 PSI), therefore the volume is bigger and thus SCFM is always a greater number than CFM.
How Much SCFM Do I Need to Run Air Tools?
Once you understand the difference between the theoretical SCFM and CFM measurements, you might be left wondering what to do with all of this information. How does it help you get the right air compressor or the right tool?
In a factory setting, where near-continuous use of the pneumatic system is expected, the best policy is to add up the CFMs of all of the pneumatic tools used. You should then compare the total to what is available from the air compressor driving them.
Ideally, the air compressor should be providing at least 150 percent of the required CFM. That way, there will always be plenty of reserve power in the system.
For home use, you do not have to invest in quite so much reserve power. But you still need to compare the numbers to ensure that you can use the tools you want to, as often as you want to, without exceeding the compressor’s duty cycle.
Here’s an example… Your air nailer needs 4 to 6 CFM of compressed air flow at a certain pressure to operate correctly. Most nailers and air tools require between 60 and 100 PSI. The stated air consumption for the nailer is likely based on the air tool running continuously. If you need to use the air nailer continuously, you will need a compressor that will continuously deliver at least 4 to 6 CFM. But with air nailers, spot use is more prevalent than continuous use.
For more information on how to size an air compressor please check out our article covering this topic.
What Does the Volumetric Flow Rate for Air Compressors Mean?
The volume flow rate is defined as the amount of fluid that passes through a link in an air compressor at any given moment. Think about the volume of air flowing between different components within an air compressor system in cubic feet per minute it takes for this newly pressurized air to travel from the tank to the end point application.
In air compressors, volume flow rate is very important because it gives you an indication of the amount of pressurized air that can pass per second through the air compressor and it’s attached air tools. This volume flow rate will indicate whether or not it is possible to deliver a certain volume of air to your end point application in a split second.
Length of pipe, number of attachments, setup and leaks are just a few of the factors that can affect the volumetric flow rate.
What Does the Pressure for an Air Compressor Mean?
The volumetric flow rate is not the only measurement you’ll need to pin down with your air compressor. It’s also important to understand the pressure, PSI, or pounds per square inch.
PSI refers to the amount of air pressure being delivered through an air compressor per inch. The vast majority of pneumatic tools require PSI ratings in the range of 40 to 90 psi to ensure successful operation. The higher the pressure, the more power the tool has to operate. By clarifying how much pressure and SCFM you need for your application or air tools, you can then establish the pressure and SCFM specification for your air compressor. For more on this, see my article on what size air compressor do I need?
PSI and CFM have an inverse relationship… meaning as there’s an increase in pressure, the CFM decreases and vice versa. This is true at the OUTLET of the compressor tank – but not necessarily true for the compressor pump itself. See my detailed article about increasing the CFM of your compressor to understand the difference.
When figuring out the total required CFM and PSI needed for a project, it’s important to keep in mind the compressor’s duty cycle. Many smaller compressors are not built to run continuously. Therefore, it cannot continuously deliver compressed airflow at the specified pressure or volumetric flow rate on a non-stop basis.
Below is a comparison table between SCFM and CFM drawn from the information provided in this article.
|Name||Standard Cubic Feet per Minute||Cubic Feet per Minute|
|Standards||Standard temperature, pressure and volume at which SCFM must be calculated||No standard conditions at which CFM must be calculated|
|Calculation||To calculate SCFM values, the air is expandad to its standard condition of around 14.7 PSI (atmospheric pressure)||To calculate CFM values, the pressure is maintained at or below 90 PSI|
|Measurement||SCFM is always a bigger number than CFM because it has a larger air volume due to being calculated at a lower pressure||CFM is always a smaller number than SCFM because it has a smaller air volume due to being calculated at a higher pressure|
|Relevance||SCFM is more relevant of how the pressurized air at a standard value will perform at the air tool||CFM is more relevant to the internal capacity of the air compressor|
Do you have a question about the difference between SCFM and CFM or air flow rates? Please add your comment here (along with photos if applicable) to help others help you with your compressor and equipment problem!
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).”
Further Reading on SCFM and CFM
Here are a few other links that help explain the overall concept of SCFM and CFM.