SCFM

SCFM; this is one topic, over and above the compressors themselves, that draws a steady stream of visitors to this site.

I'm not too surprised. Even though I think I understand the difference between SCFM, CFM, ACFM and other acronyms that have to do with the measurement of air flow, my definition may not be the one that any specific compressor vendor is using to provide flow characteristics for their machine.

The definition of SCFM is suspect due to the changing parameters from different sources. And it's how that definition applies to sizing a compressor that complicates the issue with many folks.

Compressor discharge flow measurement is confusing and fraught with error for the person that's 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 most reputable have differing definitions of SCFM, and how that information is applied in a meaningful way for us compressed air users.

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 noted that, some other reputable sources indicate that the relative humidity of the air must be zero in order for it to be measured in SCFM. Another source says that the R.H. must be at 50%, for the flow to be rated as SCFM.

Few of us have plant locations exactly at sea level. 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's specified for an SCFM rating.

For a simple definition of SCFM, the acronym 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.

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 the compressor discharge to the receiver or to the plant air mains. Others will use the acronym ACFM (Actual CFM) as the measure for air from their compressors.

And some manufacturers 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 and hasn't a lot of impact on us when it comes to determining the compressed air flow needed to do work.

As a home user or industrial user of compressed air, why do you want to know the definition of SCFM?

Really, you don't. You do need to know how much flow and pressure a specific compressor will generate for you. This to ensure that your applications have enough compressed air flow and pressure to do their work! In most cases, that's all you care about.

When you first start to seek out a 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 manufacturer's web site for that information or to obtain a contact from whom you can obtain that data.

Remembering that in compressors over 10 HP, each HP of motor capacity will generate about 4 CFM of compressed air at 90 PSI, and now knowing that number of CFM and pressure required to run your air equipment, you can now seek out a compressor to deliver that.

When you find a compressor that suits, provide them with your requirements in CFM at the pressure you need. Let the compressor manufacturer guarantee to you that their recommended unit will provide that flow and pressure, regardless of what terms they use to describe the discharge from their compressor.

Note that a compressor will always show a higher discharge flow rate at a lower pressure than it will at a higher pressure and compressor manufacturers will often highlight a high flow rate and lowlight the CFM. Remember, it will be air flow at higher pressure that you will require for your application!

Make sure, when you are discussing your compressor size with your vendor, whether they measure the flow from their compressor in SCFM or CFM or ACFM or whatever, that they assure you that you will get the CFM you need at the pressure you need to do the job you want done.

Did you know that...

Air leaks can be found by listening for them, using the tried and true method of soapy water brushed onto joints, or via an ultrasonic leak detector. Ultrasonic leak detectors can find leaks even in a noisy plant environment, and by using ultrasonics, you don't have to worry about cleaning up the soapy water when you're done testing!

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