Taking The Mystery Out Of "CFM"

by Rod Thompson – aka Rod (NH)

 

Ever wonder how to properly match your air tools with your air compressor? Thinking of buying a new compressor or new tools? Confused? If so, you are not alone. Ratings seem to be all over the place with cfm, acfm, scfm, average cfm, average scfm, cfm at xy psi, and so on. What do they all mean and how does one use them to avoid being disappointed after purchasing expensive tools or a new compressor? It can be confusing and even misleading, depending on how the information is presented by the manufacturer or distributor of the equipment.

In the paragraphs that follow I will try to explain some of the variables so that at least part of the confusion will be eliminated and one can make a more intelligent judgment about air compressors and air-powered tools for the home shop.

First rule of thumb: Never, never, NEVER compare air compressors on the basis of horsepower, especially the current crop of popular five hp machines. They are NOT all the same…that is why you see the price variation in the 5 hp category go from under $400 all the way up to about $1800 or more. Performance usually comes with a price and that certainly is true in this particular arena.

Air is a compressible fluid and can occupy different volumes depending on its’ temperature and pressure. Air at sea level has an atmospheric pressure of zero pounds per square inch as measured by a pressure gage (psig) and 14.7 pounds per square inch on the absolute scale (psia). Psia equals zero only in a perfect vacuum. Air flow is defined in terms of cubic feet per minute or CFM. The term CFM and any numbers associated with it are meaningless unless accompanied by a reference to the temperature and pressure at which the CFM is measured or stated.

One common industry standard here that you should be aware of is the use of SCFM. SCFM stands for standard cubic feet per minute. It represents air flow in CFM at standard conditions of temperature and pressure. The standard for temperature is 60 degrees F. The standard for pressure is atmospheric or zero psig (at sea level elevation). Therefore whenever you hear the term SCFM, it means air flow volume referenced to zero psig and 60 deg F…assuming the term is properly used.

Reciprocating air compressors are essentially constant volume machines at any fixed speed and discharge pressure. They suck in a volume of air through the intake valve prior to compressing and discharging that same air at a higher pressure (and temperature) through the discharge valve and into the receiver or storage tank. Air compressors are properly rated in terms of intake cfm…or more specifically, ICFM…which references the actual pressure and temperature at the intake flange of the machine. The term "free air cfm" is sometimes used in place of ICFM. It is the same as ICFM except in the instance where the addition of inlet piping and inlet filters restrict the airflow to the compressor head. This rating is a function of the volumetric displacement and efficiency of the machine as well as the speed…and is therefore independent of the location.

I have never seen this particular term used in small home shop compressors but it is common, actually necessary, in large industrial machines in the several hundred or several thousand horsepower category. The reason for this is that the compressor rating is a constant (only one rating in ICFM) but the SCFM performance is different depending on the physical location of the machine. The exact same compressor located on Denver, CO will have a lower SCFM performance than if it were located in New York, NY…due to the lower atmospheric (barometric) pressure at Denver’s much higher elevation above sea level. Similarly that identical compressor will have a lower SCFM performance if it is located where the ambient temperature exceeds 60 deg F…due to the higher temperature.

Fortunately though, the deviations in temperature and atmospheric pressure from standard conditions in many locations represent only small corrections to the air flow and the distinction between ICFM and SCFM is not commonly made. The term ICFM is sometimes replaced by ACFM for "actual" cfm so be aware. It means the same thing though…cfm at the stated reference conditions existing at the intake flange of the machine. Careful though. I have seen the term ACFM used to imply flow at the compressor discharge conditions. That usage is grossly incorrect and can lead to major discrepancies.

You will sometimes see compressors listed in terms of piston displacement or displacement cfm. No reciprocating compressor can achieve a throughput equal to the piston displacement. The delivery is always LESS than the displacement because of the volumetric efficiency of the machine. Typically the volumetric efficiency of reciprocating machines is around 80% but varies with the actual construction and decreases with increases in discharge pressure on the same machine. The percentage represents the ratio of ICFM to displacement cfm. If you know the bore, stroke and RPM of your compressor, you can calculate the displacement cfm. For single stage compressors, use the total displacement of all pistons. For two stage compressors, use the displacement of the first stage ONLY. The formula looks like this:

Displaced cfm = bore area x stroke x rpm/1728

Where bore area = 3.14 x piston diameter x piston diameter, in square inches and stroke is in inches

Air compressors are properly rated, then, on conditions at the intake and the performance is commonly expressed as SCFM or simply CFM instead of the more precise and correct ICFM or ACFM. Chalk it up to laziness, typos, simple imprecision or maketing ignorance…you pick. You will often see air compressor ratings expressed as different CFM at different psi for the same machine. For example, 8.6 CFM at 40 psi, 6.4 CFM at 90 psi, etc. This is STILL ICFM (or loosely SCFM) even though the S, I or A may be dropped from the designation. The flow is at the intake and the referenced pressure, in this instance only, is someplace else…at the compressor discharge. The reason the flow decreases due to increasing discharge pressure for the same compressor is due to the decrease in volumetric efficiency with increasing pressure…characteristic of all reciprocating machines. The clearance volume in the cylinder at piston top dead center is a constant and the air trapped therein expands back into the cylinder during the intake stroke. This air takes up space in the cylinder and decreases the space available for air being sucked in through the intake valve. The higher the discharge pressure, the more space is taken up by this air as it expands and the throughput of the compressor is decreased.

Second rule of thumb: Air compressors are properly rated in terms of ICFM or, more loosely, SCFM and NOT plain old unqualified CFM…which could mean displacement cfm if one wants to be really misleading. This is the ONLY way performance of different air compressors can be compared on any kind of sound technical basis. This is also an important distinction that will become apparent after the explanation of tool ratings.

I hope you are with me at this point because it gets worse with tool ratings!

Not unlike air compressors, air-powered tools are rated on compressed air conditions at their inlet…that is, the point at the throttle valve of the tool. Here again, the rating can be stated as CFM, ACFM, SCFM, Average CFM, Average SCFM, CFM at xy psi, etc.

It is critically important to note that the conditions at the tool throttle are greatly different than at the inlet to the compressor. An industry standard of 90 psig at the tool throttle is common. I have seen the air consumption of some autobody type sanders specifically referenced to 60 psig at the throttle but that is an exception. If there is no stated pressure in the tool rating, you should assume it is the standard 90 psig. I can’t recall ever seeing any specific temperature in the rating of air tools, so unless stated otherwise, you should assume it is 60 deg F. Unless it is very specifically stated as SCFM (and it generally is not) you should take the tool rating to mean CFM referenced to 90 psig and 60 deg F. This number needs to be converted to SCFM in order to be compared (loosely) with compressor capacity in the proper manner.

For the mathematically curious, here is the method to convert to SCFM:

SCFM = CFM x [520/(T+460)] x [(P+14.7)/14.7]

Where T=temperature in deg F and P=pressure in psig

Example: For a common 60 deg F, 4 CFM at 90 psig throttle pressure is equivalent to 28 SCFM

Surprise! That low number of "4 CFM at 90 psig" stated on the package is really a whopping 28 SCFM that your compressor may "see". What you thought was well within your "6.4 CFM at 90 psig" compressor rating is NO LONGER SO and you run out of air, even with your compressor running continuously, when this tool is used for any reasonable duration. This is especially true if you attempt to run such a tool at the most productive operating point, the rated throttle pressure of 90 psig. The 28 SCFM is well beyond the sustainable capacity of even the most expensive true five horsepower compressor. It would take about a 10HP compressor installation to run this tool at rated conditions continuously.

For an example of this, see the following specification sheet for one of Dynabrade’s automotive pneumatic sanders. Dynabrade is one of the very few pneumatic tool manufacturers to provide this level of detail relative to tool air consumption.

Further complicating the matter of tool air consumption is the use of the word "average" in some ratings. You should be extremely cautious when you see the term "average consumption = 4 cfm" or something to that effect. In the marketing of pneumatic tools to the DIY home shop market, typically having compressors in the 4-10 SCFM range, it is advantageous for the manufacturer, seller or reseller to advertise the tool at the lowest possible consumption by using an unspecified duty cycle to modify the continuous rating downward. A similar downgrading of rated air consumption can also be achieved by running the tool at an unspecified throttle pressure less than the industry standard of 90 psig. Unless it is very specifically defined, you have no way of knowing what "average" really means…and it is very unlikely that any sales personnel are going to be able to adequately explain it to you. Buyer beware.

For example, consider the above "4 CFM at 90 psig" tool used for only one minute out of every three. In this case the tool would be operated for one minute and then be stopped for two minutes, then it would be operated for one minute again and so on. The "average" air consumption of this tool is now only "1.3 CFM at 90 psig". Get the picture? This 1.3 CFM average still is the same tool requiring 28 SCFM at any particular instant...or an average of 9.3 SCFM if used only one minute out if every three. Is this all misleading? Yes it is, but it’s not untrue.

Typically, for the home shop, tools such as disc sanders, grinders and sandblasters are the ones that notoriously overcome the compressor’s ability to keep up unless they are used on a very intermittent basis. Paint spray guns are not quite as bad since they have somewhat of an intermittent use by nature due to triggering the gun on and off. Plus, if pressed, you can always spray vehicles a panel at a time rather than all at once in order to best utilize the reserve capacity in your storage tank between the compressor cut-in and cut-out settings. You can, indeed, exceed your compressor’s capacity for short periods this way…but that period will be fairly short unless you have a relatively large storage tank.

Third rule of thumb: Pneumatic tools are rated for air conditions at the throttle inlet to the tool. Make sure, as much as you can, that the rating is expressed as SCFM on a continuous basis when comparing tool demand with the capability of your compressor. Otherwise, you may have to make unwanted compromises in your use of the tool. If at all possible, compare the SCFM requirement of the tool to the SCFM capacity of the compressor. A tool requirement that exceeds the compressor capacity can only be used on a part-time basis, not continuously. The duration of the part-time is a function of the size of the air storage tank and the settings of the pressure switch controlling the compressor motor.

It's not always necessary to operate tools at the manufacturer’s rated conditions. For example, a die grinder will work when the throttle pressure is less than 90 psig. The speed will be less and the air consumption will be less, but it will still work, at least to some extent. But it won’t have the power and performance that it was designed for and is capable of. In fact, I would wager a bet that most home shop tools are NOT operated at their rated conditions because of the hidden pressure drops that are unavoidable between a wall mounted station regulator and the inlet to the tool itself. Air hose, fittings and quick couplers all work to make the pressure at the tool less than is registered on a pressure gage back at the station regulator. This difference can be significant. A 20 psig drop or more is not uncommon. Anyone who disbelieves this should install a pressure gage right at the tool inlet and check things out. Operate the tool and notice the difference between the readings at the regulator and the tool. Do this with the tool under a reasonable load if it is a sander or something similar. A check under no load will give a high free speed and is not accurate.

Ingersoll Rand has determined that tools operated at the rated 90 psig inlet rather than at a reduced 70 psig will have an increase of 37% in productive work. The air consumption will increase about 30% in going from 70 psig in to 90 psig at the tool inlet.

Fourth rule of thumb: If you think your pneumatic tools are not performing as well as you expected, install a pressure gage at the inlet and determine actual conditions while they are being used. You may be surprised at the result. Try increasing your regulator setting as necessary to get the proper pressure at the tool and see what a difference it makes. Remember, it doesn’t really matter what the pressure gage at the regulator shows. It’s the pressure at the tool inlet that counts.

Conclusion:

If you have a small home shop compressor, don’t expect more from it than it is capable of. If you are down around the 6 SCFM area, don’t bother with pneumatic sanders, etc. unless you want to do a lot of waiting for your compressor to catch up with you…and possibly overheating in the process. Or accept less than desired performance from the tool. Do what I did…buy a decent electric, variable speed, random orbital sander and a 4" or 4.5" electric angle grinder. You’ll be happier, more productive and your compressor will thank you for saving it for less demanding tasks.

Rod
Jun 2007

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