Difference Between SCFM For
       Tools And Compressors

The questions what is the difference between SCFM for tools and compressors was asked recently by a visitor to this site.

The visitor went on to say, "most air nailer compressor combos have ratings on them. The labels say that the nailer requires 4-6 SCFM but the compressor is rated 3 SCFM at 40 PSI and 2 SCFM at 100 PSI. Are the measurements of air different between the two, or do you shoot a nail and wait for the compressor to catch up?"

My response For the record, to my way of thinking, it is SCFM (standard cubic feet per minute) of air that is pre-compressor, and I use CFM as the term for air once it has been compressed and is flowing out the compressor discharge coupler and down the air hose to my air tools.

One reason for this is that Standard Cubic Feet of air is measured at one atmosphere (14.7 PSI), a lot less than the pressure that is coming out of a compressor.

Here's a page on this site where you can get more info on SCFM and CFM if you wish to know more about these measurements of air.

Compressor Discharge Pressure

Your compressor specifications state that at a discharge pressure of 40 PSI, this compressor should continuously deliver 3 CFM of compressed air. This same compressor will deliver 2 CFM of compressed air at a pressure of 100 PSI.

I say should, because that compressor may not be rated for continuous duty, and if that is the case, you have to watch out for the compressor duty cycle. Many smaller compressors are not built to run continuously, therefore, it cannot continuously deliver compressed air flow at the specified pressure and flow rate, on a non-stop basis.

Your air nailer needs 4-6 CFM of compressed air flow at a certain pressure to operate properly. You did not indicate the nailer air pressure requirements, but I expect it is between 60 and 100 PSI range. The stated air consumption for the nailer is likely based on the air tool running continuously. So, if you need to use the air nailer continuously, you will need a compressor that will deliver at least 4-6 CFM continuously, though with air nailers, spot use is more prevalent than continuous use.

Undersized Air Compressor

So why does a store sell a combination unit in which the air compressor is quite likely undersized for the demand of the air tool they include in the kit?

A couple of reasons I believe. The first is that the compressor is the more expensive of the two components, so by including a smaller capacity air compressor in the combo kit, the cost is lower to the manufacturer. There is intense pricing competition for small, DIY type air compressors, at the retail level.

The second reason is that the target market for these combo units is typically the home DIY person, and the tools will most likely be used sporadically. The manufacturer does not expect that this user will be running the tool either commercially or continuously.

The way the compressor is sized, you will fire the air nailer a few times, and then the compressor will kick back in to deliver more air and you will wait for it to catch up. That is not acceptable in a production enviroment.

And, as long as the gauge pressure on the tank is at, or above, the minimum operating pressure of the nail gun, even though the compressor is still running, you can still nail.

If the output pressure drops below the MOP (minimum operating pressure) of the nailer, your nails might not seat properly.

If it is a big nailing job you have to watch out for the compressor duty cycle. You do not want to burn it out the air compressor.

Oh, and one other thing. Make sure you check the compressor oil regularly. A new compressor will use quite a bit of oil on break in.

Hope this helps!

And this additional information came in from an anonymous contributor:

SCFM is rated at standard conditions. The Compressed Air & Gas Institute (CAGI) and the American Society of Mechanical Engineers (ASME) standards, which are 14.7 PSIA, 68 Deg. F and 36% relative humidity...and you must be at sea level too.

Unless the Air you're compressing is exactly this, you're getting less real CFM that rated SCFM....

You won't be nailing at the maximum capacity, and hard woods will present a problem.

You need a bigger compressor, or lower rated tool.