# Air Compressors

1. Sep 8, 2010

### jimff

I have an air compressor rated for 3.7 cfm @ 90 psi. what would the cfm be @ 40 psi? Is there a simple formula I can use?

2. Sep 8, 2010

### Q_Goest

I have to believe this is a recip, so CFM doesn't increase much at the lower pressure. Might go up a bit because of various factors but not much.

3. Sep 9, 2010

### Jobrag

Carry on running at 90 PSI and drop the pressure down through a regulator to 40 PSI and you should get about (slightly less then) twice the volume after the regulator. Very very rough calculation using Boyles Law and assuming that the temperature doesn't alter much.

Last edited: Sep 9, 2010
4. Sep 9, 2010

### RonL

You didn't provide rpm so I could not get the volume of the compressor.
The shade tree method I use would be,....Assume 14.7 psi atmosphere, divide that into 90 psi for a compression ratio of 6.122.
Divide 90 psi by 40 psi to get 2.25, then multiply by 3.7 cfm to get flow volume of 8.325 cfm at 40 psi.

I'm surprised by one answer above.

Ron

5. Sep 9, 2010

### Q_Goest

Manufacturers of compressors provide a CFM rating which denotes the displacement, nothing more. The inlet pressure for this application is assumed constant, so regardless of discharge pressure, the inlet density times CFM determines the flow. The primary factors that reduce this capacity are void volume (volume of gas left inside the cylinder at the end of a piston stroke) and the piston ring blowby which should both be relatively small contributors to the reduction in flow. Being this is assumed to be a constant displacement machine, the flow doesn't decrease much as discharge pressure drops. Only an increase/decrease in inlet density would cause a signficant change in flow rate.

6. Sep 9, 2010

### RonL

Guess we said the same thing in different ways, so a free flow of air at no compression, will be 22.65 cfm at whatever rpm the 3.7 cfm@ 90 psi was taken.

7. Sep 9, 2010

### Jobrag

Is the cfm value taken as actual or standard?

8. Sep 9, 2010

### Q_Goest

Hi Ron. I understand your interpretation, but that's not what is typically meant for the small air compressor market (or even fairly large air compressors). When manufacturers talk about CFM, they aren't refering to the CFM at 90 psi which is what I believe you're suggesting. They're refering to "free air flow" as it's sometimes called. That's basically just the actual displacement of the machine. If they quote the CFM at a higher pressure, the machine will generally have a higher flow at lower pressure, but that has to do with gas that's re-expanding and leakage past piston rings. For a machine with a free air flow of 3.7 CFM at 90 psig, it may increase to 4 or slightly more CFM at 40 psig, but that's because the machine loses capacity due to leakage and recycle (void volume) as discharge pressure is increased.

Here's an example of a recip compressor similar to the one I'm assuming is being referenced by the OP.
http://www.gastmfg.com/pdf/piston/specsht/6h6l.pdf [Broken]

Note the graph of pressure versus free air flow near the bottom of the page. This is a 2 stage recip with a flow of about 3.5 CFM at 90 psig and 4.0 CFM at 40 psig.

CFM is actual. The compressor actually displaces X CFM, so you have to determine SCFM by comparing to actual conditions. If the temperature and pressure of the air being drawn into the compressor is at standard conditions, then the compressor is compressing that amount of air in SCFM. So for the 3.7 CFM compressor taken as the example, if the air going into the compressor is at standard conditions the flow is 3.7 SCFM. So it doesn't matter what the discharge pressure is; if the inlet conditions are standard, the flow in CFM is also SCFM.

Manufacturers do this to 'cover their a**' so to speak. They are just giving the displacement, and it is up to the user to determine what the inlet density is. The CFM is not CFM at the discharge pressure (ie: it is not 3.7 CFM at 90 psig and 70 F) it is 3.7 CFM at inlet pressure and temperature.

Last edited by a moderator: May 4, 2017
9. Sep 10, 2010

### RonL

Thanks Q_Goest,
When is being wrong a good thing ? When it brings out an answer that helps in giving a better understanding for the question ask.
I went to my shop and looked at the specs of two compressors purchased for a project I have been working on (very slowly), they are as follows.

5 HP, Air Compressor Pump
Air delivery: 1050 RPM
15.1 CFM @ 40 PSI
13.5 CFM @ 90 PSI
12.3 CFM @ 115 PSI
145-PSI max.

I lost sight of the fact that RPM is the most controlling factor for volume.

Thanks
Ron