Increase SCFM
       Compressor Flow


This writer wants to know how to increase SCFM of their air compressor as the output is not enough for spray painting purposes.

The question: I understand that the rate of SCFM depends on many factors (temp, humidity, etc). However, I am trying to drive a Car Paint gun that states 8 SCFM rating. My compressor runs at 5.7 SCFM @ 40 PSI.

Now, I could buy a new compressor to increase that, but I don't really do this task very much, and was curious if I could do any of the following to increase SCFM?

Run two compressors, borrow my neighbors and connect them to a T and drive a mainline out that would connect to the filters and the gun?

Or could I connect a 10 Gallon tank that is connected from my main compressor and connects to the spare tank. I would set the main compressor to 90PSI, and put a regulator on the spare tank for 40PSI which the gun calls for?

Or if I am not thinking right at all please let me know what might make the most cost effective solution.

My response: Hi Michael: Thanks for your question. You say your paint gun states "8 SCFM" at 40 PSI.

Just FYI, to my mind and definition, compressed air isn't SCFM, it's CFM, since I understant that SCFM refers pre-compressor Standard Cubic Feet per Minute. But besides that....

Your present compressor will give you 5.7 CFM at 40 PSI. What pressure and flow will your neighbor's compressor provide? Is it big enough to give you all the flow you need at the 40 PSI?

You can certainly run two compressors to the same reservoir. As you point out, you'll need to adjust the cut out and cut in pressures of the two compressors to have them work properly.

Having an extra air reservoir that fills when the paint gun isn't running will be helpful. It's hard to say how long you can paint with the air from that reservoir before you empty it and your compressor supply pressure drops to cut in and the compressor has to run again to try and fill the tank.

Michael's Response: Thanks a lot for the great information, you are right, I did not say but I can't run the guns currently because I don't have enough SCFM.

About putting the T in the system:

1.) Would I just use a common 1/4 NPT fitting and run both compressors to it?

1.a.) In regards to the pressure switches, that would be a concern only if I did this as a full time thing, but I could just set both compressors to 90PSI and let them run when the air is low correct? I just need to paint parts of the car about 1 - 3 hours tops before shutting down.

2.) Should I have an additional tank that receives the air from both tanks then draw air from it. I would assume I could just punch a hole in the tank, thread the fitting and use a T to input air, then use a regulator to set the PSI out of the spare tank?

3.) Would it make any sense to punch a hole in my current compressors tank towards the bottom, and thread it for an NPT coupling through which another compressor would connect. This would fill the tank on top of the other compressor. I am not sure if threading new holes in the side of tanks is safe since the metal seams thinner compared to the fittings that are in it from the factory.

Thanks again for all the help.

Another visitor commented: Michael, your little compressor can not obviously compress air at the rate the paint gun can consume it. So how do you run your paint gun then?

You don't mention it, but I'm thinking that you have a compressor that fills an acumulator tank, and that would be your available air supply. The volume of the tank is your CF (cubic Feet of storage). Depending on the size of the main tank, you will be able to use the paint gun at 5.7 SCFM until the tank was drawn down to the "cut-in" pressure (when the compressor would automatically turn on). At that point, you might as well take a break, let the compressor re-fill the tank and then rest (cool off and let the moisture condense).

By adding a second tank into the system, you have increased the system storage capacity (@ 90 psi), and will be able to run the paint gun for a longer interval. Put a regulator between the last air tank and the gun, set at the tool presure (40 psi). You will still face the same problem when the system capacity is drawn below what the compressor can produce.

I would definitely add a second tank, the larger the better. Just remember that it will take your compressor longer to fill the system, but more air volume will be available to your tools, and the compressor will not cycle as often.

If this doesn't give you enough "paint time", then borrow your good neighbor's compressor and "TEE" it into the system. The combined SCFM outputs of the compressors will run the paint gun, and fill up both tanks.

This is what I recently did in my shop when I added a second compressor (I posted it on this site). Because the compressor's were not identical in size and HP, the pressure switches had to be replaced with adjustable one's, so that they both came on and off at the same time. In your case, as this is not an every day project, you can manually play with the compressors to get them going at the same time.

I hope that this helped you.