Compressor output question

[NCarolina]

Hey All,

I am looking at a couple of compressors to replace the one I currently have. The two I am looking at have pretty similar rated output numbers at 90 psi, 6.5 and 6.1 cfm but pretty different numbers at 40 psi, 6.9 and 8.3 cfm. What aspect of a compressor would cause them to output pretty similar numbers at 90 psi but much different at 40 psi? I don't know too much about compressors since I've only ever used small ones for running small tools up until now.

 

[BukitCase]

Only thing that comes to mind (assuming same motor horsepower) is that maybe both are 2 CYLINDER compressors, but the one with higher volume at LOWER pressure could be a SINGLE STAGE 2 cylinder (both cylinders work in parallel, so twice the possible pumping volume) where the other one may be a 2 STAGE, twin cylinder - those have the cylinders plumbed in SERIES - the larger diameter one is first in line, that feeds "pre-compressed" air to the SECOND cylinder, which is smaller in diameter so it takes the same FORCE to move, but STARTS at greater than atmosphere, and pumps up from THAT.

Easy enough to tell if this is the case if you know what to look for - at first glance, both cylinders look the same size - but one of 'em will actually be a smaller BORE, but normally will have deeper cooling fins. Look at 'em from the side, you'll see more "air" with a smaller "solid" looking center part, this is the SECOND stage.

There are other ways to tell if this is the case, but that's a fairly easy way... Steve

 

[TRWham]

Three explanations come to mind:
1. Different testing methods. There really is no standard.
2. Typographical error in one set of specs.
3. A difference in re-expansion volume yields a real difference. Piston compressors suffer a small volume of air trapped in all the clearances within the cylinder that cannot be exhausted at the end of the compression stroke. That air re-expands in the cylinder before the pump can take in any air on the intake stroke. Higher discharge pressure means more re-expansion and this is why piston compressors produce less output at higher pressure. Reed valves will usually have more clearance volume than disc valves. A larger bore might also have a greater clearance volume.

 

[larry_g]

Educate yourself a bit here;

http://www.cagi.org/pdfs/CAGIAirCompressorHP.pdf

And yes, there are standards in place. However many tend to ignore them or interpret them in their own way.

lg
no neat sig line

 

[NCarolina]

Thanks for the info guys. Unfortunately I think I ended up with the wrong compressor after making some incorrect assumptions. I assumed if the specs showed similar output at 90psi that they would have similar outputs at 40 psi but that doesn't seem to be the case. Now I have a compressor that won't keep up with what i need. Guess you live and learn

 

[Citation]

I wouldn't assume that either would have been noticeably better than the other. You are just at the limit of a 120v compressor. What tool was too much?

 

[NCarolina]

Larry, thanks for the link there is some good info in there for me to bone up on.

Citation, the main tool I use on the compressor is a drywall hopper gun for shooting down decorative concrete overlays. It doesn't require much pressure but needs a lot of volume.

 

[Tallpilot]

Could adding a receiver tank help?

 

[Citation]

Larry, thanks for the link there is some good info in there for me to bone up on.

Citation, the main tool I use on the compressor is a drywall hopper gun for shooting down decorative concrete overlays. It doesn't require much pressure but needs a lot of volume.

Keep in mind that the pump doesn't see the regulated pressure, it sees the tank pressure. Thus if your tank pressure is at 90psi both compressors are delivering about the same peak air flow.

 

[The Cobbler]

I have a texture gun that I use with a compressor too. it's backwards in that a hoppers is low pressure, high volume, and a compressor is low volume high pressure.
any larger jobs I did I would bring a larger compressor, smaller jobs/repairs I used a hot dog compressor . I also put on a valve to shut off the air suppy so I could let the pressure build up .