How to figure the CFM of a air compressor

Here's a method published by Ingersol Rand on estimating compressor capacity based on tank pump up time. It's a pdf file. Just be sure to use absolute pressures (gage pressure+14.7) when using the method.

For your particular case, I would run the calculation between 100 psig and 160 psig to get a better idea of the capacity at useful pressures. The reason is that the capacity will always drop off slightly as the discharge pressure increases. The somewhat higher capacity at 40 or 50 psig and less is not particularly helpful in most cases. If you run it from tank empty to 160 psig, you will be getting the average overall that includes the higher capacity in that lower pressure area.

Rod

Ryan

Kind of unusual to have one rated at 0 psig (free delivery). That's a good sized compressor. I am assuming an engine driven tow behind?

I also assume that it is a reciprocating machine perhaps without any significant storage tank that you could utilize for a pump up test. The reason that recips have a higher air delivery at lower pressures is due to the variation of volumetric efficiency with discharge pressure. It decreases with increasing pressure. That's the reason you see many smaller compressors rated at xxcfm@40psi and yycfm@90psi. The rating at 40 will always be a little higher than the one at 90. This characteristic is more pronounced with single stage than with multi-stage compressors.

My guess (only a guess) would be that your compressor (probably single stage) would deliver around 500 or so cfm at 110psig if it is rated at 560cfm@zero. If it's a two stage, then you should expect better than 500. There are a number of variables so without a specific manufacturer's rating or some kind of test, it is only approximate to estimate the capacity at different pressures. You know the general trend but not all the specifics. That's some sanding job you got there if you need that much air :o).

Rod

Ryan

Forget volumetric efficiency. I assumed that you were talking a recriprocating machine. You apparently have a centrifugal compressor...Joy is a major manufacturer of such machines. Centrifugals and screws operate on entirely diffferent principles than recips. The term volumetric efficiency is, as far as I know, only appropriate for recips. It is a function primarily of the cylinder clearance volume, the compression ratio and some other ill-defined variables such as internal leakage, valve pressure drop, etc. I am not familiar with the technical details of centrifugals and screws so I can't help you out much there. What little I know of centrifugals is that they require rather complex capacity controls to maintain stability and that the constant speed capacity (cfm) can drop off quite dramatically with increasing discharge pressure.

If you are just curious on the volumetric efficiency (VE) of recips, here's the formula:

VE = 100 - c [r^(1/k) - 1] - L

VE in percent
c = cylinder clearance volume % = total vol minus piston displacement
r = compression ratio - varies with discharge pressure
k = ratio of specific heats, generally taken as 1.4 for air
L = "fudge factor" percentage to account for the ill-defined variables noted above

Once you define a number for the VE, you get the compressor capacity by the following (recip only):

CFM = PD x RPM x VE

PD = piston displacement (single stage = total disp, multi stage = 1st stage disp only)

It is generally best to use the manufacturer's rating point(s) and talk only in generalities and "estimates" when you get away from those data points. Of course there is no substitute for actual test data over a wide range of operation. Unlike with recips though, manufacturers usually publish entire performance curves over the stable capacity range for centrifugals. You would really need to have such specific performance curve(s) for the machine to get a decent handle on the numbers involved. You should be able to get something from Joy, given the nameplate data...at least catalog predicted performance if not actual certified performance.

Rod

3Yc, like all Westinghouse compressors, is a very tough machine. It doesn't look like it, but it is pressure lubricated by a plunger pump off the center of the crankshaft.