Hey roger the compressor guru. and a cfm question

Thanks guys!1

Well, I'm not Roger but I'll try to address some of your questions. The term cfm means cubic feet per minute but without some qualifying statement, the term really is meaningless. This is because air is a compressible fluid and the volume is dependent on temperature and pressure. When manufacturer's rate their compressors in terms of just cfm they are being very sloppy with the terminology. What they really mean is cfm referred to the conditions of pressure and temperature existing at the actual intake opening of the machine. This is properly called intake cfm of icfm. It is also sometimes referred to as free air cfm.

Scfm is standard cubic feet per minute. It is the volume rate of flow when referred to standard conditions of temperature and pressure. In this country, that means 60 deg F and sea level atmospheric pressure of 14.7 psia. If you happen to have your compressor located at sea level on a mild temperature day, icfm is close to being the same as scfm providing you do not restrict the intake with piping or filters, especially dirty filters. If you happen to be located in Denver, CO for instance, the icfm is the same but your compressor rating (in terms of scfm) is degraded by about 20% all other conditions being the same. This is caused by the higher altitude at Denver.

Your rating of 4.5 cfm @ 100 psig means that the compressor will suck in 4.5 cubic feet per minute of "free surrounding air" when the discharge pressure is 100 psig. I assure you it does NOT mean 4.5 cubic feet of 100 psig air per minute at the discharge. There is a large difference.

All reciprocating compressors (I assume reciprocating here) have a volumetric efficiency characteristic that decreases with increasing discharge pressure. Conversely, this efficiency increases with decreasing discharge pressure. At 90 psig discharge, the compressor would suck in slightly more than 4.5 cfm of free air but not much more. At 40 psig it would be noticeably more but is not accurately predictable without detailed design/test data from the manufacturer. It could, for a guess, increase to say 5.5 cfm of free air at 40 psig but this is just a guess. You know one thing though; it can only approach and not reach or exceed the volumetric displacement of the machine at zero psig discharge. That would be bore area x stroke x rpm in the correct units.

Rod

Thanks

IF the volumetric efficiency did not change with discharge pressure the capacity would be 4.5 cfm at ALL discharge pressures, including 40 and 90 psig as well as 100. However it does change, but the extent to which it does is unknown without very detailed manufacturer information specific to the actual machine. That is why I estimated it to be 5.5 at 40. It probably would be in the area of 4.6 or 4.7 at 90 since that is quite close to the 100 rating point of 4.5. These are both the inlet flows and the discharge flows since no air is lost or gained in the process. The reference point for both is temperature and pressure existing at the inlet. I can't stress this enough.

If you wanted to know the actual conditions existing at the discharge of that same compressor (with standard conditions at the inlet) they would be 100 psig, 475 deg F (assuming no cylinder cooling) and 1.04 actual cfm at those local conditions. If the temperature were reduced to 60 deg F in a perfect aftercooler with no pressure drop, that same cfm would now be 0.58. You can see that there is a big difference in cfm for the same flow, depending on the reference conditions. In this case, with perfect cooling, 4.5 scfm goes in and 0.58 acfm comes out. The 0.58 acfm is the SAME as 4.5 scfm for these conditions.

All this doesn't really help you out with your flows at other discharge pressures though since that requires that the volumetric efficiency change of the particular machine be defined...and it is not.

For some discussion on cfm terminology, see this link.

Rod