Air compressor CFM ratings

says:

When a compressor pumps one "CFM" (cubic foot per minute), that means the intake port inhales one cubic foot of "free air" (air at atmospheric pressure, which is 0 psig) per minute. (Note: A CFM does not mean in any sense the compressed volume.)

Randy

All depends on what company made it... If it's a Sears then 3.2 CFM is what it would produce wired for 440 turning about 10K rpms and measured just before it blows apart from the stress...

If it's a good unit then it should mean the airflow at the exhaust port of the compressor head prior to any fittings, with the compressor running at specific rpms and usually measured with NO air inlet filtration.

Same method is used for a water pump except you have to add in any incoming pressure/vacuum as well.

Not correct. The capacity of an air compressor is measured at the compressor inlet with the compressor running at a specified speed and an open discharge port.

Centrifugal water pump capacity varies with the pressure difference between the inlet and outlet so a centrifugal pump is specified by a set of graphs ( pump curves) that show the flow rate along the X axis and pressure differance along the Y or vertical axis. They vay with the speed of the impeller. Positive didplacement pumps such as lobe pumps or "Mono"pumps will deliver a flow rate porportional to impeller speed into any pressure difference provided the drive has sufficient horsepower. Of course the maximum pressure is limited by the pressure rating of the system.

That would be the measurement of SCFM at atmospheric pressure. IE the amount of air that the compressor will pass at 14.7 psi. To get a CFM rating at a specified pressure you have to regulate the output of the compressor so that it actually develops that pressure. Then measure the airflow.

The problem is that it all depends on the manufacturer and which way they actually measure.

SCFM - The amount of air that the compressor will suck in at a set rpm with open ports at sea level pressure (14.7 psi) at 68 degrees temperature and 36% RH. Expressed as only a number IE- 25 SCFM

CFM@ XX PSI- The amount of air the compressor will pump at that pressure with 14.7 psi at the inlet. This is due to the volumetric efficiency losses from the increased pressures. Measured at the output of the compressor to eliminate the calculations of the VE losses inside the unit. These are the ones you see on most consumer rated units -

3.5 CFM @ 90 psi, 2.8 cfm @ 120 psi It is not a linear table though as the VE losses vary with temperatures and pressures

ACFM - This is measured in actual use. Take a known volume tank, pump air into it while measuring the time it takes to get to a set pressure. Use a nice equation to calculate out what the compressor is actually doing.

"Air gets denser as you compress it, less dense as you heat it. In order to provide accurate flow measurement to an end user or to provide an "apples to apples" comparison of flow, that variability has to be removed.

So a "standard" cubic foot was created based on a standard pressure and temperature.

Typically being 1 atmosphere and temperature of either 68F or

15C. In other words, 1 scf is the space that 1 cubic foot of air occupies at atmospheric pressure and a standard temperature. At 90 psig that same cubic foot takes up a lot less space than it did at atmospheric pressure so 1 cfm @ 90 psi contains 7 scf.(standard cubic feet)

In real life, cfm is the useful output rate and it is a measure of exactly how many cubic feet leave the compressor at the rated pressure in a minute. There is no regard for the density of that cubic foot or whether or not my compressors cubic foot contains as much air as your compressor. That's where scfm comes in - it equalizes the playing field, providing standardized correction to capacity claims at 110 psig versus those at 90 or 150 psig for instance.

In reality no one using compressed air to drive a tool really cares about scfm - we're all concerned about how many cfm at my required pressure can I get so I know I can drive this air wrench etc. As long as I get 30 cfm of 90 psi air and my wrench works I don't care how many scf that air contains."

And to test for rated flow you connect up the inlet to a known water source (for fire apparatus you are supposed to draft with no inlet pressure) Run the pump up to the correct rpm and measure the output flow. If all is well then you switch to the next rpm/pressure level and measure again. It is one of my least favorite things to do. Set up the test tank. Connect up the suction, put on hearing protection, start pumping water. Do your initial test. Then run the pump for an hour while testing every

15 minutes. Then move up to the next test. I will say that running some of the older rigs when your testing at the 250psi level can be interesting. With the engine doing all the work it does place a lot of strain on them. The old Detroits REALLY scream then.

We used to measure the old piston rigs the same way.

As far as air compressor ratings go, the only difference between CFM and SCFM is that SCFM attempts to standardize the inlet temp, pressure and humidity. CFM is still a measure of volume at more or less atmospheric pressure and temperature, not the volume of the pressurized air at the outlet. I suppose there may be exceptions for specialized pumps, but not for typical shop air compressors.

Tossing pumps for water, an incompressible fluid, into the discussion only confuse the matter.