Anyone know how to calculate air compressor CFM easily?

[darkostoj]

I got this older 3phase 4 cyl compressor with a 7.5 hp I just got up and running.

I'm just curious if there was a way to measure CFM output? Wondering how this thing stacks up to other compressors out there in case I something better comes along eventually.

I was thinking if I know the gallon tank size, starting pressure (0), and ending pressure how many seconds it takes to fill could be converted possibly?

I do know that if I leave a 1/4 air hose open with no restriction the compressor stays running and maintains 120psi

 

[PCustoms]

Posted yesterday

Dumb compressor question o' the day

We've been worried about the health of our IR UP6-5-125 5 hp rotary screw comp w/80 gallon reservoir. Either we have some serious leaks around the brewery that I can't find, the comp is not outputting enough air--but used to--or it's something wrong with our canning line that keeps shutting down...

www.garagejournal.com

 

[andyvh1959]

so no label or plaque on the compressor pump itself stating the output per rpm?

 

[darkostoj]

so no label or plaque on the compressor pump itself stating the output per rpm?

I was able to find a math formula which used the time to fill the tank, the total volume, and the finishing psi. I worked it out to 29cfm @ 160psi. Took 3:08 to fill a 60 gallon tank to 160psi. I also ran it without the air filters and it did it 3 seconds faster if anyone was curious??

I'm pretty happy with that, seems to be worth the time invested and about $500 in a VFD, fittings, belts, valves, etc

 

[vga]

Tank vol/gal x 0.134 x max psi op range / div by pump up time

 

[mm08822]

Make sure your calculations are using the correct units, eg.: temp in Rankin or kelvin, absolute pressure.

 

[nadogail]

Displacement of each cylinder X number of Cylinders X RPM, should get you in the ballpark.

 

[johninct]

I am just curious on how the VFD is working with the compressor, start/stop?

 

[darkostoj]

I am just curious on how the VFD is working with the compressor, start/stop?

I just used the factor pressure switch that opens/closes contact. Then wired those 2 wires to the FWD and GND on the VFD and configured it so when those two wires have continuity motor runs and when it breaks it shuts off. I started messing with getting a 4-20mA sensor for a PID and all that but it seemed unneccessary and also since it's a splash lube compressor having it run less than spec RPM seemed no smart.
The compressor calls for 700 rpm, but the factory pullies on the setup were only turning it at 400rpm at 60hz (1740 motor rpm). I brought the Hz up to 80 and the motor to around 2100 rpm which put the compressor around 620rpm. Still a little lower than spec, but anything above that was tripping the VFD so I just left it since the jump to a VFD that is higher than 10hp (for my 7.5hp motor) gets a lot more expensive. I have it setup with a slow start too. I'm pretty happy with it, the difference between a 15cfm and 28cfm compressor is really huge for me and worth the effort