How can I Esctimate Air Comp CFM for Vacuum Pump?

[Doug Hobkirk]

I have an old air compressor - maybe 5-10 gallons, large belt (rides flat - no V or lip on pulley!), runs very quietly. I bought it used and it have no specifications.

I am considering trying a Joe Woodworker venturi style vacuum pump. His two models requiring 2.1 and 4.8 CFM or compressed air at 80 PSI.

My pop-off is set to around 80PSI. How can I estimate how much CFM it generates?

[I doubt it will meet the requirements, but I've often wondered how I could test this or get an approximate idea.]

[Cliff Rohrabacher]

In a phrase "you can't"
Not unless you are going to calculate:
1.) the geometry of the venturi (YAh every aspect of the geometry of the venturi is really important)
2.) the port or tubing under the venturi for dynamic flow in the viscous regimes
3.) the flow of air over the venturi (CFM and speed and pressure)

You have a lot of math ahead of you. And that's just the minimum you have to do.

However you can do it the easy way.
1.) rig a slant tube monometer to a chamber.
2.) run what ever you have as a venturi with all the equipment etc you plan to drive it with.
3.) draw vacuum on that chamber
4.) Measure the vacuum you draw on the monometer.

Then make adjustments where ever it seems most logical to you to tune it up.

[Jamie Buxton]

I think Cliff is trying to answer a question which Doug didn't ask -- which is what the performance characteristics of the venturi vacuum pump are. That's not at question. The vendor says what it needs is 2.1 or 4.5 cfm at 80 psi.

Doug, the size of the compressor's tank has nothing to do with how many cfm it can produce. The cfm is set by the motor power and the design on the air-compressor itself. I'd start by looking at the specs on new machines. I'd look for ones with the same horsepower as the motor on your machine. You should be able to see a horsepower spec on a nameplate riveted to the motor. With a pop-off set for 80 psi, yours is probably a single-stage compressor.

[David DeCristoforo]

"...I'd look for ones with the same horsepower as the motor on your machine..."

You might also try an internet search (Google?) for the make and model of the compressor. I have found specs for pretty old stuff archived on various mfgr web sites. Also this: http://www.truetex.com/aircompressors.htm might be helpful...

YM

[Jason Beam]

Or ... Make a 12" cube out of lexan and stick a balloon in it. Then time how long it takes to fill the box! :P

(Mostly joking, but there is some truth to it)

[Brad Shipton]

I contemplated the air powered version from Vacupress, but ended up going with their standard system since my air compressor was under powered. Darryl from Vacupress told me their 3cfm air model would generally require a 1.5hp compressor and the 5cfm version needed 3hp or more to work reliably. My old 5gal John Deere air comp was only 1hp.

Brad