air compressor cfm

Jack345

Member
How is cfm figured? I have a 5hp compressor with 60 gal tank that provides 9.7 cfm @ 100 psi. Need to know what the cfm would be at 20, 25, 30 psi. Thinking about purchasing a hvlp spray gun & have not decided on which model. Thanks
 
Think a lot of that is wishful thinking on the part of ad writers. I use all kinds of stuff on my compressor. Sometimes have to pause to wait for compressor to catch up, but oh, well.
 
If you know the free air cfm you can go proportionately from there to the 9.7 @ 100. I'm not sure if the reduction of volume is linear but it should get you close. Estimate the free air: Displacement of pump X RPM. If it's a two stage pump use only the first stage displacement.
 
I don't know how they convert the figures but I do know a compressor that delivers 4cfm at 90 psi will keep up with me spraying paint. At 9.7 at 100 psi you have nothing to worry about.
 
Hi Mr. Jack345,
Compressor CFM will not be linear, especially on a single stage unit. As pressure builds, resistance to piston movement loads the motor, AMPs increase, RPM decreases, and CFM is reduced. The best way would be to measure it if you had one already (begin with 0 PSI and a stop watch with a known tank volume), if you are looking to purchase a unit, good manufacturers will state it on their documents pages or a call to tech support. The link I hope to post below has a couple of nice calculators, however, you will have to know inputs.

Typically, if a given unit will supply the needed CFM at 90PSI, it will do a little or a good bit more at lower pressures that one would typically use for paint. One issue that the solution may cost as much or more than the compressor (depending on need) is supplying dry and oil free air. However, there are many options to suit different needs but will almost always be something to contend with.

George

http://www.egnergy.com/calculator.html
egnergy website calculator for CFM and such
 
OK jack here is were to start. Several replys have already given you just about all you need to know. Never ever buy an oiless machine. Buy as much as you can afford. 240 power is more power efficient. I have several portable air compressors and four of them are Emglo units. Today the name is Jenny air piroducts. Cast iron pump blocks and will last a life time with air filter and oil changes.They also give very accurate CFM output and performance. Now true CFM is measured by how much air the pump is pulling In and how long it takes to pump it up to a specific pressure. Google [ Evaluating true horse power & CFM ratings of air compressors. True Tex]. Guy is a PHD and gives a pretty good lecture.
 
Everything you wanted to know. Just scroll down 3 or 4 sheets to the chart.

http://ingersollrandcompressedair.com/assets/images/manuals/Helpful%20Hints.pdf
 
Not to hijack this thread but I've got a "head-slapper" regarding air compressors:

We have an expensive Atlas Copco 15hp compressor that is way more than our small shop needs (over $10,000 new). Few months ago had a problem with it that resulted in a $500 service call due to a busted internal switch. Guy all but accused us of breaking it ourselves. (We've never had the cover off). Then he said the dryer's shot. Couple thousand to fix that. We said "no", we'll just run without one and we put an auto-drain on it.

Well, yesterday my machinist said the compressor is down, can't reset it, can't power off the control panel, nothing. Rather than dump more money into it, I went shopping online for a compressor this AM. Figured a 5hp 60 gal would serve our needs for under $1k. Most of the use is the auto-drawbar on the Bridgeport and a blowhose for clearing chips. Don't need 80+ cfm for that! Was about to pull the trigger on a purchase order when I decided to pop the panel on the Atlas to see if it was something simple. The panel "key" was hanging on something under a sticky note on the front panel. Lifting the sticky note, I saw an big RED E-stop button. Sure enough, I pulled the E-stop, and it fired right up. :oops: I had management primed to spend a couple thousand to put us back in business so I'm going to try and say that I saved us big bucks today :lol:
 

ggb3... educate this dummy... if your compressor it pumping straight into a tank with a check valve... isn't it only 'pushing' against the pressure in the line between the head the the check valve... ... would that be THAT much resistance to overcome for a 5hp compressor

Quincy tells me the pump makes 4 cu.ft. for each h.p. of the motor
i.e. 2 hp = 8 cf
optimum compressor speed is less than 900 rpm

Quincy says my # 310 compressor specs are

2-3 hp
3.5" bore
2" stroke
2-stage
my rpm is 840 (read on a tach)
ACFM at 175# 3.9/9.63 (depending on h.p. ??)

never tried sandblasting with it but figured it would do it with the 5 hp

anyway... my question about the compressor feeding thru a check valve... wouldn't it prevent such a pressure build up to limit cfm

thanks
John
 
The check valve is to prevent air from coming back from the tank to the compressor when the bleeder in the pressure switch opens and dumps the air in the line between the tank and compressor (the hiss you hear every time the compressor stops). That lets the compressor start without being under load. Once the compressor is up and running, it is pushing air through the check valve and into the tank, so it is effectively running against the tank pressure
 
(quoted from post at 22:56:54 01/27/17) The check valve is to prevent air from coming back from the tank to the compressor when the bleeder in the pressure switch opens and dumps the air in the line between the tank and compressor (the hiss you hear every time the compressor stops). That lets the compressor start without being under load. Once the compressor is up and running, it is pushing air through the check valve and into the tank, so it is effectively running against the tank pressure
should have said unloader valve rather than bleeder. sorry.
 
Actually there is a differential pressure constant but it isn't much. It's the result of overcoming the spring pressure in the valve that makes it
shut when you are finished filling; maybe a # or 2 if that much. Funny on that valve. I put in the first in my entire life just a couple of weeks
ago. Tank is advertised as 6.5 hp (really 4) 60 gallon tank. Bleed off had been getting longer and longer. About $6 on the www.
 

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