How to estimate air compressor CFM?

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Old 07-20-11, 07:01 AM
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How to estimate air compressor CFM?

I got a compressor, the chinese type, which is supposedly 2.5HP, with a 6 gal tank, with a max PSI of 125. The restart PSI value is about 80. And the label say 8 CFM.

I'm using it to paint, with a spray gun which is rated to use 4 SCFM.
Well, the compressor barely keeps up. It turns on at about 80 PSI, and from that, it doesn't fill again until a couple of minutes after I stop using the spraygun.

So, the label is lying or my compressor is broken. I bet more for a simple misleading label.

Is there a way to estimate based on HP and Tank size and pressure how much air, in CFM, a compressor could provide?
 
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Old 07-20-11, 07:20 AM
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Welcome to the forums

Not sure what the electricity available in Mexico is but here most household receptacles are 120 volts with the occasional 240. Typically, a 120 v air compressor will not be able to pump enough air to handle a paint sprayer, the capacity for that comes with the bigger 240 volt units.
 
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Old 07-20-11, 09:04 AM
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Manufacturers of air compressors and pneumatic tools lie worse than used car salespeople. The horsepower ratings they assign to compressors are vastly inflated and the air consumption of the tools is vastly under rated.

Compressors actually have two CFM ratings; displacement, which is simply the cylinder volume multiplied by the number of working strokes per minute and delivered air which takes into account the volumetric efficiency of the compressor. Delivered air will always be less than displacement and with single stage compressors it will always be more at lower pressures. If you could get the real horsepower figures you would find that a single stage compressor will deliver about 3-4 CFM per horsepower. To get closer to the true horsepower reading you can multiply the voltage by the current and then divide by 1000, this isn't exact but it will get you in the ball park.

Pneumatic tools are rated in average CFM consumption rather than actual consumption. The average consumption takes into account that the tool is rarely used continuously and the rating is then calculated back to a cubic feet per minute rating. An impact wrench will typically be used only for a few seconds per minute so receives a rating of 4 CFM average consumption but the actual consumption during the time it is running is closer to 30 CFM. Spray guns, other than really small detail guns or air brushes, take around 10 to 20 CFM while the trigger is pulled, the manufacturers are hoping that you only spray in short bursts with a significant time in between bursts when they advertise anything lower in consumption.

Remember also that all filters, regulators and hoses MUST be sized for the actual instantaneous air flow and not to the average air flow. The single biggest problem with pneumatic tools is caused by using too small of hose or filters and regulators.
 
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Old 07-20-11, 09:13 AM
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While the air pump and motor might be marginal for spraying, you really need a bigger tank!
Paint guns have some of the highest cfm usage and unlike sanders/grinders a paint gun won't work with reduced air. I'm a little surprised at the guns low cfm rating - most are 2-3 times that amount.

I have done some painting with a 1 hp 11 gallon tank compressor although a 20 gallon tank or larger would be better. A few things you can do to possibly get by with your smaller compressor is to thin the paint a little extra [make sure you allow for this so you don't get runs on what you are painting] the other is to spray in smaller sections with frequent breaks to allow the air compressor to catch up.
 
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Old 07-20-11, 10:24 AM
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I have found the inexpensive & compact oil-less compressors are useless for most jobs that require a constant supply of air. They can pump up a car tire or power a nail gun at a moderate pace but are totally inadequate for paint spraying.

I have a 3hp/230volt, 60 gallon upright compressor rated for 10 cfm at 90 psi. It can power a automotive touch-up gun without trouble. A full size, high pressure automotive spray gun is borderline and it can not keep up with a HVLP spray gun.
 
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Old 07-20-11, 02:32 PM
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I didn't even think about the OP's compressor possibly being one of those oil-less compressors

Pilot Dane, I'm surprised your compressor has so much trouble spraying. I know paint guns require as much or more cfm than other pneumatic tools but my 6 hp 60 gal compressor has always kept up with anything I've thrown at it. I've painted several vehicles with it using a HVLP conversion gun with absolutely no problems. I don't remember the cfm rating but doubt it is any higher than yours.
 
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Old 07-21-11, 07:23 AM
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Thank you very much for all your comments.

Mitch17:
Voltage at home is the same here and there. 120 standard, but can have 220 if you ask for it to the electricity company.

Furd: your explanation has been very helpful, now I understand that these numbers
are not that simply, and the manufacturer might be just lying or taking some weird parameters.

My compressor is not this, but it looks identically, just with another brand:
Portable Air Compressor - 2 HP, 8 Gallon, 125 PSI

Now, this has a more credible CFM numbers: [email protected], [email protected] That sound like the real capacity of my compressor.
May be the "8 cfm" my compressor says, is like at 15 PSI... !? lol

Do you think the hose length might affect the overall compressor performance?
I have mine with a 50ft, 3/8" hose with 1/4 inlets.
I wonder if that might be too large, or too wide.



Pilot Dane, Marksr:
I also was amused about that big compressor, and that it barely keeps up.
I know that if I had that compressor I would dare to paint a car!
I've seen car paint shops with smaller compressor that those you have, of course, they won't provide top of the line finishes.

I did not say it before, but I use my compressor for wood finishing.
So, requirements are not very high. You can pause with confidence and just wait the compressor to keep up.
In my work with that spraygun and compressor, it does keep up, but if I pain continuosly for 10 or 15 minutes, it just never fully catches up. The pump never shut off again until I stop.

Can I ask you... when you paint with that compressor, say, with your gun at 40 PSI.... how much time can you spray before the pump turns on?
 

Last edited by cesarvegamx; 07-21-11 at 08:08 AM.
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Old 07-21-11, 10:57 AM
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My small air compressor is a sears, 1hp, 11 gallon tank. Before I bought my big stationary compressor, I used the small one to paint a half dozen or so vehicles. For the most part it would keep up with a quart of paint although it might not shut off until after I refilled the quart cup. I probably used a little more thinner than I would with my bigger compressor.

An air compressor kicks on when the air in the tank goes below the preset level. Problems occur when the working pressure goes below the needed air pressure. I'm not sure how long you can spray before the pump cuts in. I've always been more concerned with whether or not the paint was atomizing correctly than whether or not the air compressor was running.
 
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