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Compressor fill times and drain times

Discussion in 'Airbrush Compressors' started by Robbyrockett2, Jun 2, 2018.


  1. Robbyrockett2

    Robbyrockett2 Air-Valve Autobot!

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    This has come up a lot lately so here it is
    Heres an easy calculator for your compressor fill time from empty
    http://www.egnergy.com/calculator.html

    To calculate fill time while spraying just subtract .5cfm from the compressors input cfm for an airbrush, 1.1cfm for a fan brush or the listed cfm for a spray gun.
    And remember to set start pressure at when it kicks on, not zero.
    Use your compressors cfm rating at 40psi if you have it

    Bear in mind, many european compressors and chinese jobs are sometimes rated by the air they take in, this is a no load rating that means very little in the real world. Its output at 40psi can be 2/3rds or even half of that.
    If you arent sure, simply compare to brand name american compressor with the same motor power and if the compressor your looking at seems way higher cfm then chances are it's rated this way (use the numbers from the american job for an estimate).

    For drain times

    ((PSI x tank volume in Cf) / 14.7) / cfm consumed

    or

    ((PSI x tank volume in L) /14.7) / lpm consumed

    This is to drain the entire tank.

    To determine if the compressor is adequate (assuming the normal 50% duty cycle)

    Use the difference in pressure between full and kick on in the above formula in the place of PSI
    If fill time while spraying is equal to or greater than your result you are good to go.
    If its a little less you're OK (assuming your not constantly spraying)
    If its half or below...its gonna struggle... but could be ok if the drain time is all the longer you intend to spray for or if you do a lot of broken up sprays for detail work at 20psi and less.
    If its a fourth, forget it , you will definitely hate your compressor.




    These will give you rough estimates, figuring the real world numbers down to the second would be way too complicated for this purpose.

    Hope this helps some people.
    doc1 and J000seph like this.
  2. Robbyrockett2

    Robbyrockett2 Air-Valve Autobot!

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    EDIT EDIT EDIT
    Sorry , late night

    To determine if the compressor is adequate (assuming the normal 50% duty cycle)

    Use the difference in pressure between full and kick on in the above formula in the place of PSI
    If fill time while spraying is equal to or less than your result you are good to go.
    If its a little more you're OK (assuming your not constantly spraying)
    If its double or more...its gonna struggle...

    If fill time while spraying is in the negative, you will definitely hate your compressor but could be ok if the drain time is all the longer you intend to spray for or if you do a lot of broken up sprays for detail work at 20psi and less.
  3. Robbyrockett2

    Robbyrockett2 Air-Valve Autobot!

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    Of course you have exceptions like 100% duty cycle models that just give a little more air than you need, or the lack of frequency and duration of you're spraying that can put you into the good zone if it's on the edge but it's a good start.
    After you run the numbers on a few compressors you'll see the relationship and for the most part just be able to look and estimate hwether its suitable or not.
  4. J000seph

    J000seph Detail Decepticon! Very Likeable!

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    I have a question that I hope is on topic for this thread. Other artists may benefit as well possibly. I currently don’t own a large air compressor. I have seen that for some spray guns it is possible to provide the necessary cfm by connecting two small compressors to an extra air tank. I’ve heard/read that it “doubles” your cfm. I was recently looking at a spec list for the different California Air Tools compressors. I own two. Both 1hp. One is a 4.6 gallon tank, the other is a 2 gallon tank. The 4.6 has a higher cfm. With increased air storage, wouldn’t it be a little better than doubled cfm? Do you have a formula for that? Haha
  5. Robbyrockett2

    Robbyrockett2 Air-Valve Autobot!

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    The extra tank accomplishes sort of sycnhronizing the two. If you link them together with just a hose the tendency is for one to do slightly more work.
    still just a hose "T" will work but expect one of them to run more than the other.

    That aside, yes just add the output of the two and treat them as one. treat all tanks as one......it's impossible to predict which tank will get drawn from slightly more , so it can throw off drain times slightly, but not enough to notice the difference with this kind of estimating anyhow.

    More storage wont increase cfm, just drain time. only more compressor will increase cfm.

    So with a t hose its just a 6.6gal with whatever the combined CFM ratings are. cant remember if CA compressors give you no load numbers or not though.

    You could real world test them and see how close the fill while spraying or fill from empty numbers are and get a better idea of actual output.
    Last edited: Jun 3, 2018
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