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  1. #1
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    Default Need help calculating compressor CFM

    Looking to buy a new compressor. Ingersoll Rand 5hp 80 gallon. Literature says CFM is 14 at max pressure which is 175 psi. I only need 100 psi for the machine it will be hooked to. Is there a way to calculate what the cfm will be at 100psi?

    Thanks in advance.
    Jonathan

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    Calculating CFM

    You probably would be better off to ask Ingersoll Rand.

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    Did a little research on this and wrote this up. If you like or not, it's free.

    Compressor CFM Calculation
    --------------------------
    1. Drain tank
    2. Record number of seconds to fill tank.

    CFM = G x 0.536 x PSIG / S

    CFM = Cubic Feet per Minute of Compressor
    G = Tank capacity in gallons
    PSIG = Gauge pressure reading on tank
    S = Seconds to fill tank

    Example:
    Time to fill tank = 11 minutes and 11 seconds
    PSIG = 155
    G = 120

    CFM = 120 x 0.536 x 155 / ((11 x 60) + 11) = 14.85

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    Thanks for the replies.
    1- Ingersoll Rand isn't open on the weekend and I'm trying to put a budget together for a new shop for a meeting Monday morning, so just trying to get close.
    2- Thanks for all the work but that doesn't help as I don't have the compressor to test. I'm trying to spec a new compressor and can't buy more than I need right now.
    I found this formula online and it seems to make sense.

    P=K*(1/V)
    90psi=K*(1/5cfm)
    Solving for K, we get a value of K= 450
    V=K*(1/P) so V=450*(1/35psi)= 12.86 cfm

    So my situation works like this I think assuming the manufacturers specs of 175psi and 14 CFM:

    175=K(1/14)
    K=2450.9
    so
    V=2450(1/100)
    V=24.5

    I think that seems right if I have 14 CFM at 175 then this says I'd have 24.5 CFM at 100.
    Which is just about perfect for what I need. So I don't need to go up to a compressor larger than this 5hp 80 gallon which would be a lot more money.

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    Nah it doesn't usually work like that. I have a 5 HP IR compressor rated at about the same CFM as the one you're looking at and have actually tested it via pump-up timing. Here's the results of my testing to 187 psi.

    20191228_153851.jpg

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    https://www.northerntool.com/shop/to...0491_200500491

    This links to a page where, at the bottom, is a comparison of (5) different compressor models showing CFM at max pressure and at 90PSI. Not much difference, certainly not the approx. 100% delta you calculate.

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    I would have hazarded an estimate of 4 CFM/HP @ 100 psi, so ~20 CFM and 3 CFM/HP at 175 psi (~15 CFM) for a 2 stage recip.

    If you are running close to that, switch from on-off to unloaded control mode, so it alternately compresses and idles to maintain tank pressure, it’ll be happier that way. When more lightly loaded, conventional on-off becomes better.
    Last edited by MrSteve; 12-28-2019 at 03:52 PM. Reason: Formatting

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    Quote Originally Posted by Pivot View Post
    Thanks for the replies.
    1- Ingersoll Rand isn't open on the weekend and I'm trying to put a budget together for a new shop for a meeting Monday morning, so just trying to get close.
    2- Thanks for all the work but that doesn't help as I don't have the compressor to test. I'm trying to spec a new compressor and can't buy more than I need right now.
    I found this formula online and it seems to make sense.

    P=K*(1/V)
    90psi=K*(1/5cfm)
    Solving for K, we get a value of K= 450
    V=K*(1/P) so V=450*(1/35psi)= 12.86 cfm

    So my situation works like this I think assuming the manufacturers specs of 175psi and 14 CFM:

    175=K(1/14)
    K=2450.9
    so
    V=2450(1/100)
    V=24.5

    I think that seems right if I have 14 CFM at 175 then this says I'd have 24.5 CFM at 100.
    Which is just about perfect for what I need. So I don't need to go up to a compressor larger than this 5hp 80 gallon which would be a lot more money.

    Nope. The CFM rating is the amount of air the compressor inhales at Standard atmospheric pressure. The output pressure doesn't have much effect, particularly with a two-stage. It'll be marginally more efficient, but it ain't going from 14 up to 24.5 CFM!

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    Keep in mind that the I/R 5 h.p. is not a 100 percent duty cycle compressor, meaning "Continuous operation".
    Trying to get a higher C.F.M and lower pressure continuously will require The compressor to be reset to a lower shut off pressure.
    As when pressure starts to exceed 100 P.S.I the C.F.M will in turn start to decrease. You would have to reset/replace the pressure control switch to cycle at say 90 P.S.I on and 100 Off, problem is the compressor is going to cycle very often if it's at maximum C.F.M flow and then there's the duty cycle issue again.
    I am not sure of your required total C.F.M @ 100 P.S.I, but it's better to match the compressor output and duty cycle to air requirements.

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    If you only need 100psi, why not buy a single stage compressor. You can get pretty good CFM from a single stage and they usually go to 125psi.

    Here is a Jenny single stage 5hp 80 gal tank rated at 23cfm @ 125psi. Jenny J5A-80V 5-HP 80-Gallon Single-Stage Air Compressor - 230V, 1-Phase

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    Default Handy compressed air formulae

    Here's a link:

    USEFUL_FORMULAS_FOR_COMPRESSED_AIR.pdf

    There's standards for compressed air calculations. Never go by manufacturer's specs without vetting them. ALL comsumer grade and prosumer grade air compressor unit specs are exaggerated if not outright lies. The only package air compressor units whose specs you can trust are big factory sized units far beyond the wallets of mere small shop owners.

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  14. #12
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    Quote Originally Posted by Pivot View Post
    Looking to buy a new compressor. Ingersoll Rand 5hp 80 gallon. Literature says CFM is 14 at max pressure which is 175 psi. I only need 100 psi for the machine it will be hooked to. Is there a way to calculate what the cfm will be at 100psi?

    Thanks in advance.
    Jonathan
    .
    probably ......
    piston can displace 14 cfm but after efficiency or leaks its usually less
    .
    max 175 psi is just that but efficiency usually lower
    .
    been my experience a 5hp compressor cannot continuously run a 1/2hp air grinder needing 5cfm at 90 psi. basically you need at least 10hp to run a 1hp air tool continuously usually need more
    .
    i used to do maintenance on big air compressor reed valves. they always leaked a bit and as they got older they leak or blow by more. percentage of leaks all over the place, anywhere from 20 to 90% depends on compressor of course (and max pressure) and gets worse as it gets older. yes air compressors waste 90% and sometimes more of input energy. 10hp in maybe less than 1hp out
    .
    some of waste is in the air tool used. that is some air tools need more cfm to get same hp. not unusual for some to need over 200% more cfm
    .
    and if you got pipe or hose air leaks you can be wasting 98% of energy that is only get 2% of energy out at the tool the other 98% is wasted energy usually heat and noise


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