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Thread: True CFM using 2 compressors.

  1. #1
    88GT Guest

    Default True CFM using 2 compressors.

    I have 2 IR 60 Gallon compressors, each rated somewhere around 13-14 CFM @ 40. I can hook them both together when I need alot of air. Complete refinishing being one of my main reasons. My GTi takes upwards of 16 CFM idealy.
    My question is, will this increase the CFM? I was told at Nace a copuple years ago "no". I thought it would close to double the CFM, but does it increase the CFM even a little? I never seem to working the compressors extremely hard doing completes. I'm just wondering. Any CFM scientists out there?

  2. #2
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    it will if ur air lines are up-sized after the compressors, cause u can only get so muck air out that 3/8 fitting on the side of the compressor tank. other words, link your compressors together with a 3/4 line to compensate for both compressors- 3/8 +3/8= 3/4. i think that this will get u going. the only other way to get your cfms up is to get your pressure up, which isn't a good idea with the 140 psi models. i am not an expert, just my experiences and advice
    Last edited by dixie1776; 12-23-2006 at 06:20 PM.

  3. #3
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    Ya also need to alter the pressures they turn on/off at. Have one come on at 90 or 100psi & the other at 80. Have one turn off at 120psi & the other at 110.. or somewhere in that region.
    If they are set "the same" ...given that most sensors are just a little bit different you'l end up with "duelling compressors" where one will be switching on & off within about 2 seconds of the other.
    & put one way check valves on the outlets so when one is pumping between 115-120psi it won't being filling the other tank & causing it to bypass the on/off settings
    Hope that made sense to ya..:confused: :cool:

  4. #4
    88GT Guest

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    Quote Originally Posted by ZRX61
    Ya also need to alter the pressures they turn on/off at. Have one come on at 90 or 100psi & the other at 80. Have one turn off at 120psi & the other at 110.. or somewhere in that region.
    If they are set "the same" ...given that most sensors are just a little bit different you'l end up with "duelling compressors" where one will be switching on & off within about 2 seconds of the other.
    & put one way check valves on the outlets so when one is pumping between 115-120psi it won't being filling the other tank & causing it to bypass the on/off settings
    Hope that made sense to ya..:confused: :cool:
    That does make sense, and I thought to alter the kick in/ kick out preasures. One of them runs more than the other until I get them both below the kick in pressure. I have 3/4 copper in the shops, but just a 3/8 hose conecting the 2 for the time being, since there is a distance between the 2

  5. #5
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    Logically if you set the air kick on/kick off pressure 20 or even 10 psi higher in one compressor than the other and the cfm usage was close to what the cfm of air being used then the second compressor would never kick on. I'd say set the kick on kick off pressure of the two air compressors 5 psi or less apart in order for both compressors to work in tandem.

    88GT - Yes two air compressors of the same cfm will put out very close to double the cfm of each individually with both compressors hooked to the same air line. I see no reason for larger air lines (hard lines or air hoses). A normal 3/8" air hose with 1/4" fittings will pass close to 25 cfm. If you have a tool in a small bodyshop using more than 25 cfm then you have a REAL air hog. None of the normal air tools in a bodyshop use anywhere near 25 cfm with the exception of maybe a larger sandblaster run at high pressure.

  6. #6
    88GT Guest

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    Quote Originally Posted by Phil V
    Logically if you set the air kick on/kick off pressure 20 or even 10 psi higher in one compressor than the other and the cfm usage was close to what the cfm of air being used then the second compressor would never kick on. I'd say set the kick on kick off pressure of the two air compressors 5 psi or less apart in order for both compressors to work in tandem.

    88GT - Yes two air compressors of the same cfm will put out very close to double the cfm of each individually with both compressors hooked to the same air line. I see no reason for larger air lines (hard lines or air hoses). A normal 3/8" air hose with 1/4" fittings will pass close to 25 cfm. If you have a tool in a small bodyshop using more than 25 cfm then you have a REAL air hog. None of the normal air tools in a bodyshop use anywhere near 25 cfm with the exception of maybe a larger sandblaster run at high pressure.
    Thanks Phil. It just didnt make sense to me that they said I still have 14@40 or what not. I find that if I need alot of air, like for polishing or DA sanding, I can work alot longer with the 2 hooked up, than with just one.
    That was actually the reason I got my PLUS gun...the low CFM requirements. Wouldnt trade it for any other gun now though.

  7. #7
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    previous poster is right, 3/8 air hose will pass close to 25 cfm, but at higher pressures. take a water pipe for example, if u have 1/2 " pipe running both your cold and hot, while your taking a shower and someone flushes the toilet, the water will get really hot(cause the toilet takes part of the water from the line)---but if your like me and don't like the temp change, you run 1/2" for your hot, and 3/4" for ur cold----this way, you have more water(or cfm)for your cold water so it'll fill the toilet without changing the temperature in the shower.---------------not tring to be in disagreement, just an example to support my advice

  8. #8
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    Dixie, this is an interesting thread and one worth pursuing. I see two problems with you water line analogy. first of all we're talking about air and not a liquid. Air is a gas which we all know can be compressed but a liquid can't be compressed. But that aside, in your analogy of the comode taking water away from the shower head means that when the commode is flushed while the shower is in operation the water usage exceeds what the system is capable of supplying. I understand what your saying about the water lines it makes good sense to add volume capacity to the cold water line to increase available water supply to both the shower and the toilet. What that in reality is doing is changing the dynamics of the water delivery system to where the usage doesn't exceed delivery capacity.

    But in the air system discussed earlier the actual usage at no point normally should exceed delivery capacity that the 3/8 hose and 1/4 fittings are able to flow - especially - at normal higher pressure. The actual air usage in the previous post by 88GT was a little higher than 13 cfm which is close to half of what the air line/air hoses are capable of flowing. If we were discussing air usage which exceeds the capacity of the air lines, say 35 or 40 cfm then larger air lines would have to be used to increase the volume capacity of the air delivery system. BUT at 35 - 40 cfm of air usage it would exceed the maximum delivery capacity of the two air compressors which are around 13 cfm each. I'm not nit picking your post, just trying to make sense of what we're dealing with.

  9. #9
    88GT Guest

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    It sounds like I'm not depriving the GTi of the 15+ cfm. In fact for something like a hood, I don't bother connecting them, unless they just happen to be connected already.

  10. #10
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    hey phil,
    i was tring to get the point accross that more cfm flow would be easier accomplished with an increased air line size( this is what i was doing with the example, i know that the properties of a gas are different but cfm flow example would be the same)if you hook up both the compressors in sequence with the line size that was designed for a single compressor,you would be drawing half the air from each compressor, but if you were to double the line size,you would be drawing all the air from both compressors thus doubling the cfm, meaning no disrespect to anyone on this board,but someone posted a ? so i was tring to help out with my experiences, after all , that is what this board is for right?

  11. #11
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    Dixie, no problems. Your information is just a valid as mine. I was just trying to reason my way through how the air system works. A friendly discussion about a subject that I find interesting.

  12. #12
    88GT Guest

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    Quote Originally Posted by Phil V
    Dixie, no problems. Your information is just a valid as mine. I was just trying to reason my way through how the air system works. A friendly discussion about a subject that I find interesting.
    No offense taken at all Dixie. Thanks for the input.
    Phil, I did quite a bit of research on air supply before I plumbed all my air. There is alot to consider, but unfortunately I couldn't find much info on CFM. I would like to believe I have at least 18 CFM and it seems I do. I don't seem to have too little air for the GTi, even doing completes, but the compressors both run constantly when polishing DA sanding or blasting for long periods of time. I just let them build back up from time to time. They are both 100% duty cycle, but I don't like them running for an hour straight.

  13. #13
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    i reread your other post, i think that we're both tring to say the same thing , to get the cfm up with the same air line size, you must raise the pressure, thus forcing more air through the line, right?
    the other thing i was talking about was about how to get the cfm up without increasing the pressure as i think the compressor he's talking about is single stage and max psi i think is around 140.
    me personally, i have a IR 2 stage 80 gal with a 60 gal kicker tank just for the volume, as the compressor only comes on for 45 seconds to a minute at the time.before the kicker tank was added,the compressor was only on for around 20-30 seconds at the time.oh yeah, the compressor pumps a lil over 25 cfm per minute

  14. #14
    88GT Guest

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    Quote Originally Posted by dixie1776
    i reread your other post, i think that we're both tring to say the same thing , to get the cfm up with the same air line size, you must raise the pressure, thus forcing more air through the line, right?
    the other thing i was talking about was about how to get the cfm up without increasing the pressure as i think the compressor he's talking about is single stage and max psi i think is around 140.
    me personally, i have a IR 2 stage 80 gal with a 60 gal kicker tank just for the volume, as the compressor only comes on for 45 seconds to a minute at the time.before the kicker tank was added,the compressor was only on for around 20-30 seconds at the time.oh yeah, the compressor pumps a lil over 25 cfm per minute
    Yes they are single stage. I figured, rather than sell the one and get a bigger 2 stage, I would be cheaper off getting one more and making a way to connect them together if needed. Another reason is, I have 2 seprate buildings, so I could keep the plumbing as short as possible by locating one at each building. Would anyone have opted for 1 bigger compressor rather than 2? Just wondering

  15. #15
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    i personally perfer the two stage, but i think its all personal preference

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