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Thread: air compressors - single stage vs two stage

  1. #1
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    Default air compressors - single stage vs two stage

    A poster a couple days ago posted a message (the son of the guy who started and run Eaton Compressor Manufacturing) and what he said totally threw out just about everything I thought I knew about single stage compressors vs two stage compressors. I now have a whole new respect for single stage units. I was always under the impression that two stage units were more efficient and pumped more air --- definately not the case.
    I can't believe I didn't figure the whole thing out a long time ago.

    Most single stage pumps are two pistons and cylinders of the same diameter. Both cylinders pump the same amount of air independently, that goes directly into the air receiver tank. On the other hand a two stage pump has two cylinders but one cylinder is larger than the other. Air is sucked into the first cylinder (the larger one) and the air is compressed, that compressed air is then pumped into the second (smaller cylinder) where it is compressed even more. From the second cylinder the air is then pumped directly into the receiver tank. So logically if you set a single stage unit next to a two stage unit both having basically the same piston displacement and pump rpm the single stage unit is going to pump more air and logically is a more efficient air pump. While it is true that the two stage pumps are designed to pump air to a higher cut off pressure than the single stage units. Most two stage units are designed to cut the pump off at 175 psi while the single stage units are designed to shut off around 125 psi.

    I'm now questioning if there is any REAL advantage to a two stage unit compared to a single stage unit of comparable quality and piston displacement. While the two stage unit will pump to a higher pressure which means more air reserve in the receiver tank the single stage pumps more air to make up for the less reserve capacity. It should also be kept in mind that the two stage is going to take considerably longer to pump up to 175 compared to a single stage unit pumping more air to only 125 psi. The single stage will cycle more often but the two stage pump will run quite a bit longer per cycle compared to the single stage unit. So to me the advantages of a two stage is cancelled out in favor of a good quality commerical single stage unit.

    NONE of the air tools in a body shop require air of more than 90 psi. So air coming out of an air tank at 175 psi is just going to have to be regulated down to 90 psi anyway. The extra air pressure in a two stage unit is wasted in the type of work we do in an autobody/paint shop.

    I think where the single stage units got/and get a bad rap is that most home owner compressors are not real good quality single stage units that have electric motors and pumps running at twice the rpm that a quality made commerical single stage unit would be running at. For those little single stage home owner units doubling the rpm means almost doubling the air output for that inexpensive small unit. What they are doing in reality is sacrificing quality and longevity for an inexpensive unit that to the unitiated appears to be an excellent buy with a good cfm output. The down side is those home owner units will self destruct in a fraction of the time it would take a quality commerical unit to wear out and reach the end of its useful life (or just get rebuilt, quality units warrant a rebuild when needed. Not just throw it away and buy another one).
    Last edited by Phil V; 12-14-2007 at 09:14 AM.

  2. #2
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    Is there such a thing as a single stage that gives a lot of cfm?
    All the higher cfm units I've seen are two stage.

    Remember, work is work.
    A given motor size can only compress so many cubic inches of air,
    wether it's one large cylinder, or two smaller ones.
    The power it takes is closely the same,
    disregarding small efficiency differences of course.
    It only takes a small diameter increase to double the volume.
    So even if a cylinder looks only slightly larger, it's much more air.

    Most single stage units can't maintain the 90 psi min for air tools.
    In fact, most fall well below that point before comming on again
    once they stop running.:cool:

  3. #3
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    Quote Originally Posted by Phil V
    The single stage will cycle more often but the two stage pump will run quite a bit longer per cycle compared to the single stage unit. .
    That's not necessarily true.
    You have to compare compressors of equal cfm at the same psi.
    Cycle times are easily adjusted by on-off adjustments at the
    pressure switch, you can set a two stage lower if you want to,
    and it will reduce running times.
    The old Eatons were 175 psi, the newer ones cut off at 150.
    At least the one I just bought 2 yrs ago does.
    If you lower your 2-stage to match, it'll outperform your single stage.

    The Eaton 5hp single stage is 19.5 CFM @ 100 psi
    The Eaton 5hp 2-stage is 17 CFM @ 175 psi
    At almost twice the psi, the two stage is surely doing better cfm wise.
    That 17 will be much higher at 100 psi. :cool:
    Last edited by JCCLARK; 12-14-2007 at 09:53 AM.

  4. #4
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    Quote Originally Posted by JCCLARK
    Most single stage units can't maintain the 90 psi min for air tools.
    In fact, most fall well below that point before comming on again
    once they stop running.:cool:
    What air tools need 90 PSI minimum other than maybe a blaster?

    I purchased a single stage, 80 gallon compressor in 1970 and ran a 5 man body shop with it for 20 years. Of course single stage was the norm back then but I'm still running the same compressor and I'm hoping to get 30 years out of it. Actually it generates very little water and oil but I don't work it as hard these days.

  5. #5
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    My mistake, I shouldn't have said minimum.
    90 psi is the recommended pressure on a lot of my air tools.
    At least that's what it states on their spec sheet.
    But you can run them on less, I don't know why you would though.
    My sanders and impact wrenches don't work near as well on less.:cool:

  6. #6
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    Default I would never go back to a 120psi system...

    Having 175psi in the system means that you have much more air in your reserve. The delta between 90 psi and your reserve's psi is much better with a 175pis unit.

    This gives a better operating region for your pump to catch up with consumption. It makes a hell of a difference when using impact guns, sanders, grinders and sandblasters...

    Painting is not such a big deal for compressors...
    My 2 cents worth...
    Serge

  7. #7
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    So are there any single stage compressors out there that
    pump out more than 19 cfm at 90 PSI???

    If not, then how come?:cool:

  8. #8
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    Actually ALL the major commercial/industrial compressor manufacturers (Quincy, Curtis, Champion, Saylor Beale, Atlas Copco etc etc) make and sell single stage compressors in the 2 to 15 hp range, many rated in excess of FIFTY CUBIC FEET OF AIR PER MINUTE.

    Single-stage compressors are fine up to 150 psi. Higher pressures will require a two-stage unit. A single-stage compressor typically will have a higher CFM rating because the cylinder draws in air and compresses it with every rotation. A two-stage model compresses the air up to an intermediate pressure in one or more cylinder(s) and then passes it on to another cylinder to finish the job. Because the air is typically passed through an intercooler between stages, a two-stage compressor is more efficient at higher pressures.

  9. #9
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    Serge, relying on reserve air pressure in the receiver tank is kind of temporarily "cheating the hangman" isn't it ? My idea of what makes a good air compressor (besides build quality) is fast recovery time. All the air in reserve is not going to do any good when the air in the tank is used up and the compressor pump isn't pumping enough air to keep up with demand.

  10. #10
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    I have a two stage but a good single stage will easily top out at 135 psig which is what the Campbell-Hausfeld "Quadzilla" that's a very good buy for the buck will top out at.

    If you really need a lot of air then rotary screw is simply the way to go period. You can hardly hear it running and it's very efficient from both the space and electricity point of view. Much better than rotary piston. Anything bigger than 7 1/2 HP should, in my opinion, be rotary screw.

  11. #11

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    Quote Originally Posted by JCCLARK
    So are there any single stage compressors out there that
    pump out more than 19 cfm at 90 PSI???

    If not, then how come?:cool:
    This is the highest single stage I've seen from the home centers. Anyone have one of these?

    http://www.lowes.com/lowes/lkn?actio...065&lpage=none

    Eaton's 4.5hp/60gal rates 18 and 14 @ 90psi??? Not sure what that means.

    http://www.eatoncompressor.com/catal...747/172983.htm

  12. #12
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    Quote Originally Posted by JCCLARK
    The Eaton 5hp single stage is 19.5 CFM @ 100 psi
    The Eaton 5hp 2-stage is 17 CFM @ 175 psi
    At almost twice the psi, the two stage is surely doing better cfm wise.
    That 17 will be much higher at 100 psi. :cool:
    Actually, with a 2 stage compressor you don't gain much volume at the lower psi. Don't forget that the cyls are in series so if you're not using the second piston to boost the pressure all you're doing is passing the air from the first cyl to the second, essentially doing nothing. A 2 cyl single stage has the pistons moving air in parallel, meaning each piston is moving a seperate amount of air.

    There are lots of large single stage compressors. The one in our shop now is 10hp single and the previous shop I was in was about the same.

    Yes, you can increase your reserve in a given size tank by increasing the pressure, but I would suggest it's more efficient to have a larger storage vessel. I suspect that there is greater energy required to get to the higher psi than to fill a larger or second tank with the same additional volume at the lower pressure.

    Good informative discussion.
    Steve G.

  13. #13
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    Default Ok...

    Now I am totally confused!
    My 2 cents worth...
    Serge

  14. #14
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    Quote Originally Posted by Serge
    Now I am totally confused!
    What part?

    If you're running a high pressure situation you are able to squeeze more cubic feet of air (at atmospheric pressure ) into a given size container.

    The numbers are arbitrary, but let's say that you have a 100 cu ft container with air pressurized to 125 psi. At atmospheric (or room) pressure that same amount of air might need 500 cu ft to contain it. If you increase the pressure to 175 you might have 800 cu ft of atmospheric air in the tank. But you could also contain 800 cu ft of air at 125 psi by having a 150 cu ft tank instead of the 100 cu ft tank.

    When I say it uses less energy, there is additional energy required to increase the pressure from 125 to 175 to add that volume. To keep it at 125 psi and add the volume by using more storage does still use more energy to compress that addition 300 cu ft of air to 125, but it will be less than raising the pressure to 175 to increase the volume.

    It's hard to visualize, and I don't know the math or physics formulas, but the one indicater is the heat. Two stage compressors running 175 always have an after cooler. That heat in the air over and above the heat of air at 125 psi is lost energy.
    Diesels engines cranking at about 300 rpm with compression pressures of 350 psi will ignite diesel fuel injected into them. 175 at 1000 rpm would probably ignite ether. At 125 you can't ignite anything.

    As I read this back I realize I probably didn't clear anything up, but I tried. Conceptually, it's not an easy thing to explain or to grasp.
    Phil's got a good explanation of the series/parallel cylinder arrangement at the beginning of this thread.
    Steve g.

  15. #15
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    everything I have read (besides this) the 2 stage always out performed single stage. I remember years ago doing some DA sanding with a small 30 gallon 2 stage unit and I never had to stop. It ran constantly, but it kept up. My 30 gallon single stage wouldnt have come close to doing that

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