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  #31 (permalink)  
Old 12-26-2009, 09:27 AM
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Quote:
Originally Posted by Old Fool
Tubetek, I think you are missing the true point.


His mind is set with a misconception that will live on for eternity.

You don't have a clue and you would jump on the wagon with anyone who disagreed with anything I said, you chose your name well!

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  #32 (permalink)  
Old 12-26-2009, 10:51 AM
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Quote:
Originally Posted by oldred
You don't have a clue and you would jump on the wagon with anyone who disagreed with anything I said, you chose your name well!
Old Red, you are so mistaken, it is you that doesn't have a clue and rides on the everyone else is doing it wagon. I am really surprised you are even on this site as Hot rodders generally are ones that dont follow the masses.

I would agree with you or anyone else when they know what they are telling others.,,but your misconception on this topic is not correct. You have offered no scientific evidence to state other wise. Because a bunch of low budget body shops use 2 stage compressors you consider them the best.
A body shop would use a rotary (you do know what that is) IF they could afford it.. They don't because they dont have the $$ to invest in top line equipment.
2 stage compressors have been the norm for years. Back in the early days there were not many hi capacity single stage compressors, so to get capacity a person had to step up to 2 stage. After many years of 2 stage being the way to go,it became "bible" that they are better. Not science, just legend.
Today single stage air compressors are readily available that perform equal to the more expensive 2 stage compressors up to the 100-120 psi range. Above that a secondary stage is required to increase the pressure,, NOT the volume.
by your way of thinking the new 400psi compressors for nail guns will surely replace the 2 stage you are so set on being the best
.More pressure is better, NOT. show me one air tool used in a body shop that has a manufacturer recommended operating pressure of over 100psi.
Why make heat to get 175 psi air when you dont need the pressure. You point out how the 2 stage has cooler air because it has a piece of fin tube between the first and second stage. That thing is there to cool the air so it doesnt super heat during the higher pressures it will be subjected to
It has nothing to do with 2 stages being cooler, quite the opposite it is hotter if not inter cooled.
capacity at 100 psi in scfm is the same whether it is made in a single or multiple stage process.

your 35 years working on air compressors does not impress me,I too did the same thing for 30 years,single stage, 2 stage, screw, vane, rotary turbine, along with piston, gear, vane, centrifugal, multi stage centrifugal pumps, operating with psi ranges up to 12000 psi.

You can continue to spread your misconception without any more of my input, it does not good to try and correct your misconception

One thing we can agree upon is we are not going to agree on this subject..
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  #33 (permalink)  
Old 12-26-2009, 03:17 PM
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Believe what you like, but telling me I am "so mistaken" without offering anything but opinion is doing exactly what you have accused me of. I think everyone else here knows the real facts so say what you want I am not going to be drawn into another useless argument because you are disputing common compressor knowledge not my opinion. Did you bother to check any of the links? I am not going to try to prove anything to you by searching for info so do a Google search and look it up for yourself. So far the the only thing either of you two have provided (besides some childish name calling and jabs from you) is opinion based on what you believe. The fact that even the compressor manufacturers explain about the better efficiency of two stage vs single stage pumps was simply explained away by saying it is "just **** written by idiots", Really? Ok you evidently know more about it than even the manufacturers.

Just a short quote from that hydraulics/pneumatics site,

" A two-stage (actually it said multi-stage, I did not simply copy/paste) unit compresses from inlet to discharge pressure in two operations - generally passing the air through an intercooler to remove some of the heat of compression between each stage. This SAVES POWER (sound suspiciously like more efficient?) and keeps the compressor's operating temperatures lower."


The other site states that the efficiency in a two-stage air compressor is, in their words, MUCH HIGHER than in a single stage air compressor. It also states that a two-stage air compressor produces MORE CUBIC FEET OF AIR PER HP THAN A SINGLE STAGE COMPRESSOR which results in lower operating costs, they then point out that with a two-stage air compressor less heat is generated.

Actually lower peak temperatures are generated because part of the total heat is removed during the cylinder exchange of the compressed air.


That's just a couple of places but there are lot's more, all are part of the compressor company conspiracy to mis-lead everyone and are just as wrong I suppose.

I guess since it is just **** coming from idiots it don't mean much anyway.




Once before when discussing two stage vs single another fellow put it like this, it is much easier for a compressor to compress 1 atmosphere to 4 by doing it in two stages rather than all at once, that's what spreading out the load means and why that mechanical advantage truly is an advantage. By removing some of the energy stored in the air as heat before further compressing it by a smaller more mechanically advantaged piston less total resistance will be met during both stages in the two stage vs the single stage being required to do it all in one operation and without the benefit of being able to remove some of the stored energy (heat). You could compress air to much higher pressures, although heat removal would have to be dealt with, with a single stage compressor of a given HP rating but in order to do so the CFM would have to be reduced drastically by using a smaller pump or reducing RPM. If the CFM was not reduced then the CFM/pressure/HP required would quickly exceed the HP limit. The two stage, by more efficiently using the available HP, CAN easily produce these higher pressures while still maintaining a higher CFM rate. My point is that if you tried to increase the cut-out pressure of a single stage compressor, of say 5 HP, to about 160 PSI or higher it would overload the motor by trying to produce that much pressure at a CFM rate equivalent to what a two stage could easily do with the exact same power. How can the two stage do this? By using it's available power much more efficiently.

Let's try an example. A single stage compressor of 18.1 CFM@90 PSI (Let's assume these specs are correct and use the compressor that started this) producing 5 HP will be at very near the limit of the motor's power curve at cut-off, if you tried to increase the pressure much higher at all while still producing that much CFM (or even the the amount the CFM would normally drop as pressure increases) you would quickly exceed the motor rating. A two stage would have no trouble at all producing that much pressure while still maintaining an equivalent, or even higher, CFM rating using the same motor. Don't believe that? Just check out a few compressor specs. The single stage and two stage compressors of the same HP will be producing very similar CFM numbers at the single stage cut-out pressure but the two stage can continue to increase pressure (while still maintaining CFM) without exceeding the motor power rating while the single can not. Again if it were not for the fact that the two stage uses it's avaliable power more efficiently it could not do this. To put it bluntly a 5 HP two stage can easily produce 16 CFM or so at 175 PSI but a single stage would smoke that 5 HP motor long before reaching that kind of pressure and CFM, even if it could deal with the extremely hot discharge air. A single stage is more efficient? Really?




Ok, I said earlier that not all two stage compressors use the high pressure and some will have cut-off settings similar to the single stage but apparently that is not done much anymore as I could not find an example of one still available, the one I had mentioned has a 175 PSI setting unless the owner wants a setting of 150 PSI. I can not see how this would have any bearing on the difference in efficiency of the two stage vs single from a mechanical perspective but I was evidently mistaken about this being fairly common, at least in recent years. Still I was mistaken on that point so take it however you like.

Last edited by oldred; 12-27-2009 at 10:06 AM.
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  #34 (permalink)  
Old 12-26-2009, 05:13 PM
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Quote:
Originally Posted by oldred

" A two-stage (actually it said multi-stage, I did not simply copy/paste) unit compresses from inlet to discharge pressure in two operations - generally passing the air through an intercooler to remove some of the heat of compression between each stage

This SAVES POWER (sound suspiciously like more efficient?) and keeps the compressor's operating temperatures lower."
Okay, I can see how at a given pressure you might have slightly cooler air. But I can't understand how this saves energy. The energy was already lost to heat. Essentially, you're using your power to heat up the garage.

Are you suggesting it's easier to compress cool air than hot? I don't this this would be true and it seems like the whole thing would be a wash anyway.


Quote:
Originally Posted by oldred
The other site states that the efficiency in a two-stage air compressor is, in their words, MUCH HIGHER than in a single stage air compressor. It also states that a two-stage air compressor produces MORE CUBIC FEET OF AIR PER HP THAN A SINGLE STAGE COMPRESSOR which results in lower operating costs, they then point out that with a two-stage air compressor less heat is generated.
It seems like this is demonstrably false. For every example of a two stage being more efficient at 90psi, there seems to be a counter example. Even if you cherry pick your data, you'd only be able to say "slightly more efficient."

Quote:
Originally Posted by oldred
Once before when discussing two stage vs single another fellow put it like this, it is much easier for a compressor to compress 1 atmosphere to 4 by doing it in two stages rather than all at once, that's what spreading out the load means and why that mechanical advantage truly is an advantage.
I don't think this is correct. TubeTek's explanation of the physics of mechanical advantage and work was more on target for this situation.

Quote:
Originally Posted by oldred

You could compress air to much higher pressures, although heat removal would have to be dealt with, with a single stage compressor of a given HP rating but in order to do so the CFM would have to be reduced drastically by using a smaller pump or reducing RPM. If the CFM was not reduced then the CFM/pressure/HP required would quickly exceed the HP limit. The two stage, by more efficiently using the available HP, CAN easily produce these higher pressures while still maintaining a higher CFM rate. My point is that if you tried to increase the cut-out pressure of a single stage compressor, of say 5 HP, to about 160 PSI or higher it would overload the motor by trying to produce that much pressure at a CFM rate equivalent to what a two stage could easily do with the exact same power. How can the two stage do this? By using it's available power much more efficiently.

Let's try an example. A single stage compressor 18.1 CFM@90 PSI (using the specs from the compressor that started this) producing 5 HP will be at very near the limit of the motor's power curve at cut-off, if you tried to increase the pressure much higher at all while still producing that much CFM (or even the the amount the CFM would normally drop as pressure increases) you would quickly exceed the motor rating. A two stage would have no trouble at all producing that much pressure while still maintaining an equivalent, or even higher, CFM rating using the same motor. Don't believe that? Just check out a few compressor specs. The single stage and two stage compressors of the same HP will be producing very similar CFM numbers at the single stage cut-out pressure but the two stage can continue to increase pressure (while still maintaining CFM) without exceeding the motor power rating while the single can not. Again if it were not for the fact that the two stage uses it's avaliable power more efficiently it could not do this. To put it bluntly a 5 HP two stage can easily produce 16 CFM or so at 175 PSI but a single stage would would smoke that 5 HP motor long before reaching that kind of pressure and CFM, even if it could deal with the extremely hot discharge air. A single stage is more efficient? Really?
I don't think anyone is arguing that the things are much more efficient at high pressures. But at 90psi, the 2-stage design doesn't seem to have much benefit beyond slightly cooler operating temps, which are of dubious benefit.

I'm not trying to attack you. Your posts were incredibly helpful to me in choosing and setting up my compressor (ironically, the IR single stage.) But I think you may be wrong here. I have zero engineering expertise, but a little background in physics and it seems like the 2-stage is offering what we sometimes call a 'free lunch.'

There are whole (very successful) industries built around providing useless technology. Ab machine manufacturers spring to mind. And their explanations have the same tenuous ring as the 2-stage compressor companies'. "Get rock hard abs in three minutes a day with no strain or effort because our machine is SO much more efficient than situps."

I'd also bring up homeopathy and stereo cables, but I'd probably get flamed
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Old 12-26-2009, 07:36 PM
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Originally Posted by wyomingclimber
Okay, I can see how at a given pressure you might have slightly cooler air. But I can't understand how this saves energy. The energy was already lost to heat. Essentially, you're using your power to heat up the garage.

Are you suggesting it's easier to compress cool air than hot? I don't this this would be true and it seems like the whole thing would be a wash anyway.




It seems like this is demonstrably false. For every example of a two stage being more efficient at 90psi, there seems to be a counter example. Even if you cherry pick your data, you'd only be able to say "slightly more efficient."



I don't think this is correct. TubeTek's explanation of the physics of mechanical advantage and work was more on target for this situation.



I don't think anyone is arguing that the things are much more efficient at high pressures. But at 90psi, the 2-stage design doesn't seem to have much benefit beyond slightly cooler operating temps, which are of dubious benefit.

I'm not trying to attack you. Your posts were incredibly helpful to me in choosing and setting up my compressor (ironically, the IR single stage.) But I think you may be wrong here. I have zero engineering expertise, but a little background in physics and it seems like the 2-stage is offering what we sometimes call a 'free lunch.'

There are whole (very successful) industries built around providing useless technology. Ab machine manufacturers spring to mind. And their explanations have the same tenuous ring as the 2-stage compressor companies'. "Get rock hard abs in three minutes a day with no strain or effort because our machine is SO much more efficient than situps."

I'd also bring up homeopathy and stereo cables, but I'd probably get flamed

No, it's not offering a free lunch at all but it is easy to see how it could be mis-understood. Remember the example I gave of a car with one gear? At extremes you could choose to have a low gear and plenty "pull out" torque but no high speed, or you could choose a higher gear and have a higher top speed but no pulling power, adding another gear greatly increases efficiency and allows you to have both. The same principle extreme applies since you can have a single stage pump producing high CFM but only at low pressure or you can have high pressure but only with low CFM, it can not pump high pressure and high volume at the same time. A compromise must be made then so with the car you could choose a gear that would allow an acceptable, but compromised, top speed while still allowing enough torque to start you moving. With the single stage compressor the compromise comes by setting the CFM@pressure to closely equal the motor HP (well nearly so anyway) at the desired cut-out pressure, any higher pressure at that CFM would exceed the motor HP.

Quote:
Originally Posted by wyomingclimber
I don't think anyone is arguing that the things are much more efficient at high pressures. But at 90psi, the 2-stage design doesn't seem to have much benefit beyond slightly cooler operating temps, which are of dubious benefit.


A two stage compressor with a like size motor will produce a similar CFM as the single stage at the single stage cut-off point but at that point the single would have nearly maxed out the motor and can not pump any higher while the two stage will be using only a much smaller percentage of the available power allowing it to continue to a much higher peak pressure while still maintaining CFM (I am aware that heat plays a major role in selecting the single stage cut-off point but that does not change the fact that the motor is still nearly maxed out at that point and would overload if you tried to go higher at the same CFM). So it is not just at higher pressures, the two stage will be producing similar CFM but using less HP at the same pressure as the single at cut-off. Any argument that the two stage is producing significantly lower CFM at the single stage cut-off pressure would not make sense at all because that would mean excessively long recharge times, which simply is not the case with a two stage and I don't think many here would believe that it is! As far as the web site claim that the two stage is much more efficient, much more or slightly more, the fact that the two stage can pump a similar CFM to a much higher pressure on the same HP as the single stage, which could not reach that pressure, demonstrates efficiency, whether it is considered "much" or "slight" depends on who is considering and their definition of "much" and"slight".

Last edited by oldred; 12-27-2009 at 08:17 AM.
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