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Topic Review (Newest First)
12-26-2009 07:36 PM
oldred
Quote:
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".
12-26-2009 05:13 PM
wyomingclimber
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
12-26-2009 03:17 PM
oldred 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.
12-26-2009 10:51 AM
Old Fool
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..
12-26-2009 09:27 AM
oldred
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!
12-26-2009 09:23 AM
oldred Tube, there's no point in us discussing this because the first part of your reply speaks volumes,

" a person can go to any number of sites, manufacturer and otherwise, and find claims that 2 stage compressors are more efficent than single stage."

They build the things but they don't know what they are talking about? OK.

And going back to school?

"Been there already. Didn't particularly care for it, but I ground thru to the finish."

That statement says volumes about your attitude!

It was boring to you I guess, sounds like you learned a lot,


"The reason is real simple. In a 2 cyl compressor, for example, you have 2 cylinders pumping air to the tank with a single stage, and one cylinder pumping to the tank in a 2 stage, while the 2nd cylinder is just pumping to the cylinder that's actually supplying the tank.

"Higher discharge temp = more wasted work = less efficiency."


By your own description of the two stage pump you think it is less efficient because it wastes energy as heat and is also wasteful by pumping air from one cylinder into another before pumping it into the tank. The single stage however is MORE efficient because it pumps directly into the tank and bypasses that wasteful step. MY, my now why didn't I and all the engineers who designed these things think of that?


You keep asking me to explain why a two stage is more efficient but you already dispute the very reason it is and think that step is just wasted energy so what's the point?


Again there is no point in arguing about this because you will disagree with anything I say and you are convinced your flawed logic is right in spite of what the manufacturers are telling you and in spite of the accepted engineering data. It is easy to twist and downplay what was said no matter how much sense it might make like when I said,

"First the single stage pump is a lot simpler and cheaper to build with a heck of a lot fewer parts and as you did correctly say they are quite sufficient for most small shops."

You replied


When built to equal quality standards, there should be very minimal difference in the cost of manufacture of a single stage or 2 stage pump of a given hp rating. There's few additional parts in the 2 stage, and the cost of manufacture of those parts would be partially offset by the fact that the displacement of the 2 stage will be consistently less than that of the single stage. Less total displacement = smaller components = reduced cost of manufacture.

In a 2 stage pump, we're not talking about some device that needs powder metal 4340 rods or hypereutectic pistons to survive, but rather one that operates at max pressures not much greater than cylinder pressures in a cranking but not running gas engine. When garden variety materials will get the job done, the smaller of 2 similar devices will almost always be cheaper to produce.



How can anyone look at the differences in a simple single stage pump and a two stage with it's higher parts count and think they are built at the same cost? I never said they had to be made of exotic materials or even different material, I simply said they are more complex and require a greater number of parts and thus cost more to build.


I think most everyone here knows better so what's the point? Again by your logic two stage pumps are not as efficient as a single stage and two stage pumps just waste energy and cost more while producing less air. So, by your thinking the ONLY reason for the existence of two stage compressors is to produce higher pressure, so I guess that makes the two stage compressors with a low shut-off pressure a total waste then.


A quick check for some manufactures' specs may be of more interest to some people than just arguing back and forth about mechanics, Eaton compressor has comparable two and single stage outfits, their 5 HP single stage compressor is rated at 14 CFM@100 PSI (it is called an 18 CFM 5 HP in the model name but they clearly point out that is pump displacement and they clearly state it is rated at the 14 CFM@100 PSI), The nearest model two stage with the same 5 HP motor is rated at 17 CFM at 175 PSI. What you are saying conflicts with just about everything that is normally accepted about compressors not just my opinion and disputes even the manufacturers, a couple of examples here,

One of many who according to you are ALL just plain wrong-

http://www.grainger.com/production/i...compressor.htm

and maybe this engineering site has it all wrong too,

http://www.hydraulicspneumatics.com/...luidPowerAcces

So I guess to sum it up everyone here who bought two stage compressors just wasted their money and would have gotten better performance from a single stage? It costs no more to build a two stage pump despite the fact is has far more parts and is much more complex? Because they produce LESS cfm a two stage compressor's sole purpose is to produce higher peak pressure despite some of them not doing so? A two stage pump simply wastes energy with that useless extra step, wonder why the engineers didn't catch that? Like I said before you write well but your logic is seriously flawed!
12-26-2009 05:01 AM
Old Fool
Quote:
Originally Posted by TubeTek
Not true at all. Nothing I said would constrain both compressors to have the same displacement. In fact, the typical single stage compressor will most always have greater displacement than a 2 stage designed for the same horsepower.



Real world numbers don't show this statement to be true. Comparing IR's current 10hp single and 2 stage offerings (to get into compressors larger than any homeowner type units and the BS performance specs that often come with them) shows the single stage produces 36cfm @ 100psi, and 33.8cfm@125 psi. The 2 stage version produces 34.2cfm when ordered in the 125psi configuration. All measurements based on free air delivery. If both units were operating at 105/125 cut in and cut out, the difference in air delivery would be difficult if not impossible to measure by commonly available methods.



What, specifically, make the 2 stage pump more efficient?



Not sure what sort of subversive intentions you might think I have, but I'm all ears and waiting for a logical explanation of this efficiency advantage, based on solid engineering principles. Since I got my degree in Mechanical Engineering from NC State a bit more than 36 years ago, I'm reasonably qualified to recognize the difference in an explanation based on engineering facts and one based on **** written by marketing type idiots that's typical of about 99% of the technical "facts" published today in reference to a wide range of equipment.



When built to equal quality standards, there should be very minimal difference in the cost of manufacture of a single stage or 2 stage pump of a given hp rating. There's few additional parts in the 2 stage, and the cost of manufacture of those parts would be partially offset by the fact that the displacement of the 2 stage will be consistently less than that of the single stage. Less total displacement = smaller components = reduced cost of manufacture.

In a 2 stage pump, we're not talking about some device that needs powder metal 4340 rods or hypereutectic pistons to survive, but rather one that operates at max pressures not much greater than cylinder pressures in a cranking but not running gas engine. When garden variety materials will get the job done, the smaller of 2 similar devices will almost always be cheaper to produce.



I stated clearly that I was excluding any application that actually needs air pressures in the upper range of pressures achievable only by a 2 stage pump. And so far, what you're talking about is "fitness for purpose" and not efficiency.



You're still discussing fitness for purpose, and not efficiency. You've selected the ability to produce high pressure as a criteria for acceptable performance when, in fact, that high pressure is seldom if ever needed in any normal shop application.



And you seem to be dreaming up things I never said. Still waiting for that expalnation of increased efficiency.



Not true by any stretch of the imagination. Spreading out the load, as you call it, enables the pump to reach a higher pressure. It does not make the pumping operation more efficient. Simple example.... I can't lift a 300# load 6 ft in the air by lifting it straight up. But I can roll it up an inclined ramp until its raised 6 ft with no problem. Does the ramp make the operation more efficient? No, it only provides a means of mechanical advantage. Technically, the process becomes less efficient when the ramp is used because rolling the load requires the effort necessary to overcome the rolling friction as well as the effort required to raise the load. The longer and more gentle the ramp angle, the more inefficient the operation becomes due to friction losses, even though the effort input within a particular expanse of time might be less. Similarly, the more spread out the load to enable compressing air to higher pressures, the less efficient the overall operation becomes due to increased frictional losses. Actual numbers from manufacturers on single and 2 stage compressors bears this out so long as both are operating within the acceptable pressure range of single stage compressors.



Still waiting for that explanation of enhanced efficiency. Lacking that, there are plenty of applications that use higher pressure air available only from multi-stage compressors. But, when you can look up the delivery specs on any legitimately rated and comparable single stage and 2 stage compressor, and see that the single stage will typically produce slightly more free air in the 100 psi range, why would you want to spend the additional money on the 2 stage if you don't need tank pressures higher than 125-130 psi?



Been there already. Didn't particularly care for it, but I ground thru to the finish. Maybe that's why I know the definition of efficiency while you obviously don't.

FWIW, a person can go to any number of sites, manufacturer and otherwise, and find claims that 2 stage compressors are more efficent than single stage. I've looked at a lot of these sites, and read a lot of these claims, but I've yet to see the first legitimate explanation of WHY they're more efficient. For that matter, I've yet to see ANY attempt to back up the claim of higher efficiency. That leads me to believe its most likely marketing hype. The fact that the operative word is "efficiency", one of the world's most overused marketing buzz-words, tends to confirm that suspicion in my mind.
Tubetek, I think you are missing the true point.


His mind is set with a misconception that will live on for eternity.
12-26-2009 01:42 AM
TubeTek
Quote:
Originally Posted by oldred
I don't think I have ever seen so much mis-information so eloquently put, you write very well!

Tube with all due respect you got almost nothing right and the reason is simple, pump efficiency.

[2 compressors with equal hp, running at the same speed, similar pump construction but one being single stage and the other two stage, the single stage will put out more CFM every time.]

That logic is totally flawed because it assumes both compressor pumps are the same displacement for a given HP and are running at the same speed which is almost never the case
Not true at all. Nothing I said would constrain both compressors to have the same displacement. In fact, the typical single stage compressor will most always have greater displacement than a 2 stage designed for the same horsepower.

Quote:
and a 5 HP two stage will produce more CFM per HP than a 5 HP single stage.
Real world numbers don't show this statement to be true. Comparing IR's current 10hp single and 2 stage offerings (to get into compressors larger than any homeowner type units and the BS performance specs that often come with them) shows the single stage produces 36cfm @ 100psi, and 33.8cfm@125 psi. The 2 stage version produces 34.2cfm when ordered in the 125psi configuration. All measurements based on free air delivery. If both units were operating at 105/125 cut in and cut out, the difference in air delivery would be difficult if not impossible to measure by commonly available methods.

Quote:
The statement that the single stage is more efficient is just plain wrong, the two stage is much more efficient than a single stage and because of this two stage compressors are able to use a larger CFM pump arrangement than a single for a given HP.
What, specifically, make the 2 stage pump more efficient?

Quote:
Your intentions are obvious and I am probably wasting my time but I will try to explain why the two stage has an efficiency advantage over the single stage.
Not sure what sort of subversive intentions you might think I have, but I'm all ears and waiting for a logical explanation of this efficiency advantage, based on solid engineering principles. Since I got my degree in Mechanical Engineering from NC State a bit more than 36 years ago, I'm reasonably qualified to recognize the difference in an explanation based on engineering facts and one based on **** written by marketing type idiots that's typical of about 99% of the technical "facts" published today in reference to a wide range of equipment.

Quote:
First the single stage pump is a lot simpler and cheaper to build with a heck of a lot fewer parts and as you did correctly say they are quite sufficient for most small shops.
When built to equal quality standards, there should be very minimal difference in the cost of manufacture of a single stage or 2 stage pump of a given hp rating. There's few additional parts in the 2 stage, and the cost of manufacture of those parts would be partially offset by the fact that the displacement of the 2 stage will be consistently less than that of the single stage. Less total displacement = smaller components = reduced cost of manufacture.

In a 2 stage pump, we're not talking about some device that needs powder metal 4340 rods or hypereutectic pistons to survive, but rather one that operates at max pressures not much greater than cylinder pressures in a cranking but not running gas engine. When garden variety materials will get the job done, the smaller of 2 similar devices will almost always be cheaper to produce.

Quote:
This simplicity and cost effectiveness comes at the expense of efficiency however because the single stage must make a major compromise between peak pressure and recovery time, besides it would run into heat problems above about 135 PSI without the benefit of the two stage inter-stage cooler.
I stated clearly that I was excluding any application that actually needs air pressures in the upper range of pressures achievable only by a 2 stage pump. And so far, what you're talking about is "fitness for purpose" and not efficiency.

Quote:
With a single stage pump and a given HP you can have high CFM or high pressure but you can not have both high CFM and High pressure at the same time, if you use a large displacement pump to produce high volume at low tank pressures you would exceed your HP limit before reaching the desired shut-off pressure. By using a smaller pump a higher peak pressure could be attained but at lower pressures the CFM would be low and the motor just loafing along. The compromise must be made then for the HP available between CFM at lower pressure and peak pressure. An example I have given in the past is a car with only one gear, you could have high torque and low top speed or high top speed but low torque or a compromise could be made between the two to make the most of the available HP. The two stage gets around this to a certain degree by using the small and large piston arrangement which allows for higher CFM at higher pressures but because the compression load is spread out it can do this while still reaching a higher peak pressure.
You're still discussing fitness for purpose, and not efficiency. You've selected the ability to produce high pressure as a criteria for acceptable performance when, in fact, that high pressure is seldom if ever needed in any normal shop application.

Quote:
You seem to be saying that as if it means nothing, maybe the engineers who designed these things are just stupid?
And you seem to be dreaming up things I never said. Still waiting for that expalnation of increased efficiency.

Quote:
That is the very reason a two stage works so much more efficiently, by spreading out the load.
Not true by any stretch of the imagination. Spreading out the load, as you call it, enables the pump to reach a higher pressure. It does not make the pumping operation more efficient. Simple example.... I can't lift a 300# load 6 ft in the air by lifting it straight up. But I can roll it up an inclined ramp until its raised 6 ft with no problem. Does the ramp make the operation more efficient? No, it only provides a means of mechanical advantage. Technically, the process becomes less efficient when the ramp is used because rolling the load requires the effort necessary to overcome the rolling friction as well as the effort required to raise the load. The longer and more gentle the ramp angle, the more inefficient the operation becomes due to friction losses, even though the effort input within a particular expanse of time might be less. Similarly, the more spread out the load to enable compressing air to higher pressures, the less efficient the overall operation becomes due to increased frictional losses. Actual numbers from manufacturers on single and 2 stage compressors bears this out so long as both are operating within the acceptable pressure range of single stage compressors.

Quote:
If your reasoning were true there would be little need for two stage pumps with their extra complexity and expense. Trying to argue that the only reason for two stage pumps is the higher peak pressure of the usual 175 PSI would make no sense at all because not all of them go that high, some choose to pump more CFM at a lower pressure.
Still waiting for that explanation of enhanced efficiency. Lacking that, there are plenty of applications that use higher pressure air available only from multi-stage compressors. But, when you can look up the delivery specs on any legitimately rated and comparable single stage and 2 stage compressor, and see that the single stage will typically produce slightly more free air in the 100 psi range, why would you want to spend the additional money on the 2 stage if you don't need tank pressures higher than 125-130 psi?

Quote:
Don't know where you got your info but you need to go back to school.
Been there already. Didn't particularly care for it, but I ground thru to the finish. Maybe that's why I know the definition of efficiency while you obviously don't.

FWIW, a person can go to any number of sites, manufacturer and otherwise, and find claims that 2 stage compressors are more efficent than single stage. I've looked at a lot of these sites, and read a lot of these claims, but I've yet to see the first legitimate explanation of WHY they're more efficient. For that matter, I've yet to see ANY attempt to back up the claim of higher efficiency. That leads me to believe its most likely marketing hype. The fact that the operative word is "efficiency", one of the world's most overused marketing buzz-words, tends to confirm that suspicion in my mind.
12-25-2009 09:38 PM
wyomingclimber Oldred:

Regarding efficiency. If you compare a single stage and two stage compressor that pulls 30 amps, it doesn't appear that the two stage flows more air @90psi.

So are you saying the efficiencies accrue at higher pressures? And if so, does this matter if you never use more than 90psi? (i.e. a car with only one gear is fine if the roads are so bad you can only drive 10mph.)

TubeTek makes an interesting point regarding thermodynamic law. Energy cannot be lost--put in 30 amps of electricity and you get 30 amps worth of air, vibration, and heat, right? That means a more efficient compressor would have to produce less of one or both of those unwanted byproducts.

I've never been able to wrap my mind around why two stage compressors are better. Having said, that, there are a lot of things I can't wrap my mind around
12-25-2009 04:44 PM
oldred
Quote:
Originally Posted by Old Fool
Man thought the earth was flat for 100's of years.

Your user name says it all.
12-25-2009 04:44 PM
oldred
Quote:
Originally Posted by TubeTek
Sorry guys, but any claims that a 2 stage compressor is "more efficient" than a single stage of similar construction is purely a myth.

2 compressors with equal hp, running at the same speed, similar pump construction but one being single stage and the other two stage, the single stage will put out more CFM every time.

The reason is real simple. In a 2 cyl compressor, for example, you have 2 cylinders pumping air to the tank with a single stage, and one cylinder pumping to the tank in a 2 stage, while the 2nd cylinder is just pumping to the cylinder that's actually supplying the tank.



Simple reason for this as well. The 2 stage runs the same quantity of air thru 2 compression cycles. Each cycle represents work input to the air. Some of the work is stored as potential energy in the form of compressed air, while the rest is converted to heat, either carried in the air or rejected thru the compressor casting, piping, intercooler piping, etc. Take a specific volume of air and compress it once, and you'll have some discharge temperature. Now run that same chunk of air thru a 2nd compression cycle, and it'll get hotter every time. Each compression stroke represents a combination of useful and wasted work, so when you compress the same air twice you will always be adding the wasted work to that air as well as the useful work. Compress it once and push it to the tank, as in the case of a single stage, and you've added less heat, and less wasted work, to that volume of air.

For most uses, whether its a body and paint shop, or a machine and fab shop like my shop, a single stage compressor will do anything that needs to be done via air, and it'll do it cheaper both on a monthy operating cost and on a long term maintenance cost, because the single stage will typically be operated at a lower temperature and lower level of mechanical load. For some reason though, there's been such an effective job of selling people on the need for 2 stage compressors that its difficult to even find large cast iron single stage pumps today.



Then, out of curiosity, I tried the IR, and then the Quincy, separately, just to get some idea of how much air each was putting out. Nothing else using air at the time. Laid a chunk of steel on the blaster's pedal and let each one run until their pressure was no longer dropping. IOW, would one hold a continuous 35psi and the other a continuous 70psi, or whatever, since it would give some reasonable measure of the output of each.

One was the inherent better efficiency of a single stage, primarily due to less work being wasted as heat.

I don't think I have ever seen so much mis-information so eloquently put, you write very well!

Tube with all due respect you got almost nothing right and the reason is simple, pump efficiency.

[2 compressors with equal hp, running at the same speed, similar pump construction but one being single stage and the other two stage, the single stage will put out more CFM every time.]

That logic is totally flawed because it assumes both compressor pumps are the same displacement for a given HP and are running at the same speed which is almost never the case and a 5 HP two stage will produce more CFM per HP than a 5 HP single stage. The statement that the single stage is more efficient is just plain wrong, the two stage is much more efficient than a single stage and because of this two stage compressors are able to use a larger CFM pump arrangement than a single for a given HP. What you are saying sure is going to come as a surprise to a lot of engineers out there that have had it wrong all these years! Your intentions are obvious and I am probably wasting my time but I will try to explain why the two stage has an efficiency advantage over the single stage.


First the single stage pump is a lot simpler and cheaper to build with a heck of a lot fewer parts and as you did correctly say they are quite sufficient for most small shops. This simplicity and cost effectiveness comes at the expense of efficiency however because the single stage must make a major compromise between peak pressure and recovery time, besides it would run into heat problems above about 135 PSI without the benefit of the two stage inter-stage cooler. With a single stage pump and a given HP you can have high CFM or high pressure but you can not have both high CFM and High pressure at the same time, if you use a large displacement pump to produce high volume at low tank pressures you would exceed your HP limit before reaching the desired shut-off pressure. By using a smaller pump a higher peak pressure could be attained but at lower pressures the CFM would be low and the motor just loafing along. The compromise must be made then for the HP available between CFM at lower pressure and peak pressure. An example I have given in the past is a car with only one gear, you could have high torque and low top speed or high top speed but low torque or a compromise could be made between the two to make the most of the available HP. The two stage gets around this to a certain degree by using the small and large piston arrangement which allows for higher CFM at higher pressures but because the compression load is spread out it can do this while still reaching a higher peak pressure.


"The reason is real simple. In a 2 cyl compressor, for example, you have 2 cylinders pumping air to the tank with a single stage, and one cylinder pumping to the tank in a 2 stage, while the 2nd cylinder is just pumping to the cylinder that's actually supplying the tank."


You seem to be saying that as if it means nothing, maybe the engineers who designed these things are just stupid?

That is the very reason a two stage works so much more efficiently, by spreading out the load. If your reasoning were true there would be little need for two stage pumps with their extra complexity and expense. Trying to argue that the only reason for two stage pumps is the higher peak pressure of the usual 175 PSI would make no sense at all because not all of them go that high, some choose to pump more CFM at a lower pressure. Don't know where you got your info but you need to go back to school.
12-25-2009 03:44 PM
Old Fool Man thought the earth was flat for 100's of years.
12-25-2009 02:02 PM
TubeTek Sorry guys, but any claims that a 2 stage compressor is "more efficient" than a single stage of similar construction is purely a myth.

2 compressors with equal hp, running at the same speed, similar pump construction but one being single stage and the other two stage, the single stage will put out more CFM every time.

The reason is real simple. In a 2 cyl compressor, for example, you have 2 cylinders pumping air to the tank with a single stage, and one cylinder pumping to the tank in a 2 stage, while the 2nd cylinder is just pumping to the cylinder that's actually supplying the tank.

If you want to get an idea of which compressor is most efficient, check the discharge temperature of the air coming from the single and 2 stage units. The 2 stage will always have the higher discharge temperature. Heat rejection is the most common and most accurate method of measuring wasted work. Higher discharge temp = more wasted work = less efficiency.

Simple reason for this as well. The 2 stage runs the same quantity of air thru 2 compression cycles. Each cycle represents work input to the air. Some of the work is stored as potential energy in the form of compressed air, while the rest is converted to heat, either carried in the air or rejected thru the compressor casting, piping, intercooler piping, etc. Take a specific volume of air and compress it once, and you'll have some discharge temperature. Now run that same chunk of air thru a 2nd compression cycle, and it'll get hotter every time. Each compression stroke represents a combination of useful and wasted work, so when you compress the same air twice you will always be adding the wasted work to that air as well as the useful work. Compress it once and push it to the tank, as in the case of a single stage, and you've added less heat, and less wasted work, to that volume of air.

For most uses, whether its a body and paint shop, or a machine and fab shop like my shop, a single stage compressor will do anything that needs to be done via air, and it'll do it cheaper both on a monthy operating cost and on a long term maintenance cost, because the single stage will typically be operated at a lower temperature and lower level of mechanical load. For some reason though, there's been such an effective job of selling people on the need for 2 stage compressors that its difficult to even find large cast iron single stage pumps today.

We've got 3 compressors at our shop. One is a 5hp 2 stage IR, made before the US discovered manufacturing in China, which we've always run at a cutout pressure of 175-180 psi. Because my dad has been in the heavy equipment business for about half a century, the pressure is set that high so it doesn't take half a day to fill a big tire to 120 psi. Its piped into the shop air system, but valved off from that system unless we happen to need it for something unusual.

The 2nd compressor is a Quincy single stage made in the mid 50's. 2 cyl pump, rated for use with a 5, 7.5, or 10hp motor. Its got a 3hp single phase 1750 motor on it because it was originally bought for use in a state highway maintenance garage in a remote mountain location where the only electrical service into the building was a 100 amp single phase.

The third is a 2 stage 10hp 3 cylinder radial. Its the one that's normally used for general shop air supply. We normally keep it set for 140 cutout and 105 cut in pressures.

The 10 went down one day when I was bead blasting a bunch of parts. I opened the valve and started the 5hp IR. Found it couldn't quite keep up with the blaster. Almost, but not quite. So I started the 3hp Quincy as well, and finished the job.

Then, out of curiosity, I tried the IR, and then the Quincy, separately, just to get some idea of how much air each was putting out. Nothing else using air at the time. Laid a chunk of steel on the blaster's pedal and let each one run until their pressure was no longer dropping. IOW, would one hold a continuous 35psi and the other a continuous 70psi, or whatever, since it would give some reasonable measure of the output of each.

The IR ran a while and eventually settled out at 66psi. Then the Quincy settled at 62psi. Very little difference in output, since both were discharging thru the same orifice and both pressures were measured with the same gauge.

Remember though, the Quincy is only 3hp while the IR is 5hp. Two things going on here to make their performance so nearly the same. One was the inherent better efficiency of a single stage, primarily due to less work being wasted as heat. The other is that the slow running speed of the Quincy made its pumping efficiency even higher than normal via reduced intake pressure losses, and even less than normal heat input into the air due to the reduced speed necessary for it to play nice with the 3 hp motor.

Obviously, if you have a need for high pressure air, the single stage isn't the way to go. OTOH, its real difficult to find any sort of tool or other air operated device that's designed for an operating pressure greater than 90psi. Personally, if I ran across someone who had a quality 10hp single stage in good shape, and they had a need for a 2 stage, I'd swap 'em my 10 horse in a heartbeat.

Regarding the 5hp IR single stage mentioned above, the 18 cfm is a very rational output number. 4 cfm per horsepower isn't unusual at all for a single stage. Dad's got a number of portable compressors from small to pretty big, all with CFM measured at 90psi output. Little 100 cfm jobs, driven by a 25hp diesel, up to a 1600cfm driven by a 400hp diesel. Right across the board, even though the collection is made up of several brands, the CFM number is right around 4x the engine horsepower number.
12-25-2009 10:37 AM
oldred
Quote:
Originally Posted by Old Fool
I too have the same IR and have had excellent results with it.

He obviously has a **** ** for IR, and anyone that does not agree with him

Ok maybe I am just making up all that stuff and maybe these 4 guys in this SAME thread are just being know it all's too?

http://www.hotrodders.com/forum/comp...=emerson+motor


Plus I got a call about 3 months ago about one here locally that has the same problem and I have seen this at least twice on other sites, but then I guess the "old know it all" should not tell anyone to keep from hurting feelings!
12-25-2009 10:30 AM
oldred
Quote:
Originally Posted by Old Fool
Yes that was discussed thouroughly in thread http://www.hotrodders.com/forum/resp...air+compressor. Post 42 and 43 are the proof in the pudding.



You picked out what you wanted from that other post to try to make your point but go back and look at everything I said about that thing before calling me names. I already admitted in this very thread that I came on too strong about it in my first reply to this thread and I clarified that I meant it was, as I said, a "rip-off" because of exaggerated claims and not that it was necessarily a bad compressor. I have said repeatedly that it was a decent outfit and have told more than one they would be happy with it BUT I also said it is not all they claim it to be and if that hurts your feelings that's too bad.
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