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Old 12-26-2009, 02:42 AM
TubeTek TubeTek is offline
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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.

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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.

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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?

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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.

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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.

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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.

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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.

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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.

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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.

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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?

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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.

Last edited by 302 Z28; 12-26-2009 at 04:36 PM. Reason: Violation of guidelines. Please see: general board guidelines.
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