Hot Rod Forum : Hotrodders Bulletin Board - View Single Post - Air Compressor for Automotive Painting
View Single Post
  #29 (permalink)  
Old 12-26-2009, 05:01 AM
Old Fool's Avatar
Old Fool Old Fool is offline
Registered User
Last photo:
Join Date: Jan 2008
Location: spokane,wa.
Posts: 1,365
Wiki Edits: 0

Thanks: 8
Thanked 42 Times in 36 Posts
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.

Last edited by 302 Z28; 12-26-2009 at 03:43 PM. Reason: Violation of guidelines. Please see: general board guidelines.
Reply With Quote