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Old 12-26-2009, 09:23 AM
oldred oldred is offline
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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-

and maybe this engineering site has it all wrong too,

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!

Last edited by oldred; 12-27-2009 at 06:18 PM.
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