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Old 12-25-2009, 05:44 PM
oldred oldred is offline
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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.

Last edited by oldred; 12-25-2009 at 07:31 PM.
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