What limits the speed of an air compressor?

[Downunder Bob]

A well thought out and executed experiment, However i can't help thinking it's a lot of work for a very small gain, particularly in the hobby set up, However you had the pleasure of testing your ideas, well done. The old story whatever floats your boat.

 

[strantor]

Amazing amount of time, effort and knowledge invested here.
Thanks for sharing it with us!

-brino

Time & effort sure, knowledge is questionable.

I agree, fantastic effort and report, thank you. Please do post the final configuration for our enjoyment too.

Will do!

The only problem I have with this whole project is in use the compressor will run in a very narrow psi range.

I don't understand your meaning. It runs in the same PSI range as any other commercially available single stage compressor.

Does the cost of this conversion justify the apparent small increase in output?

For me, yes. I had most of the parts on hand. For most people, probably not. If you had to go out and buy everything, absolutely not.

I was involved in compressed air systems for a major Glass container Mfg. we ran 3500 plus ( yep 3500) horsepower of compressors 24/7 to maintain the forming machines. I constantly looked at more efficient means of providing compressed air. Two methods stood out. One was to use a variable inlet controlled centrifugal machine as the system control. The other was my favorite but corporate didn't like the initial cost. It involved conditioning the intake air to the compressor by cooling & dehumidifying. This resulted in your choice of two results. One being a reduction of total horsepower or the other being is the increase of total air volume. Both gave you clean DRY air and reduced downstream treating of the air & maintenance to the forming machines caused by wet air. So to suggest an addition to your project, condition your intake air using a window air conditioner getting the air down to 30% humidity and 50/60 deg temp this set up should also pressurize the intake slightly also. I know this works I've seen it.

I'll keep it in mind, thanks!

Very cool thread. Thank you for all the effort and data.

I used two "A" coils from a house central air unit, soldered together as my condenser. Yours looks really great too. One of my favorite simple formulas...P*V=P*V

Your HVAC coils are certainly better than this. Now that I have a wealth of data I might try subbing my old A/C condenser coil back into the mix and see if I suffer any loss of CFM. Hooray for gas laws! If you add temperature to the mix (P1V1/T1=P2V2/T2 ) you start to get answers about missing CFM at higher pressures.

A well thought out and executed experiment, However i can't help thinking it's a lot of work for a very small gain, particularly in the hobby set up, However you had the pleasure of testing your ideas, well done. The old story whatever floats your boat.

I am likewise a bit underwhelmed with the gains. However if you regularly do something requiring lots of CFM at not much pressure (like HVLP painting) this could be very beneficial. With 18.5CFM@40PSI, this 3HP compressor fills a niche that otherwise would demand a 7.5HP compressor.

 

[Downunder Bob]

Very cool thread. Thank you for all the effort and data.

I used two "A" coils from a house central air unit, soldered together as my condenser. Yours looks really great too. One of my favorite simple formulas...P*V=P*V

I imagine your condenser is also going to cool the air so your favourite formula should read P*V/T = P*V/T .

 

[nnam]

I think the gain is rather good. We can't expect magic. However, with better cooling of out going air, the tank actually store more air.

Better cooling of the compressor head and active filter oil, you may get higher rpm as long as you watch the temperature.

Furthermore, put a larger motor can push it higher.

On the graph, I don't get the up and down, should it be smooth. I am not talking about noise, but the second graph where constant HP appears to show up and down CFM. Can you explain that?

 

[nnam]

on another note, a larger tank solves most of the problem, I think.

 

[strantor]

I think the gain is rather good. We can't expect magic. However, with better cooling of out going air, the tank actually store more air.

Better cooling of the compressor head and active filter oil, you may get higher rpm as long as you watch the temperature.

Furthermore, put a larger motor can push it higher.

On the graph, I don't get the up and down, should it be smooth. I am not talking about noise, but the second graph where constant HP appears to show up and down CFM. Can you explain that?

I don't think this thing needs a bigger motor. In some of my testing I ran for 20+ minutes at the VFD's "redline" rather than the motor's, running the motor at 150%+ of its rated power, and it never even got warm.

Ideas of oil pumps, jets, etc. still plague my thoughts despite prior warnings about piston/rod inertia. This compressor is NOT balanced. In some frequency bands (ranges of RPM) it threatens to shake the unistrut wonder apart. Curious thing though, it seems smoother and happier the faster it goes. In fact, at its rated RPM it's pretty shaky, but at 125% of rated RPM it purrs like a kitten. Maybe it would like to go 150%? 200%? It's almost cheap enough to warrant exploring.

On the graph I left little notes explaining the humps. I was opening a valve and purging all the air from the tank.

 

[strantor]

on another note, a larger tank solves most of the problem, I think.

I disagree; let's discuss. The way I see it, the only reason compressors have a tank is so that you can periodically, briefly, get more CFM than the fixed-speed machine can deliver. You exceed its flow capacity at a cost; a trade-off - you get it at lower pressure. Ever ran your ugga-dugga until it just won't dugga anymore and just hisses at you? You've bleed your tank down to like 20PSI and now you must take a smoke break while the compressor replenishes. This is the "wasted capacity" that plagues the typical compressor. While you're out there giving yourself cancer, the motor is putzing along at the only RPM it knows; you paid for 3HP or 5HP or whatever, but for the first half of your cigarette you're only getting 1HP; maybe 2?

The large tank is (IMO) a poor compromise of a solution. And scaling the tank upwards seems to be popular but I don't understand why. If you're exceeding the CFM capacity of your pump, then you're exceeding the CFM capacity of your pump, plain and simple. It doesn't matter how big your tank is; you're going to drain it down, and when you do, you're going to have to wait for it to recharge. The bigger the tank the longer it takes to drain down AND the longer your smoke breaks have to be. Small tank, less ugga-duggas between smoke breaks, and shorter smoke breaks. Either way, same number of ugga-duggas to get the job done, and in the same amount of time.

Now introduce the constant HP concept and maybe, just maybe, your ugga never ceases to dugga. Maybe pressure drops and it slows, the but the motor rises to meet the CFM demand with no smoke breaks atall. I don't know; it will be tested...

So with this setup I think I favor my 26gal tank. If given a choice I may have even gone smaller to save space. AND if it were only say 10gal, it would be ready to go at full system pressure less than a minute after plugging it in. It would be ready before I even found my air tool and that one QD that I have, that gets passed between all the tools because none of them ever has a friggin QD on it despite the fact I buy at least a dozen every time I go to HF... but I digress.

Agree? No? Let's talk it over. I suspect there's something I'm missing, as I quite often do, when I look around and see that everyone making compressors (and making money by making compressors) is making them with bigger tanks and people seem to want bigger tanks. What am I not seeing?

 

[Larry$]

If your primary goal has been to quiet the thing down, putting it someplace to do that is the cheapest, easy solution. Just make sure it has plenty of cooling. I've got a 25hp screw in a small sound conditioned space, with lots of air flow generated with the air to air cooler. It has now run that way for over 12,000 hours, must work! I've got quite a few motors on equipment that run on 400 cycle power 14,000+-rpm but designed for that. Our motors on VSDs are all designed for that. I would guess that the limiting factor in running at other than design speed is over heating at slow and balance/bearings/lube at high.
Splash lube "systems" have a small "scoop" on the crank that splatters the oil around. At design speed it will throw some oil high enough to lube the rings. As speed is slowed will enough oil get to the rings? Heat damages oil, best to use a full synthetic.
Some oil & water (condensate) always exit a compressor. They cause damage to many things that air is used for. Treat your air to limit the damage done. Cool it and separate the water, filter out the oil and dirt, regulate the pressure to the lowest level that works. Compressed air is expensive.

 

[nnam]

On the graph I left little notes explaining the humps. I was opening a valve and purging all the air from the tank.

Thanks. I was reading via a phone, I couldn't see the text, and forget to check it again. If you worry about wear, start with very clean inside, even gasoline or whatever safer to clean it up. Then try some experiment, then check the oil against a very clean piece of paper and flash light to check for metal. I think there are better ways, and even more costly, which I say, not recommend due cost, oil analysis.

If the balance is an issue, I think try solving it may help.

 

[nnam]

Agree? No? Let's talk it over. I suspect there's something I'm missing, as I quite often do, when I look around and see that everyone making compressors (and making money by making compressors) is making them with bigger tanks and people seem to want bigger tanks. What am I not seeing?

The efficiency reduced if the motor starts and stops often. A larger tank would help if you increase the kick off pressure. So say you use 90 psi on your tool, and you want to kick it off at 90 psi, that means, if the pump can handle that CFM of your tool, it would kick off, and you can keep working.

A larger tank keeps it longer before it kicks off again, improve efficiency.
It also helps the motor with longer life. The vfd with constant torque helps alot in that regard.

It's only a problem if you empty the tank. In that case, why not keep it full, and have an auto water bleeding valve, not sure if that solves the rust issue.