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Small high pressure air compressor ideas

Post questions and info about pneumatic (compressed gas) powered cannons here. This includes discussion about valves, pipe types, compressors, alternate gas setups, and anything else relevant.
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Unread postAuthor: Technician1002 » Sun Mar 06, 2011 1:03 am

BigBang J wrote:But this method would require 2 of the same compressors, right?
because if you have 2 different compressors and one doesn't pump quite as much as the other you would either starve the second compressor in the series, or you would over pressurize the second compressor in the series. Is this correct?


Not correct. As you compress a gas, the volume becomes smaller. It may have been correct for 2 water pumps in series.

Two comressors in series often have at least a 8:1 displacement difference between stages for a high pressure use and about 4:1 for medium pressure.

For example, the first stage would compress 9X from 1 atm to 9 ATM and the second stage would then go 9X more from 9 atm to 81 ATM Note the smaller second stage has a huge load..

Here is a typical 2 stage compressor. The first stage is high volume. The second stage has a smaller piston and often a shorter stroke along with thicker cylinder walls to handle the pressure.
Image
Medium pressure compressor. Delivers 200 PSIG.

Many shop compressors have a smaller ratio where the first stage only pumps to 4 or 5 bar and the second delivers below 300 PSI.

Stacking two identical compressors in series does not make a two stage compressor.
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Unread postAuthor: saefroch » Sun Mar 06, 2011 3:08 pm

What Tech is saying is very correct. I have two pumps, with the same stroke length connected in series. One has a 1" diameter head, the other a .5" diameter head. The 1" diameter takes in atmospheric pressure, and compresses up to about 200psi. The .5" compresses that air to 1,000psi. The .5" diameter pump has something like 4 times the displacement of the 1" pump, on average.
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Unread postAuthor: Lockednloaded » Wed Mar 09, 2011 2:18 pm

Quick Idea I want to run by you:

I want to but one of those "250psi" mini air compressors, but not the super cheap plastic ones. I'm buying a solid metal one that costs about 30$. My plan is to reduce stress on the mini compressor by inputing a regulated LP stream of air. Would there be room for the pressurized input? would it even be worth the effort/money?

Here's the compressor on Amazon
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Unread postAuthor: Gun Freak » Wed Mar 09, 2011 7:44 pm

I don't think there is a spot to pressurize the input, unless you modify it a bit. A little fitting could probably be epoxied into place somewhere. But the word on the street is those things aren't very good and will die out with repeated use that a spudder would put it through. The pressurized input should help a bit though...
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Unread postAuthor: BigBang J » Fri Mar 11, 2011 10:07 pm

Ahh, Tech and Froch thank you for explaining that to me I wasn't thinking it through all the way. Really you would want a higher volume compressor feeding a lower volume compressor.

"I have two pumps, with the same stroke length connected in series. One has a 1" diameter head, the other a .5" diameter head. The 1" diameter takes in atmospheric pressure, and compresses up to about 200psi. The .5" compresses that air to 1,000psi. The .5" diameter pump has something like 4 times the displacement of the 1" pump, on average."

saefroch, were you just using an analogy here or do you really have that compressor? I was wondering because I wanted to know if it was moterized and what size motor it uses.
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Unread postAuthor: saefroch » Fri Mar 11, 2011 10:50 pm

Not motorized, but I'd love to have a motorized compressor. It'd be much better designed than what I was talking about.
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Unread postAuthor: Technician1002 » Sat Mar 12, 2011 12:18 am

A common misconception is that feeding pressure into a compressor will reduce the load on it. The opposite is true. It is counter intuitive, but the higher volume of air in a cylinder requires higher horsepower to compress it. It has to do with work performed. A compressor with the inlet sealed off pulling a vacuum uses little power and provides little output.

1 atm into the inlet provides a load on the compressor. Providing 2 atm increases the load.
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Unread postAuthor: BigBang J » Sat Mar 12, 2011 10:26 pm

yeah I've been wondering why people have been saying that it helps decrease the load on the motor. It seems to me like you said, it would compress more air but require more power.
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Unread postAuthor: saefroch » Sat Mar 12, 2011 11:27 pm

The load on the compressor is related to the force differential across the pump head. With a compressor drawing vacuum, there is almost no force differential. I've observed this with my pump. I can draw a very significant vacuum, and it's obvious, because the pumping action becomes effortless.

However with regards to increasing inlet pressure, I guess it depends on how you define "load on the motor." Increasing inlet pressure while keeping max outlet pressure constant will reduce the number of rotations, but increase the amount of time the compressor motor is applying a high torque.

At lower inlet pressure, only the end of the stroke is high-torque.

Increasing inlet pressure will of course require more power, keeping in mind that P=W/t, but also that P=IV. If you're at all impatient, increase input pressure.
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Unread postAuthor: BigBang J » Sun Apr 10, 2011 9:17 pm

hey guys what do you think about this for a cylinder for a high pressure air compressor. I have not completed the drawing, but I have access to high speed internet today so I thought I would go ahead and upload it now.

Hey, you guys know anything about the shoebox compressor on youtube? Are they up for sale?
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Unread postAuthor: saefroch » Sun Apr 10, 2011 9:33 pm

With a stroke length that small, you'd probably want to look into a "zero dead volume" pump head design. It seems I may be missing something, because all I see moving is that one component, the pump head...
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Unread postAuthor: Lockednloaded » Sun Apr 10, 2011 10:32 pm

BigBang J wrote:Hey, you guys know anything about the shoebox compressor on youtube? Are they up for sale?


They do sell them, but it takes forever to get one, and they're far from cheap
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Unread postAuthor: BigBang J » Mon Apr 11, 2011 2:43 pm

Lockednloaded wrote:
BigBang J wrote:Hey, you guys know anything about the shoebox compressor on youtube? Are they up for sale?


They do sell them, but it takes forever to get one, and they're far from cheap


Yeah I found the website and took a look at the waiting list, its quite long.
And something like $430.

So I am looking to make a moterized pump to at least 1000psi.
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Unread postAuthor: BigBang J » Mon Apr 11, 2011 2:51 pm

saefroch wrote:With a stroke length that small, you'd probably want to look into a "zero dead volume" pump head design.


Yeah, I think I will change the piston head to a zero dead volume design.


saefroch wrote:It seems I may be missing something, because all I see moving is that one component, the pump head...


The oring on the piston slides to open and close the intake through the piston, and the check valve on the left side opens and closes.
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Unread postAuthor: al-xg » Mon Apr 11, 2011 3:14 pm

This situation is slightly more complicated than depicted, putting two shop compressors in series does not help max pressure. But putting two fridge compressors in series (feeding air into the casing) puts pressure on both sides of the piston head, and that is a different situation.
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