High pressure air compressor

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Mitchell the idiot
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High pressure air compressor

Postby Mitchell the idiot » Wed Jan 06, 2016 2:09 pm

Thanks to the administrators for including me on this forum. This is my first post, so here it goes. I was looking around the internet for an inexpensive way to get 4500 psi of air to fill air rifles and the portable fill tanks I take with me when I go huntin' and target shooting. The most inexpensive way I found was a compressor called the Shoe Box which is available through most all of the online airgun stores. Inexpensive is a relative term though, it retails for $1200.00! So I got to thinking (that is scary in itself) for $1200.00 I am gonna try to come up with a solution on my own, or in the immortal words of Joe Cocker "try with a little help from my friends". The issue I see with most of the HP compressors is the minuscule volume they put out. Filling an 80cc or 100cc rifle tank is no big deal, but after watching a video on YouTube where a guy added 600psi to a medium sized portable fill tank with his new Shoe Box and it took him two and a half hours. That seemed a little excessive. The Shoe Box uses a standard shop compressor to precharge the unit with 85psi then it uses two stages of compression to get the pressure up to 4500psi. The two stages of compression are pretty high ratio. If the the two stages are of equal compression ratio and discounting for pumping losses and relative humidity and any of the other parameters, the ratio is at least 7.5:1. Adding in the the variables I'm sure it is more like 8:1 or more. My Idea is to increase the pressure to the input of the compressor to like 600psi using a large gas cylinder such as a nitrogen cylinder and pressurize that with 3 or 4 refrigerator compressors which can output up to 850psi. I would then regulate the output of the gas cylinder to 600psi that would then be the input to the first stage of the compressor. Using my simple figuring that would mean that I would only need two stages of compression at 3:1 each to get my 4500psi. With that in mind the cylinders could be quite a bit larger and the stroke quite a bit shorter. Which would cut down on frictional losses and reduce the drive mechanism size and complexity. What I'm wanting to know is did I miss anything in the VERY general design and is it really feasible.

Glenn Ford
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Re: High pressure air compressor

Postby Glenn Ford » Wed Jan 06, 2016 4:06 pm

High pressure compressors are expensive for a very good reason, 4500psi air can do a lot of damage. If you are interested in what a few scuba divers do to boost gas pressure take a look at this publication http://www.airspeedpress.com/newbooster.html
Whatever you do don't underestimate the potential for injury from a home built booster.

Russ Hanscom
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Re: High pressure air compressor

Postby Russ Hanscom » Wed Jan 06, 2016 9:04 pm

I have worked with industrial sized high pressure compressors to 4000 psi. Stresses and horsepower requirements go up with volume and pressures. Not sure your complex process is worth the effort, but it could be interesting. I would watch for a surplus industrial type of compressor designed for the conditions, might save a lot of effort.

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Re: High pressure air compressor

Postby steamin10 » Fri Jan 08, 2016 9:43 pm

Back in the day when Aluminum bottles first appeared, everybody had 2500 psi rigs for air, and had to change to higher pressure rigs, or get booster packs. These boosters ran off of low pressure air, like a tire filler of respectable cfm, or took high pressure air, and drove the booster with it to top off the dive tanks. As time went on, they fell from use, since they were too inefficient compared to a purpose built dive air compressor.

For the small amount of high pressure air you are dealing with, I would not be afraid to tackle building a Booster, that is but a twin cylinder opposed air compressor, much like the singles built for early steam engines. It is simple math to determine the ratio of drive to driven, given that both cylinders will be equal pressure at the start of the cycle, from the air feed, (not ambient) but the over bearing drive piston will drive the smaller piston until it maxes out on shaft pressure. This maximum is designed in, and can be compound for greater speed, if you account for the gradient during the fill.

This is old news to me, but breathing air, presents some special purity problems, not unlike keeping air oil free because of ignition of oils in a pressure atmosphere with O2. I did not read the site promoted, but scanned the first page or so, and find it interesting. Thanks for the link, I will be reading up on that site in the near future.
Big Dave, former Millwright, Electrician, Environmental conditioning, and back yard Fixxit guy. Now retired, persuing boats, trains, and broken relics.
We have enough youth, how about a fountain of Smart. My computer beat me at chess, but not kickboxing
It is not getting caught in the rain, its learning to dance in it. People saying good morning, should have to prove it.

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Re: High pressure air compressor

Postby hammermill » Sat Jan 09, 2016 1:02 pm

many air gun friends use small shoebox type compressors to charge thier storage bottles. basic you run compressed air in and then boost the pressure. scuba and scba compressors are not uncommon but seem to be large hp and 3 phase.

some more digging on the air gun forums will locate you plans for building a shoebox booster.

it is always considered good form to add a general location to you id. some one hands on may be close by

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Re: High pressure air compressor

Postby choprboy » Tue Jan 12, 2016 6:54 pm

If you are not adverse to governement auctions (as some items may not be as described/gutted/non-fuctional), watch the army/navy/air force surplus sales. There are often high pressure compressors for far less than $1200. May be listed as nitrogen/argon compressor (high pressure for airplane avionics cooling), flame thrower regeneration compressor (recharging pressure tanks), SCBA compressor, etc. One of the local paintball fields has an old 4-cylinder diesel-driven trailer mounted compressor, bought for a couple hundred bucks, for filling air tanks onsite.

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Re: High pressure air compressor

Postby david5605 » Wed Jan 13, 2016 12:41 pm

Fire departments fill their own air bottles. They have both station based and pumps on the trucks for the job. I wonder if you could buy them used somewhere? I'm sure they are VERY expensive new.

My biggest concern with a home brew setup would be fittings/tubing that could take the pressure.

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Re: High pressure air compressor

Postby steamin10 » Wed Jan 13, 2016 5:10 pm

David: High pressure fittings in small sizes are available all over the market. Most often used is stainless tubing, and compression fittings. Rubber hose is available from many sources, even airless paint suppliers. Airless paint units can see 6k psi in operation, so parts are widely obtainable.
Big Dave, former Millwright, Electrician, Environmental conditioning, and back yard Fixxit guy. Now retired, persuing boats, trains, and broken relics.
We have enough youth, how about a fountain of Smart. My computer beat me at chess, but not kickboxing
It is not getting caught in the rain, its learning to dance in it. People saying good morning, should have to prove it.

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Bill Shields
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Re: High pressure air compressor

Postby Bill Shields » Fri Oct 14, 2016 5:40 pm

look around for paint ball fittings...very cheap and safe. Swagelok makes fittings that will handle the pressure if assembled correcty.

I had one of those compressors for a few years...after spending literally months looking around for a used one that was running, I broke down and just purchased one....for paint ball tank filling.... a bauer - cost me around $4K and was NOT for breathing air. It was 4 HP and ran on split phase 220 but you can get them to run on IC engines also.

the reciprocating boosters work well if you don't want a lot of volume. They are called 'air amplifiers'

https://www.highpressuretech.com/produc ... fiers.html

if someone that is making breathing air (scuba or emergency air tank) has a running compressor, most of the time they are not going to let it go.

as mentioned, some of the old bombers had these units on board for opening / closing the bomb bay doors..finding one that actually RUNS is a real trick. I looked at a 1/2 a dozen before I finally gave up...nobody would guarantee that they would run.

curtis cutter
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Re: High pressure air compressor

Postby curtis cutter » Sun Oct 16, 2016 10:51 pm

I know this is an old thread but is it necessary to use 4500 PSI or would two 2250 PSI bottles work? The reason I say this is many fire departments are going to the 4250 bottles from the 2250's and you may find surplus 2250 PSI compressors.

Mr Ron
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Re: High pressure air compressor

Postby Mr Ron » Wed Jan 18, 2017 2:47 pm

I hope your signature "Mitchell the idiot" is not a true statement. You must thoroughly know the workings of pneumatics, compressors and fittings. Working at those high pressures is like working with explosives. A fitting that lets go under pressure is like a bullet from a gun and a ruptured tank is like a bomb. I wouldn't touch a project like that with a 10' pole. I remember seeing a film of a water heater exploding when the pressure relief valve was disabled. A cast iron tank completely destroyed the house it was in while a steel tank took off like a rocket and went through the roof. and these tests were at much lower pressures; typically household pressures. The purpose of the film was to illustrate the danger of a malfunctioning safety valve.
Mr.Ron from South Mississippi

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Re: High pressure air compressor

Postby Harold_V » Thu Jan 19, 2017 2:21 am

Guess we'll never know. This individual made a showing the day following his post, and has never returned since. More than a year. Pretty safe to say, he'll likely never return.

Wise people talk because they have something to say. Fools talk because they have to say something.

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