Air entrainment into compressor other than intake

[e1cycle]

Greetings . First post here . Nice Site!
I have a question about How much outside air can get into a compressor when its running . In other words, and hypothetical, if the intake air was to be hosed and pulled totally from a special huge storage tank with a special "XYZ" mix totally void of any air inside , at atmospheric pressure , then pressurized by the compressor , into the dive tank. How much out side air would find it way into this "hypothetical" XYZ mix closed loop system through the running compressor itself.
Any guesses ?

 

[captain]

None, depending how huge is the huge storage tank the compressor would start to create a vacuum in it and eventually would not be able to suck any more XYZ gas from it.

And the point of the question being what exactly.

 

[Cave Diver]

And the point of the question being what exactly.

As a guess, I'd say they wanted to use the compressor as a mixing station or a booster pump.

If that is the goal, there are better, safer ways to accomplish that. More info would be helpful in giving a full answer.

 

[rjack321]

Seems like they don't understand the basics of their compressor... everwhere downstream of the 1st stage valve and/or intake hole is under pressures >1 ATA. Gas isn't going to move up gradient, so the answer is none. Some of XYZ gas might leak out of course.

Its not hard to use a compressor as a helium or premixed gas booster. I do all the time to scavange leftover gases. But based on the need to ask this question, I'm concerned that this might be beyond the OP's understanding for awhile.

 

[e1cycle]

Thanks for the replies What we want to do is use a quality made , portable scuba compressor use the intake and hose it into a helium balloon of 6500 cu ft and draw out the gas and re -tank it . At 10 cfm it would take about 11 hrs with one unit, 5 with 2 working. Even if it took 24 hours it would still be worth it .
If the compressor is to leaky we would put it in a air tight enclosure and purge with helium and keep a constant pressure on it at 1 pound with a helium cylinder. All the coolers and hose connections would be plumbed air tight to out side the enclosure. .
But is this necessary ? are quality scuba compressors air tight enough to work this way in the open air ? Thats what we're trying to find out . Any help is much appreciated .

 

[rjack321]

It will work depending on how high a pressure you wish to achieve though. It will take a toll on the compressor rings because helium is so small there is alot of blowby (like having low compression in a car engine).

6500cf is twenty-two 290cf 2400psi "T" cylinders. Its going to take about 12hours of continuous running a 10cfm compressor. If you have 2 10cfm units alternate running each unit 1 hour at a time to allow it to cool down.

You might find this Macy's day parade ballon is less that 100% helium inside BTW. That's not your compressor leaking, its Macy's trying to not spend any more $$$ than what it takes to float the ballon. They could cut the helium with methane for instance, so obviously gas quality will be unknown. I wouldn't use it for diving without a full chemical analysis of the gas which is going to be pricey. Probably to the point where its not worth this whole endeavor.

 

[e1cycle]

Thanks !
When you say "toll on the rings" , about how may re tank cycles would you expect it to last in this application?
But over all your saying the compressor could run in the open air and not infuse air into the system ? Are they that tight ?
If so then We could go to a Higher CFM / bigger gas powered unit because we were looking for small electric units so we could enclose them. This would be really good news! if we knew for certain it didn't have to be enclosed and the helium didn't get contaminated.

No this is all helium gas , no mixed stuff . 98 % pure welding gas. .

 

[rjack321]

Have you ever run a compressor of any kind before??? Gas leaks OUT of compressors not IN.

2% contaminants is not welding grade and that's potentially huge if part of that is carbon monoxide from filling the ballons. If they used a gas powered blower for instance. CO is a significant contaminant in the low part per million range. 2% is 2 parts per 100, so 2 orders of magnitude higher.

 

[Cave Diver]

Thanks !
When you say "toll on the rings" , about how may re tank cycles would you expect it to last in this application?
But over all your saying the compressor could run in the open air and not infuse air into the system ? Are they that tight ?

As long as you have the intake sealed well there shouldnt be any other places it could leak. If that was the case, it would be useless having intake filters on a compressor.

 

[e1cycle]

Thanks I have a better understanding of Scuba compressors. And It sounds like we may be able to do this. You guys know whats doable , What compressor would you recommend now that we dont need an enclosure to isolate the compressor.

BTW , I service and rebuild Industrial type piston comps as part of my duties at work. but, I've never messed with Dive or extremely High PSI 3 -5 stage. I know the ones I work on would suck real bad at this job being basically swiss cheese when helium is concerned , Open vents to the crank case , sealing around manifolds, head gaskets and such and that is what prompted the pressurized enclosure idea.. So thats why I ask questions. Sorry for seeming dumb to you. I'm sure dives are much better built, but dont know first hand HOW much better built. and if there was a way for air to migrate into the gas flow, or gas escaping from anything other than a failure. Thanks for putting up with my ignorance.