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Initial thoughts on high pressure gun using fridge pump

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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Initial thoughts on high pressure gun using fridge pump

Unread postAuthor: Deactivated 1 » Sun Oct 29, 2006 10:18 am

Hi. I just thought it could be interesting to design a gun based around a spent CO2 bottle and a copper barrel. As for the valve I'm undecided. (Actually, I may have posted about this a while ago) But I just thought, to get the best use out of it, one might use a fridge compressor to pump the CO2 tank up to a respectable pressure (with plain old free air).

See here and here. I don't know what pressures you could expect from an average fridge pump, since I just wanted to post my initial thoughts in the hope that someone else might have something interesting to add.

Here’s the idea: a 20 oz CO2 bottle as a chamber (Mentioned in another thread), which could be pressurised to a very high pressure with a fridge compressor. This saves going to a store and having it pressurised with a crappy flow restricting valve - you could do it however you wanted with safety in mind. Then, the valve that restricts the highly pressurised air in the CO2 tank from the barrel would have to be one that can be opened and closed quickly. Using up an entire tank at this higher than average pressures would be wasteful, not to mention loud. I think a portable semi-auto could be possible. I'll put more thought into such a valve. I looked into BB gun valves, but they seem a little ghetto. I might be wrong though. I'll look into those more when it isn't midnight.

Any thoughts? I'm sure you're all thinking I'll blow myself up, but just because I'm talking in broad terms here doesn't mean I'll build safely if at all in the future.
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Unread postAuthor: Recruit » Sun Oct 29, 2006 11:01 am

It is illegal to remove the compressor in most countries because doing so
releases the freon.
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Unread postAuthor: mark.f » Sun Oct 29, 2006 12:58 pm

Evacuate the freon into an empty coke bottle using an automotive vacuum pump. Won't make it any more legal, but at least you'll sleep better knowing you didn't pollute the environment.
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Unread postAuthor: Deactivated 1 » Mon Oct 30, 2006 12:05 am

There's no need to swat me down with the legality of it all - The guns themselves are illegal where I live, and the environment is going to hell anyway.

Has anyone else converted a fridge compressor into an air compressor? Can you comment on whether or not my idea would be feasible? I'll probably never build it, but I've had an idea in mind for a while and this is one step closer to my hardware store if it would work.

Does anyone know of a valve I could use that actuates really quick? Apparently, CO2 BB guns use a mechanism which hits a valve open with a spring loaded hammer, allowing the CO2 to escape for a short time before it pushes the valve closed again. Despite the crappy airflow of such a design, the thousands of PSI pushing against the projectile due to the pressure of the CO2 accounts for it.

So, If I can pressurise a spent CO2 container with air and find some sort of fast actuating valve, it would become semi automatic and portable. This is assuming I could develop very high pressures with the fridge compressor, otherwise the poor flow of the valve would reduce the performance too much.

Thoughts?
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Unread postAuthor: pyrogeek » Mon Oct 30, 2006 1:07 am

I have used CO2 before, and have a little knowledge about it. It is stored as a liquid, so as a result the tank tends to stay around 800-1000PSI depending on the temperture. The bulk tanks (like what are used for welders and soda fountains) are rated to 1800PSI, so it won't get anywhere near that, because the burst disk will rupture before that happens.
The older refridgerators used, I want to say R12, quit using it probably around the early '90s since cars quit using it around that time. That refridgerant also acted as a lubricant, so if you use a pump from a refridgerator that used it, you will need to find a way to lubricate it. I don't know what kind of pressure a pump like that could make, but I don't think the tubing they use is capable of carrying close to 1000PSI.
If you could safely dispose of the old refridgerant though, and you had a larger tank (maybe an old #20 propane tank?) then you could just use something like a piston valve and the lower pressure, and hopefully it would do the job for you. Something like 200PSI and a pretty free flowing valve seems like it would do pretty well.
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Unread postAuthor: pyrogeek » Mon Oct 30, 2006 1:13 am

Also, note that the first link mentions how slow the pump pumps. The writer said it may take 1/2 hour for it to pressurize a fire extenguisher as it's tank. THAT SOUNDS WAY TOO SLOW!
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Unread postAuthor: Recruit » Mon Oct 30, 2006 2:51 am

Get a big tank and let it fill when you are asleep.
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Unread postAuthor: Deactivated 1 » Mon Oct 30, 2006 5:50 am

Yeah it does seem slow. I had imagined filling only a small container to a really high pressure, although that would take just as long as a larger tank to a relatively low pressure. Thanks for the info on the tanks. I'll have a look into high pressure piston valves.

I'd like to see this all work out. It would really just be an oversized gas powered BB gun in terms of operation.

Edit: Okay, so I've been starting at the animated diagram of a piston valve here for a while, and initially I thought it would be impossible to open and close it quickly. I.e it would discharge the full pressure from the chamber through the barrel in one shot. However, if the equalisation hole (Hole drilled through piston to allow air to flow into the chamber) was really large - maybe half the diameter of the piston itself - It might just seal up again quickly.

I'm doubtful though, as the chamber should only seal back up once the projectile has left the barrel. At this point, it becomes really easy for the air to escape out the barrel - there's no blockage. But I want it to go through the piston and push it closed again...

Another downside is that I can't use a linear design with a piston valve. A linear design would be better, as it would be easier to fit inside a replica gun.

I hope someone understands what I said about the piston valve :P Does anyone think that would work, or alternatively can anyone think of a different valve that would let a small volume of air discharge from a highly pressurised chamber?

Check out the general design for BB gun valves here. Note that this design would be inappropriate in a 100-200psi gun due to the poor airflow. (One thing that confuses me is how the hell a spring loaded hammer can whack the valve stem in hard enough to counteract the 800 or so psi acting on the other side)
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