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Opinions on 12v compressors

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: Gippeto » Wed Feb 20, 2008 8:31 pm

I had a 20 pound propane bottle in mind.

To get 4 hours, I calculated your chamber at 7.5 cu.in. , figured your approximate flow rate and went from there. I did not have a tank handy so I guessed at the dimensions. Should be reasonably close.

I think 250 psi should be safe, but will have to hydro test the tank to make sure.



Just found your link to the test pressure. 2.5 mpa or 362psi.
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Unread postAuthor: rcman50166 » Wed Feb 20, 2008 8:48 pm

I just bought a compressor for my BBMG project. It is an ok compressor to use but it is very slow to accumulate pressure in a large container. I'd recommend using it if the air demand is low or short in duration. Now the trouble is making a portable air tank to use with it. I assume that you may want to use the compressor for a mobile charge station. Getting a tank to be small enough to be portable or even wieldable is difficult; especially with PVC. If you need large amounts of compressed air over one charge I would recommend it but if you have a high demand I would recommend using a 3000 psi nitrogen tank; typically used for paintball.
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Unread postAuthor: POLAND_SPUD » Wed Feb 20, 2008 9:11 pm

@well the flow is acctualy higher because I haven't included all the fittings, Ts etc. just the pipe that is the main part of the chamber... sorry I should have included this information...

are you going to use three of them at once ? this would be a good idea

@ rcman - what type of compressor have you bought? a shop compressor or the same as I have?
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Unread postAuthor: Gippeto » Wed Feb 20, 2008 9:14 pm

This is more a case of "I have most of the stuff so I can put it together for coffee money."

I will have a use for semi portable, high pressure air of course, but 250 psi is only a start. As testing progresses, I will be needing more....a whole lot more. :D

Paint ball tanks are too small, they would give ~3 full power shots each.(48oz)


Polan_spud,

Higher flow is good news. :) Do you have a rough estimate? Units do not matter, I can convert.(cu.in./min or cm3/min) Thanks.

I would probably have two hooked together in the shop and keep one as a spare.
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Unread postAuthor: POLAND_SPUD » Wed Feb 20, 2008 9:36 pm

hmmm let see.... two 1/2" galvanised/maleable iron Ts, four 1/2" nipples and one 1/2" female-to-female fitting + inside volume of 1/2 QEV :D
rough guestimate --> additional 15 - 30cc (?)

so about 280 - 300 cc at 300 psi /minute

if you use two of them at the same time that will be up to 600 cc at 300 psi / minute
:D
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Unread postAuthor: Gippeto » Wed Feb 20, 2008 9:44 pm

75 minutes to fill a 27 litre bottle to 250psi using one compressor or 38 minutes with two. This seems very reasonable, especially considering the cost. :)

Thanks.
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Unread postAuthor: Ragnarok » Thu Feb 21, 2008 5:21 am

There is always the possibility of hitching a regular DIY compressor up to the intake on the fridge ones - as well as a check valve to bypass the fridge pump and allow the DIY compressor to do the first fill at a higher rate.

At 120 psi, that means that it's delivering 9 times the air through the pumps.
Including the higher initial fill rate, that would bring 38 minutes down to well under 4.
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Unread postAuthor: Hotwired » Thu Feb 21, 2008 8:03 am

I've been hearing that fridge compressors develop an oil mist to keep the parts lubricated in the sealed system.

When its opened up and being used as an air pump the oil will start being deposited in whatever you're pumping up. Which could be a good thing. But chances are it's a lot more oil than would strictly be needed for lubricating parts.

An oil filter sounds like it'll be required.
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Unread postAuthor: POLAND_SPUD » Thu Feb 21, 2008 8:39 am

yes hotwired you are right, fridge compressors develop an oil mist...Every month I add about 3 ccm of oil to the air intake
I don't have any air filter on my compressor and the internal surface of the gun is lubricated - it doesn't look messy and i haven't noticed any performance drop so I don't bother... the gun also releases a little of this oil mist when firing... I suppose it is good becase the internal surface won't rust but I am aware that after several months of using it to pump a tank there would be some oil on the bottom of it... (possibly putting the tank upside down would prevent this ??)
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Unread postAuthor: Gippeto » Thu Feb 21, 2008 10:28 am

Ragnarok, are you thinking boost pump / pressure amplifier?

That is on the project list too. (next 2-3) Do a google search for "jetsam baby booster" they have a pdf with a tear down and parts list.

Very simple and similar to some of the stuff I work on. :D
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Unread postAuthor: POLAND_SPUD » Thu Feb 21, 2008 10:45 am

I think he suggests to use a normal shop compressor to pump the first 120 psi and then to use the fridge compresor to pump to 250 psi....
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Unread postAuthor: Ragnarok » Thu Feb 21, 2008 10:58 am

POLAND_SPUD wrote:I think he suggests to use a normal shop compressor to pump the first 120 psi and then to use the fridge compresor to pump to 250 psi....

Not exactly, but that's part of it.

Air at 120 psi is 9 times denser than air at atmospheric pressure. If you put that air through a fridge pump, then it will pump 9 times the volume in the same time.

With the right setup, you could use the shop compressor for the first 120, then pump the denser air from it instead to get higher air volumes through the fridge pump in the same time. Instead of 280-300 cc per minute, you've got 2500 cc or more. That's a big difference.

The flow of even one low end shop compressor could supply 3 or 4 fridge compressors with this denser supply. If you can get said compressors cheaply, then you've got quite an arrangement. That would fill a 27 litre container in under two minutes I guess.
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Unread postAuthor: spanerman » Thu Feb 21, 2008 11:01 am

Long shot....but if you were at realy high pressures could this oil cause dieseling?
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Unread postAuthor: Ragnarok » Thu Feb 21, 2008 2:00 pm

@spanerman: Only if the temperature is high enough. As fridge compressors don't compress fast enough to do that, you're hardly in danger.

You need to compress air to a high enough pressure seriously fast - in the order of a few dozen milliseconds - to achieve such an effect.
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Unread postAuthor: Gippeto » Thu Feb 21, 2008 2:30 pm

Ok, I did not have much time earlier, I had to get to work.

You are suggesting to pre-charge the storage tank to 120psi (through the check valve), and then supply the fridge compressor with 120psi inlet air.

Thus using the fridge compressor as an electrically driven boost pump.

I can think of no reason why this would not work.

A very good idea. :)
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