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home made compressor (i have reached 1800psi)

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home made compressor (i have reached 1800psi)

Unread postAuthor: maggotman » Thu Dec 18, 2008 10:46 am

diving pumps are stage pumps if you got a regular compressor and fet it into the intake of a fridge compressor it would step up the pressur insted of goting from say 0 to 500 psi in one go if you made more than one step each time and cooled the air could it be posible to store the air in liquid form
what pressur does air turn liquid ? just a thaught
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Last edited by maggotman on Sat Jan 03, 2009 10:28 am, edited 2 times in total.

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Re: home made compressor idea

Unread postAuthor: ALIHISGREAT » Thu Dec 18, 2008 10:57 am

maggotman wrote:diving pumps are stage pumps if you got a regular compressor and fet it into the intake of a fridge compressor it would step up the pressur insted of goting from say 0 to 500 psi in one go if you made more than one step each time and cooled the air could it be posible to store the air in liquid form
what pressur does air turn liquid ? just a thaught


LOL liquid air, well air is a mixture of gases so not all would liquefy at the same point.... but air is 78% nitrogen and liquid nitrogen boils at -196 C at atmospheric... but i'm not sure i think the boiling point is higher with pressure (hence pressure cookers)
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Re: home made compressor idea

Unread postAuthor: Ragnarok » Thu Dec 18, 2008 11:17 am

maggotman wrote:what pressur does air turn liquid ? just a thaught

At 20 degrees C, I think it's in the region of about 12,000 psi to start liquefying the nitrogen in the air, although I may well be out, because I cannot find one sodding nitrogen phase diagram for the right temperature/pressure range.*

To start liquefying the oxygen, it would be about... 17,000 psi.
It should not have to be said that these pressures are NOT recommended.

*Instead, I'm relying some of my old calculations and DYI's figures, which can be found here: http://www.spudfiles.com/forums/viewtop ... tml#139978
Although I formerly mentioned oxygen's liquefication pressure as 40 ksi, I've amended that, based on better science.
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Unread postAuthor: cheeseboy » Thu Dec 18, 2008 11:31 am

I think its technically possible if you had about 7 fridge compressors to spare and a chamber capable of holding these pressures, which is really not worth the risks involved in pressures this high. The rate of power increase in spud guns with pressure is a curve the increase in velocity levels out as the pressures get higher (sorry for the bad description, can't find a picture)
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Unread postAuthor: Ragnarok » Thu Dec 18, 2008 11:34 am

Oh yeah, that's the problem I forgot to mention. Fridge compressors can't be used in stages like that, they're not designed for it.

I have designed a compressor that does work like that, and it would manage about 150 bar (2200 psi), but even that would take a heck of a lot of work to build, and a good storage tank.
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Unread postAuthor: POLAND_SPUD » Thu Dec 18, 2008 1:59 pm

Fridge compressors can't be used in stages like that, they're not designed for it.
the second part is perfectly ok...
but why can't they be used like that...? I suppose they can...

of course putting them in several stages would be madness and would result in death or serious injuries...

but putting ~50 psi at their air input would be ok IMO
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Unread postAuthor: maggotman » Thu Dec 18, 2008 2:05 pm

i have 3 co2 extinguishers rated up to 200bar
i was looking for about 500 psi

basically i want to store gass as a liquid so i can have hi pressure on demand
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Unread postAuthor: POLAND_SPUD » Thu Dec 18, 2008 2:29 pm

I suppose that 500 psi from a fridge compressor with an air inlet hooked up to a regular shop compressor should be achievable... I use the word 'suppose' because I've never tested such setup so it's more like 'yeah... I think it might work'

(note that some fridge compressors can reach and go past 500 psi mark... I've done it several times but it goes without saying that using them at 500 psi lowers their service life as more stress is put on the parts)...

you might find it difficult to get liquid gases at 500 psi :wink:
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Unread postAuthor: Ragnarok » Thu Dec 18, 2008 2:43 pm

POLAND_SPUD wrote:but why can't they be used like that...?

For the same reason that feeding extra pressure into a bike pump inlet wouldn't increase it's output - I can only put so much force on it.

Same thing for the compressor pump motor. It has an upper limit to the force it can generate, and thus an upper limit to the pressure that can be created.nMost pumps cannot be used in stages like that, as they each have an upper limit.

However, that said, there are reasons why you would want to hitch a shop compressor up to the input on a fridge compressor. Providing compressed air at 120 psi to the input would mean the pump was pumping air through it at 9 times the rate, which would drastically reduce fill times.

@maggotman: I say this with the utmost emphasis on your safety.

DISREGARD ANY IDEAS YOU HAVE ABOUT STORING AIR AS A LIQUID, IT IS NOT SAFE.

For one thing, the pressures are dangerously high. For another, liquid oxygen is also one of the hardest things to store, given how rapidly anything in contact with it oxidises/rusts/tarnishes.

Also, CO2 extinguishers rated to 200 bar usually have a 190 bar blow out safety, meaning they can't really be used above about 150 bar if you want to avoid any chances of the safety blowing - which unless you have a replacement, is the end of the cylinder. I'll assume you have no replacements, as you weren't even aware it was fitted with one.

150 bar is about 2200 psi. Not anywhere near what would be required to store air as a liquid.
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Unread postAuthor: maggotman » Thu Dec 18, 2008 3:39 pm

ime going to feed the fridge with a compressed air and see how that goes first
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Unread postAuthor: POLAND_SPUD » Thu Dec 18, 2008 4:11 pm

@ rag hmm now lets assume that you are in a sealed room filled with air at 10 bar... and you have a pump cappable of pumping up to 10 bar.... you pump the gun to the maximum...

what would be the pressure inside the gun ?? 10 bar or more ?? :-D


EDIT
what you are missing here rag is that fridge compressors have thick casing.... why does it have to be so thick ?? my guess is that the inside of the case is a part of the circut in fridges... which means that pressure inside the compressor case is equal to pressure at the air inlet...

I assume that increasing pressure at the air inlet = increase of pressure acting on the other side of the piston = compressor can pump to higher pressure since the pressure differential will stay the same but output pressure is higher than normally

that's my reasoning here... I can't find any other reason why would anyone manufacture them with such a thick casing... that would be waste of money
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Last edited by POLAND_SPUD on Thu Dec 18, 2008 4:48 pm, edited 2 times in total.
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Unread postAuthor: Ragnarok » Thu Dec 18, 2008 4:28 pm

maggotman wrote:ime going to feed the fridge with a compressed air and see how that goes first

Do you ever get the feeling that no-one is listening to you.......?

POLAND_SPUD wrote:@ rag hmm now lets assume that you are in a sealed room filled with air at 10 bar... and you have a pump cappable of pumping up to 10 bar.... you pump the gun to the maximum...

That is an entirely different situation to using compressors in series.

Also, by simple logic, the pressure in the launcher is still only 10 bar above ambient, making no performance difference if fired within the room. Leaving the room to shoot would cause explosive decompression, and probably also decompression sickness, thus making the point entirely moot. :tongue10:
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Unread postAuthor: POLAND_SPUD » Thu Dec 18, 2008 4:38 pm

@ rag check my edit it's perfectly ok... assuming that I am right about the inside-of-the-casing-pressure-being-equal-to-pressure-at-the-air-inlet theory :wink: (please if it'll turn out that I am right let's use this name to reffer to it)

Do you ever get the feeling that no-one is listening to you.......?
:roll:
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Unread postAuthor: Ragnarok » Thu Dec 18, 2008 4:54 pm

POLAND_SPUD wrote:I can't find any other reason why would anyone manufacture them with such a thick casing... that would be waste of money

I know the reason you've missed.

The casing on fridge compressors is thick to deaden the sound of the motor. People don't like noisy fridges.
Noisy fridges is a bigger waste of money than thick walled compressors, because no-one will buy a fridge that makes too much noise.

Simple.
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Unread postAuthor: POLAND_SPUD » Thu Dec 18, 2008 6:14 pm

well that's your opinion... unless someone tests it we won't know for sure
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