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home made compressor (i have reached 1800psi)

A place to ask general spud cannon related questions.
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Unread postAuthor: Daltonultra » Tue Jan 06, 2009 9:39 pm

maggotman wrote:i think it was a lg one


Was it possibly this one:

Image
Image

If it is that one, am I right in assuming the smaller tube is the output?

BTW, if you're gonna make a steel coax to fire at those pressures, you will have to use a bronze piston seal. Any kind of plastic is gonna blow right through.
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Unread postAuthor: D_Hall » Tue Jan 06, 2009 11:42 pm

john bunsenburner wrote:Are you sure of that D_hall? Plainly theoretically speaking, it should be possible, though i am guessign it is not very practical or we would be using it(we as in humanity) :?:

Sorry, it is not possible even in theory. Just because you think it "should be possible" does not mean that it actually IS possible.

If you compress air at room temperature to Possitively Insane pressures, all you get is a very dense gas. You do not get a liquid. To turn air into a liquid you also need to refrigerate.

Again, look up the phrase "critical temperature."

Here, I'll even provide the wiki link...

http://en.wikipedia.org/wiki/Critical_temperature



edit to add: A fun thought experiment....

You have a pressure vessel half full of water and half full of steam ("water vapor" if we're at room temperature). Start to heat the water up. What happens? More water boils. The pressure increases. OK.... But two other things happen. The density of the steam increases while the density of the water decreases.

Keep heating.

Density of steam continues to increase while density of water continues to decrease.

Keep heating....

At some point, you'll find that the steam is just as dense as the water. How then can you tell the difference? Answer: You can't. Your water isn't really water anymore. It's all just really dense steam!

This happens at about 374 C (705 F). Note that steam turbines tend to run using steam at these temperatures. Why? Because they can *GUARENTEE* that there's no water in it (water droplets will damage turbine blades (see: http://en.wikipedia.org/wiki/Steam_turbine)). Since there's no water in it, it is refered to as "dry steam".

This phenom happens for pretty much all liquids/gases. As I'm sure you can imagine, it happens at different temperatures for different materials. It just so happens that for nitrogen and oxygen (the two primary constituents of air), this happens at cryogenic temperatures.

Note that CO2 is a fun one. CO2's critical temperature is only slightly above room temperature (31 C / 88 F). Thus, CO2 can be dancing around the critical point while it sits in a tank in your pocket or whereever.
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Last edited by D_Hall on Wed Jan 07, 2009 12:00 am, edited 3 times in total.
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Unread postAuthor: Gippeto » Wed Jan 07, 2009 12:26 am

BTW, if you're gonna make a steel coax to fire at those pressures, you will have to use a bronze piston seal. Any kind of plastic is gonna blow right through.


Really? I'm using teflon, and have pressurized to ~1000psi, with no signs of extrusion problems.

Seat pressure @ 1000psi is ~1370psi.

I'll check again, but I do believe this is well below the pressure required to extrude room temp teflon.
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Unread postAuthor: D_Hall » Wed Jan 07, 2009 9:31 am

And gee, I routinely use hydraulic systems that run at 3500 psi... Viton o-rings.
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Unread postAuthor: john bunsenburner » Wed Jan 07, 2009 12:16 pm

what about the whole thing about boiling points increasing as pressure rises???One of us has something wrong though i am very certain it is me but i want to understand this. I always learnt that if you increase pressure you increase the boiling point that means that if you have oygen compress it insanely and then let it cool to room temperature, you should have liquid O2? Is that not the principle of a pressure cooker having the pressure rise so that water boiles at a higher temperature than normally?
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Unread postAuthor: psycix » Wed Jan 07, 2009 3:56 pm

you have oygen compress it insanely and then let it cool to room temperature, you should have liquid O2?

No, supercritical O2.
O2 is already beyond its critical point on room temperature, so it will never turn into a normal liquid.

After a quick search on google:
* Critical temperature : -118.6 °C
* Critical pressure : 50.43 bar
If the temperature higher then -118.6 C, increasing the pressure will only grant you a supercritical fluid.
http://en.wikipedia.org/wiki/File:Phase-diag2.svg
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Unread postAuthor: maggotman » Wed Jan 07, 2009 4:01 pm

i have 3 empty 02 tanks all rated to 200 bar
if i stored 100 bar air in them would that be an ok saftey factor
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Unread postAuthor: ALIHISGREAT » Wed Jan 07, 2009 4:11 pm

maggotman wrote:i have 3 empty 02 tanks all rated to 200 bar
if i stored 100 bar air in them would that be an ok saftey factor


the 200bar is with a safety factor... so 200 bar would be fine.
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Unread postAuthor: maggotman » Wed Jan 07, 2009 6:09 pm

just had a closer look it seems to have a safety valve in the valve.
its nice and light aluminum and will just fit in a rucksack

me needs to get a new fridge pump to abuse :twisted:
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Unread postAuthor: POLAND_SPUD » Thu Jan 08, 2009 10:30 am

yeah it's 200 bar with a safety factor... but when people say they want to use some tank this ussually means - an old tank... you can't tell whether there is rust inside or not.. what is more, air contains water... so after some time water will start to consensate inside the tank...

you plan to use a fridge compressor to get 100 bar and I know yours broke down at 1800 psi, which is more than 100 bar. However, you don't know how well it would perform in the long run.. I suppose htat overheating might be a problem here
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Unread postAuthor: john bunsenburner » Thu Jan 08, 2009 12:25 pm

Why not use a lubricant to cool it, i find it irgonic that fridge compressors overheat...
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Unread postAuthor: maggotman » Thu Jan 08, 2009 1:12 pm

the tanks are aluminum they will not rust
well the fridge that i have got lined up has a fan to cool the gas so i will face it at the tank and i might run 2 at a time if i can get more
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Last edited by maggotman on Thu Jan 08, 2009 5:54 pm, edited 1 time in total.

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Unread postAuthor: Lentamentalisk » Thu Jan 08, 2009 5:09 pm

So if one were to make a steam cannon, using supercritical fluid, what would happen when the valve is opened?
I know that with very very hot water, when the valve is opened, it all boils off, but in the process, a lot of energy is used to boil the water. If the water were stored in its supercritical fluid state, then would there be any energy used in state change, as it turns into a gas, or what?

By the way, D_Hall, that little mental exercise was absolutely fascinating! I love learning weird concepts like that.

Edit: Maggotman, is there any way you could press the spell check button or something like that before you post? Your spelling has been horrendous, making it hard to understand what you are saying at points.
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Unread postAuthor: maggotman » Thu Jan 08, 2009 5:53 pm

sorry about that ime just being lazy
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Unread postAuthor: psycix » Thu Jan 08, 2009 9:58 pm

So if one were to make a steam cannon, using supercritical fluid, what would happen when the valve is opened?
I know that with very very hot water, when the valve is opened, it all boils off, but in the process, a lot of energy is used to boil the water. If the water were stored in its supercritical fluid state, then would there be any energy used in state change, as it turns into a gas, or what?

You can look it up in the phase diagram.
When you are in the supercritical zone and drop off pressure (and a little temperature due to the decompression of the gas), move downwards in the diagram, and that leads you into the gaseous phase.
I have no idea whether it takes, gives or does not take/give any energy to go from supercritical to gas.
I think that it does not give or take any energy as the supercritical fluid phase looks the most like the gas phase.

EDIT:
Bling idea, I know the answer:
Looking back on D-halls though experiment, the water needed to boil in order to become supercritical. Thus I think I can confirm that there is no energy gained or taken when going from superciritcal to gas. They are one the same energy level.
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