# Thread: Gas exerting pressure on gas?

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## Gas exerting pressure on gas?

"One gas can not, does not exert pressure on the other gas. The gas exerts pressure on the surface area of the container it is held in." I read this in another persons post on the site and I can't quite believe it. If two different gasses are in one container and both at the same temperature, and gas A at temp x exists at 100 psig and gas B at the same temp exists at 200 psig, gas A would be all liquid while gas B would exists as liquid and vapor. (lets say the container is 2 thirds full of liquid for this theoretical situation). The post I was reading said because the two gasses did not mix or interact with each other that something else would happen... I didn't really understand what he was getting at. Is my theory true?

The post im referring to http://hvac-talk.com/vbb/showthread....a-system/page3
It was by icemeister

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Originally Posted by TheL33tOne
"One gas can not, does not exert pressure on the other gas. The gas exerts pressure on the surface area of the container it is held in." I read this in another persons post on the site and I can't quite believe it. If two different gasses are in one container and both at the same temperature, and gas A at temp x exists at 100 psig and gas B at the same temp exists at 200 psig, gas A would be all liquid while gas B would exists as liquid and vapor. (lets say the container is 2 thirds full of liquid for this theoretical situation). The post I was reading said because the two gasses did not mix or interact with each other that something else would happen... I didn't really understand what he was getting at. Is my theory true?

The post im referring to http://hvac-talk.com/vbb/showthread....a-system/page3
It was by icemeister
You need to read up a little on Dalton's Law of Partial Pressures.

http://en.wikipedia.org/wiki/Dalton%27s_law for starters.

And, icemeister is a smart dude. He knows of what he speaks and those in the know try to learn from him.

Here is another excellent thread on the topic.

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## interesting

Originally Posted by jpsmith1cm
You need to read up a little on Dalton's Law of Partial Pressures.

http://en.wikipedia.org/wiki/Dalton%27s_law for starters.

And, icemeister is a smart dude. He knows of what he speaks and those in the know try to learn from him.
That's wild, so the molecules "go in between each other" kind of and they can both exist as a liquid and gas at the same time? So it would be a totally different animal if there was a barrier between them, like a sealed pneumatic cylinder with no rod, and gas A on one side and gas B on the other side, in that case the piston would slam to one side. I learn something new each day. - David

5. Originally Posted by TheL33tOne
That's wild, so the molecules "go in between each other" kind of and they can both exist as a liquid and gas at the same time?
yes, but the math for the gas laws only really works with non condensable gases. If one or more of the gases in the mix is condensable under the conditions present, the math goes out the window.

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## Thank you taking your time to teach me

Originally Posted by mark beiser
yes, but the math for the gas laws only really works with non condensable gases. If one or more of the gases in the mix is condensable under the conditions present, the math goes out the window.
Thank you taking your time to teach me, i have a totally unrelated idea / question that you may find interesting
I have an idea, that i cant post in detail in public because its not patented yet, but its for liquid pressure amplification, similar to hy-save's device http://www.hysave.com/lpa-retrofit-f...on-efficiency/ however i believe my device is way better. (its smaller, doesn't need a tank, so on..) The only thing i can think of that could stop my device from working, mind you i have not yet built a prototype or done any testing so there could be other things i have not considered, would be if it is impossible to re-compress gas into a liquid after it went through my pump. My pump wont leak, there are no seals, and it will never cavitate. it has the ability to take in liquid and gas and use positive displacement to send the mixture on its way to the liquid line heading to the evaporators. So my pump is not the issue, what i do not know is if when my pump pulls in liquid refrigerant on the intake stroke and in the process a bit flashes off inside the pump, would it be possible for my pump to (on the output stroke) compress it back into liquid before sending it to the evaporator. or is it stuck as gas at this point.

to keep things simple (for me.. heh) lets just say you have bubbles in your liquid refrigerant line going to the display cases and you add 15 to 20 psig to the line in a nice even manner, do the bubbles re condensate making the line solid liquid again? or does trying to compress that gas heat it to the point where it fights you trying to compress it. i personally dont think it would add that much heat to it and fight you, but my friend thinks so. my friend also has just patented his own device similar to hy-saves and is in the near future about to complete his prototype.

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i think i get it now, once one becomes a liquid all bets are off, because two gasses in one tank i can picture being at the same pressure no matter what because there in the same state, but if you had a gas and water the gas will exert pressure on the water like the atmosphere on a lake which tends to prevent the water from just evaporating like it would tend to do in a vacuum. of course things get more complex in a live refrigeration system with all the constant changes going on.

8. Originally Posted by TheL33tOne
"One gas can not, does not exert pressure on the other gas. The gas exerts pressure on the surface area of the container it is held in." I read this in another persons post on the site and I can't quite believe it. If two different gasses are in one container and both at the same temperature, and gas A at temp x exists at 100 psig and gas B at the same temp exists at 200 psig, gas A would be all liquid while gas B would exists as liquid and vapor. (lets say the container is 2 thirds full of liquid for this theoretical situation). The post I was reading said because the two gasses did not mix or interact with each other that something else would happen... I didn't really understand what he was getting at. Is my theory true?

The post im referring to http://hvac-talk.com/vbb/showthread....a-system/page3
It was by icemeister
This is all wrong. The gas molecules collide with each other. The average distance that they travel before successive collisions is called the "mean free path". The idea (theory) that the gasses behave independently, not interacting in any way with each other, will work to account for Dalton's Law of Partial Pressures, but this is not what is actually occurring. This theory will not account for the real behavior of gasses, things such as diffusion rate, speed of sound, or even the production of sound waves per se. In reality the molecules collide, transferring momentum between them. The chain reaction that follows, along with the Law of Conservation of Momentum, insure that the momentum of a given molecule in free flight at some given instant will be transferred to the surface of the container, even if not by that molecule directly. Think in terms of billiard balls. During the break the momentum of the cue ball is transferred to the rack of balls, which afterward fly toward rails of the pool table. The combined momentum of all of the balls will be equal to the initial momentum of the cue ball. A measurement of the pressure along the rails will be the same whether the cue ball struck the rack squarely, or whether it missed the rack completely and struck the rails itself. Either theory would accurately account for the pressure exerted on the rails, but the correct theory would be the one that is based in reality, i.e. that the momentum was conveyed via multiple collisions.

Gasses can and do exert pressure on each other. If you introduce nitrogen into a refrigeration system with its normal charge of refrigerant in place (compressor off), then the nitrogen will be initially concentrated near the point of entry, and will be a pure gas at the point of entry. Diffusion throughout the system could take hours or even days depending upon the system geometry and density of the refrigerant vapor in the system. Until that occurs there is pressure being exerted between the two gasses. This is the only thing that prevents each gas from instantly filling the entire volume.

As the gasses begin to diffuse the picture gets a little more blurry. You'll have gradients of the single gas densities and these will overlap from one end of the system to the other so that you will have a mixture of gasses and their mixed ratio will also form a gradient. The ratio of gasses in any vanishingly small volume will vary from one such volume to the next, until at some point in time the mixing becomes more or less uniform. This latter case is referred to as state of equilibrium, and is the state most often referred to in discussions of Dalton's Law, even though it is probably the least common condition encountered in the real world.
Last edited by hvacrmedic; 09-01-2012 at 01:58 AM.

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