# Compressed gases questions

• posted
Just curious, I'm not transferring any gases. I was just watching the
guy fill our propane cylinder and it got me to wondering.
Here's the first situation: Let's say I have two gas cylinders of
equal size. One is filled with a gas that is liquid at room
temperature when compressed, the other is empty. The cylinders are
upright. Now the cylinders are connected with a pipe so that the gas
from the full one flows into the empty one. As the gas starts to fill
the empty cylinder it cools enough that it liquifies and rains into
the cylinder. But once the pressure equalizes the rain stops and the
end result is that both cylinders are at the same pressure but that
the originally full cylinder will have more liquid in it.
Here's the second situation: Everything is the same except the
empty cylinder is much larger than the full one, maybe ten times as
much volume. Once the pressure is equalized between the two cylinders
will the much larger cylinder now have more liquid than the smaller
one? I think it will.
Thanks,
Eric
• posted
There are commerically-available cylinder heaters to empty one cylinder into another. works with anything that is partially liquid at room temperature and elevated pressure. You have to weigh the cylinder that is being filled to make sure it is not OVERfilled.
Jon
• posted
Just curious, I'm not transferring any gases. I was just watching the guy fill our propane cylinder and it got me to wondering. Here's the first situation: Let's say I have two gas cylinders of equal size. One is filled with a gas that is liquid at room temperature when compressed, the other is empty. The cylinders are upright. Now the cylinders are connected with a pipe so that the gas from the full one flows into the empty one. As the gas starts to fill the empty cylinder it cools enough that it liquifies and rains into the cylinder. But once the pressure equalizes the rain stops and the end result is that both cylinders are at the same pressure but that the originally full cylinder will have more liquid in it. Here's the second situation: Everything is the same except the empty cylinder is much larger than the full one, maybe ten times as much volume. Once the pressure is equalized between the two cylinders will the much larger cylinder now have more liquid than the smaller one? I think it will. Thanks, Eric ========================================
As the higher pressure gas flows into the second cylinder that cylinder will warm up from the heat of compression so I don't think there will be any "rain". The first cylinder will cool from the heat of evaporation as liquid evaporates to replenish the head pressure. Once the tank pressures and temperatures equalize any liquid will stay where it is, since to make it move you have to supply heat of evaporation to the liquid to make vapor, and remove that same amount of heat to condense vapor where you want the liquid to end up. Cylinder volumes only matter in the time to reach thermal equilibrium.
• posted
Good luck wading into Thermodynamics. I had less trouble with Quantum Mechanics. -jsw
• posted
Greetings Carl, I forgot about the empty cylinder heating up. But if it is large enough I still think some gas would liquify before the tank heated up too much. There must be a mathematical way to figure it out. I might need to do a little research. Eric
• posted
Did you read the reference I gave you to latent and specific heat calculations?
-jsw
• posted
?Que?
If the pressure is high enough to keep the more volatile gas in the liquid state then, aside from a tiny transfer as pressure is equalized, you'll wind up with two containers of (mostly) distinct liquids. If the vent between the two is maintained long enough you'll get about a 50/50 mix in each container .... basically a layer of liquid propane floating on top of water.
Not terribly interesting ....
Oh yea ... is there an 'alt.fat.NOacceptance'' group ? :-)
• posted
in fact used as such in some places . The thing to remember is that as the liquid propane enters the empty cylinder it expands - and boils some off - and chills the empty bottle . The inlet valve acts as an expansion valve , just as in a cooling system . That chilling effect is why we can fill small bottles from bigger ones - but it must be done with a bottom or liquid tap into the big tank so it supplies liquid propane only . We used to fill Dad's camper tanks this way from the big tank that fed our house before we got natural gas .
• posted
What carl says is correct and what you say is correct. As the gas boils off it cools. The heat from the cooled gas has to go somewhere so it heats up the cylinder the gas is expanding into. So I think what happens if the empty cylinder is much larger than the full one is that the gas will chill and condense and rain into the empty cylinder until the increased pressurizitation of the cylinder heats it up enough to stop the gas from condensing. I wonder when that happens? Eric
• posted
No but we could make one.

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