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Thread: CO2 mysteries

  1. #1
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    CO2 mysteries

    Last year I attended the Micromatic Draft Beer Dispense program and was told that keeping your CO2 tank inside the chiller box would yield 30% less product due to the tank & gas being cold. Keep the tank outside the box was the message. I don't see how this is possible as the CO2 is a liquid that boils off gas as the pressure is lowered--temperature having very little to do with it. Lower temperatures just means that there is more (very little more, BTW) CO2 left in the tank when it's "finished" than there would be at a higher temperature. Bring that same "empty" tank out of the box and warm it up and you'll push another 10 beers. No big deal, and certainly not anything like 30% less. Can someone explain to me their logic?
    On a related topic, and one I'm less sure about: We fill our small CO2 cylinders from large ones. The large 50 pounders have a "syphon tube" that extends to the liquid in the bottom of the tank. Past practice (don't know where this came from) has been to chill the small receiver tanks before filling from the larger one. Something about getting more CO2 into the tank. Does this make sense? Thanks for any help!

  2. #2
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    pV=nRT

    Play with pV=nRT and see what happens, keeping in mind that if there is CO2 liquid in the cylinder, the CO2 gas will be at the "dew" point, so to speak, unless you're above the critical point or the cylinder is empty enough. Liquid CO2 normally boils off as a gas inside the cylinder at room temperature. If you substantially lower the temperature, then it will boil off a little less intensely inside the cylinder. Consider dry ice, as well: In a freezer, it turns to gas more slowly than at room temperature. Within the hump on the diagram, the CO2 is present as both a liquid and a gas.



    By the way, do you brew on either a Bohemian or a Traditional system? Just curious...
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    Last edited by crassbrauer; 10-30-2007 at 07:59 PM.

  3. #3
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    Thanks for the reply Crassbrauer! I know the ideal gas laws, but here I'm dealing with phase transformations from liquid to gas. CO2 in the cylinder is liquid and will evaporate/boil until there is ONLY gas left. THEN and only then will the ideal gas laws be at play and only then will the CO2 temperature have any real effect. Why would one of the world's great draft specialists tell me that my CO2 won't last as long in the dispense box? I don't get it. And why would cooling a cylinder before filling make any difference in its capacity to hold CO2?

  4. #4
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    I get what you're saying, but if two phases are present in the cylinder, there's always some "boiling off" going on. pV=nRT does actually help in this case, because with R and n being equal, T, V and p change. If T drops substantially, i.e. from room temperature to 5 C, p and V must drop as well. This doesn't require that the gas has to be in a non-saturated state.

  5. #5
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    Had to go back through some of my books to look at the Ideal gas law to be certain but the T is in Kelvins (C + 273). This means that a change from room temperature at 20 C (293 K) to 5 C (278 K) would yield only a 5% change in T. Though it does change the formula, it is not a huge change.

    Dave

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    I have found that with a 50 lb CO2 tank in a cold environment and a serious draw of gas, there can be quite a bit of frozen CO2 remaining inside the cylinder. It won't come out on demand until several hours later after it thaws and vaporizes. Perhaps this is what Micromatic was referring to.

  7. #7
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    gitchegumee>

    even though it's in equilibrium inside the tank, the gas you draw (unless you are drawing liquid CO2) still follows the ideal gas law. What it really says is the supply pressure from the tank is lower. But since we only use ~15psi, it's no big deal if the tank pressure drops from 850 to 800psi, or 600psi if we care.

    However, evaporating liquid CO2 to gas does take up energy (just like boiling water into vapor is an endothermic reaction; so is true for CO2), and keeping a CO2 tank inside the cooler is basically removing the energy needed to the tank to evaporate the CO2. obviously it still evaporates inside the cooler, but as one can imagine, with lower efficiency.

    overall, it's just not the most efficient way of using your CO2 (or cooler space!)

  8. #8
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    Right. Phase changes require a large amount of energy. They are exploited in a large number of industrial processes for this reason, e.g. air conditioners / coolers, steam heating, etc., etc.
    Last edited by crassbrauer; 11-01-2007 at 05:04 PM. Reason: can't type

  9. #9
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    Thanks for the replies. I agree with what everyone is saying. I know that the liquid CO2 boils off (with energy input), and then the gas follows the ideal gas laws. So now back to my question: Assuming that we're NOT making dry ice in the cylinder (good point), WHY WOULD A CYLINDER IN A COOLER DELIVER SIGNIFICANTLY LESS (they quoted 30%) THAN A CYLINDER OUTSIDE THE COOLER. I mean, the amount of gas doesn't change! Five pounds is five pounds. I'm using the gas in exactly the same way! And why would chilling a cylinder before filling it make any difference? Perhaps a hot cylinder would vaporize the incoming liquid to a high pressure, thereby blocking further liquid addition? That's the only thing I can think of to justify this practice. Thanks for all the help!

  10. #10
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    Yes, 5 lbs is 5 lbs and there is no extra gas left in the cold cylinder when it is spent. So maybe the 30% difference is not the total amount of CO2 released, but instead the slower (by 30%) rate at which the liquid vaporizes into gas to push the beer if it is at dispense temperature and has to scrounge to get enough heat to vaporize the liquid CO2.

  11. #11
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    I think you're right, Moonlight. Everyone else, too for that matter. Once the tank is "empty" in the cooler, there might be a trickle once warmed, but again as Schlosser notes that difference is tiny. I think the fine folks at Micromatic may have made an error. Most of this stuff they know very well from practical experience, so I'm perplexed that on this issue they seem to be wrong. Thanks for the help!

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