Questions for someone who understands gases (Physics major?)

[ASantiago]

Scenario: Two identical corny kegs (A and B), each containing 4 gallons of beer from the same batch. The kegs are both in the same kegerator and at 40F. Each keg is connected to a CO2 tank. Both CO2 tanks are identical, filled out the same, connected to the kegs using identical hardware and hoses, and both set to 5 psi.

The only difference between these two set ups is that the CO2 tank for keg A is kept at 76F (say, outside the kegerator), while the CO2 tank for keg B is kept inside the kegerator with the kegs.

Q1: Will the amount of CO2 molecules per cubic inch be higher in CO2 tank B than in tank A?

Q2: At 5 psi, will the beer in keg B absorb more CO2 than the beer in keg A because of the higher density of the molecules due to its tank being kept at 40F vs 76F?

I keep my CO2 tank in the kegerator and my beers are over-carbonated. Before I start moving kegs around (the CO2 tank is behind them), stirring up all the sediment in them, I'd like to know if moving the tank outside the kegerator has a good chance of reducing the carbonation in the beers, all other factors being kept the same.

Thanks in advance for any answers and discussion.

 

[blizzard]

The kegs will be the same pressure because the regulator sets the pressure of CO2 in the kegs at 5 psi. Consider this - if you had a full CO2 tank and set the regulator at 5 psi your beer would be carbed the same as if you had a half-full tank carbed at 5 psi. The key is the regulator.

 

[DSorenson]

Well the problem is that you are describing a sealed system. No matter which case, you are dealing with a specific mass of CO2. This simplifies the problem.

Since your volume is fixed and you have a set mass of CO2, we know that the only effective change will be the density and the pressure in the system. In case A where the tank is outside the tank, we will notice a gas density shift between the compressed CO2 outside the fridge and the pressurized keg inside the fridge. I have heard it posed that putting your CO2 tank outside of your keggerator helps with things like CO2 utilization, but I am not convinced and here is why:

The system is dynamic and reaches equilibrium between draft pours (and is finding equilibrium under force carbing situations). The warm air that enters the keg goes through a change in density once it passes the fridge walls (becomes more dense). The psi just beyond the regulator should be the same in either case, since presumably the gas has enough time to cool. The ideal gas law should be enough to demonstrate this concept.

PV=nRT, Where P is pressure, V is volume, n is the number of moles, R is the ideal gas constant and T is temperature. Inside the keggerator just past the regulator we have a presumably exact pressure limit. The temperature inside is the same in either case (since no matter what, the gas will quickly reach thermal equilibrium). R is constant, as is the number of moles in use. We also know that we have a fixed volume. Nothing should change...

As you force carb your beer, however, the distribution of pressure inside the keg changes. This is expected and should remain the same in each case.

I however yield to those who might be more experienced. This is just my intuition.

 

[ten80]

It doesn't matter if the tanks are inside our outside of the kegerator, I think this is much simpler than you think, lets work it out starting at the keg end:

1. Both kegs (and the beers) are at the same temperature.

2. Because of this, the CO2 in the headspace of each keg will be at the same temperature.

3. The pressure in the headspace of each keg is set by the pressure setting on the regulator. Even thought the hose leading outside of the kegerator may be warm on the outside of the kegerator, CO2 has very little heat capacity, therefore outside temperature will not transfer to the head space of the keg connected to this line.

Therefore, even if there is more pressure in the CO2 cylinder outside of the kegerator because it is warmer, the pressure in the kegs is set by the regulator connected to the CO2 cylinder. And because the gas inside the kegs is at the same temperature, the solubility of the CO2 in each keg is the same.

 

[max384]

EDIT: Crap. I misread your question. I was answering based on the kegs being at different temps, not the CO2 tanks.

Q1.

The warmer keg will have less CO2 molecules than the colder one.

We get this from the ideal gas equation: PV=nRT

If we solve for n (moles of the gaseous substance), we get:

(PV)/(RT)=n

To simplify things, we'll skip SI units, and set the gas constant to 1, because it really doesn't matter. The purpose here is to illustrate that with colder temps, you'll be left with more moles, all else equal, not to actually find any real values. We'll set the values to the following:
P = 5
V = 5
R = 1
T = 1 (in the cold example)
T = 10 (in the warm example)

If the temperature value is one, we get:

(5*5)/(1*1)=n
n = 25

If the temperature value is ten, we get:

(5*5)/(1*10)=n
n = 2.5

As you can see, as the temperature rises, there are less moles of gas, with a constant pressure and volume.

Q2.
Water is more soluble to some gases (CO2 included) at cooler temperatures. Thus, the colder the water temp, the more CO2 will be absorbed into solution. I'm not sure if the higher concentration of CO2 molecules in the colder keg also plays a role, or if it's just the solubility of the beer. It may be both.

 

[max384]

Scenario: Two identical corny kegs (A and B), each containing 4 gallons of beer from the same batch. The kegs are both in the same kegerator and at 40F. Each keg is connected to a CO2 tank. Both CO2 tanks are identical, filled out the same, connected to the kegs using identical hardware and hoses, and both set to 5 psi.

The only difference between these two set ups is that the CO2 tank for keg A is kept at 76F (say, outside the kegerator), while the CO2 tank for keg B is kept inside the kegerator with the kegs.

Q1: Will the amount of CO2 molecules per cubic inch be higher in CO2 tank B than in tank A?

Q2: At 5 psi, will the beer in keg B absorb more CO2 than the beer in keg A because of the higher density of the molecules due to its tank being kept at 40F vs 76F?

I keep my CO2 tank in the kegerator and my beers are over-carbonated. Before I start moving kegs around (the CO2 tank is behind them), stirring up all the sediment in them, I'd like to know if moving the tank outside the kegerator has a good chance of reducing the carbonation in the beers, all other factors being kept the same.

Thanks in advance for any answers and discussion.

Okay, now that I figured out the actual question, let me re-explain. Using the same equation PV=nRT, you'll see that the pressure will decrease as the temperature decreases. I'll illustrate this by solving for P with a change in T, using the same fake values from before, and 1 for the number of moles (n):

P=(nRT)/V

Colder temp:
P=(1*1*1)/5
P = 0.20

Warmer temp:
P=(1*1*10)/5
P=2

The reason I walked you through this is because the CO2 bottle left in the fridge would read slightly lower (not sure if it would be neglible or not in actual practice) if it were set to 5 psi while outside the fridge. So, as long as you set the regulators each to 5 psi while in temperature equilibrium with their stored temperature, there won't be a difference between the two kegs.

 

[rnm410]

I won't waste your time:

1. Select the system as the kegs
2. The conditions are the same in the system
Outcome. The carbonation will be the same
Note. The only variance is, keg A will require more energy (time) to reach steady state as defined by final carbonation. But is negligible, compare the specific heats and masses of water/gasses.
Tip. Select the proper system.

Credibility, I went to school for this.

Refer to this chart alternately.
http://www.homebrew.com/pdfs/CO2chart.pdf

 

[max384]

I won't waste your time:

1. Select the system as the kegs
2. The conditions are the same in the system
Outcome. The carbonation will be the same
Note. The only variance is, keg A will require more energy (time) to reach steady state as defined by final carbonation. But is negligible, compare the specific heats and masses of water/gasses.
Tip. Select the proper system.

Credibility, I went to school for this.

Refer to this chart alternately.
http://www.homebrew.com/pdfs/CO2chart.pdf

Why would this be the case? I'd assume that the C02 would be in temperature equilibrium by the time it reached the keg.

 

[ten80]

Hah! Three great explanations of the same phenomenon!

One thing you didn't mention was whether you were dealing with two kegs of the same beer or two different beers. For example a Heffeweizen may seem perfectly carbonated at 14 psi and but an imperial stout at the same temperature would seem very over-carbonated because the style is better suited to fewer volumes of CO2.

If it is the same beer in each keg, then you have an issue with a CO2 leak in one of the kegs, which would prevent that keg from fully carbonating.

 

[ten80]

Why would this be the case? I'd assume that the C02 would be in temperature equilibrium by the time it reached the keg.

I think you are saying the same thing, i.e. the energy lost from the "hot" CO2 as it enters the kegerator is negligible.

 

[rnm410]

Why would this be the case? I'd assume that the C02 would be in temperature equilibrium by the time it reached the keg.

I think you are saying the same thing, i.e. the energy lost from the "hot" CO2 as it enters the kegerator is negligible.

Sums it up perfectly. Negligible.

 

[max384]

OK. Just making sure I wasn't missing anything.

 

[Misplaced_Canuck]

Sums it up perfectly. Negligible.

Pretty much. As you first connect, the external tank's line will have slightly warmer gas than the internal tank. But overtime, the rate of transfer is slowed down, the line gets chilled, and the CO2 comes in at a very similar temperature.

Don't forget that CO2 coming out of the line is not the same temperature as inside the tank, as liquid CO2 (inside tank) loses heat when it is decompressed. (the old PV=nRT). Hence why you see chunk of dry ice when using a CO2 extinguisher.

MC

 

[judsonp]

The CO2 temperature doesn't matter at all, at least for long-term (equilibrium) behavior. The keg will continue drawing CO2 from the tank until the partial pressure of CO2 in the beer matches the regulator pressure. Since the beers are at the same temperature, this will be the same mass of CO2 for both. (Since your CO2 comes in a sealed tank, mass of CO2 is what matters.)

The only thing that really changes when you put a CO2 tank in a cold environment instead of a warm one is that the tank gauge reads differently. That's because CO2 is stored as a liquid, so the tank pressure is whatever the CO2 vapor pressure is -- and that changes with temperature.

 

[StLouBrew]

I keep my CO2 tank in the kegerator and my beers are over-carbonated. Before I start moving kegs around (the CO2 tank is behind them), stirring up all the sediment in them, I'd like to know if moving the tank outside the kegerator has a good chance of reducing the carbonation in the beers, all other factors being kept the same.

Thanks in advance for any answers and discussion.

I'm just wondering if neither of the two scenarios you posed have anything to do with the problem you're trying to solve. Having just gone through a "right-sizing" process myself with my beer lines, I'm just wondering if you've taken that variable into consideration?

Cheers

 

[raouliii]

Independent of the previous gas discussions, I believe there are more pertinent questions.

How in the hell are you getting truely overcarbed beer while applying 5psi at 40degF beer temperature?

Was this set it and forget it or was it burst carbed at some higher pressure?

How are you defining "overcarb"?

Is it just foaming when poured or does it have residual, very active CO2 after the pour?

What beer line diameter and lengths are you using?

 

[judsonp]

How in the hell are you getting truely overcarbed beer while applying 5psi at 40degF beer temperature?

I missed that part earlier. There is no way to force-carb at 5 psi at 40 F and actually get overcarbed beer. (There's no way to force-carb at 5 psi at any temperature and get overcarbed beer.)

Probably you're seeing symptoms of an unbalanced system that just happen to look like overcarbonation (e.g., foaming) or you're not actually carbonating at 5 psi (e.g., you're using a higher pressure first or you have sugars fermenting in your keg).