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Old 12-13-2012, 10:03 PM   #1
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Default dispensing vs carbonating pressure

Being new to kegging, I have more than a few questions, some of which could be viewed a rather "dumb" or not.

So, I plan on carbonation of my ales at around 2.2 volumes CO2. The kegs will be stored in my basement utility room, which sits at around 58F - perfect (IMO) for Scottish Ale and porter.

The BrewSmith calculator recommends around 18 psi for carbonation to 2.2 volumes CO2 at 58F

SO........if 58F is also my serving temperature, would I leave the CO2 bottle connected and regulator set at 18 psi for the duration of drinking the keg dry?

BTW, until I get a beer tower, I am using picnic faucets - crude, but useable!

Thanks,
Charlie

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Old 12-14-2012, 03:50 AM   #2
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58 is a little warm for pouring 2.2 volumes. You'll need about 15' of 3/16" ID serving line to pull it off but given the higher temp as it comes out of the faucet, you'll lose quite a bit of that carbonation quickly. Yes, leave it at that pressure the whole time.

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Old 12-14-2012, 03:55 AM   #3
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Thoghts in my head the same. Trying to explain it like Bobby. I'm on a different plain.

Bobby does not steer wrong. Listen to him, He sings a true song.

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Old 12-14-2012, 04:31 AM   #4
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Quote:
Originally Posted by Bobby_M View Post
58 is a little warm for pouring 2.2 volumes. You'll need about 15' of 3/16" ID serving line to pull it off but given the higher temp as it comes out of the faucet, you'll lose quite a bit of that carbonation quickly. Yes, leave it at that pressure the whole time.
OK, I have 4' of line from keg to faucet. What volume of CO2 would be more appropriate for dispensing at 58F? And where do I find this information?

Thanks,
Charlie
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Old 12-14-2012, 05:05 AM   #5
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OK, this is what I managed to find in terms of calculations, please correct me if I am incorrect, or there's something that I have overlooked.

L = (P-(H/2)-1)/R

P= psi keg pressure
H= height of faucet above the mid-line of the keg
R= resistance of serving line

I am using Bevflex 200, which is vinyl, so should have an R value of around 2.7.

This gives me a length of 5.9ft or 5' 11"

Given that my keg and faucet will sit at 58F, there should be no issue with temperate differentials between keg and faucet.

So why 15 feet as recommend? I am not questioning your expertise, but would like to learn why you feel that 15 feet would be better. That way, I can install 15 feet before I attempt to dispense any beer!

Thanks,
Charlie

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Old 12-14-2012, 01:42 PM   #6
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Quote:
Originally Posted by CButterworth View Post
So why 15 feet as recommend? I am not questioning your expertise, but would like to learn why you feel that 15 feet would be better. That way, I can install 15 feet before I attempt to dispense any beer!
Line resistance isn't a fixed number, it's strongly dependent on the density/viscosity/velocity of the fluid going through it. As a result, flow rate and temperature will both dramatically change the resistance you see in your system.

It's a similar scenario when you drive your car on the highway and the drag from air resistances kills your gas mileage. All other things being equal, if you drive at 80 MPH you're going to see much lower gas mileage than if you drive at 60 MPH. This is because the faster you go, the more wind resistance you're going to see. On a cold day, when the air is denser, it's going to be even worse. It's the same with the beer in your tubing: if the beer is flying through the lines, the resistance is going to be higher. If the beer is ice cold, the resistance is going to be higher. So you can see the limitations in fixed resistance values.

To actually calculate resistance per foot of tubing you'd use a complicated equation, not a fixed number.

The balancing equations and resistance values you're referring to are for bars, and assume you're pouring ice cold beer at a high flow rate (1 gal/min I believe) which makes them basically irrelevant for home kegerators where we like our beer warmer and we pour slower.

In most cases, the standard 5' that comes with a kit is not enough at ~12-14 psi, and will result in continuous foaming.

Most of us have found that 8-12 feet of standard 3/16" ID tubing is plenty for normal serving pressures at about 40 degrees. The pour may be on the slow side, but it will be foam free. In your case, with the higher serving pressure, you'll probably need 15 feet as Bobby suggested.
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Old 12-14-2012, 02:51 PM   #7
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It struck me last night that Boyle's Law is totally applicable here:

Gas solubility is inversely proportional to temperature. So, if I want 2.2 volumes of CO2 in my beer, the warmer the beer, the greater the pressure required.

The downside of this is that the greater the keg pressure, the greater the serving pressure and the greater the flow rate (maybe to the extent of being able to subdue rioters on a bad Saturday night in Paris!!!!)

By using a longer serving line, the resistance of the line increases, which will offset the higher serving pressure.

DOES THE ABOVE SOUND ABOUT RIGHT?

so, should I buy 20ft 3/16 line and shorten it empirically? It is not expensive, or can shorter lengths of beer line be joined? I would guess that the latter is not recommended due to adding turbulence in the beer flow to the faucet (as well as sanitization issues).

This whole kegging thins is extremely interesting, and hopefully very tasty.

Thanks,
Charlie

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Old 12-14-2012, 02:56 PM   #8
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You've nailed it.

Except that Boyle's Law (part of the ideal gas law) is applicable to gases only, not to gas dissolved in a liquid. For that, you need to look at Henry's Law.

I would do exactly that, buy 20 feet (since it's so cheap). If it pours fine, great! If it's way too slow, trim it off a foot at a time until it's good.

You're right, it's not the best idea to join multiple lengths of tubing because additional interfaces/restrictions can lead to turbulence and thus foaming.

Kegging is awesome. Enjoy it.
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Old 12-14-2012, 03:19 PM   #9
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Great explanation of the reasoning behind this. I was just going to say, 'Because warmer beer foams more!' but the scientific reasoning is appreciated!

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Old 12-14-2012, 06:16 PM   #10
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One of the points I made too briefly was that even if you get a nice controlled pour through the appropriate line length, the nature of warmer carbonated liquids is that they don't hold on to CO2 as readily as colder.

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