Kegging Calculations

[Wacki]

Hi all,
This is my first time kegging and I want to make sure i'm doing my line length and serving pressure calculations correctly, and i'm accounting for altitude if needed (as i live around 5500ft). I understand how to determine my carbonating pressure to achieve a specific CO2/volume result using this web calculator/chart, which says I want to put my keg on 11.2psi for 2.5 vols of CO2 at 38* F...which it is sitting at now for about 4 days. I remember reading that I dont need to adjust this PSI for altitude (please correct if I'm wrong).

To find my line length, I used this calculator, using 11.2 as my pressure inside the keg, 2.2 as resistance/ft, and 1.5ft as the height from the center of my keg to the tap. This gives me a line length of ~4.75ft, which is close enough to the line that came in my kit.

I think I've done everything good up to this point (please correct me if I am mistaken), but I'm confused about the process now to put the keg on serving pressure after two weeks (the sheet I used also doesn't account for altitude, does that matter?), should I:

- turn off and remove the gas line from the keg
- purge the excess pressure in the keg (All the way??)
- re-connect the gas line at serving pressure (which I'm calculating to be around 7psi, based on a spreadsheet I found here (4.75ft hose length, 3/16 diameter hose, 1.5ft height, 9 second fill time)
- connect beer line
- pour and enjoy beer??

Thanks for any comments or critiques in advance!

 

[mrsoltys]

Hi,
I'm the creator of the second spreadsheet mentioned. While i've never kegged a home brew, I do know a lot about fluids and love beer, so I wanted to contribute a bit to the discussion.

First: about Serving Pressure vs Carbonating Pressure. I think you'll want to keep your pressure at 11.2 the whole time your servering your keg (not backing off to 7psi). This will keep the beer properly carbonated the whole time, vs reducing the pressure which will make the last bit of beer flatter as the carbonation comes out of the beer over time.

Second: About hose length. The first calculator you used gave you 4.75 ft, while my calculator gives you a length of 7.5 ft (9 second pour at 11.2 psi) or 9 ft (10 second pour at 11.2 psi). The equations use different assumptions (the first being more back of the envelope, while I'd argue that mine might be a little more theory-based). I'm obviously going to say that I think my equation is a little more accurate (Although i haven't tested it experimentally), but you can do a simple test: Start with a 7.5 ft hose, and if your beer pours in 9 seconds, you're good to go. If it comes out too slow, trim it down towards 4.75 ft. (It's harder to work backwards... starting at 4.75 ft and if your beer pours too quick and foamy, add hose length).

Third: About altitude corrections. I just had a good debate with some other fluids guys about this. We're all pretty sure that you don't need to do any correction to the carbonating pressure for elevation, but if we're wrong and the beer taste a bit under-carbonated then the correction would be to add 2.7 psi. (don't forget to increase your hose length).

I should note: It takes about 10 days for the carbonation to reach equilibrium. If the beer tastes under carbonated right at the start, give it some time before upping the pressure, and maybe go in small 1 psi increments.

 

[mrsoltys]

Third: About altitude corrections. I just had a good debate with some other fluids guys about this. We're all pretty sure that you don't need to do any correction to the carbonating pressure for elevation, but if we're wrong and the beer taste a bit under-carbonated then the correction would be to add 2.7 psi. (don't forget to increase your hose length).

After a bit of reading, i see that we were wrong here. Carbonating pressure tables assume sea-level. You'll need to add approx. 1 psi per 2000 ft above sea-level for proper carbonation. Don't forget to lengthen your hose.

So:
at 5500' for a reading of 11.2 psi, set your CO2 pressure to 13.9 psi. Then run a 9' hose for a 9 second pour time (assuming still 1.5 ft from keg center to tap, and using my calculator). Finally, enjoy some beer.

Note: No elevation correction is needed for calculating the hose length.

 

[zachattack]

After a bit of reading, i see that we were wrong here. Carbonating pressure tables assume sea-level. You'll need to add approx. 1 psi per 2000 ft above sea-level for proper carbonation. Don't forget to lengthen your hose.

Yup, since the regulator reads psig you'll need to compensate. Add 2.5 psi for your elevation, the absolute CO2 pressure will be the same but the gauge pressure will be a bit higher.

Most people on the forum have found that the line length calculators don't give great results for a number of reasons. It's nice that this spreadsheet has some alternate formulas, but remember you might want to serve a beer at a high pressure down the road. I'd grab 15 feet of 3/16" ID tubing (if you're using standard vinyl) and plan to trim it down a bit if the flow is too slow. For most people 10 is fine, but given your elevation and the fact that your pressure needs to be a bit higher I'd try 15.

mrsoltys, for homebrewing the only differences are that some of us like to be able to serve a variety of carb levels, we generally keep the beer warmer (~40 deg F) and we don't mind a slower pour. Most of us just buy longer lines than we need and go with it. As you pointed out, the standard "XX psi per foot of tubing" assumptions break down once you change the temperature and flow rate.

 

[JuanMoore]

Keep in mind that all of those calculators and spreadsheets give you the MINIMUM length that they think won't cause foaming, based on a bunch of assumptions. If the assumptions are wrong, you'll need a longer line. If you ever decide to try serving a beer at a warmer temp or higher carb level than you used for the calculations, you'll need a longer line. The only side effect of longer lines is a slightly slower pour. It's also much easier to trim a foot or two off of an extra long line than it is to replace a line that's too short. For all of these reasons, I'll echo zachattack's suggestion to buy a longer line than you think you'll need.

First: about Serving Pressure vs Carbonating Pressure. I think you'll want to keep your pressure at 11.2 the whole time your servering your keg (not backing off to 7psi). This will keep the beer properly carbonated the whole time, vs reducing the pressure which will make the last bit of beer flatter as the carbonation comes out of the beer over time.

Not only that, but the lower pressure will also cause CO2 to come out of solution in the lines, forming pockets of gas, which will cause the first pour of every drinking session to be foamy (until the carbonation reaches equilibrium with the lower pressure).

Second: About hose length. The first calculator you used gave you 4.75 ft, while my calculator gives you a length of 7.5 ft (9 second pour at 11.2 psi) or 9 ft (10 second pour at 11.2 psi). The equations use different assumptions (the first being more back of the envelope, while I'd argue that mine might be a little more theory-based). I'm obviously going to say that I think my equation is a little more accurate (Although i haven't tested it experimentally), but you can do a simple test: Start with a 7.5 ft hose, and if your beer pours in 9 seconds, you're good to go. If it comes out too slow, trim it down towards 4.75 ft. (It's harder to work backwards... starting at 4.75 ft and if your beer pours too quick and foamy, add hose length).

There's no one ideal pour speed. Virtually all of the other calculators/spreadsheets/equations out there for line balancing assume ~1gal/min. That works ok for commercial systems where the beer is kept very cold and the carbonation levels are at or under ~2.7 vol. The warmer or more carbonated the beer is, the slower the pour needs to be in order to prevent foaming. This is why so many homebrewers don't trust those line balancing calculators.

If you want a real challenge, try creating a spreadsheet that accounts for the relationship between the temperature, carbonation, fluid velocity, and foaming. Ideally the inputs would be desired serving temperature, desired carbonation level, ID and material of serving line, and the outputs would be the required serving pressure, and the line length that would give the fastest fluid velocity that doesn't cause excessive foaming.

Since this becomes such a complex animal at higher temperatures and pressures, many people opt for extra long lines. The only side effect is a slightly slower pour, and the benefit is being able to serve beers at a wide variety of temperatures and carbonation levels without foaming. Since the line resistance decreases as the fluid velocity decreases, the time it takes to fill a pint even with 15'-20' long lines isn't significantly longer than with 5'-7' lines.

BTW, I really like your spreadsheet. It's much better and more accurate than anything else I've seen. It also doesn't assume that line resistance is a constant for a given type of beer line, which drives me crazy.

 

[mrsoltys]

BTW, I really like your spreadsheet. It's much better and more accurate than anything else I've seen. It also doesn't assume that line resistance is a constant for a given type of beer line, which drives me crazy.

Thanks!

If you want a real challenge, try creating a spreadsheet that accounts for the relationship between the temperature, carbonation, fluid velocity, and foaming. Ideally the inputs would be desired serving temperature, desired carbonation level, ID and material of serving line, and the outputs would be the required serving pressure, and the line length that would give the fastest fluid velocity that doesn't cause excessive foaming.

I love challenges. I think this is possible. The problem is when "excessive foaming" occurs depends a lot on the brew in question, and so some educated guess on velocity (flow rate) is needed.

 

[Wacki]

Thanks all for your input, I agree with JuanMoore, I liked the intricacy of your spreadsheet. I'm a quant by day and its always cool to see the complicated side of something I dont understand.

So in all I need to add 2.75psi while carbing to adjust for altitude, make my beer line about 15ish ft long to account for potential for warmer temps and higher carb'd beers, and keep the serving pressure the same as the carbing pressure.

Since I am not changing the pressure when serving the beer, do I need to do any special process like I mentioned before like purging?? It seems unnecessary and all i should have to do is hook up the beer line and drink beer!! Or am I missing something else??

Thanks again!

 

[ShockedHop]

Mrsoltys

I was just coming to post a link to your spreadsheet for others. I too am a first time kegerator owner. I tried other formulas that I found but nothing compares to actual science! I used your spreadsheet and now I have perfect pours every time. Thanks!!!