Balancing a nitro system

[jwbeard]

I'm in the process of cutting new lines for a keezer, and am trying to calculate the proper line length for a nitro faucet. I used to get way too much head on stouts poured from my nitro system (1.5-2 inches, easily), and turning the pressure down gave a better result (~1 inch) but invariably resulted in the keg going flat since it didn't have enough applied CO2 pressure. I was running 7 ft lines on a kegerator that, on a good day in the summer, could only get down to 45º or so.

I'll probably be running the new system (a keezer) at 42-44º, but can't quite figure out how to calculate the proper line length for a stout (assuming 42º, 2 vol works out to 7.8psi). I read through this excellent resource, but the formula in Appendix C that purports to give you the proper applied pressure for a blended gas system (75 N2/25 CO2) seems to want me to put the keg under 75 psi to get 7.8 psi of applied CO2 pressure:

Applied = (Ideal CO2 Pressure + 14.7)/.25 -14.7
Applied = (7.8+14.7)/.25 - 14.7
Applied = 22.5/.25 - 14.7
Applied = 90 psia - 14.7
Applied = 75.3 psig of N2/CO2
​

Since most everything I've ever read has said blended systems do well at 30-40 PSIA, what am I missing here? I know the stout restrictor plate adds a bit of resistance, but I can't imagine it is enough to compensate for an extra 60 psi over what most 12" systems can comfortably handle. Not to mention 75 psig seems absurdly high... Thoughts?

[Edit] It occurs to me that 2 vol is a bit high for a stout (it's what the program I use, Beer Alchemy, suggested...), but even at 1.5 volumes the formula still suggests 53.3 psia of beer gas so the problem persists...

 

[jwbeard]

For the benefit of subsequent brewers, my (uneducated) thoughts re the question I posed -

After some digging online, I found that Guinness supposedly has 1.1-1.2 volumes of CO2 when it's served off tap. After plugging those volumes in for 42 degrees, I got the following results:

Volumes_CO2 Reading__Beer Gas Reading
1.0________-3.614________29.644
1.1________-2.476________34.196
1.2________-1.339________38.743
1.3________-0.204________43.284

That is, after adjusting for the partial pressure of the CO2 in the beer gas, the proper line pressure for beer gas at 1.2 volumes at 42º would be 38 psi, which is right in the range for the 30-40 psi of pressure I've seen recommended for beer gas systems. I think the volumes CO2 recommendations I've seen online (namely, 1.7-2.3 volumes) are just out of whack with the recommended beer gas pressures - those ranges are probably just fine for some stouts and porters, but if you set your 75/25 beer gas system to 30 psig at 42º you're not going to get 1.7 volumes (it's actually less than one atmosphere of CO2 pressure, working out to 11.85 psia or -2.85 psig, with the negative 2.85 pounds being supplied by nitrogen in the system).