I'm just starting to purge kegs with fermentation gas, and couldn't find anything about the effects on increased keg pressure of purging effectiveness. It seems important, so I wanted to share.
TL;DR: Don't try to purge a keg with fermentation gas while the keg is under pressure. Either hook the unpressurized keg to spunding valve output, or turn up the pressure as late as possible to pressurize after purging is pretty much complete.
The not-as-short version (I can elaborate if needed, but I'm writing this on a phone while a 2yo jumps on my head, so this is all I've got for now.): I'm going to assume the numbers from @doug293cz 's post in Keg purging with active fermentation for easy comparison. There are a few caveats at the end*, but they don't massively impact the results, and there's no way around them.
I'm going to assume 1ATM of additional pressure thanks to spunding. This is ~14.7psi. It's a convenient number for the math as the amount of ideal gas in a volume doubles if the pressure doubles from 1atm to 2atm.
The good:
By pressurizing the keg to 1ATM with that nice CO2, your O2 concentration is halved. (The mass of O2 available to damage your beer, sadly, is the same.) So instead of starting at 210,000ppm, we're starting at 105,000ppm. Sweet!
The bad:
Unfortunately, the gas we're diluting from here on has twice the mass of the non-pressurized case. So if a bubble leaving the system and getting replaced by pure CO2 was diluting our old O2 concentration by 0.99999, now we're diluting by 0.999995. Looks small.
The ugly:
Due to the nature of compounding, the effect is huge. If we had 440L to purge a 25L keg pressurized to 1atm*, the result is 26.1ppm instead of 4.8ppb in the unpressurized case. That's almost 5,500x worse!
Conversely, if you purge an unpressurized keg and the use the last 25L of CO2 to pressurize it, you'll end at 13ppb. Much closer to the original case. The headspace after transfer would have 5.2x the base case O2 mass in it, although you could release the pressure post-transfer to get down to 2.6x.
*Caveats. There are some factors not covered/adjusted for above.
Conclusion: The ideal case may be to purge an unpressurized keg from the spunding valve output. Transferring might cause foaming issues, though, so spunding on the keg output at the end of fermentation may be more practical.
Note than making any connections after starting will introduce O2, even if the lines are purged, and unpurged lines would be really bad. (A 1m x 5mm ID tube could add 165ppm of O2 to a purged keg...)
TL;DR: Don't try to purge a keg with fermentation gas while the keg is under pressure. Either hook the unpressurized keg to spunding valve output, or turn up the pressure as late as possible to pressurize after purging is pretty much complete.
The not-as-short version (I can elaborate if needed, but I'm writing this on a phone while a 2yo jumps on my head, so this is all I've got for now.): I'm going to assume the numbers from @doug293cz 's post in Keg purging with active fermentation for easy comparison. There are a few caveats at the end*, but they don't massively impact the results, and there's no way around them.
I'm going to assume 1ATM of additional pressure thanks to spunding. This is ~14.7psi. It's a convenient number for the math as the amount of ideal gas in a volume doubles if the pressure doubles from 1atm to 2atm.
The good:
By pressurizing the keg to 1ATM with that nice CO2, your O2 concentration is halved. (The mass of O2 available to damage your beer, sadly, is the same.) So instead of starting at 210,000ppm, we're starting at 105,000ppm. Sweet!
The bad:
Unfortunately, the gas we're diluting from here on has twice the mass of the non-pressurized case. So if a bubble leaving the system and getting replaced by pure CO2 was diluting our old O2 concentration by 0.99999, now we're diluting by 0.999995. Looks small.
The ugly:
Due to the nature of compounding, the effect is huge. If we had 440L to purge a 25L keg pressurized to 1atm*, the result is 26.1ppm instead of 4.8ppb in the unpressurized case. That's almost 5,500x worse!
Conversely, if you purge an unpressurized keg and the use the last 25L of CO2 to pressurize it, you'll end at 13ppb. Much closer to the original case. The headspace after transfer would have 5.2x the base case O2 mass in it, although you could release the pressure post-transfer to get down to 2.6x.
*Caveats. There are some factors not covered/adjusted for above.
- The beer itself will contain more CO2 when under pressure. This reduces available purge gas.
- The fermenter headspace will contain more CO2 when under pressure. This reduces available purge gas.
- 25L seems like an over-estimate for corny keg volume. E.g. if you assume 23L, the base case goes down to 1ppb.
- Any sweeping action in the keg will be reduced due to lower velocity in the pressurized case.
- The choice of pressure will have huge implications. E.g. spunding at 1/3atm (4.9psi) only results in 78x O2 concentration.
Conclusion: The ideal case may be to purge an unpressurized keg from the spunding valve output. Transferring might cause foaming issues, though, so spunding on the keg output at the end of fermentation may be more practical.
Note than making any connections after starting will introduce O2, even if the lines are purged, and unpurged lines would be really bad. (A 1m x 5mm ID tube could add 165ppm of O2 to a purged keg...)
Last edited: