Purge keg with fermentation CO2

[slurms]

Short question... If I connect the off-gas of my fermenter to an empty keg, will it produce enough CO2 to completely (within reason) replace all air with CO2? What about if I connect two kegs in series, will it be enough to flush both of them all of air?

Longer story... Currently, I ferment in a fermonster where I modified the lid to have a liquid and gas QD and a floating diptube. The gas from the fermentation is directed to the diptube QD of a keg full of sanitizer, so the ferm gas pushes all liquid out and fully purges the keg which then receives the beer later. This is all well and good, but I was thinking about trying to switch things up (for a reason, don't like cooling the fermonster because sometimes it does not form a good seal and I think takes in air..)

Definitely not a new idea, but Ultimately what I was thinking was this. Will have the fermenter, one dry-hop keg, and one serving keg. Sanitize the dry hop keg and hang a bag of hops in there and seal it up. Use the ferm gas to purge the empty dry hop keg (hopefully it produces enough CO2 to do this). Then move the beer to the dryhop keg, then after dryhopping, move it to the second keg to be tossed in the keezer (that will be purged with either the ferm CO2 or just liquid-filled and CO2-tank emptied.

I'm sure someone has done this or currently does this. Any thoughts? Much appreciated.

 

[day_trippr]

Thanks to the work of HBT's resident gas physics guru @doug293cz the answer to the basic question is known: a five gallon batch of middling gravity will produce sufficient CO2 to purge a five gallon keg, and a ten gallon batch can purge a pair of 5 gallon kegs (which I've been relying on since he made his work known, with unqualified success).

I expect a five gallon batch cannot fully purge two kegs, however, but perhaps Doug can provide guidance...

Cheers!

 

[slurms]

Appreciate the response. I remember seeing a post working out that info, and it must have been from doug. I didn't think it would be enough to purge 2 kegs, but worth crowdsourcing the answer.

Probably not worth risking oxidation with trying to purge 2 at once. CO2 is probably one of the cheaper things in this hobby given how long a 20lb tank lasts. Don't mind using some for a quality beer.

 

[mac_1103]

I expect a five gallon batch cannot fully purge two kegs

If a five gallon batch with OG = 1.050 and FG = 1.010 produces enough CO2 to purge one five gallon keg, then it stands to reason that a five gallon batch with OG = 1.100 and FG = 1.020 produces enough CO2 to purge two five gallon kegs. So the answer is to brew bigger beers!

 

[odie]

I would think the rate of co2 production is so slow as to not eject the o2 in the keg but instead the co2 will slowly mix with the o2 and will slowly reduce the ratio of o2 to co2 but not notably.

Better would be to completely fill the keg with water/sanitizer blend, filled to the rim. let the co2 production push the liquid out and then you would have nothing but co2 remaining.

but then, what happens when you open the keg to fill it with beer? Unless you can reverse transfer via the dip tube?

 

[day_trippr]

I would think the rate of co2 production is so slow as to not eject the o2 in the keg but instead the co2 will slowly mix with the o2 and will slowly reduce the ratio of o2 to co2 but not notably.

Fortunately, @doug293cz's work shows your thinking is wrong.

Cheers!

 

[slurms]

I think the idea is that it produces enough CO2 during fermentation that the attached keg's O2 ppm is small enough to be considered negligible.

My current practice is to fill a keg with sanitizer and push it out with the ferm gas. I am trying to make it so I can dry hop without opening the fermenter and get O2 into it.

 

[mac_1103]

I would think the rate of co2 production is so slow as to not eject the o2 in the keg but instead the co2 will slowly mix with the o2 and will slowly reduce the ratio of o2 to co2 but not notably.

It does mix. It would mix even if the rate of CO2 production was much higher. It works because the volume of CO2 produced is large enough (>400 L) to reduce O2 to ~5 ppb by mixing.

 

[IslandLizard]

My current practice is to fill a keg with sanitizer and push it out with the ferm gas.

In that scenario you can easily push out the sanitizer from 2 kegs. But I would do them successively, one at a time.

BTW, I haven't tried to push out a sanitizer-filled keg with ferm gas, yet.

 

[slurms]

BTW, I haven't tried to push out a sanitizer-filled keg with ferm gas, yet.

It works well, as long as everything is sealed up nice and tight and your fermenter can handle a few psi I guess. Can get at least 2 kegs purged that way, goes quick at peak ferm.

Issue with this is I still would have to open the first keg to dry hop it, defeating the purpose of fully purging it in the first place.

 

[mac_1103]

Issue with this is I still would have to open the first keg to dry hop it, defeating the purpose of fully purging it in the first place.

Put the hops in an empty sanitized keg with a floating dip tube. Purge with ferm gas. Do a closed transfer from your FV to the keg.

@doug293cz's epic post on CO2.

 

[slurms]

Put the hops in an empty sanitized keg with a floating dip tube. Purge with ferm gas. Do a closed transfer from your FV to the keg.

Yep, that is my plan for today's brew.

Not sure if you dry hop this way. But do you just toss the hops in once sanitized and not worry about the hops in the residual sanitizer? Or rig a bag to hang from the lid somehow and drop it in later?

 

[doug293cz]

I would think the rate of co2 production is so slow as to not eject the o2 in the keg but instead the co2 will slowly mix with the o2 and will slowly reduce the ratio of o2 to co2 but not notably.

The analysis assumes continuous complete mixing with the current gas content of the keg, so it is a worst case analysis. Even then the residual CO2 concentration is 5 ppb or less.

In practice there is not continuous complete mixing, and there is significant "sweeping" of the starting keg atmosphere by the flowing CO2. This makes the residual CO2 concentration even less than what my worst case analysis predicts.

Brew on

 

[Climb]

Two things to be mindful of:
1. If you are purging a keg filled with liquid using fermentation CO2 AND using a fermentation chamber AND your fermenter and wort are warmer than the current temperature setting of the fermentation chamber, the fermenter will cool causing the wort and air in the fermenter to shrink in volume thus sucking back liquid from the keg into the fermenter. Always be sure your fermenter and the wort it contains are colder than the ambient temperature of the fermentation chamber OR leave the blow off hose hanging in air until a temperature equilibrium is established.
2. If purging a keg filled with a water based liquid, be mindful that the liquid will contain dissolved oxygen.

 

[mac_1103]

But do you just toss the hops in once sanitized and not worry about the hops in the residual sanitizer?

OK, people will probably find all sorts of fault with this, but FWIW... I drain as much liquid as possible by tilting the upside down keg so the residual sanitizer pools over the cut off gas dip tube which has an open QD on it. Then I put the lid on and blow air from my oil-less compressor through the liquid keg post with an inline sanitary air filter to dry it out further (open QD still on the gas post). Then I open it up again and drop the hops in commando before hooking it up to purge.