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Keg purging with active fermentation

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processhead

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I seem to recall reading the CO2 is denser than air and will displace the air in a container. Not sure if that is true since air is made up partially of CO2.

If it is true, it would seem that under the right conditions and avoiding turbulence while filling, one could displace the air out of a keg with CO2 gas.
 

doug293cz

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I seem to recall reading the CO2 is denser than air and will displace the air in a container. Not sure if that is true since air is made up partially of CO2.

If it is true, it would seem that under the right conditions and avoiding turbulence while filling, one could displace the air out of a keg with CO2 gas.
Watch this. CO2 is about 3.6 times lighter than Br2, and about the same as NO2 (later in the vid.) Br2 mixes completely in ~30 minutes, NO2 (and thus CO2) much less than that. Even if you only get partial mixing, the amount of O2 picked up is way too much for good beer stability.


Brew on :mug:
 
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Mer-man

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Hey guys and gals, I just wanted to offer that I can share the spreadsheet. Just PM me your email address and I will send it over.
 

murphyslaw

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Two questions on purging kegs this way:

1. Do you need to take the poppet out of the keg posts/QD's? I imagine not, since its just gas passing. There's a gas QD on one side (to the fermenter) and a liquid QD on the other (to the jar of starsan), right?

2. Can I use this keg of co2 to cold crash my fermenter (fermonster) without oxidation? I'd have to take off the liquid QD to avoid sucking back sanitizer. I'd probably also have to build up pressure in the keg with bottled co2, otherwise my fermenter may just cave in. But if I do that, as soon as I hook it up to my fermonster, the pressure from the keg may blow the stopper out. So maybe the answer is hooking up bottled co2 to the keg at a very low pressure, like 1psi? Not sure if this is a good idea with a plastic fermenter.
 

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Two questions on purging kegs this way:

1. Do you need to take the poppet out of the keg posts/QD's? I imagine not, since its just gas passing. There's a gas QD on one side (to the fermenter) and a liquid QD on the other (to the jar of starsan), right?
It just goes in. No need for anything special other than QDs.

2. Can I use this keg of co2 to cold crash my fermenter (fermonster) without oxidation? I'd have to take off the liquid QD to avoid sucking back sanitizer. I'd probably also have to build up pressure in the keg with bottled co2, otherwise my fermenter may just cave in. But if I do that, as soon as I hook it up to my fermonster, the pressure from the keg may blow the stopper out. So maybe the answer is hooking up bottled co2 to the keg at a very low pressure, like 1psi? Not sure if this is a good idea with a plastic fermenter.
If you mean to pull CO2 back into the headspace of the fermenter, you can, but you'll draw a vacuum on that keg...which will stop the process. You have to have something to replace the drawn off CO2 from the keg. If it's air, well, then your purged keg will increasingly have air and O2 in it, and you'll be pulling that into your headspace.
 

murphyslaw

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If you mean to pull CO2 back into the headspace of the fermenter, you can, but you'll draw a vacuum on that keg...which will stop the process. You have to have something to replace the drawn off CO2 from the keg. If it's air, well, then your purged keg will increasingly have air and O2 in it, and you'll be pulling that into your headspace.
Right....So I guess you could use the mason jar setup on the other end of the keg? Not very KISS, but it gets you both a fully purged keg and cold crash w/o oxidation.
 

leesmith

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Do any of you guys know how long it takes to purge a serving keg with fermentation gas from the fermenting vessel?

Water purged vs. just hooking it up without doing any “pre” purge?
 

mongoose33

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Do any of you guys know how long it takes to purge a serving keg with fermentation gas from the fermenting vessel?

Water purged vs. just hooking it up without doing any “pre” purge?
About as long as fermentation proceeds. Here's a link to discussion/formulae about how effective different approaches are to purging a keg. But how effective this is depends on how much CO2 is produced, which is dependent on the amount of sugar in the wort and the degree of attenuation.

http://www.********************/bre...rging-transferring-stabilizing-finished-beer/

How effective pressurized purging of a keg will be depends on the pressure; purging at 30psi will work faster/better than 10psi simply because you're increasing the ratio of CO2 to O2 in the keg at the higher pressure, then venting it.

********

Here's what I do: I fill a keg w/ Star-San, then push it out with CO2. This produces a pretty darned low-O2 environment in that keg. Then I connect it to the fermentation gas output of my fermenter to continue to purge out whatever O2 remains in that keg. Overkill? I don't know. All I know is that fermentation gas is free, I might as well use it.

And starting so low as a result of a star-san purge means I will truly get very, very close to 0 ppm O2 in the keg when I start filling it.

Sometimes I'll hook up a gas QD on the keg and run it into a blowoff jar so I can monitor using the bubbling to know when I'm about done.

Like so; the fermenter is covered up with blankets, but the ferm gas line comes out of the fermenter to the black QD, then another line from the gray QD to the blowoff jar.

purgingkeg.jpg
 

doug293cz

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About as long as fermentation proceeds. Here's a link to discussion/formulae about how effective different approaches are to purging a keg. But how effective this is depends on how much CO2 is produced, which is dependent on the amount of sugar in the wort and the degree of attenuation.

http://www.********************/bre...rging-transferring-stabilizing-finished-beer/

How effective pressurized purging of a keg will be depends on the pressure; purging at 30psi will work faster/better than 10psi simply because you're increasing the ratio of CO2 to O2 in the keg at the higher pressure, then venting it.

...
The article linked in the quoted post is mostly a direct "cut 'n paste" of the third post in this very thread, which I wrote. Credit is given at the end of the article, and there is some useful added information.

Brew on :mug:
 

leesmith

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The article linked in the quoted post is mostly a direct "cut 'n paste" of the third post in this very thread, which I wrote. Credit is given at the end of the article, and there is some useful added information.

Brew on :mug:
Do you think it’s possible that an already co2 tank purged keg could be 100% o2 purged with pure co2 from being connected to a fermentation blow off in a short period of time; maybe an hour or so?
 

SanPancho

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is nobody else using a spund on their serving keg? the pressure in the system keeps krauzen down. only takes maybe 2-3psi to suppress the foam from blowing off. and prevents o2 coming back in when primary finishes and i'm too busy/not paying attention. and the low psi is compatible with plastic carboys.
 

doug293cz

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Do you think it’s possible that an already co2 tank purged keg could be 100% o2 purged with pure co2 from being connected to a fermentation blow off in a short period of time; maybe an hour or so?
No. First, there is no such thing (thermodynamically) as 100% purged. Second, in one hour, the fermenter will only produce about 4 - 5 liters of CO2.

Brew on :mug:
 

doug293cz

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Ok, a "5" gal corny has an actual volume of about 5.35 gal, so 2 of those plus 1.5 gal headspace in the fermenter totals 12.2 gal, or 46.2 liters. Assuming you have 20 liters of wort at 1.050 that finishes at 1.010, you would generate ~440 liters of CO2 during fermentation. After some multiple gas dilution cycles the final O2 concentration is about 15 ppb (0.015 ppm.) So, purging (2X) 5 gal kegs with a 5 gal ferment appears to give acceptable results.

Brew on :mug:
 

SanPancho

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@doug293cz along lines of preceding question, is there a general rule or guideline to be had on the sugar to purge volume ratio? Maybe something like “1kg sugar at 80%AA is enough to purge X liters from ambient condition”. i guess you need to state what the acceptable ppb limit is tho.
 

doug293cz

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@doug293cz along lines of preceding question, is there a general rule or guideline to be had on the sugar to purge volume ratio? Maybe something like “1kg sugar at 80%AA is enough to purge X liters from ambient condition”. i guess you need to state what the acceptable ppb limit is tho.
Probably could come up with something like that. Anyone else interested?

Brew on :mug:
 

SanPancho

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hey mr @doug293cz - have been contemplating the co2 purge number stuff a bit more. maybe a simpler question i can ask is what sort of volume ratio would be considered sufficient to purge a particular vessel? i.e. 5x the vessel size in co2 would be sufficient to get to 10ppb or whatever. im thinking this should be more simple to get a good rule of thumb.

at that point ,then you just need to know a pound of extract is like 1/2lb co2. convert to volume, etc. etc.
 

Transamguy77

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This is an idea I had wanted to try and now I see it will work. Now I wanted to try using a spunding valve, can I just put that on the final keg instead of an airlock?
 

JLeuck64

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I have only recently started fermenting in the keg and using that CO2 produced to purge the serving keg. About the same time I started that process I also started using a regulator (or spunding valve) to ferment under pressure so I am trying to figure out a couple different techniques at the same time lol!

Using the CO2 to purge the serving keg works awesome! I try to think in simple terms and I believe the serving keg does get purged of O2 eventually, but heck even if it doesn't when I transfer the beer in to the keg, that column of gas is pushed out and the little head space is 100% filled with CO2.

Fermenting under pressure is awesome too! The beer gets carbonated naturally :bigmug:

20200430_230213.jpg
 
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Dusan Kovacevic

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Probably could come up with something like that. Anyone else interested?

Brew on :mug:
Just here to let you know there is still interest in this topic and anything that could help the rest of us that are not very good at maths would be greatly apreciated. I'm sure there are many people lurking here without commenting. Plus what you have going on here is very valuable and you yould be doing public service by dumbing it down a bit.
Cheers
 

renstyle

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Just here to let you know there is still interest in this topic and anything that could help the rest of us that are not very good at maths would be greatly appreciated.

I'm sure there are many people lurking here without commenting.
That's me! (Lurker)

At the start of fermentation, give the batch a few days for the yeast to eat all the O2 in the ferm with a blow-off/airlock.

Has anybody tried the trifecta of:

1) StarSan purge of serving keg - only 3-ish oz air trapped in headspace
2) using additional ferm CO2 to bring the O2 concentration down to the low-ppb range in the 3oz headspace
3) using still more ferm CO2 to spund the batch to get a jump start on carbonation? Even 2-3 PSI would be good.

The calculations for most batches show 400+ L of CO2 generation can be expected.

Looks like that should be more than enough gas to do all three?

Trying to map this out on a schedule, I dunno if I'm capable of determining the time it takes for the O2 levels in the ferm to decrease to the point where the gas QD can move from an airlock to pushing StarSan, then how long to wait after the serving keg is fully purged before switching to a spund situation?
 

day_trippr

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I have only recently put together a CO2-capture/keg-purge system and have done just one pair of kegs off a pair of fermenters. Started with "empty" but still-wet Star San sanitized kegs and went from there. Given the prodigious volume of CO2 produced by this particular batch (11 gallons of 1.107 wort that finished at 1.028) I'm pretty confident those kegs were as close to pure CO2 as the method allows - all tubing is EVAbarrier so uber low O2 ingress factor.

But a week or so later someone posted they started with a keg filled with Star San and used the fermentation gas to push it out, apparently thoroughly. Which is pretty cool. I thought it might put significant pressure on the yeast, but even at the very beginning with say 20 inches of Star San in the keg, that's less than 0.75 psi, which I would hope isn't going to be an issue...

Cheers!
 

renstyle

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Given the prodigious volume of CO2 produced by this particular batch (11 gallons of 1.107 wort that finished at 1.028) I'm pretty confident those kegs were as close to pure CO2 as the method allows - all tubing is EVAbarrier so uber low O2 ingress factor.
I don't have the maths to back it up, but I'm tending to agree with you. ;-) That EVAbarrier tubing is good stuff, using it also.

But a week or so later someone posted they started with a keg filled with Star San and used the fermentation gas to push it out, apparently thoroughly.
IIRC, is was less that 24hrs to purge a nearly-full StarSan keg using only ferm CO2. That's where my timing thoughts came into play.

Use a regular blow off/airlock for the first few days, say 3-4?

Then switch to gas QD to purge the StarSan serving keg ~ 18-24hrs.

3oz of air at 21% concentration, is 0.63-ish oz of O2 starting in the headspace. This will get diluted with ferm CO2 as the StarSan is purged, not 100%, but pretty low.

Maybe less than 12hrs before switching to spunding?
 

Vale71

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There is no reason to have any headspace in the keg when purging. Just fill it till StarSan solution starts coming out of the PRV and you'll have little to no air trapped in the keg.
 

doug293cz

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There is no reason to have any headspace in the keg when purging. Just fill it till StarSan solution starts coming out of the PRV and you'll have little to no air trapped in the keg.
Nope. Due to the design of the lid, when liquid comes out of the PRV, there is a 3 fl oz air bubble trapped in the lid. I personally measured this (as have others.)

It is possible to cut the gas "dip" tubes shorter, so that when the keg is tilted there is no air trapped when liquid comes out the gas port.

Brew on :mug:
 

renstyle

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Nope. Due to the design of the lid, when liquid comes out of the PRV, there is a 3 fl oz air bubble trapped in the lid. I personally measured this (as have others.)

It is possible to cut the gas "dip" tubes shorter, so that when the keg is tilted there is no air trapped when liquid comes out the gas port.

Brew on :mug:
Trimming the gas dip tubes shorter sounds like a good idea, anything that could minimize headspace while not changing functionality of the keg. :cool:
 

twd000

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hey mr @doug293cz - have been contemplating the co2 purge number stuff a bit more. maybe a simpler question i can ask is what sort of volume ratio would be considered sufficient to purge a particular vessel? i.e. 5x the vessel size in co2 would be sufficient to get to 10ppb or whatever. im thinking this should be more simple to get a good rule of thumb.

at that point ,then you just need to know a pound of extract is like 1/2lb co2. convert to volume, etc. etc.

just want to echo this question on dilution ratio - "how many multiples of the vessel volume is needed to get O2 down to __x ppb O2?"

I posted a thread about yeast-oxygen scavenging to purge kegs: "can I use "yeast oxygen scavenging" to purge an empty keg?"
 

twd000

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so I interpret the chart and table as relating to a pressurized "vent and purge" scenario, right? So not applicable to my question

I think the equation for continuous dilution is relevant here:
210000 ppm * ((25 L - 0.0001 L) / 25 L)^4390000 = 0.005 ppm

so if I want to achieve 100 ppb O2 (a common target in LODO packaging), I need ~ 360L of fermentation gas. Interestingly, that equation is insensitive to the 0.0001 L bubble size. Even changing it to 1-liter bubble size gives you the same result, as it simplifies to the number of volume replacement cycles
 

doug293cz

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so I interpret the chart and table as relating to a pressurized "vent and purge" scenario, right? So not applicable to my question

I think the equation for continuous dilution is relevant here:
210000 ppm * ((25 L - 0.0001 L) / 25 L)^4390000 = 0.005 ppm

so if I want to achieve 100 ppb O2 (a common target in LODO packaging), I need ~ 360L of fermentation gas. Interestingly, that equation is insensitive to the 0.0001 L bubble size. Even changing it to 1-liter bubble size gives you the same result, as it simplifies to the number of volume replacement cycles
Sorry, misunderstood the question. I see what you are looking for now.

Actually the result does depend on bubble volume, especially as the bubbles get within a couple of orders of magnitude of the purge volume. I ran the 25L purge volume and 439L of CO2 case for several different bubble volumes. Here are the results:

1611095035052.png

Using a 1 L bubble volume will lead to slightly optimistic estimates.

Brew on :mug:
 

renstyle

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Which spreadsheet is that?

Brew on :mug:
Likely referring to the last line in post #3: " Coming next, the spreadsheet to allow you to do your own calculations. "

Epic post btw. :cool:

Edit: I read thru the thread again, cuz it was fun. I'd hit up @Mer-man, see if he still has the spreadsheet he whipped up available:

Hey guys and gals, I just wanted to offer that I can share the spreadsheet. Just PM me your email address and I will send it over.
 
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Dusan Kovacevic

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Likely referring to the last line in post #3: " Coming next, the spreadsheet to allow you to do your own calculations. "

Epic post btw. :cool:

Edit: I read thru the thread again, cuz it was fun. I'd hit up @Mer-man, see if he still has the spreadsheet he whipped up available:
Yeah, I see he was sharing that spreadsheet so I tought someone has it.
Also I wonder if he shared it with any brewing software?
 
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