Purging keg with CO2 from fermentation: liquid or gas post?

[CyberFox]

If I connect tubing from the fermenter to a keg to purge it with CO2 from fermentation, what would result in better purging: attaching it to the keg's gas post or liquid post, or does it matter?

 

[IslandLizard]

If I connect tubing from the fermenter to a keg to purge it with CO2 from fermentation, what would result in better purging: attaching it to the keg's gas post or liquid post, or does it matter?

Yes it does matter.
Without a doubt, connect to the liquid post. That has the long diptube going all the way to the bottom, resulting in better mixing and purging the air.

[Added]
The outlet, the gas post, should have a QD on it with tubing going into a jug with Starsan (or water), similar to a blow-off tube, and the function of an airlock. That also prevents air from going back into the keg when fermentation has slowed or has completed.

 

[CyberFox]

That's what I figured, but I just wanted to confirm. Thanks.

 

[IslandLizard]

That's what I figured, but I just wanted to confirm. Thanks.

I've added the airlock functionality to that post, just in case.

 

[CyberFox]

I've added the airlock functionality to that post, just in case.

Thanks for that. I imagine that I could also use a spunding valve on the gas post as an alternative to the blow-off, correct?

 

[IslandLizard]

Thanks for that. I imagine that I could also use a spunding valve on the gas post as an alternative to the blow-off, correct?

Only if your fermenter can hold pressure, otherwise the CO2 will escape through gaps in the fermenter (e.g., lid connection) and not purge your keg.

Set it to a lowish threshold pressure, perhaps around 3-10 psi. It will even carbonate your beer somewhat too.

 

[CyberFox]

Only if your fermenter can hold pressure, otherwise the CO2 will escape through gaps in the fermenter (e.g., lid connection) and not purge your keg.

Set it to a lowish threshold pressure, perhaps around 3-10 psi. It will even carbonate your beer somewhat too.

Yep, I'll be using a keg as a fermenter, so no problem there. Thanks for the help.

 

[IslandLizard]

Yep, I'll be using a keg as a fermenter, so no problem there. Thanks for the help.

How much headspace will the primary keg have, in regard to krausen and potential blow off? You don't want that going into the serving keg you're purging.

 

[CyberFox]

How much headspace will the primary keg have, in regard to krausen and potential blow off? You don't want that going into the serving keg you're purging.

I'm using a 10 gallon keg for fermentation and do a max of 6.5 gallon batches (crazily hopped IPAs), so no worry about headspace.

 

[IslandLizard]

I'm using a 10 gallon keg for fermentation and do a max of 6.5 gallon batches (crazily hopped IPAs), so no worry about headspace.

Excellent!
Dry hop sacks hanging from a magnet in the headspace?

 

[CyberFox]

I've actually read about that technique, but I've just been opening the lid, quickly tossing in the hops and closing the lid, then purging with CO2. I partly do this to dry hop without a bag for better extraction. You think the magnet trick would be that much better for preventing oxygen?

 

[IslandLizard]

I've actually read about doing that technique, but I've just been opening the lid, quickly tossing in the hops and closing the lid, then purging with CO2. I partly do this to dry hop without a bag for better extraction. You think the magnet trick would be that much better for preventing oxygen?

Same here. I've been reading about it and trying to find a way that works well.

Like you, I briefly open the keg while streaming CO2 into the headspace. I then quickly drop in a "CO2 purged bag" containing hop pellets, and purge quite a few times.

I guess you are using a floating dip tube with one of those filter baskets on the entrance of the line?

The potential issue I see with a bag full of hops hanging in the headspace for a few days, while fermentation proceeds, is it being flushed with 10s of gallons of CO2 at low room temps. Maybe it doesn't harm the pellets or as much?

 

[CyberFox]

Same here. I've been reading about it and trying to find a way that works well.

Like you, I briefly open the keg while streaming CO2 into the headspace. I then quickly drop in a "CO2 purged bag" containing hop pellets, and purge quite a few times.

I guess you are using a floating dip tube with one of those filter baskets on the entrance of the line?

The potential issue I see with a bag full of hops hanging in the headspace for a few days, while fermentation proceeds, is it being flushed with 10s of gallons of CO2 at low room temps. Maybe it doesn't harm the pellets or as much?

That's exactly the concern I had about that technique: the effect on the hops hanging in the headspace. I don't know one way or another, but if I had to guess, I don't really see how it would harm them.

Yeah, I'm using a FLOTit 2.0 floating dip tube, which is pretty nice. I dry hop without a bag and then I cold crash for a few days to drop the hops before transferring. It works well.

 

[IslandLizard]

Yeah, I'm using a FLOTit 2.0 floating dip tube, which is pretty nice.

I recently saw them being reviewed at HomeBrewFinds. It looks like it does that job very well.

 

[JRHDBrew]

I do fermenter connected to gas post of keg and a liquid line from the liquid post to a bucket. I start with the keg half full of star San. As fermentation begins it pushes all the star San through the liquid dip tube into the bucket. Then starts to blow bubbles. Then during cold crash the keg acts as a bladder to catch any star San sucked back.

 

[DeMalt]

Yep, I'll be using a keg as a fermenter, so no problem there. Thanks for the help.

If you fill the keg to be purged with starsan and push it out with the co2 you will have a sanitized keg and zero oxygen. Works great and zero chance of oxygen in the keg

 

[CyberFox]

If you fill the keg to be purged with starsan and push it out with the co2 you will have a sanitized keg and zero oxygen. Works great and zero chance of oxygen in the keg

That's a good way to do it, but I'm planning to use the crazy amount of CO2 from fermentation to purge the keg. I've heard that it does a better job of purging than pushing out sanitizer, but I can't remember the numbers. I also just make up one gallon of sanitizer per batch and it only takes me a few seconds to sanitize the inside of a keg, so I'm good there.

 

[JRHDBrew]

Same here. I've been reading about it and trying to find a way that works well.

Like you, I briefly open the keg while streaming CO2 into the headspace. I then quickly drop in a "CO2 purged bag" containing hop pellets, and purge quite a few times.

I guess you are using a floating dip tube with one of those filter baskets on the entrance of the line?

The potential issue I see with a bag full of hops hanging in the headspace for a few days, while fermentation proceeds, is it being flushed with 10s of gallons of CO2 at low room temps. Maybe it doesn't harm the pellets or as much?

My neighbor is the COO of a local craft brewery and has been awesome in dialing in process. We’ve talked about dry hoping. In order of preference for him: 1. Suspended bag (CO2 won't hurt the hops, oxygen will) 2. Open lid, Drop them in and purge by pushing CO2 through the liquid post 3. At high Krausen, tops them in a get that lid back on ASAP. He is to not drop the hops so they splash. That high Krausen is an insulation blanket of CO2 and is producing CO2 fast enough it wil quickly expell any O2 introduced. Method 3 is clearly only good for early dry hop additions.

 

[Wagon_6]

How long does it take fermentation to purge a keg?

 

[JRHDBrew]

How long does it take fermentation to purge a keg?

That ultimately depends on several factors. I fill my 5 gallon keg with 2.5 gallons of star San. Generally that is all displaced out of the keg in 24 -36 hours or so. Generally said I would say it is well purged before you reach high Krausen.

 

[Bobby_M]

... I'm planning to use the crazy amount of CO2 from fermentation to purge the keg. I've heard that it does a better job of purging than pushing out sanitizer, but I can't remember the numbers....

I can see how it's better by using less starsan and taking advantage of all that free CO2 but I can't wrap my head around the full starsan push being any less effective at removing O2.

 

[Group W]

Me too Bobby.

When filling a keg with CO2 from the fermenter, what about the air that is initially in the top of the fermenter? Seems like assumptions must be made about purity of the CO2.

 

[JRHDBrew]

I can see how it's better by using less starsan and taking advantage of all that free CO2 but I can't wrap my head around the full starsan push being any less effective at removing O2.

Ultimately, aren’t they both doing the same thing. One is just purging star San and O2 and the other is just purging O2. I use the star San method as that is my sanitizing of the keg as once fermentation is done I do a closed transfer to that keg. Beer replacing the CO2.

 

[Red over White]

Here's the way I look at working towards achieving low oxygen in freshly cleaned kegs. An unmodified keg is known to trap up to 3 oz of air in the lid area, in addition to the dissolved oxygen in the sanitizer (8 ppm minimum). How much oxygen comes out of the sanitizer into the co2 headspace while pushing it out is an unknown to me, but some inevitability has to. Trimming the gas tube and filling the keg 99.99% through the beverage out post, with the keg on an angle forcing all the remaining air out of the gas in post is probably the best we can do there. Adding NaMeta in the sanitizer to scrub the oxygen from it, is likely the next best procedure. Some of this is discussed in this thread.

https://www.homebrewtalk.com/threads/keg-purging-with-active-fermentation.628658/

 

[CyberFox]

I can see how it's better by using less starsan and taking advantage of all that free CO2 but I can't wrap my head around the full starsan push being any less effective at removing O2.

Red over White just beat me to posting this thread. I'm basing my comments off of doug293cz's posts (mainly posts 3 and 20).

He said that the "worst case O2 levels left in a keg purged with the output of an active fermentation" is 0.005 ppm (5 parts per billion).

He also said that, due to the 3 fluid oz of headspace not being able to be be filled with liquid, "doing a liquid purge is almost equivalent to doing 5 purge cycles at 30 psi. Thus you still need 8 more 30 psi purge cycles to get down to the ~100 ppb O2 range." This results in 930 ppm of oxygen remaining.

 

[IslandLizard]

He also said that, due to the 3 fluid oz of headspace not being able to be be filled with liquid,

IIRC, there are methods to remove that last air bubble underneath the lid too.

What I've been doing is pushing out about a pint first, then purge that small headspace 5-10 times at 30 psi, before pushing the bulk out.
I push it out quickly, at 20-25 psi, and reduce the pressure as the keg empties. I end up at around 10-12 psi when it starts blowing foam.

I also clean, then purge a few kegs at a time, so the Starsan goes into the next (clean) keg, filling from the bottom up with an opened picnic tap.

 

[Broken Crow]

I put a disconnect on the gas post with a short drain hose, lock the PRV valve open, push the starsan in the liquid post and while it's spilling over the top, I immerse as much of the lid as I can before pulling up on the bale to close it, then I pull off the gas QD, and then I close the PRV so there's only that tiny edge left... Then I hook my gas to the liquid post to maximize bubbling and repeatedly pull the PRV (spraying starsan everywhere) under the theory that since the bubbles contain only CO2, I can use them to push out the last remaining O2.

 

[CyberFox]

IIRC, there are methods to remove that last air bubble underneath the lid too.

What I've been doing is pushing out about a pint first, then purge that small headspace 5-10 times at 30 psi, before pushing the bulk out.
I push it out quickly, at 20-25 psi, and reduce the pressure as the keg empties. I end up at around 10-12 psi when it starts blowing foam.

I also clean, then purge a few kegs at a time, so the Starsan goes into the next (clean) keg, filling from the bottom up with an opened picnic tap.

True. I was just explaining how using CO2 from fermentation is more effective at removing oxygen than a liquid purge by themselves. You can take extra steps to remove the 3 fluid oz air bubble with a liquid purge, but that's just one more step to take and a little extra CO2 to use. Overall, I like the idea of using CO2 from fermentation because it requires less steps, less CO2, less sanitizer, and is more effective at removing oxygen than liquid purging. That's what I'll be sticking with.

 

[DuncB]

I've heard that it does a better job of purging than pushing out sanitizer, but I can't remember the numbers.

No it doesn't,
@doug293cz has done the maths and in a thread on purging kegs with ferment gas.
Pushing out starsan is even less oxygen provided you start purge after fermenter oxygen used up or purged.
Some keg types can also be completely filled with starsan.
Other option is starsan sanitise then fill keg with water and sod met and purge that with ferment gas.
Leftover liquid in keg may deactivate any further oxygen as sod met antioxidant. I have no numbers re the sod met concentrations.

 

[CyberFox]

No it doesn't,
@doug293cz has done the maths and in a thread on purging kegs with ferment gas.
Pushing out starsan is even less oxygen provided you start purge after fermenter oxygen used up or purged.

So I misunderstood doug293cz's posts I mentioned above and liquid purging results in even less than 0.005 ppm (5 parts per billion) of oxygen?