Pressurized ferment / CO2 purge question

[McKnuckle]

Hi all,

For those in the know, please validate my understanding of this proposed setup... (It's staged for review and not yet in action):

The 3 gallon keg on the left is the fermenter, and the 2.5 gallon keg on the right is the beer's future serving vessel.

A jumper is connected from the fermenter's gas IN to the serving keg's liquid OUT, feeding the latter CO2 from the active fermentation. The spunding valve is on the serving keg's gas IN port, terminated with an airlock.

CO2 will first fill the fermenter headspace, then find its way up the tube to the serving keg. Once it fills the serving keg, the gas will build roughly equal pressure throughout the system until it exceeds the spunding valve's 10 psi setting, and I'll see bubbles.

When fermentation is done, I can count on the two kegs being equalized at 10 psi. Then I can remove the gas jumper, connect a liquid jumper, move the serving keg below the fermenter, reduce the spunding valve psi setting, and beer should flow.

Am I okay here?

 

[day_trippr]

Looks legit. The only thing I'd change would be to tube the spunding valve exhaust to the bottom of a vessel with a few inches of water to cover instead of that "lock", but that's a nit. Carry on

 

[hopjuice_71]

I do something similar but with a couple of variations. My future serving keg is totally filled with starsan. I let the fermentation go in the fermentor keg for a day or so so that the headspace is largely purged of CO2 (QD gas out used as blowoff). Then I connect the fermentor gas in to the serving keg gas in with a jumper; serving keg liquid out has a QD and tube to a bucket. The CO2 generated by the fermentation very quickly (like a couple of hours) pushes all of the starsan out of the serving keg thereby purging it. I let this go a couple more days (or so - extra purging) then disconnect the completely purged serving keg (including its blowoff) and put the spunding valve on the fermenting keg to capture the last of the fermentation CO2. Requires a bit of vigilance but it works fantastically well. After the fermentor keg is cold crashed I just transfer liquid out to liquid out with the spunding valve on the serving keg gas in.

Once the serving keg is empty of starsan you could change up the connections - fermentor gas in to liquid out - and put a spunding valve on the serving keg, if you wanted everything at the same pressure. I haven't found this necessary, however.

Cheers

 

[McKnuckle]

...a vessel with a few inches of water to cover instead of that "lock"...

When actually in use, that thing is filled halfway with water, just like a standard airlock. It is included and works perfectly with the SpundIt valve. Thank you for the reassurance!

 

[hopjuice_71]

Oops, just saw in the title that you were looking for a pressurized ferment, which my setup is not. Your setup looks good for this, though.

 

[day_trippr]

When actually in use, that thing is filled halfway with water, just like a standard airlock. It is included and works perfectly with the SpundIt valve. Thank you for the reassurance!

Ah, ok. Never seen one of those, looked full of air so assumed it was some spin on the "silicone airlock".
That should work just fine

Cheers!

 

[doug293cz]

Yes, you will be fine. I analyzed this scenario in a post here.

Brew on

 

[McKnuckle]

I've seen that epic post Doug, thanks for linking to it!

Actually, I was less concerned with purging the receiving keg as I was with having the spunding valve on the receiving keg, rather than the primary, in terms of creating a pressurized environment in both kegs. But then I imagine that gas will find the path of least resistance until it can't go anywhere else, so the entire system should pressurize equally, and fairly quickly, during the early stages of fermentation.

 

[day_trippr]

On the plus side there's little chance your spunding valve will get plugged up with krausen

Cheers!

 

[Vale71]

Any change in pressure will propagate at the speed of sound, so I'd say that counts as fairly quickly.

As for gravity transfer at the end of fermentation, you'll actually need two connections for this to work. You'll need to connect the liquid post of keg A with the liquid post of keg B and the gas post of keg A with the gas post of keg B. This is because you also need to provide a return path for CO2 from the receiving keg to the donor keg to keep pressure equalized throughout the transfer process. You'll probably also need to vent a little CO2 from the receiving keg after placing it lower than the donor keg to actually get the liquid transfer started (to give it a little nudge, so to speak) and then quickly connect the two gas posts to keep the transfer going by gravity.

 

[McKnuckle]

...you'll actually need two connections for this to work. You'll need to connect the liquid post of keg A with the liquid post of keg B and the gas post of keg A with the gas post of keg B. This is because you also need to provide a return path for CO2...

Ah, but this is why this entire operation confuses me so much. I have a hard time visualizing the physics involved. I got the idea from this thread, where @mongoose33 talks about doing the same thing. There's even a photo! But I think I may have overlooked that there's CO2 connected to the fermenter in his pic...

If both kegs are initially at 10 psi, I connect the liquid ports and then lower the pressure on the spunding valve (still attached to receiving keg) to 8 psi, the liquid in the donor keg should start to flow into the receiving keg.

The spunding valve would then hold that pressure differential until all of the liquid has descended, followed lastly by the CO2 in the donor keg's headspace. At that final point, the donor keg will be empty, and the receiving keg will be full of liquid and headspace CO2 held at 8 psi.

But you're saying that the pressure will equalize to 8 psi quickly in the donor keg, and the liquid will then stop flowing. And at that point, I can connect the gas ports and use the established siphon to finish it off (see last post in the linked thread where that is mentioned).

Luckily I can hook up CO2 and push it all out if this process becomes a SNAFU!

 

[Vale71]

But you're saying that the pressure will equalize to 8 psi quickly in the donor keg, and the liquid will then stop flowing. And at that point, I can connect the gas ports and use the established siphon to finish it off (see last post in the linked thread where that is mentioned).

Luckily I can hook up CO2 and push it all out if this process becomes a SNAFU!

Yes, that's exactly the idea behind gravity transfer. CO2 must be transfered between receiving and donor keg at the same rate at which beer is transfered from donor to receiving in order to maintain equalized pressure or the transfer will stop prematurely. In any case it's good to have bottled CO2 at hand should it not work out.