Keg purging with active fermentation

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Barleymist

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I am using two fermzilla all rounders 15g and three corny kegs to store co2. I made an inline spunding valve with gas poppets on either end. This way all O2 is consumed by fermenting beer. Co2 sounds like a jet stove for past three days, I had no idea beer makes this much co2. I could purge 100 kegs. My question is how many times should I pull keg post to purge each keg at 20psi. Do I even need to purge? The O2 should all be absorbed via diffusion by the fermenting beer within x hours? I now have three kegs at 20psi so I can use them to push beer out of zillas. I have no need for my CO2 tank and regulator now. The benefit of being inline is I can push beer out of second zilla. With a gas jumper cable and a spunding valve a third port for beer out is needed. The only downside is my perlick faucet will leak below 9 psi so I will either need to hook up my co2 tank inline via gauge port, or use my picnic tap when the psi drops that low.
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renstyle

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Referring back to the @doug293cz very detailed 3rd post in this thread...

Twenty purges at 20psi will get the O2 concentration down to 10 parts per billion (ish). So.. a few more than twenty, which would be further down the list. ;)
 

DuncB

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If this is your method to store CO2 seems expensive to fill all of those kegs and just have them full of gas, you could get more PSI in each one to increase your "store".

Why not just have one fermzilla or keg with sugar and yeast in and a spunding valve on it on a T with the other limb going to your kegerator or keg that you are purging ( or combinations therof). When CO2 production slows just inject a bit more sugary water and yeast nutrient in. Or you could just ferment it to the highest pressure your Corney would take and then use an inline regulator to your kegerator / keg you are purging.

Was a method i considered during lockdown if CO2 ran out. Really wanted some inline filters that could catch bacteria / yeast in the CO2 to prevent cross contamination. I believe these are available.

Friend has a plato and I couldn't believe how many litres of CO2 were produced by a 23 litre ferment.
 

renstyle

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Why not just have one fermzilla or keg with sugar and yeast in and a spunding valve on it on a T with the other limb going to your kegerator or keg that you are purging ( or combinations therof). When CO2 production slows just inject a bit more sugary water and yeast nutrient in. Or you could just ferment it to the highest pressure your Corney would take and then use an inline regulator to your kegerator / keg you are purging.
That's an interesting idea, using a corny keg as a fermenter of sugar wash+scads of nutrient. Many folks have stated that several types of yeast can handle pressures in excess of 100psi, but their beer making flavor performance is reduced. Wouldn't be a problem for just making CO2 tho, and pretty cheap.

I too am keeping this idea in the playbook for the next CO2 supply shortage! LOL

Cornys (the cans themselves) can handle over 130psi, and most PRV's on the ball locks are the grey 100psi type. The next most common is the green PRV, which blows at 65psi.

For the sake of a thought experiment, fill the initial "gas storage keg" with water/starsan. So full that you have only a few mL of headspace filled with air mix.

So, say you did a gas to gas jumper from the ferm vessel to this storage keg initially:

Pressurize the tiny amount of headspace in that liquid tank. Doing it this way would reduce the purges needed to get the headspace O2 concentration down pretty low (math fails me on Monday evenings, but I'm sure this could be calculated using post #3 and a good pot of coffee). You'd also lose A LOT less CO2 gas by purging that headspace first.

Then add the bev to bev jumper from the storage keg to either: 1) another keg or 2) a bucket large enuf to hold the purged liquid. Homer buckets and their ilk work.

To maximize the CO2 "stored", you'd have to remove the bev jumper once all of the liquid (or as much as you can get) has been purged. Then you can spund it to 20, 30 or even more pressure, all depends on how well you can keep that initial fermentation managed. :D

In that situation, I'd switch to a ferm-gas to bev-keg jumper, and spund off the gas port on the corny to avoid any lingering liquid from mucking up the spunding valve.

I suppose you could use a non-liquid filled keg, it would just take a longer fermentation or just multiple fermentations. One to fully clear the keg down to 5 parts per billion O2, and then another fermentation to pressurize the vessel. There would still be some O2 getting pushed out of the fermenter during active fermentation, and this is where the diffusion calculations get way over my head. If you give it more time, and more CO2, you could get there.

The fermenter you are using is rated to about 35psi, so you wouldn't be able to use the corny to its full rating, but it does seem plausible this would work.
 

DuncB

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Renstyle good points. Keg King did test to failure one of their all rounder fermenters . Video below shows the pressure.
I personally don't have any Corny kegs but do know that a wheat beer I made was accidentally fermenting like crazy at 35 psi in the fermentasaurus.
There is a lot of scope for this gas self sufficiency I think.
 

renstyle

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I went straight to kegging and skipped bottling other than batches here and there that I want to share. Now I'm looking hard at a DIY counter-pressure filler using kegland carb caps and the wye to connect 2 to the top of a bottle... oh someday... what a hobby eh?

So since I did have the kegs, I've done a few batches using 3 cornys:

1) ferm vessel
2) starsan vessel (eventual serving vessel for #1) and
3) a holding keg for the purged starsan (which will become the serving vessel for the next batch).

I've used the same starsan, with modest top-ups between brews several times now, round robin, so I've got that pretty well dialed in.

My desire was to help along some lagers with some pressure, plus not needing any CO2 purge from my tank on the #2 serving vessel. So I've never spunded any higher than the 12-15psi used to push the beer from my CO2 tank.

If the yeast can handle the pressure, I don't see why it couldn't be done to whatever level that keeps the yeast going (along with nutrient additions), especially if you are using a sugar wash or other sugar source in the ferm vessel where you don't care what it tastes like! :D
 

DuncB

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I started bottling ( only a couple of batches ) and then to kegs. You would be limited to PET bottles I think with the Kegland T piece connector and a few connectors and valves could work.

I bought the williams warn counter pressure bottle filler for the excess litres from the fermentasaurus that wont fit in a 20 litre keg and for competitions, archive etc. The williams warn does send co2 to the keg as well as to the bottle to keep the system balanced and this would require a T on your gas line to achieve this and a means of turning the gas on and off to the bottle being filled.

Have to say the WW is so easy to use. Mess / spillage or waste is non existent and it really does make the process a pleasure.
I would consider the Boel Itap now as well, but it wasn't available when I bought my counter pressure filler.

It's one project after another. I use my Kegland T for pressurised liquid additions to the kegs such as finings. Works well for that.
 

renstyle

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I was scheming a way to incorporate some type of rubber/silicone stopper-bung on the bottom portion of the wye, it'll get worked out eventually... :cool: but at the same time, PET bottles for a few brews here and there for friends isn't blasphemy (to me anyway). I always make sure they have nice beer glasses for the bottle conditioned batches I've done, so they "know to pour" LOL.

<googles WW, ogles the pics> ooh, that IS a pretty piece of kit!

All the other CP fillers I was looking at were close to the price of "beer gun" offerings, so since I keg, I figured I'd eventually go that direction for just a few bottles...

I'll keep fiddling with the kegland plastic bits, I was only out a pair of tenners... see what I can work out before I move up.

Almost forgot... bonus is that I can use one of the carb caps with a small bit of 5mm line and a PET bottle to pressure "inject" gelatin into a keg during crashing. ;)
 

Barleymist

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If this is your method to store CO2 seems expensive to fill all of those kegs and just have them full of gas, you could get more PSI in each one to increase your "store".

Why not just have one fermzilla or keg with sugar and yeast in and a spunding valve on it on a T with the other limb going to your kegerator or keg that you are purging ( or combinations therof). When CO2 production slows just inject a bit more sugary water and yeast nutrient in. Or you could just ferment it to the highest pressure your Corney would take and then use an inline regulator to your kegerator / keg you are purging.

Was a method i considered during lockdown if CO2 ran out. Really wanted some inline filters that could catch bacteria / yeast in the CO2 to prevent cross contamination. I believe these are available.

Friend has a plato and I couldn't believe how many litres of CO2 were produced by a 23 litre ferment.
I was brewing 45 gallon batches, I am downsizing to 15g to keep it fun. 9 hour brew days are intense. Kegs are extraneous now after upgrading to zillas. I charged kegs up at 25psi during active ferment purged 20 times, although not needed I think since they are connected inline so yeast actively stripped the O2. Then I dropped pressure to 15psi. 180F boil Brut Chocolate Labrador Ale is superb 9 days in. I hear lager yeast can handle higher pressure. I am enjoying Gulo high attenuation cross of english ale and french saison, instead of my typical budejovice lager house strain. I reuse my yeast so it is a balance of not stressing it out and harvesting co2. I like the idea of making a Co2 generator, then I could bump up to max pressures. You would not need to purge, just add a few cups of water and sugar to bottom and the keg would self charge and yeast would scavenge all O2... I would rather be able to drink it though. I think a slow lager strain would be ideal, for continuous co2 production. Has anyone tried adding fresh wort to beer in primary besides making heavy beers. I am wondering if I can just keep topping off the fermzillas or do I have to drink it all first. Would yeast be happy in a 50% beer/ wort solution?
 
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DuncB

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Not sure that you would ever get to drink your beer if you keep adding wort to it.
Regarding yeast, I did use White labs high pressure lager yeast to rush an NZ pilsner from grain to bottle and comp in a fortnight. Pressure fermented and got 3rd place so happy. BUT the yeast was an absolute swine to clear. As well as cold crashing and gelatine I then used Clarity F as well.
Since then have just used Opshaug Kveik hot fermented 34 C under pressure building to 25psi after day 2. No esters and very clean.
further issue with topping up is the alcohol would probably rise and this would hamper the yeast more. I don't know enough about the dynamics of yeast ferment, but cooler is slower, more alcohol is slower and poor nutrition is slower.
So perhaps just sugar and yeast, with occasional feed in a cool space would work. Could also just warm it a bit to kick up activity when you need some CO2 if you are drawing a lot from kegs or need to purge.
Somewhere I saw how many litres of CO2 per gram of sugar so that might be a useful bit of info for your drip feeding.
When I reduced the pressure in the fermentasaurus that was at 35psi and still fermenting the yeast went crazy as they got roused and had less pressure on them.
 

renstyle

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I hear lager yeast can handle higher pressure.
...
I think a slow lager strain would be ideal, for continuous co2 production.
My take on the yeast strains was that lager yeast can benefit from pressure during the 1st stage of primary to allow you to ferment at higher temps and avoid much (all?) of the ester production that these strains are known for when fermenting warm.

Lower ferm temps and long(er) cold conditioning were what was used in the past to suppress... they have a name for that. :D

Ale yeast will also benefit from a temp bump, up to a point. This is what most folks take into account when making starters. I don't cool the starter wort down to beer pitch temps, I get it down in the 70s-80s where I won't kill the yeast, then let 'er rip. I'm decanting off the starter beer and pitching only slurry.

If anybody has Chris White on speed-dial, wonder what his thoughts are?
 

DuncB

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I think it's a food thing mainly, no food no ethanol no CO2. Temperature and pressure are variables that you can adjust. If making CO2 want the biggest headspace and smallest liquid volume and cheap ingredients such as sugar and yeast.
Might be heresy to suggest it but bakers yeast is great at making gas and not much ethanol, however not a clue re pressure handling of this yeast. If your unitank has a collecting bowl, could just drop out the contents of the bowl yeast trub etc and then replace it with 500ml of sugar water and the production line would be reinvigorated.
 

Barleymist

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I make tiny beers from my spent mash that often turn out quite good. Try filling cooler mash tun with cold water overnight will prevent lacto spoilage and extract more sugar especially from 170 sparge and low mash temp. My cooler tun can fit two deer so it has very high thermal mass when full smells bad unless cooled below 80. My plan is to to keep a low pressure, low alcohol yeast "brink" that is moderately tasty in my big mouth bublers. Ferment zillas at 30 psi? Dump yeast and trub to keep all rounder from getting autolysis. My natural priming setup tolerates higher carbonation pressure since it is all at cellar temp, 55F.
 
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t^3

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I'm looking how to dry hop without introducing oxygen. Could I put the hops in a keg and connect it to the fermenter to purge it with free CO2. Would having the hops sit in the keg for a week or so be an issue? When fermentation finishes transfer into that keg. After a few days cold crash the keg then transfer to purged serving keg. Anybody try this?
 

t^3

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@DuncB Interesting idea. I have been doing some high hop doses lately, 2 oz/gal. I am using a SSbrewbucket and usually put 5.75 gallons in it since I lose so much because of the high dry hop dose. There is not much headspace left, the lid is usually covered with yeast. Makes me think the magnet/bag would sag into the wort.
 

DuncB

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The ss brewbucket has a lid? so you could do a dry run, not sure that the krausen would do that much harm to the hops when it rises.

What about one of those metal hop infusing baskets? Could that be held up above the ferment?
 

t^3

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I may try it. Do you stir the fermenter occasionally to help with the hop contact? I saw another thread where they put washers in the bag and a big magnet on the outside. May have to do that since the brew bucket is stainless.
 

DuncB

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I do magnet to magnet, then can as said move the bag around bit to swish it if it's hopped once ferment over. Not sure how you'd stir it without opening it. Could get a bit of a swirl on it by rocking the fermenter around, but I don't as I swish the bag around under the magnets control.

I think the washers or nuts in the bag was to weigh the bag down so it sank after it was released from the magnet. Not a need for this if you drag it into the ferment and drag it out to drain after ( which might reduce your loss to soaked hops).
 

RampantOctopus

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Sooo, two questions for the folks who are purging sanitizer filled kegs with co2 harvested as described in this post:

I went straight to kegging and skipped bottling other than batches here and there that I want to share. Now I'm looking hard at a DIY counter-pressure filler using kegland carb caps and the wye to connect 2 to the top of a bottle... oh someday... what a hobby eh?

So since I did have the kegs, I've done a few batches using 3 cornys:

1) ferm vessel
2) starsan vessel (eventual serving vessel for #1) and
3) a holding keg for the purged starsan (which will become the serving vessel for the next batch).

I've used the same starsan, with modest top-ups between brews several times now, round robin, so I've got that pretty well dialed in...
1. How much star san are you using (or at least starting with) -- a full 5 Gal? I'm wondering if I should start with a full 5 gal RO+Star San for the first iteration of this set up... aaaaand next question...

2. Do you find that your star san is picking up an aroma carried on the harvested CO2 (and if so, do you think it has impacted future batches)?

love this thread btw, glad to see that it's been recently revived.
 

DuncB

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The keg is full to the brim so no air space at all pre purging. So volume is dictated by keg size.

Haven't noticed any aroma pickup.

Have seen one website, can't remember which video but on the Homebrew Network re NEIPA that he sterilises keg then fills keg with Sodium Metabisulphite solution that he purges out. His theory that the little remnant of liquid and sod met in the bottom of the keg acts as an antioxidant for any oxygen that was in the water ( and came out of solution ) during the purge out and can absorb any other dissolved oxygen in the ferment when transferred across.
I haven't seen any really good science on this although perhaps the theory is good given it's use in wine making.
I expect that any of these effects would all be scrubbed away by the ongoing purge with the harvest method. Might have a benefit if the keg was purged just before being filled but I'm not sure.
 

renstyle

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1. How much star san are you using (or at least starting with) -- a full 5 Gal? I'm wondering if I should start with a full 5 gal RO+Star San for the first iteration of this set up... aaaaand next question...
I use 5++ gal of StarSan in the keg. I fill the keg as full as possible via the main lid, then seal. Then I use a 2-liter PET bottle with a carb cap and a bev-2-bev jumper to top off the keg as much as possible (tilting the keg until it runs out the gas post as well as the PRV). I'll give it the tiniest pop off my CO2 tank to seat the lid.

2. Do you find that your star san is picking up an aroma carried on the harvested CO2 (and if so, do you think it has impacted future batches)?
My keg rotation is such that the current "starSan holding keg" is serving keg <next>.

The keg I purge the starSan INTO becomes serving keg <next-one-after-that>.

I have not noticed any aromas carried with the sanitizer, but this starSan is only topped off a little each time I brew, using the 2-liter bottle. It never gets utilized for other purposes (I have a bucket of starSan for that job), just an endless round-robin into the next serving keg.

This one tank has been pushed thru the last 4 brews, and as long as I keep it sealed up pretty well, I can keep this up for a few more. :cool:
 

RampantOctopus

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I use 5++ gal of StarSan in the keg. I fill the keg as full as possible via the main lid, then seal. Then I use a 2-liter PET bottle with a carb cap and a bev-2-bev jumper to top off the keg as much as possible (tilting the keg until it runs out the gas post as well as the PRV). I'll give it the tiniest pop off my CO2 tank to seat the lid.



My keg rotation is such that the current "starSan holding keg" is serving keg <next>.

The keg I purge the starSan INTO becomes serving keg <next-one-after-that>.

I have not noticed any aromas carried with the sanitizer, but this starSan is only topped off a little each time I brew, using the 2-liter bottle. It never gets utilized for other purposes (I have a bucket of starSan for that job), just an endless round-robin into the next serving keg.

This one tank has been pushed thru the last 4 brews, and as long as I keep it sealed up pretty well, I can keep this up for a few more. :cool:
That's awesome. I kind wish I had not shortened the dip tubes I did (not enough to replace them yet) but it turns my keg stock into a slightly less flexible group. Anyhow, given the need to purge all of the star san with each fermentation, the shortened tubes all become primaries, the rest become spunding secondaries/serving kegs I guess. Though a couple more eps of brulosophy and I'll just wind up fermenting directly in pint glasses or something.

Thanks guys!
 

renstyle

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That's awesome. I kind wish I had not shortened the dip tubes I did (not enough to replace them yet) but it turns my keg stock into a slightly less flexible group. Anyhow, given the need to purge all of the star san with each fermentation, the shortened tubes all become primaries, the rest become spunding secondaries/serving kegs I guess. Though a couple more eps of brulosophy and I'll just wind up fermenting directly in pint glasses or something.

Thanks guys!
I use one of these mesh filters attached to a floating dip tube in my ferm-keg. I have two kegs with these that take turns in the chamber. That freed-up two regular dip tubes for me. :)

I also fine with gelatin in primary as I begin a cold crash, rather than after I transfer to the serving keg, to minimize pickup on the still-normal length dip tube during serving.

I dunno what I'm gonna try for dry hopping, likely drop a metal hop tube in primary a few points before hitting FG...
 

RampantOctopus

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I use one of these mesh filters attached to a floating dip tube in my ferm-keg. I have two kegs with these that take turns in the chamber. That freed-up two regular dip tubes for me. :)
That's good stuff. I do have a floatIt, but this is certainly a better priced option for me to expand the fleet as it were.
I also fine with gelatin in primary as I begin a cold crash, rather than after I transfer to the serving keg, to minimize pickup on the still-normal length dip tube during serving.
Where was this post two hours ago before I added gelatin to my secondary and cold crashed. lol. that's timing.
 

renstyle

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Where was this post two hours ago before I added gelatin to my secondary and cold crashed. lol. that's timing.
Since I've committed to doing the starSan purge to the serving keg whenever possible, I wanted to keep the business end of the dip tube as low as possible to best remove max sanitizer (and even then a drbble is always left behind, which is fine).

That meant serving from the rock-bottom of the keg, so I started cold crashing in primary so the filter on the floatie could have max effectiveness.

An advantage of crashing in a keg is that suck-back issues are reduced if not eliminated. :)
 

DuncB

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I use that mesh filter as well. Link not working at present. But is effective.
I'm using one of these to inject the finings into the primary whilst under pressure a day or two before the transfer. Or you can use it to put the finings into the transfer keg.


Much cheaper and bigger than the williamswarn version and I've also used it as a randalliser which was a bit of fun.
The fittings are directly swappable for bulkhead posts so I've put the universal post on instead of the push fit line connector at the input end.
 

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Bump...I am wondering if people have put this to practice with success.

I have been enjoying brewing 2.5 gal batches lately, but my pair of small Torpedo kegs are the bottleneck in my system. I recently picked up some floating dip tubes with the potential of fermenting in a keg and/or serving from a keg. If I can use the CO2 from fermentation, it would make me feel better than "wasting" CO2 with 2.5 gals of beer in a 5 gal keg.

I don't have room in my fermentation chamber (a 5 cu ft chest freezer) for three 5 gal kegs, but I could fit two in there. I was thinking I could ferment a 2.5-3 gal batch in a 5 gal keg. I would jumper the gas to another 5 gal keg (maybe with a spunding valve attached to that keg). Then once fermentation was done, I would push the beer into the "purged" 5-gal keg for serving.

I could also just ferment and serve from the same 5 gal keg with the floating dip tube.
 

doug293cz

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Bump...I am wondering if people have put this to practice with success.

I have been enjoying brewing 2.5 gal batches lately, but my pair of small Torpedo kegs are the bottleneck in my system. I recently picked up some floating dip tubes with the potential of fermenting in a keg and/or serving from a keg. If I can use the CO2 from fermentation, it would make me feel better than "wasting" CO2 with 2.5 gals of beer in a 5 gal keg.

I don't have room in my fermentation chamber (a 5 cu ft chest freezer) for three 5 gal kegs, but I could fit two in there. I was thinking I could ferment a 2.5-3 gal batch in a 5 gal keg. I would jumper the gas to another 5 gal keg (maybe with a spunding valve attached to that keg). Then once fermentation was done, I would push the beer into the "purged" 5-gal keg for serving.

I could also just ferment and serve from the same 5 gal keg with the floating dip tube.
The original analysis was for a 5.3 gal ferment purging a 5.3 gal keg (including headspace) + 1.3 gal of fermenter headspace. You would be fermenting about half of that, and the fermenter headspace increases to about 2.8 gal. So, you would be purging more volume with about half of the CO2 as the original analysis. This will leave you with a higher residual O2 content in the purged keg.

Brew on :mug:
 

DuncB

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yes very possible. You might want to fill the pumping keg with higher pressure by temporarily rising the pressure on the fermenting beer to say 25 psi. Get your keg filled then lower the pressure. You don't need pressure though to do a closed transfer just gravity and gas recirc.
Then you could use all of your 25 psi to serve with.
 

CascadesBrewer

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The original analysis was for a 5.3 gal ferment purging a 5.3 gal keg (including headspace) + 1.3 gal of fermenter headspace.
Is there a spreadsheet somewhere that let me do my own calculations for volume? I now see that the original calcs seems to be for 20L of beer. I assumed they were for less as most people do not ferment 5.3 gals in a keg.

My biggest question was if people have been doing this will success. I am all for theory backed up by calculations, but the real test of a model is matching it up to real world measurements...or at least successes.

I realize that I can fit a pair of 5 gallon kegs, and my 3 gallon Fermonster into my chest freezer. Maybe my best bet is to build a lid with a gas post, then just push solution out of one keg into another. I would not be able to use the spunding valve, but I should at least end up with a fully purged keg.

I don't have a spunding valve yet, but I picked up a couple pairs of gas/liquid disconnects and barbs to make liquid and gas jumper lines.
 

CascadesBrewer

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I am not sure what to expect, but I got this going:
IMG_3060.JPG

The keg on the left has a 2.5 gal batch of a Pale Ale (with Voss). It has one of the FLOTit floating dip tubes. The jumper is from the gas post on the fermenting keg to the liquid post on the other keg. The keg on the left just has a standard dip tube on the liquid post and my spunding valve on the gas post.

I am not sure if I will just serve out of the fermenting keg, or do a transfer into the other keg, in which case the second keg is just for playing around this time. I am leaning toward just serving out of the fermenting keg.

I picked up the parts for the spunding valve off Amazon. All the connections seem fine, but the spunding valve seems to leak a little at the valve part no matter where the knob is set. I might have to send that part back. I probably should have been more patient and got the Kegland spunding valve...but that Amazon next day thing got me.

In theory I should have carbonated beer ready to drink in a week or so. I am fermenting this at room temp because the orange character of Voss was a bit too strong in the last batch I fermented at 85F.
 

DuncB

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So you have already purged the right keg of the starsan and are now just pushing CO2 from the left pale ale keg out of the gas and into the liquid, then gas out via the dodgy spunding valve.

Don't forget to get the pressure set that takes into account the fermenting temp. The Dr Hans calculator good for that .


ONce you feel that the right keg has had enough flushing and is at say 25 pse then remove it and put spunder on your fermenting keg. If you think it's really leaky then leave it off and then check pressure periodically and use it under supervision only. I had a dodgy leaking metal spunder and it was fine during active but then lost all the gas once ferment finished. Pressure will need to be around 25 psi at 20 celsius for 2.x vols. see calculator.

Cold crash once ferment finished without spunder on and then do the closed transfer to the other keg beer to beer and gas to gas with bit of gravity help as well.
 

CascadesBrewer

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So you have already purged the right keg of the starsan...
Nope...my understanding of the posts at the start of this thread talk about just using CO2 from fermentation to flush out oxygen from a keg. If I was using my fermentation chamber (chest freezer) I would not have room for 3 kegs, so I am trying this. Also, I am not sure how often I would have 3 free kegs to support chaining that many kegs (though I could use a bucket to catch the StarSan if fermenting in this tub).

Note, I thought about filling the second keg with StarSan and maybe I will play with that in the future. It seems that the common process is to just connect the kegs at time of pitching yeast...but then all the air in the headspace of the fermenter will be the first to fill the "purged" keg. Then from there you have to rely on some dilution to bring the oxygen levels to an acceptable level.
 
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