# Cold Crashing bucket fermenter while hooked up to a pressurized corny keg

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#### Beerandgranite

##### Active Member
TLDR: Is the volume of co2 in a corney keg (5.5gallons) at 1-2psi enough to offset the loss of pressure in the fermentor (6.5 gallons) when crashing 5-5.75 gallons of wort from 65-33 degrees?

At risk of overcomplicating things, i am trying to find a method other than the balloon full of co2 to prevent oxygen ingress while cold crashing using a plastic bucket fermentor. I have equipped my bucket fermentors with spigots and gas posts (the latter used for the blowoff as well to pressurize while doing closed transfer) and they seem to be able to reliably hold 1-2psi for long periods of time (multiple days). my method is to:
• connect the fermentor via the gas post to the gas side of corney keg filled with starsan solution
• Purge the keg via fermentation
At this point, i would like to attach a spunding valve set to 1-2psi to the corney keg liquid post until fermentation is complete. the idea would then be to cold crash using the pressurized head space in the bucket, as well as the pressurized volume of the corney keg to prevent oxygen ingress when the pressure drops. Is the volume of co2 in a corney keg (5.5gallons) at 1-2psi enough to offset the loss of pressure in the fermentor (6.5 gallons) when crashing 5-5.75 gallons of wort from 65-33 degrees?

Interesting idea. If you have time you can look into PV=nRT and figure it out. Off the top it feels like you'll not use anywhere near a full psi of CO2 from a 5 gallon keg but of course it should be verified. That's not an exact answer of course but maybe gets you started if you end up having to DIY the answer.

Actually you'll need to start with finding the volume change of the beer itself. There are some online calculators for that as well. That'll next be able to tell you the pressure change.

Sheesh there was a time I could do this super fast. I'm now trying to remember the details.

You could also do it the old fashioned way and try it (with water).

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Interesting idea. If you have time you can look into PV=nRT and figure it out. Off the top it feels like you'll not use anywhere near a full psi of CO2 from a 5 gallon keg but of course it should be verified. That's not an exact answer of course but maybe gets you started if you end up having to DIY the answer.

Actually you'll need to start with finding the volume change of the beer itself. There are some online calculators for that as well. That'll next be able to tell you the pressure change.

Sheesh there was a time I could do this super fast. I'm now trying to remember the details.

You could also do it the old fashioned way and try it (with water).
Awesome thanks, I knew there had to be some kind of formula out there, I just had no idea find it. I span my wheels on google for a solid fifteen minutes and decided it was to specific a question for search bar.

I’ll see where this leads me. I’d prefer to come to a theoretical answer, as I don’t know I would be able to determine success by running a test. If the fermenter collapses then it’s an obvious fail, however if it doesn’t it’s not an obvious success. I guess if it doesn’t collapse and I can still get a hiss out of the keg it would mean it likely worked? Although I don’t know there’s even much sounds the comes out of the PRV on a keg at 1psi.

Measure the diameter of the lid of the fermenter and use that to calculate the square inches of lid. Multiply that number by the psi you intend to use and decide if your fermenter lid will hold the pressure. My wild guess is that you will find the lid laying on the floor some distance from the fermenter and a mark in the ceiling from where it hit.

PV = nRT does not take dissolution into account. As you cool wort/beer the solubility of CO2 goes up. Wouldn't it be easier to cold crash in the keg, where you can pressurize it a lot more?

PV = nRT does not take dissolution into account. As you cool wort/beer the solubility of CO2 goes up. Wouldn't it be easier to cold crash in the keg, where you can pressurize it a lot more?
That would be my choice, transfer to the keg, apply CO2 at the calculated PSI for the beer style and target temp, then cold crash. Kill two birds with one CO2 tank.

That would be my choice, transfer to the keg, apply CO2 at the calculated PSI for the beer style and target temp, then cold crash. Kill two birds with one CO2 tank.
Thats what i am currently doing. works well enough, however there is a lot of yeast carry over doing it this way. when i clean out a spent keg, there is a cup's worth in the bottom. I eventually plan to start clarifying my beers and figured cold crashing before transfer would be a good first step. I also need to figure out how to get gelatin into a closed system

I'm too lazy/stupid to do the math here but I can tell you empirically that a cold crash takes up about 1.5-2 gallons of CO2 in 48 hours.

Since a keg cannot collapse to give that up, the pressure has to be high enough that it can give up about 2 gallons of CO2 before the pressure drops to zero PSIG.

I'm too lazy/stupid to do the math here but I can tell you empirically that a cold crash takes up about 1.5-2 gallons of CO2 in 48 hours.

Since a keg cannot collapse to give that up, the pressure has to be high enough that it can give up about 2 gallons of CO2 before the pressure drops to zero PSIG.
I can do the rigorous math, but don't have time for the next few days. If we take Bobby's empirical estimate of 2 gal, then we need 7.3 gal of CO2 at 14.7 psi absolute (0 psi gauge), compressed into the 5.3 gal volume of the keg. A quick application of PV=nRT gives the pressure as: 14.7 pisa * 7.3 / 5.3 = 20.25 psia => 5.55 psig.

Brew on

I can do the rigorous math, but don't have time for the next few days. If we take Bobby's empirical estimate of 2 gal, then we need 7.3 gal of CO2 at 14.7 psi absolute (0 psi gauge), compressed into the 5.3 gal volume of the keg. A quick application of PV=nRT gives the pressure as: 14.7 pisa * 7.3 / 5.3 = 20.25 psia => 5.55 psig.

Brew on
Hmm, I wonder how the balloon solution works without turning inside out. I would guess the volume would be mostly similar between a Mylar ballon and a keg, and I couldn’t imagine more than 5psi in one. My assumptions may be wrong

Hmm, I wonder how the balloon solution works without turning inside out. I would guess the volume would be mostly similar between a Mylar ballon and a keg, and I couldn’t imagine more than 5psi in one. My assumptions may be wrong
The balloon is variable volume, rather than variable pressure (like a keg). You can remove all of the CO2 volume from a balloon, vs. a keg where you have 5.3 gal of CO2 remaining. Thus you only need about 2 gal volume with a balloon.

Brew on

Yeah, the balloon only needs to be about 2 gallons of capacity and it can be sucked completely flat. I developed the Cold Crash Guardian product with a 1 gallon bladder bag and it would often get sucked down flat within the first 2 days of a crashing. We upgraded to 2.5 gallon bladder and haven't had it completely empty yet.

A bucket fermenter can't really take more than 1psi before the the lid and bottom bulge out so you really can't make it happen with keg.

You could also do it the old fashioned way and try it (with water).
I don’t think doing this with water would really be useful. I think water is significantly less compressible than either wort or beer.

I don’t think doing this with water would really be useful. I think water is significantly less compressible than either wort or beer.

Perhaps, but I doubt it. Enough to matter, anyhow, which turns it into an opinion. Still - you'd know if it definitely didn't work, or easily worked. A quick test wouldn't be a precision test, just getting an idea if you're even close.

Keg vs. balloon, stating the obvious, aside from the keg being 2 times the size (if I understood the balloon correctly at 2.5 gallons) would be the pressures involved. When the balloon is full I wonder what pressure it's at? 1 or 2 psi (meaning of course 1-2 over atmosphere), as could be the keg described in the first post, or more like 0.5 or even less.

Put simply if the 2.5 gallon balloon suffices, a 5 gallon keg certainly should. Especially if it's under a greater pressure.

Also I'm positive you're right it fills and deflates but - cold crashed beer doesn't drop 2 gallons in volume so I'm curious where that balloon's CO2 went? Obviously I'm missing something.

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You wouldn't want to use an actual latex balloon but rather something like mylar. My cold crash bags are much thicker, but they also do not stretch and create pressure either. That's why I call them bladders rather than balloons.

The point with the keg is that you can't even consider the 5 gallons of CO2 that it holds because as soon as the pressure equals atmospheric (0 PSIG), no more CO2 can be drawn out. You'll need about 2 gallons of CO2 more and that's where Doug's answer of about 5psi comes from. The problem is that you would need a regulator to drop that 5psi down to about .5 psi or you'll blow the lid off the bucket if it isn't already leaking at that pressure.

The balloon's co2 goes into the fermenter's headspace (because the CO2 that was in there and in the balloon while warm contracts when it gets cold) and also dissolves into the beer.

Also I'm positive you're right it fills and deflates but - cold crashed beer doesn't drop 2 gallons in volume so I'm curious where that balloon's CO2 went? Obviously I'm missing something.
The volume of the gas sucked into the fermenter while cold crashing will be less than the volume that was in the balloon during fermentation. Remember pV=nRT. T is reducing. So p and V will as well.

Source: am engineer

The volume of the gas sucked into the fermenter while cold crashing will be less than the volume that was in the balloon during fermentation. Remember pV=nRT. T is reducing. So p and V will as well.

Source: am engineer
Ok, let's get pedantic.

The volume reduction of the gas (at constant pressure) going from 20°C (68°F) to 0°C (32°F) in percent is:
Cold Volume = Warm Volume * 273.15°K / 293.15°K = 0.9318​
Volume Lost = Warm Volume - Cold Volume = Warm Volume * (1 - 0.9318) = Warm Volume * 0.0682​
Percent Lost Volume = 100% * 0.0682 = 6.82%​
The volume reduction of the beer is: 0.9984, or 0.16%

So if we start with 6 gal of beer, and 2 gal of headspace in the fermenter, and cold crash from 20°C to 0°C the volume reduction at constant pressure would be:

Volume Delta = 6 * 0.016 + 2 * 0.0682 = 0.23 gal​
Well that's hardly any - why do I need more buffer volume than that?

Well, the solubility of CO2 increases at lower temperatures. At 20°C, the beer will hold 0.84 volumes of CO2. At 0°C, the beer can hold up to 1.59 volumes of CO2. Both temps assume headspace is 100% CO2 at 0 psig. The difference is 1.59 - 0.84 = 0.75 volumes. The additional 0.75 volumes of carb represents 0.75 * 6 gal = 4.5 gal of CO2 (at 0 psig & 0°C.)

So, gas and liquid volume shrinkage account for 0.23 gal, but additional CO2 absorption to get to equilibrium at cold crash temps would require 4.5 gal. It's the CO2 reabsorption that really requires the most make up CO2.

Now CO2 reabsorption takes time to get to equilibrium, so ~2 gal of make-up needed for a 2 day cold crash seems reasonable. If you cold crashed for a couple of weeks you would need about 4.75 gal of CO2 make up.

Brew on

CO2 solubility determined from equation 2.1 in the attached PDF

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The volume of the gas sucked into the fermenter while cold crashing will be less than the volume that was in the balloon during fermentation. Remember pV=nRT. T is reducing. So p and V will as well.

Source: am engineer

Right, and like the density change, you are correct but it's not nearly enough of a difference to explain it all, is what I'm trying to say. I just went for the really short version.

Ok, let's get pedantic.

The volume reduction of the gas (at constant pressure) going from 20°C (68°F) to 0°C (32°F) in percent is:
Cold Volume = Warm Volume * 273.15°K / 293.15°K = 0.9318​
Volume Lost = Warm Volume - Cold Volume = Warm Volume * (1 - 0.9318) = Warm Volume * 0.0682​
Percent Lost Volume = 100% * 0.0682 = 6.82%​
The volume reduction of the beer is: 0.9984, or 0.16%

So if we start with 6 gal of beer, and 2 gal of headspace in the fermenter, and cold crash from 20°C to 0°C the volume reduction at constant pressure would be:

Volume Delta = 6 * 0.016 + 2 * 0.0682 = 0.23 gal​
Well that's hardly any - why do I need more buffer volume than that?

Well, the solubility of CO2 increases at lower temperatures. At 20°C, the beer will hold 0.84 volumes of CO2. At 0°C, the beer can hold up to 1.59 volumes of CO2. Both temps assume headspace is 100% CO2 at 0 psig. The difference is 1.59 - 0.84 = 0.75 volumes. The additional 0.75 volumes of carb represents 0.75 * 6 gal = 4.5 gal of CO2 (at 0 psig & 0°C.)

So, gas and liquid volume shrinkage account for 0.23 gal, but additional CO2 absorption to get to equilibrium at cold crash temps would require 4.5 gal. It's the CO2 reabsorption that really requires the most make up CO2.

Now CO2 reabsorption takes time to get to equilibrium, so ~2 gal of make-up needed for a 2 day cold crash seems reasonable. If you cold crashed for a couple of weeks you would need about 4.75 gal of CO2 make up.

Brew on

CO2 solubility determined from equation 2.1 in the attached PDF
Nice!

I'd started working it through as well. 65 to 33 degrees (F, converted to Rankine), was a reduction of 6.095%. The reduction of water at those temps (I will maintain that it's close enough to beer for this) was very slight, almost negligible at these relatively low temps, at 0.133%. That's as far as I got. I was not going to get into solubility! You win the internet today for that extra work!

Not countering what you've said with my different % results, just reinforcing it actually. Hope it's taken that way.

Not countering what you've said with my different % results, just reinforcing it actually. Hope it's taken that way.
I think our results are consistent, as I chose a slightly larger temp delta than you did, so my volume changes are slightly larger.

Brew on

An alternative is the \$14.00 PLAATO suck back protector. You can blow some CO2 into a mylar balloon and attach to the inlet of the suckback protector that way any suck back will be only CO2. Below is the Plaato suckback protector. It just goes inline with the fermenter bung and existing airlock. Attach a mylar balloon with co2 to the side with a rubber band and your done. Total cost under \$20.

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Ebay can be your best friend. I am crashing 100 gallons in a custom fermentor. This particular blanketing regulator is capable of 7-10 WC (Water closet) which is 0.25 to 0.36 PSI. Any overpressure is vented. During crash there is no chance of blow off. Unfortunately I started cold crash with the vent line in a beaker with star san . It sucked it about 2 cups light star san solution but at least it was not oxygen. 2 cups of diluted star san are probably going to have minimal impact on taste of 100 gallons. The total amount of star san was less than a 1/4 oz in the 2 cups that were sucked back.

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