Hey brainiacs. I ferment 2.5 gallon batches in 5 gal corny kegs. Plenty of headroom. Before I cold crash I was going to remove the airlock and replace it with the corny lid and purge with co2, then again add co2 to pressurize the tank to reduce any vacuum during the crash. Any ideas on how much pressure to use? Thanks.
Positive pressure cold crash
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kombat
Well-Known Member
I just set mine to the usual 12-15 psi and leave it hooked up. Carbonate while cold-crashing/clearing.
Whatever pressure, it's still going to be absorbed. Why not just hook it up at under 5 pounds? That should be low enough...
Anyhowe
Well-Known Member
Good point, maybe 5 psi of beer gas as the N would not be absorbed.
Sure. You could waste nitro if you wanted. Or just put more than 1 pound of co2 on it and no worries. Actually, if you were confident in your keg seal, you could start by purging then hitting it with 20 #'s of co2 and disconnecting, cold crashing, and end up a bit negative... and still be ok.
I think you're really over thinking this, and I have terribad OCD. So you're probably really really over thinking this...
Anyhowe
Well-Known Member
Quite the opposite. I am really not thinking much about it, that is why I asked here and didn't spend the time to figure it out myself. I am sure that one (with scientific tendancies like those water gurus on that other thread) could actually Calculate the reduction due to cooling.
You say 20 psi, I assume that was just a wag or have you done it? I was just curious because it is likely fine to do nothing and just have some negative pressure in the corny. But I thought that it might be cool to try to find the balance point.
A corny won't hold a vacuum. What problem are you trying to solve here? Do you feel there would be a negative impact to cold crashing at atmospheric pressure? Are you bottling? If serving from a keg, why not just carb it and serve it in the corny you're crashing in?
Anyhowe
Well-Known Member
I usually crash in the fermentation cornet then rack to a conditioning/serving corny. Just trying to keep o2 out of the beer when crashing
There's really no need to rack it again, just go ahead and serve from the same keg you're crashing in. My opinion anyway.
He ferments in the corney keg. He wants to crash that keg, then siphon the beer off the trub into another keg after crashing.
I think people are missing that part. The initial keg is full of trub.
@OP
Since there's a decent size head space in those kegs (2.5 gallon) there's also a decent amount of CO2 in it, even at modest pressures. Don't compare this to a full keg with only a quart of headspace. You only need to compensate for the contraction of the 2.5 gallon of liquid, and what is being absorbed by the beer during your cold crash to retain a high enough positive pressure to maintain the seal. And even if the seal gets lost, I doubt there is much air seeping in around the lid.
Since there's a decent size head space in those kegs (2.5 gallon) there's also a decent amount of CO2 in it, even at modest pressures. Don't compare this to a full keg with only a quart of headspace. You only need to compensate for the contraction of the 2.5 gallon of liquid, and what is being absorbed by the beer during your cold crash to retain a high enough positive pressure to maintain the seal. And even if the seal gets lost, I doubt there is much air seeping in around the lid.
Ahh right, he's fermenting in them.
Yes the amount of CO2 in the headspace would be plenty protection against oxidation while crashing. I wouldn't pressurize at all, or even seal the lid. Actually, I'd keg it first, then crash and carb in one shot, which is exactly what I do except that I ferment in carboys or sanke kegs.
Yes the amount of CO2 in the headspace would be plenty protection against oxidation while crashing. I wouldn't pressurize at all, or even seal the lid. Actually, I'd keg it first, then crash and carb in one shot, which is exactly what I do except that I ferment in carboys or sanke kegs.
rnm410
Well-Known Member
I can run these calcs if you are still interested.
Anyhowe
Well-Known Member
Yes! Thank you.
I would prefer to crash and use gelatin in the primary before transfer to the conditioning and serving keg. It would be great to know roughly what pressure I should add after to fermentation is complete such that I am close to equal after the crash.
Again, thank you.
rnm410
Well-Known Member
For the case from 60*F to 40*F
Specific volume change of the liquid is in the hundredths of thousandths difference, i.e. vacuum formed do to liquid contraction is negligible.
The negative pressure relative to outside the keg due to cooling of the gas in the head space is -0.6 psi.
This calculation neglects the amount of new gas entering, which is about the volume contained in a 12 oz can. And also neglects how much gas from positive pressure will be absorbed by the liquid.
Can you post a picture of your setup?
I may have a solution for you.
Anyhowe
Well-Known Member
Well there is really no picture to be had or taken. Simply a 5 gal corney keg wth 2.5 gallons of beer in it sitting in the fridge. I was gong to put in the lid and shoot it with x psi to get a positive charge then remove the gas. The idea was to balance the contraction due to cooling to end up relatively neutral after the crash.
rnm410
Well-Known Member
Achieve and maintain minimal positive pressure, therein lies your answer. I believe the minimum ours regs can supply is about 2psi. This will have no negative effect on yeast health, possible small amounts of carbonation will form.
Possibly flush and pressure the keg prior to transferring to seat the orings.
tagillis
Active Member
Hey masonsjax, Would you mind expanding upon this? At what point are you at risk of oxidation, i.e. - what if there was only had 1.5 gallons of head space? I've recently heard some conflicting schools of thought on cold crashing induced oxidation. Just trying to gather more info and perspectives.
rnm410
Well-Known Member
Maybe this will have some value here. https://www.homebrewtalk.com/showthread.php?t=44344
If there's co2 in there it is heavier than air and should settle on the surface creating a protective layer. If you're not actively splashing the beer around, you should be fine. I've had beers sit in primary for a year in a PET carboy at room temp and taste perfectly fine. A few weeks cold crashing in a sealed keg is nothing to worry about IME.
Anyhowe
Well-Known Member
Fwiw my main concern has nothing to do with oxygenation or carbonation for that matter. It is just a matter of science wondering how much positive pressure one needs in my situation such that the reduction in pressure due to the contraction caused by cooling is accounted for.
rnm410
Well-Known Member
Already stated 0.6 psi. Which equates to a draw of about 12 oz of gas volumetrically.
Warning: there are allegedly sentient beings that still believe in CO2 blankets.
That is all.
Cheers!
That is all.
Cheers!
Warning: there are allegedly sentient beings that still believe in CO2 blankets.
That is all.
Cheers!
I for one believe in CO2 blankets, use them all the time. They just don't exist with another gas present in the same space.
kombat
Well-Known Member
Be careful with this. With most of my kegs, they'll leak with such low pressures. Most of mine need to be pressurized to at least 5-8 psi to seal completely, otherwise they'd slowly leak out all your CO2. What I'm trying to say is, unless you've got a tightly-sealed keg, I don't think it's possible/practical to try and keep it pressurized at 2 psi without losing a lot of CO2 in the process.
Anyhowe
Well-Known Member
Interesting. Given that 1 psi equates to about 2.3 feet of 'head' elevation, how is it that people are sucking quantities of starsan into their carboys through a blowoff tube during the crash? .6 psi could only raise the starsan about a foot or 1/2 the height of the carboy.
kombat
Well-Known Member
Keep in mind that the liquid is contracting too, creating an additional suction effect.
Anyhowe
Well-Known Member
Ok now I am starting to think about it... But only a little. The density of water at 70 degrees is roughly 0.998 At roughly 40 degrees it is 1.0. (An insignificant difference?) As it cools below 40 degrees it actually begins to become less dense and as you all know expands on its way to freezing at about .934 or 9% less dense than room temperature water. So I have doubts about the above comment.
Likely then the most significant contraction amount is possibly due to the absorption of the gas into the liquid?
Wow. This is really going down the rabbit hole now...
How did you calculate this? Unless I' missing something the initial specific volume of the CO2 is dependent on both pressure and the temperature and therefore not completely defined here. Without knowing the specific volume the mass of CO2 cannot be calculated.If you assume a pressure at the beginning of your calc you will still need to account for the (massive) amount of CO2 absorption into the beer. Fact is if you pressurise to 10psi and come back with NO TEMPERATURE CHANGE head pressure will be almost nill in a day or so (probably a couple days on a 1/2 gas 1/2 beer keg). Decreasing the temperature speeds up the CO2 absorption process and reduces the volume of the gas present.
Without a positive pressure cold crashing a sealed keg with that large of a CO2 volume does not sound like a good idea to me. Keg may not bust but you WILL have a negative pressure in the keg.
No. Why would this particular CO2 stratify but the remainder of the CO2 in the atmosphere stay mixed and not you know create a "blanket" on the surface and kill you?
Anyhowe
Well-Known Member
Yea facts tend to do that sometimes.
So I think I will add 10psi and let folks know if there is positive, negative or no pressure in my corney after a 2 day 36 degree crash.
I believe the calculation is underestimating the volume change in CO2 and incorrectly calculating the pressure change.
This is what I do. The first pint I pull will have some trub but after that it is good to go, and if there is a little haze it will fall in a week or so as well.
If the purpose of this is simply not to carbonate before racking...... you do realize that the beer already will have a carb level of around ~1 volumes due to CO2 absorbed during fermentation?
For reference the last couple brews I have cold crashed in the fermenters, pressure transfered from my fermenters to kegs that contain CO2 stones. I then store these close to freezing in the conditioning freezer attached to low pressure CO2. After a couple days/weeks I'll connect the OUT of the keg to the OUT of a clean keg and pressure transfer the fully carbonated beer to the new keg. I have had no issues with this transfer. 1. I don't need as many carb stones and 2. I get to leave an additional amount of trub behind and not worry about kicking it up when moving kegs.
rnm410
Well-Known Member
Thermo tables and assumed ideal gas law.
http://www.engineeringtoolbox.com/water-specific-volume-weight-d_661.html
So basically what y'all came to terms here, is to force carbonate...
http://www.engineeringtoolbox.com/water-specific-volume-weight-d_661.html
So basically what y'all came to terms here, is to force carbonate...
Can't use the thermo tables for CO2. Tables assume that the gas is saturated at the associated temperature and pressure and it isn't.
rnm410
Well-Known Member
I didn't.
rnm410
Well-Known Member
What did you use for CO2 then?
Assumed ideal gas law, PV=mrT.
Used the headspace alone as the system. Gas constant and mass will not change, equated state 1 to state 2to determine pressure with constant volume. For volume "draw" I assumed pressure would remain that of ambient and gas would have to cross the control volume. Just a quick simple dirty calc, theoretical but good enough for guidelines.
If I were concerned about gas draw when crashing in a keg. I would hook up the gas out post, on the tube out or threaded portion zip tie a co2 filled balloon to about the size of two fists. It will not carbonate.
If you crash with 10-14 lbs, you are just carbonating as normal.
I can also comment on headspace and co2 blankets.
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