Positive Pressure To Avoid Suckback

[kgranger]

I have a Spike Unitank - they mention in the cold crashing process write up that to avoid suck back from the blowoff tube, they recommend adding about 5 psi to the tank, as the positive pressure will prevent the vacuum from forming during temp reduction. To add pressure, you have to take off the blowoff tube and add the gas manifold, sealing the fermentor.

So isn't the suck back problem already eliminated now? What is the point of the pressure pressure if we are now sealed off from the sanitizer bucket, thus having nothing to suck back? Does the pressure help the tanks integrity when the vacuum starts? I figured since it is rated at pressure limits of 15psi, a slight vacuum wouldn't collapse it, but maybe I am terribly wrong.

 

[balrog]

headspace gas will contract on lower temperature. Add CO2 (a little pressure now at warmer temp) to make up the volume lost to contraction. While you might have really good seal without additional CO2, the instant you opened it up, it would suck in air, with the oxidation risks.

 

[day_trippr]

Even more than contraction, it is the accelerated absorption that causes suck back when cold crashing. While contraction has a limit, the absorption continues...

Cheers!

 

[balrog]

Even more than contraction, it is the accelerated absorption that causes suck back when cold crashing. While contraction has a limit, the absorption continues...

Cheers!

Will the absorption continue, and cause vacuum in the headspace? I ask because I just did a ferment in keg, no spund, and put it into 40F to crash before xfr, and just bumped & removed CO2 8-10psi, and i'm wondering if it will cause vacuum since it's not "on" gas.

[Edit: and my apologies for thread jacking]

 

[day_trippr]

I don't know how far absorption will go in a truly sealed vessel - that's never come up in this context before.
Eventually equilibrium will reign, but what would be the (likely negative) head space pressure at that point?

Sounds like a @doug293cz question

Cheers!

 

[doug293cz]

I don't know how far absorption will go in a truly sealed vessel - that's never come up in this context before.
Eventually equilibrium will reign, but what would be the (likely negative) head space pressure at that point?

Sounds like a @doug293cz question

Cheers!

If we know the initial conditions: CO2 partial pressure in the headspace, the partial pressure of any other gases in the headspace, the volume of the headspace, the volume of beer, the carb level of the beer, and the initial temp of the beer; and the final temp of the beer, we can calculate the headspace pressure after equilibrium is reached (assuming a completely sealed system.) If someone has a particular case they want analyzed, I can get the answer.

Brew on

 

[day_trippr]

Ok, just for grins...I don't know anything about the specific unitank so I'll freely substitute a commonly used vessel, along with these datapoints:

- 5 gallon ball lock corny keg (8.5" if diameter matters)
- with exactly 5 gallons of, say, 1.012 neipa. That volume should end at the sidewall so maximum surface area should be maintained.
- said neipa was at the end of fermentation and dry hopping and went in the keg at 68°F to begin the cold-crash.
- 68°F beer out of the fermentor likely packs around 0.85 volumes of CO2.
- Otherwise assume the head space was filled with atmospheric content as the kegger has yet to drink the closed-transfer Kool-Aid (but s/he will )

If I missed any initial condition I'll edit this

Cheers!

 

[wardens355]

When I cold crash my better bottles, I put a couple psi of pressure with a CO2 regulator on the carboys, chill for 1-2 days, then take off pressure and transfer to keg. If I don't leave on pressure the carboys sucks in/forms a pretty strong vacuum.

The seals may be designed on the tank for pressure sealing rather than vacuum (so could risk leak in during cooling without pressure), and many times tanks aren't really made for vacuum, so could always risk deformation if the seals are designed for vacuum and one forms.

Other poster asking about potentially forming a vacuum after bumping headspace with pressure: probably will eventually form a vacuum as the air space and beer cools. I'd recommend at least bumping occasionally, or just leave on slight pressure.

 

[Bobby_M]

The flex plus will hold 15 PSI but it's not designed to withstand a vacuum so you can destroy tanks that way.

 

[doug293cz]

Ok, just for grins...I don't know anything about the specific unitank so I'll freely substitute a commonly used vessel, along with these datapoints:

- 5 gallon ball lock corny keg (8.5" if diameter matters)
- with exactly 5 gallons of, say, 1.012 neipa. That volume should end at the sidewall so maximum surface area should be maintained.
- said neipa was at the end of fermentation and dry hopping and went in the keg at 68°F to begin the cold-crash.
- 68°F beer out of the fermentor likely packs around 0.85 volumes of CO2.
- Otherwise assume the head space was filled with atmospheric content as the kegger has yet to drink the closed-transfer Kool-Aid (but s/he will )

If I missed any initial condition I'll edit this

Cheers!

You picked the absolute least interesting case. With no CO2 in the headspace, you get no additional absorption of CO2 into the beer on cooling, and will actually have some CO2 come out of solution with time. You'll get the lowest final headspace pressure when the headspace starts with pure CO2 at 14.7 psia (0 psig.)

The One of the Spike Unitanks has a 7 gal capacity. If we assume 5.5 gal of beer, then we would have 1.5 gal of headspace. At the end of fermentation with a normal airlock, the headspace would be 0 psig of pure CO2. This would be much more interesting to look at, say from 68°F to 32°F.

Brew on

 

[balrog]

You picked the absolute least interesting case. With no CO2 in the headspace, you get no additional absorption of CO2 into the beer on cooling, and will actually have some CO2 come out of solution with time. You'll get the lowest final headspace pressure when the headspace starts with pure CO2 at 14.7 psia (0 psig.)

Ok. I had keg fermented pale ale at 64, gas out went to starsan until bubbling stopped when I pulled QD and keg remained sealed. Put into 40F chamber with 8 psi CO2 attached for 30s then removed. SO 8psi CO2 in headspace never opened to environment, sitting at 40F, sealed keg.

 

[ScrewyBrewer]

The negative pressure caused by the cooled beer will cause air to get inside the fermenter if the seal is not tight. If the seal is really tight the negative pressure will distort the kettle lid. Using a sealed seven gallon fermenter I pump enough Co2 pressure to bulge the lid outward before cold crashing. The positive pressure inside the fermenter prevents suck back and/or distorting the lid.

During fermentation the blow off tube attaches to the lid using a Co2 connector. When cold crashing the blow off tube is removed and the gas post is used for injecting Co2 pressure.

 

[jack13]

I don't know how far absorption will go in a truly sealed vessel - that's never come up in this context before.
Eventually equilibrium will reign, but what would be the (likely negative) head space pressure at that point?

Sounds like a @doug293cz question

Cheers!

Regarding the headspace pressure question. I doubt this answers it, but it may be useful info nonetheless.

I cold crash in my serving kegs. I pressurize to 10 psi (that is, the serving pressue on my keezer), put in freezer anywhere from 29 to 34 F for a day before adding gelatin. When I open the pressure release before opening the keg, there's usually no hiss of CO2 coming out, nor any noise indicating air rushing in. I've always thus assumed the internal/external pressure was roughly equal after the cold crash.

So if there was 10psi of positive pressure in the keg to start, and by the end the pressures were seemingly balanced, does that mean the negative pressure created by the crash was about 10psi?

 

[ScrewyBrewer]

So if there was 10psi of positive pressure in the keg to start, and by the end the pressures were seemingly balanced, does that mean the negative pressure created by the crash was about 10psi?

Yes. Assuming the Co2 pressure hasn't leaked out through the keg's gasket or fittings, you should be ok.

 

[day_trippr]

That's the thing about cornys, you almost have to assume the lid is going to leak at negative pressure...

Cheers!

 

[Vale71]

Cornys are definitely NOT airtight at negative pressure. Sometimes it's even hard to get them to seal at positive pressure, think what will happen to the lid being pushed in by atmpospheric pressure in the opposite direction it normally gets pushed by internal pressure.
If there is no hiss when you pull the PRV you can rest assured that some air got in.

 

[jack13]

In regards to last 3 posts...

It follows that I should simply apply more positive pressure before cold crashing then, right? I could see what happens when I open the keg after cold crashing if I do 20psi, for example. If CO2 escapes then I know there was positive pressure the whole time, and thus probably no leaking. Or do 30psi for that matter.

 

[doug293cz]

Regarding the headspace pressure question. I doubt this answers it, but it may be useful info nonetheless.

I cold crash in my serving kegs. I pressurize to 10 psi (that is, the serving pressue on my keezer), put in freezer anywhere from 29 to 34 F for a day before adding gelatin. When I open the pressure release before opening the keg, there's usually no hiss of CO2 coming out, nor any noise indicating air rushing in. I've always thus assumed the internal/external pressure was roughly equal after the cold crash.

So if there was 10psi of positive pressure in the keg to start, and by the end the pressures were seemingly balanced, does that mean the negative pressure created by the crash was about 10psi?

In regards to last 3 posts...

It follows that I should simply apply more positive pressure before cold crashing then, right? I could see what happens when I open the keg after cold crashing if I do 20psi, for example. If CO2 escapes then I know there was positive pressure the whole time, and thus probably no leaking. Or do 30psi for that matter.

How much beer is in the keg? Beer volume and headspace volume need to be known in order to do a calculation. Given a "standard" ball lock corny, if one is known. then so is the other.

Brew on

 

[jack13]

How much beer is in the keg? Beer volume and headspace volume need to be known in order to do a calculation. Given a "standard" ball lock corny, if one is known. then so is the other.

Brew on

Yep, all standard corny kegs. I do 2.5 gal, 4 gal, and 5 gal batches.

 

[doug293cz]

Yep, all standard corny kegs. I do 2.5 gal, 4 gal, and 5 gal batches.

The higher the beer volume to headspace volume ratio, the larger the pressure drop. So, I'll look at the 5 gal case.

Brew on

 

[doug293cz]

Ok, I was a little slow getting to the answers. Had to find and modify an old spreadsheet. You can access the spreadsheet here. The sheet should be downloaded to Excel or LibreOffice Calc so that you can use the "Goal Seek" tool (required to get the answer for your inputs.) Inputs cells are all blue, and the calculations work on volume in liters and temps in °C. There are some conversion calcs available on the top of the sheet. (The default sheet has suitable volumes for 5 gal of beer in a standard ball lock corny.) The spreadsheet assumes that the initial temperature is the fermentation temp, and calculates the initial carbonation level for an unpressurized fermentation.

So, I ran a test case for 5 gal of beer in a standard 5 gal ball lock corny, where the initial temp is 20°C (68°F) and the cold crash temp is 0°F (32°F), and the headspace in the keg is completely purged of air and pressurized to 30 psi. The headspace pressure ends up at -5.56 psi (i.e. a partial vacuum.) You can use the spreadsheet to run other sets of initial conditions.

Brew on

 

[jack13]

Wow, very cool! Thanks!

 

[Brooothru]

If we know the initial conditions: CO2 partial pressure in the headspace, the partial pressure of any other gases in the headspace, the volume of the headspace, the volume of beer, the carb level of the beer, and the initial temp of the beer; and the final temp of the beer, we can calculate the headspace pressure after equilibrium is reached (assuming a completely sealed system.) If someone has a particular case they want analyzed, I can get the answer.

Brew on

In a practical sense, I spund at 1 ATM/14.7 psig. The lager I'm currently fermenting was at 50F. I cold crashed 5F per day down to 36F and pressure was then ~13 psig. I don't know the exact internal volume of my unitank (7 gals + headspace) nor the exact volume of the beer (no sight glass), but simple math would suggest that the drop of -14F drew a vacuum of 1.7 psi, which would suck quite a bit of Star San and O2 into the tank, if not risk implosion for a sealed but unpressurized tank.

Brooo Brother

 

[Vale71]

The only thing missing from the spreadsheed is the volume of air that will be sucked in. In this case it's almost exactly 1/2 liter of air.

 

[doug293cz]

The only thing missing from the spreadsheed is the volume of air that will be sucked in. In this case it's almost exactly 1/2 liter of air.

That's a different spreadsheet. This one is specifically for determining headspace pressure when cold crashing a seal vessel, like a keg.

Brew on

 

[doug293cz]

In a practical sense, I spund at 1 ATM/14.7 psig. The lager I'm currently fermenting was at 50F. I cold crashed 5F per day down to 36F and pressure was then ~13 psig. I don't know the exact internal volume of my unitank (7 gals + headspace) nor the exact volume of the beer (no sight glass), but simple math would suggest that the drop of -14F drew a vacuum of 1.7 psi, which would suck quite a bit of Star San and O2 into the tank, if not risk implosion for a sealed but unpressurized tank.

Brooo Brother

Spunding is whole other animal. By spunding at 14.7 psig, you have a starting carb level in the beer of 2.27-2.28 volumes, vs. a "0" pressure fermentation at 50°F which would have 1.13-1.14 volumes. An unpressurized ferment at 68°F gives a starting carb level of 0.84 volumes. The more CO2 the beer starts with, the less CO2 will be absorbed from the headspace during cold crashing, and the lower will be the final pressure drop. And, suck back only occurs with unsealed vessels.

Brew on

 

[bucketnative]

There's also a small contribution from the saturated vapor pressure of water over the beer.

 

[Vale71]

23 mbar at 20°C an only 6 mbar at 0°C? Hardly significant.

 

[bucketnative]

23 mbar at 20°C an only 6 mbar at 0°C? Hardly significant.

Well, I said it was small.

 

[eric_pwb]

Ok, I was a little slow getting to the answers. Had to find and modify an old spreadsheet. You can access the spreadsheet here. The sheet should be downloaded to Excel or LibreOffice Calc so that you can use the "Goal Seek" tool (required to get the answer for your inputs.) Inputs cells are all blue, and the calculations work on volume in liters and temps in °C. There are some conversion calcs available on the top of the sheet. (The default sheet has suitable volumes for 5 gal of beer in a standard ball lock corny.) The spreadsheet assumes that the initial temperature is the fermentation temp, and calculates the initial carbonation level for an unpressurized fermentation.

So, I ran a test case for 5 gal of beer in a standard 5 gal ball lock corny, where the initial temp is 20°C (68°F) and the cold crash temp is 0°F (32°F), and the headspace in the keg is completely purged of air and pressurized to 30 psi. The headspace pressure ends up at -5.56 psi (i.e. a partial vacuum.) You can use the spreadsheet to run other sets of initial conditions.

View attachment 665794

Brew on

I tried downloading the spreadsheet to figure out what happens in a few different scenarios I was considering, but it seems that the spreadsheet is broken, and has all the formulas missing, both in its online google sheets version and when downloaded as an excel xlsx file.

 

[doug293cz]

I tried downloading the spreadsheet to figure out what happens in a few different scenarios I was considering, but it seems that the spreadsheet is broken, and has all the formulas missing, both in its online google sheets version and when downloaded as an excel xlsx file.

Start a conversation (PM) with me. Not something to debug in this thread.

Brew on

 

[doug293cz]

The link to the spreadsheet in a previous post had a bad copy of the spreadsheet. I have fixed the bad copy, and updated the link in the previous post. Here's the correct link: https://docs.google.com/spreadsheets/d/1D2UYwwH8_yXeUl7Qa3YozSHYCBYkLOrZrKrMFvMWdDI/edit?usp=sharing

Brew on