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Thread: Vacuum created during lager fermentation

  1. #1
    Join Date
    Feb 2006
    Posts
    66

    Vacuum created during lager fermentation

    Here's a weird one for you - maybe someone has some experience with this:

    I made a light lager with a Czech Pils yeast last Friday. Fermentation was going smoothly and the temperature was around 54 degrees. I didn't stop at the brewery on Wednesday and on Thursday morning about 12" of water had been sucked up the blow-off tube. The glycol was cycling at the time, but the temperature was still around 54. I pressurized the vessel with CO2 to zero and soon afterward fermentation continued (CO2 began blowing off through the airlock).

    Has anyone ever had a problem with vacuums being created during fermentation? Occasionally if I knockout a beer at 55 and the fermentation tank is a little warm, a small vacuum will be created for a short period, but this is the first time I've had a significant vacuum so late in the process. Perhaps this happens more often but my pressure relief valve is faulty?

    Thanks for any help.

  2. #2
    Join Date
    Dec 2004
    Location
    Mukilteo, WA
    Posts
    304
    Hey, Rafters.........

    What you're seeing, I believe from the decription, is the difference in thermal properties between air and beer. If you turn on the glycol to crash the tank, or even when fermentation is complete and no more CO2 is being generated, or even if fermentation is complete and the Temp Controller is calling for more coolant, the air in the headspace will compress proportionally with its temperature.

    Basically, the beer is still holding temp at 54F, the fermentation is complete so there's no positive pressure, and the headspace gas (CO2) collapses and creates a vaccum in the headspace that sucks up a bit of water in the blow-off tube.

    Just for fun, measure how many inches of water is sucked up the tube and multiply by .434 to find out how many PSI of vaccum you're pulling. The more hedspace you have, the more vaccum is created.

    BTW - We see this all the time when we ferment a 15 Bbl batch in a 28 Bbl Fv and crash it. It'll pull about a foot or so of water.

  3. #3
    Join Date
    Feb 2006
    Posts
    66
    Thanks Diamond- the glycol-cooling-air-creating-vacuum was my first guess, but I hadn't noticed it being so pronounced before. I'll measure the column next time.

  4. #4
    Join Date
    May 2003
    Posts
    1

    PV=nRT

    The headspace cooling is causing a pressure drop faster than the CO2 is being generated by the fermentation. By the way, the amount of the pressure drop is totally independent of the headspace. 20% full or 80% full the pressure drop will be the same for the same amount of cooling.

    And be careful, I've seen larger tanks collapse this way, by sticking glycol valves.

    Matt
    Double Mountain Brewery

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