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Flash pasteuriser back pressure calculation

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  • Flash pasteuriser back pressure calculation

    Hi all,

    Just wondering if anyone has a formula for calculating the back pressure required on a flash pasteuriser. We have had some refermentation issues on our nitrogenated products, but not in our carbonated products. I'm not sure if this is because the nitrogenated is more affected by the increase in co2, and hence we're more likely to get comments back from customers, or if there is a different back pressure required due to the different gas mix vs CO2 product, i.e. we could be getting gas breakout that is preventing effective pasteurisation.


  • #2
    Pasteuriser pressures are absolutely critical as it would appear you have discovered. Firstly, you need to estimate how many PUs you require. This is based very much on the microbiological quality of the beer you are pasteurising - the types of micro-organism (yeast and say lactic acid bacteria for your soured beer / cider), and their resistance to being killed by heat, and the number of organisms of each type. Then you can calculate how many PUs are required to kill off sufficient of the most resistant organisms to give you the shelf life you require. So in the big breweries they will aim for about 1 yeast cell per 100 litres after pasteurisation to give them the shelf life they require. They probably start off about 1 cell per ml before pasteurisation.

    Having worked out how many PUs, the residence time in the holding tubes determines the temperature required to give the number of PUs required.

    Then you can look up the equilibrium pressure required to keep that level of CO2 in solution (nitrogen is almost irrelevant in the amounts that can be dissolved in beer say up to 50 ppb, and the effect on equilibrium pressure). Now you have to ensure the pressure of the pasteurising / pasteurised beer in the system is ALWAYS higher than the unpasteurised beer - in case of pinhole leaks. Have a look at the attached and be scared by the pressures required. (Actually second part, to follow)
    Attached Files


    • #3
      Part 2 of presentation - the whole presentation as one was too large to upload

      This one has example pressure figures included
      Attached Files


      • #4
        Hi Dick, thanks for your reply. You mentioned N2 is insignificant up to 50 ppb, but we would have N2 at 65-70 ppm - so presumably it's quite significant then?


        • #5
          According to an Alpha Laval chart I have, the maximum solubility of pure nitrogen at zero C (32 F) is 29ppm. The maximum we ever put into our nitrogenated keg ales or stouts was about 54 ppm. I understand that Guinness has 49 ppm (I have long since forgotten who told me that - but not a Guinness employee).

          Have a look at the following presentation. It might clear up a few other queries. Unfortunately, the example temperature / pressure / pasteurisation stage doesn't allow for nitrogen. However, taking 100 ppb O2 and 6 g / litre CO2 and zero N2 at 72 C, i.e. the same values used in the presentation, my calculator gives an equilibrium pressure of 9.2 bar.

          Having said all that, according to the calculator we used to use, at 5 g / litre (2.5 v/v) CO2, 100 ppb oxygen, 70 ppb N2, at 72 C (165 F) the equilibrium pressure is 14.95 bar.

          I got the negligible influence wrong - sorry - faulty memory obviously. The dissolved oxygen has a negligible influence according to my calculator, which I must admit I find a bit strange, but that is what it says.

          So taking my calculators result as a more valid target because it includes nitrogen, a nitro-keg stout contained 2.2 g / litre of CO2, a nitro-can ale contained up to 5.2 g / litre

          So, to give an idea of the equilibrium pressure associate with these (assuming both had 70 ppm nitrogen in, which they didn't!!)

          The nitro-keg stout had an equilibrium pressure of 7.7 bar, the nitro-can ale being 13.63 bar.

          If I assume your beer has a CO2 content of 6 g / litre, this means the unpasteurised beer in the heating zone needs to be at least 14.95 bar, which means it is should be at least 15.5 bar to ensure all the gas stays in solution, and thus the holding coil zone needs to be at an even higher pressure to ensure that when it passes into the first cooling zone (i.e. against the incoming unpasteurised beer) should there be a pinhole leak or slight ripple in pressure, it needs to be higher still - perhaps 16.5 bar.

          Does this help scare you more re pressures required?