Author Topic: Pressure Purging Equation and Homebrewing Application  (Read 1129 times)

Offline hopfenundmalz

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Pressure Purging Equation and Homebrewing Application
« on: July 04, 2015, 12:24:53 PM »
There was a discussion on how to transfer beer and minimize staling in the beer here.
https://www.homebrewersassociation.org/forum/index.php?topic=23575.msg301175#msg301175

Sean makes the statement that if you have a DO meter you become shocked as to how much purging is necessary to achieve low levels. How much is that?

There are procedures to purge vessels of toxic or explosive gases. If you search for "pressure cycle purge equation" you will get hits to some links with information. Without going into all the details and the derivation, you can estimate how many purge cycles you need to reach a level of gas concentration.

N=ln(Clow/Coriginal)/ln(Plow/Phigh)

N=Number of purge cycles
Clow=Lower concentration target
Coriginal=Original concentration, so for air that is 0.21
Plow=Low pressure, 14.7 PSI absolute for our application
Phigh=High pressure for the purging, I use 30 PSI gauge, so 44.7 PSI absolute

For our case.
N=ln(Clow/.21)/ln(14.7/44.7)

selecting a sequence of 0.1, 0.01, 0.001... we can calculate the number of cycles. Round up for your application.

Concentration   #purge cycles.

0.1   0.6671342451
0.01   2.7375696785
0.001   4.8080051119
0.0001   6.8784405452
0.00001   8.9488759786
0.000001   11.019311412
0.0000001   13.0897468454
0.00000001   15.1601822787
0.000000001   17.2306177121


So to get close to 1 ppm (0.000001) you need 11 purge cycles. State of the art canning and bottling lines have about 50 ppb TPO or less.

That will use a boatload of CO2 for an empty keg, so the technique of pushing sanitizer out saves on CO2. If the head space is purged 5 times at 30 PSI, then the headspace will have about 1000 ppm O2. The headspace is small, so there is not a lot of waste for that process.

I hope this helps.


« Last Edit: July 04, 2015, 03:57:58 PM by hopfenundmalz »
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Offline HoosierBrew

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Re: Pressure Purging Equation and Homebrewing Application
« Reply #1 on: July 04, 2015, 03:10:48 PM »
Good info, Jeff. I haven't seen the info laid out in this fashion before. Very helpful !
Jon H.

Offline hopfenundmalz

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Re: Pressure Purging Equation and Homebrewing Application
« Reply #2 on: July 04, 2015, 04:00:10 PM »
I think I will start doing 8 cycles at 30 PSI when filled.

If it is a keg with sanitizer pushed out, filled though the dip tube, then it is good to go.
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Offline denny

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Re: Pressure Purging Equation and Homebrewing Application
« Reply #3 on: July 04, 2015, 04:01:21 PM »
I think I will start doing 8 cycles at 30 PSI when filled.

that's pretty much what I do, although I don't count the cycles.  I just do it "a bunch".
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Offline HoosierBrew

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Re: Pressure Purging Equation and Homebrewing Application
« Reply #4 on: July 04, 2015, 04:10:05 PM »
I've always done it 'a bunch' too, not really counting cycles.  Might be a good idea to be a little more exact given this info.
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Offline hopfenundmalz

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Re: Pressure Purging Equation and Homebrewing Application
« Reply #5 on: July 04, 2015, 08:28:22 PM »
I think I will start doing 8 cycles at 30 PSI when filled.

that's pretty much what I do, although I don't count the cycles.  I just do it "a bunch".
I had been doing 5, but in this case 8 will be better.
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Offline Philbrew

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Re: Pressure Purging Equation and Homebrewing Application
« Reply #6 on: July 04, 2015, 08:32:19 PM »
Can someone explain how to do a proper purge cycle please.  (for a full keg)
Many of us would be on a strict liquid diet if it weren't for pretzels.

Offline hopfenundmalz

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Re: Pressure Purging Equation and Homebrewing Application
« Reply #7 on: July 04, 2015, 08:43:36 PM »
Can someone explain how to do a proper purge cycle please.  (for a full keg)
Keg is full and you want to purge O2 in the headspace.

Set regulator to 30 PSI. Pressurize head space. Pull PRV to vent, letting the gases flow until it is just about to atmospheric. Repeat for X cycles, where X gets you to a low enough concentration.
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Re: Pressure Purging Equation and Homebrewing Application
« Reply #8 on: July 04, 2015, 09:01:12 PM »
I should mention that purging techniques are pretty contentious over on ProBrewer. There's a continent that thinks that it requires less CO2 to fill from the bottom while constantly bleeding the tank from the top via a spunding valve or restricted butterfly.

I've only checked product DO levels, but it would certainly be an interesting thing to test with a sniffer.
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Offline bengelbrau

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Re: Pressure Purging Equation and Homebrewing Application
« Reply #9 on: July 04, 2015, 09:49:04 PM »
Is there any info anywhere on what concentration of O2 really matters? I've been purging an empty keg 3 or 4 times, and have noticed no oxidation, even for beers that are over a year old in the bottle.

Offline hopfenundmalz

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Re: Pressure Purging Equation and Homebrewing Application
« Reply #10 on: July 04, 2015, 09:52:28 PM »
I should mention that purging techniques are pretty contentious over on ProBrewer. There's a continent that thinks that it requires less CO2 to fill from the bottom while constantly bleeding the tank from the top via a spunding valve or restricted butterfly.

I've only checked product DO levels, but it would certainly be an interesting thing to test with a sniffer.
There are also links out there that talk about continuous purges of ducts and piping. Haven't gotten into that.
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Offline hopfenundmalz

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Re: Pressure Purging Equation and Homebrewing Application
« Reply #11 on: July 04, 2015, 10:09:23 PM »
Is there any info anywhere on what concentration of O2 really matters? I've been purging an empty keg 3 or 4 times, and have noticed no oxidation, even for beers that are over a year old in the bottle.
One of the first signs of oxidation is that hop aroma fades. Oxidation takes on many forms, it is not always cardboard. Active yeast can scavenge some O2 too. The pros take lengths to minimize the Total Packaged O2.

Sean might have insights from the pro perspective. Pro Brewers have DO meters for a reason, those aren't sheep.
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Offline denny

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Re: Pressure Purging Equation and Homebrewing Application
« Reply #12 on: July 05, 2015, 04:00:36 PM »
I think I will start doing 8 cycles at 30 PSI when filled.

that's pretty much what I do, although I don't count the cycles.  I just do it "a bunch".
I had been doing 5, but in this case 8 will be better.

Although I haven't been counting, I'd be surprised if I haven't been doing at least 8.
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Offline hopfenundmalz

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Re: Pressure Purging Equation and Homebrewing Application
« Reply #13 on: July 06, 2015, 07:33:26 PM »
I should mention that purging techniques are pretty contentious over on ProBrewer. There's a continent that thinks that it requires less CO2 to fill from the bottom while constantly bleeding the tank from the top via a spunding valve or restricted butterfly.

I've only checked product DO levels, but it would certainly be an interesting thing to test with a sniffer.
Looking at some links, it looks like purging by introducing CO2 one one end of a vessel may be more efficient on CO2.

One could purge an empty keg by attaching to the liquid out, the flow would be bottom up like you describe. How long would one purge for? That is the question? Would be a neat test for someone with a meter.
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