Keg purging with active fermentation

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Mer-man

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We all know fermentation produces 100% pure CO2 gas -- wouldn't it be great to use it for purging a keg without pushing water out first?

Others have shown smart things like purging a serving keg with a fermenting keg.

I know there's a fixed relationship of CO2 production to sugar consumption, but I imagine that a continuous flow into a keg has a different diffusion dynamic than fill/purge -- and that's beyond me.

So I am looking for two things here:
1) A scientific way to determine how much pure CO2 is necessary to "wash out" a volume containing air, and
2) An easy way to determine how much CO2 will be produced by a given volume of wort at a given gravity, assuming a specific apparent attenuation.

I think once number 1 is determined, a chart or formula could be created to make number 2 a useful thing.
And there would need to be some assumptions like flowing gas in through the "out" dip tube and venting through an airlock on the "in" dip tube.

Has anyone worked on this?
 

Brentk14

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I would say that even though it is pure, you still have a lot of yeast and what not being carried by the CO2 in the blowoff. I seem to have fairly aggressive fermentation that leads to some flow of yeast and krausen into the blowoff tube. How would you account for that, and how would you filter, or "wash" it to just have CO2?
I have thought about doing this as well, but never could wrap my head around how I could get just clean pure CO2 into a sanitized keg without contaminating it with yeast transported by the CO2. I have thought about a system kind of like what Jaybird has with his Krausen Catcher, but figured out if it's a waste of time or not.
 

doug293cz

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Watch this space. Imma write a epic response that will leave you amazed and enthused :ban: (or dazed and confused :confused:, or just whatever ... :rolleyes: :goat:)

Stay tuned

Brew on :mug:

============================================================================

Ok, you've waited long enough, here is my analysis of the OP's questions. You be the judge, is it epic or not? Worth the wait, or a big let down?

Pull up your waders, it's gonna get deep here. You might want to take this on while drinking coffee rather than beer. :D

============================================================================
We all know fermentation produces 100% pure CO2 gas -- wouldn't it be great to use it for purging a keg without pushing water out first?

Others have shown smart things like purging a serving keg with a fermenting keg.

I know there's a fixed relationship of CO2 production to sugar consumption, but I imagine that a continuous flow into a keg has a different diffusion dynamic than fill/purge -- and that's beyond me.

So I am looking for two things here:
1) A scientific way to determine how much pure CO2 is necessary to "wash out" a volume containing air, and
2) An easy way to determine how much CO2 will be produced by a given volume of wort at a given gravity, assuming a specific apparent attenuation.

I think once number 1 is determined, a chart or formula could be created to make number 2 a useful thing.
And there would need to be some assumptions like flowing gas in through the "out" dip tube and venting through an airlock on the "in" dip tube.

Has anyone worked on this?
Let's break this down into manageable pieces, and then look at them one at a time.

First question: Does the continuous flow of CO2 from the fermenter create any different dilution kinetics than the typical multiple cycles of pressurize then vent?

When we pressurize the headspace of a keg we produce a burst of CO2 gas originating at the gas in tube. This burst creates turbulence in the headspace which very effectively mixes the starting headspace gas and the added CO2. We can safely assume that the gases are well mixed prior to the vent cycle. This means we can use static, or equilibrium, math to determine the amount of dilution. We want to know solute concentration in a solution when additional diluent is added to a solution. In our case the solution is a gas solution, the solute is oxygen (O2), and the diluent is CO2. For one dilution (purge) cycle, the change in solute concentration is:
New_Conc = Prev_Conc * Starting_Amount / (Starting_Amount + Diluent_Amount)​
When working with gases in fixed volume vessels, the "amount" of gas is proportional to the absolute pressure (psia), and absolute pressure equals gauge pressure (psig) plus atmospheric pressure (14.695 psia at sea level.) This follows from the universal gas law: PV = nRT. Thus the original "amount" of gas is 14.7 psia, and the diluent amount of gas is the pressure that we add to the keg, so the dilution per cycle becomes:
New_O2_Conc = Prev_O2_Conc * 14.7 psia / (14.7 psia + Purge_Pressure)​
After we pressurize for the purge, we still have the same amount of O2 in the headspace that we started with, but the concentration is lower. Once we vent the headspace, the pressure drops back to 14.7 psia, and we have less total gas than we had before. Venting doesn't change the O2 concentration in the headspace, but since it does reduce the total amount of gas in the headspace, the amount of O2 goes down as well. The pressurize part of the purge cycle reduces the O2 concentration in the gas mix, and the venting then reduces the O2 amount. The effect of additional purge cycles is multiplicative, so the formula for multiple purge cycles is:
Final_O2_Conc = Orig_O2_Conc * (14.7 / (14.7 + Purge_Pressure)) ^ N​
Where N = number of purge cycles​
The O2 concentration in air is 21% or 210,000 ppm. If we assume that the keg headspace starts out as air, then we can calculate and plot the resultant headspace O2 concentration for various numbers of purge cycles at different pressures.

ppm O2 after purge table.png

ppm O2 after purge chart.png

So, what happens if instead of doing pressurize/vent cycles, we flow CO2 into a vessel that originally contains air? Does the flow improve the dilution and removal efficiency of O2 compared to the cyclic process? We can argue that if the CO2 inflow is fast enough that CO2 comes in faster than it can mix with the air, then it could form a sort of gas piston that would push air ahead of it towards the vent, and that this would push out more O2 per volume of CO2 than if complete mixing of incoming CO2 and existing gas occurred (as it does in the pressurize/vent case.)

The best case for non-mixing of CO2 and headspace would be if there were absolutely no internal "air" currents, such that the only mixing of CO2 with headspace gas would be via diffusion. So the question comes down to: Is the linear CO2 flow rate faster than the diffusion velocity of CO2 in air? If the CO2 flow rate were much faster than diffusion, then mixing would be limited, and continuous flow would be more efficient than purge/vent. If CO2 flow rate were much slower than diffusion, then gases would be mostly mixed, and continuous flow would not be any more efficient than pressurize/vent. If the flow rate and diffusion rates were of the same order of magnitude, then there would be significant, but not complete, mixing, making this the most complex scenario to analyze.

To start we need to get an estimate of the diffusion velocity of CO2 in air. If we limit our analysis to one dimensional flow (say from bottom to top of a keg, uniform velocity across the width), things will be much simpler, but still valid. Fick's first law of diffusion is (ref: Diffusion - Wikipedia):
Flux = -D * (𝚫Conc / 𝚫Dist)​
Where Flux is in mass/area-time,​
D is the diffusion coefficient, and​
𝚫Conc / 𝚫Dist is the concentration gradient​
If we divide Flux [mass/area-time] by density [mass/volume] we get linear velocity [dist/time] which is what we are looking for.

The diffusion coefficient for CO2 in air is about 0.15 cm^2/sec (ref: Oxygen Diffusion/Air - Cornell Composting) Now if we make some assumptions about gradients we might encounter, we can estimate a linear CO2 flow rate due to diffusion. We will use approximate numbers for simplicity, since we are only looking for order of magnitude estimates of velocity.

A corny keg has a volume of about 20 L or 20,000 cm^3, and a height of about 55 cm, leaving a cross sectional area of about 20,000 cm^3 / 55 cm = 364 cm^2. The density of CO2 at STP is about 2 g/L or 0.002 g/cm^3 (ref: Gases - Densities.) If we assume 2.5 cm of pure CO2 at the bottom of the keg, and 2.5 cm of air at the top of the keg, and a uniform concentration gradient from the bottom to the top, the CO2 gradient becomes:
𝚫Conc / 𝚫Dist = (0 - 0.002 g/cm^3) / 50 cm = -4.0e-5 g/cm^4​
The CO2 flux becomes:
Flux = -D * (𝚫Conc / 𝚫Dist) = -0.15 cm^2/sec * (-4.0e-5 g/cm^4) = 6.0e-6 g/cm^2-sec​
And finally the linear velocity of CO2 due to diffusion is:
CO2_Diffusion_Velosity = CO2_Flux / CO2_Density = 6.0e-6 g/cm^2-sec / 0.002 g/cm^3 = 0.003 cm/sec​
Next we need to determine the linear flow velocity of CO2 being fed through a keg from an active fermentation.

The reaction for fermentation of maltose is:
Maltose + H2O --> 2 Dextrose --> 4 Ethanol + 4 CO2​
Maltose has a molecular weight of 342.30 g/mol and CO2 has a molecular weight of 44.01 g/mol, so each gram of maltose fermented generates 4 * 44.01 / 342.3 = 0.5143 gram of CO2. So, if we determine how much sugar we ferment over what period of time, we can calculate how much CO2 we created and calculate an average flow rate over the cross section of a keg.

Let's work an example assuming 20 L of wort with an OG of 1.050 that achieves 80% apparent attenuation over a four day fermentation. First we have to determine how much sugar we started with. An SG of 1.050 is equivalent to 12.39°Plato, or 12.39% sugar by weight. To convert SG to plato use the following formula (ref: Brix - Wikipedia):
°Plato = -616.868 + 1111.14 * SG - 630.272 * SG^2 + 135.9975 * SG^3 @ 20°C​
Water at 20°C has a density of 0.9982 kg/L, so the weight of 20 L of wort @ 1.050 is:
20 L * 1.050 * 0.9982 kg/L = 20.96 kg​
This wort is 12.39% sugar by weight, so the weight of sugar is 2.597 kg. At 80% apparent attenuation, this beer would have an FG of 1.010, or 2.561°Plato. Since the presence of alcohol affects the SG the actual attenuation of the beer is lower (the final °Plato is higher), we must correct the final °Plato using the Balling approximation (ref: https://byo.com/hops/item/408-calcu...ion-extract-and-calories-advanced-homebrewing):
Real_Final_°P = Apparent_Final_°P * 0.8114 + Original_°P * 0.1886​
And, plugging in the numbers for our example:
Real_Final_°P = 2.561 * 0.8114 + 12.39 * 0.1886 = 4.415°P​
Thus the finished beer contains 4.415% by weight of sugar, which works out to:
Final_Sugar_Weight = 20 L * 1.010 * 0.9982 kg/L * 0.04415 = 0.890 kg​
The total sugar fermented works out to:
Fermented_Sugar_Weight = 2.597 kg - 0.890 kg = 1.707 kg​
And the total weight of CO2 created works out to:
CO2_Weight_Created = 1.707 kg_Maltose * 0.5143 kg_CO2/kg_Maltose = 0.878 kg or 878 g of CO2​
Since CO2 has a density of about 2 g/L, we created about 439 L or 439,000 cm^3 of CO2.

If we push our CO2 through the keg at a constant rate over a four day fermentation, the flow rate of the CO2 over the 364 cm^2 cross section of the keg works out to:
CO2_Velocity = 439000 cm^3 / (4 days * 24 hr/day * 3600 sec/hr * 364 cm^2) = 0.0035 cm/sec​
Damn, that works out almost the same as our diffusion velocity of 0.003 cm/sec. So, we are in the complex, hard (i.e. infeasible) to analyze regime of relative flow rates. So, what do we do now? Well, we punt, and do the worst case analysis which would assume that we get no O2 removal assist from the sweeping action of the bulk CO2 flow. As a result of doing this our residual O2 levels will be less than we calculate, so we will have a built in safety factor.

So, the answer to our first question is: Yes, the bulk CO2 flow probably helps sweep out more O2 than do simple pressurize/vent cycles, but the analysis is too difficult, so we'll just ignore the flow sweep effect, and end up with a pessimistic estimate of our final purged keg O2 levels (i.e. things will actually be better than the calculations show.)

Second question: What's the worst case O2 levels left in a keg purged with the output of an active fermentation?

So, just how do we attack a continuous slow purge flow analytically? Assume a tube runs from the fermenter to the keg liquid post, and an airlock is fitted to the keg gas post. Then every time the airlock bubbles you lose a small volume of the current gas mix (which we are assuming is homogeneous) from the keg and fermenter headspace. Let's call this volume "𝚫V", and the total volume of the fermenter headspace, keg, tube, etc. "V". Furthermore, let's call the current concentration of O2 in V "C". We then have the following:
Total O2 in V before bubble = C * V​
O2 lost to bubble = C * 𝚫V​
Total O2 in V after bubble = C (V - 𝚫V)​
Concentration of O2 in V after bubble = C * (V - 𝚫V) / V​
If C[0] is the concentration of O2 initially, then after "N" bubbles, the current concentration of O2 is:
C = C[0] * ((V - 𝚫V) / V)^N​
For V = 25 L and 𝚫V = 0.0001 L (0.1 mL), (V - 𝚫V) / V = 0.9999960. We're not getting much purging action per bubble; this doesn't look very promising yet.

So, where will we end up at the end of the example fermentation above? Well, we generate 439 L of CO2 from fermentation, and if we divide that into 0.0001 L bubbles, we produce a total of 4,390,000 bubbles. If we plug that into our formula above, and start with 210,000 ppm of O2 in V, then we have:
Final O2 Conc = 210000 ppm * ((25 L - 0.0001 L) / 25 L)^4390000 = 0.005 ppm​
Believe it or not, we reduce the O2 concentration from 21% by volume to 5 parts per billion by volume! :smack: :ban: :ban: :ban: Talk about the power of compounding!

We can only conclude that using the output of a reasonable size fermentation can very effectively purge a keg of O2.

Coming next, the spreadsheet to allow you to do your own calculations.

Brew on :mug:
 
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doug293cz

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I would say that even though it is pure, you still have a lot of yeast and what not being carried by the CO2 in the blowoff. I seem to have fairly aggressive fermentation that leads to some flow of yeast and krausen into the blowoff tube. How would you account for that, and how would you filter, or "wash" it to just have CO2?
I have thought about doing this as well, but never could wrap my head around how I could get just clean pure CO2 into a sanitized keg without contaminating it with yeast transported by the CO2. I have thought about a system kind of like what Jaybird has with his Krausen Catcher, but figured out if it's a waste of time or not.
Not sure why you are concerned about the possibility of some yeast in the CO2 that would then get into your keg. What do you think is going to happen that won't when you put beer with yeast in it into your keg? If you really want to filter out the yeast, use one of the Norcal Krausen Catchers with one of these between the catcher and the keg. (Coincidentally, my Krausen Catchers arrived in the mail today.)

Brew on :mug:
 
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Mer-man

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Yeah, I thought one could just insert a filter too.

Anyway, yeast is not a terrible concern -- one could, perhaps, imagine a situation where there is enough head space that there is no appreciable yeast blow off . . . .
 

Veets

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Epic post, doug293cz.
How about, instead of pressurizing and purging, one were to evacuate a keg and then pressurize? I'm thinking something like the vac-u-vin wine stopper and pump system, where the stopper is somehow connected to the pressure release valve on a corny. Do you think that one could pump out sufficient air by hand so that when the keg is repressurized there is minimal oxygen left?

Here's a thread on a different forum that discusses how much vacuum you can get with a hand pump, and it seems quite good.

http://www.exisle.net/mb/index.php?/topic/64087-food-preservation-technologies-you-can-use-at-home/

I suppose if we're talking about vacuum, then we'd need to consider how much resistance the springs in the poppet valves have. If trying to pull a vacuum on a corny would just open the valves and draw air in, then we'd have to talk about capping off the valves..
 

doug293cz

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Epic post, doug293cz.
How about, instead of pressurizing and purging, one were to evacuate a keg and then pressurize? I'm thinking something like the vac-u-vin wine stopper and pump system, where the stopper is somehow connected to the pressure release valve on a corny. Do you think that one could pump out sufficient air by hand so that when the keg is repressurized there is minimal oxygen left?

Here's a thread on a different forum that discusses how much vacuum you can get with a hand pump, and it seems quite good.

http://www.exisle.net/mb/index.php?/topic/64087-food-preservation-technologies-you-can-use-at-home/

I suppose if we're talking about vacuum, then we'd need to consider how much resistance the springs in the poppet valves have. If trying to pull a vacuum on a corny would just open the valves and draw air in, then we'd have to talk about capping off the valves..
I think the bigger concern is whether of not a corny keg can physically withstand a vacuum. They can take ~120 psi internal pressure, but I don't know if they can withstand 15 psi external pressure. I don't want to risk one of my kegs to find out.

If kegs will withstand 15 psi external, then you could do vacuum purging. I don't think you could get to sub 1 ppm O2 levels with a single evacuation using an inexpensive pump. But you could do it with several cycles, back filling with CO2 after each vacuum cycle. The dilution math is pretty much the same as for pressure purging,

I'm pretty sure 15 psi won't push a poppet open.

Brew on :mug:
 
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raysmithtx

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Epic post, doug293cz.
How about, instead of pressurizing and purging, one were to evacuate a keg and then pressurize? I'm thinking something like the vac-u-vin wine stopper and pump system, where the stopper is somehow connected to the pressure release valve on a corny. Do you think that one could pump out sufficient air by hand so that when the keg is repressurized there is minimal oxygen left?
This has me thinking of using my Foodsaver to try the vacuum. It will pull about 22 inches of vacuum. Connect the food saver to the Gas post with the poppet removed from the keg connector. Hook up the CO2 bottle to the Out post through a keg connector. When it reaches max vacuum, crack the CO2 tank to fill the vacuum. Remove the connector with the foodsaver still running and the CO2 still going in. Replace the poppet in the keg connector while the CO2 is still running and put the connector back on. My concern is that the lid will not hold a vacuum.

Assuming it did hold and we could get a vacuum I wonder how much oxygen would be left at 22" of vacuum?
 

doug293cz

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This has me thinking of using my Foodsaver to try the vacuum. It will pull about 22 inches of vacuum. Connect the food saver to the Gas post with the poppet removed from the keg connector. Hook up the CO2 bottle to the Out post through a keg connector. When it reaches max vacuum, crack the CO2 tank to fill the vacuum. Remove the connector with the foodsaver still running and the CO2 still going in. Replace the poppet in the keg connector while the CO2 is still running and put the connector back on. My concern is that the lid will not hold a vacuum.

Assuming it did hold and we could get a vacuum I wonder how much oxygen would be left at 22" of vacuum?
Yeah, I forgot about the lid. The lid has an area of 8 - 9 sq in, so at full vacuum (14.7 psi) the force pushing the lid in would be 117 - 132 lb. Good chance it will leak.

At 22" of vacuum, you would have about 210,000 ppm * (30 - 22) / 30 = 56,000 ppm of O2. You'd have to do multiple, sealed cycles to get to less than 1 ppm.

Brew on :mug:
 

RevKev

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doug293cz that was a very nice read. A one way valve and inline filter would definitely allow this to work. Just worry about having this as a closed system and I'm not sure what pressure the PRV releases at and if that's below the burst pressure of a typical carboy.
 

doug293cz

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doug293cz that was a very nice read. A one way valve and inline filter would definitely allow this to work. Just worry about having this as a closed system and I'm not sure what pressure the PRV releases at and if that's below the burst pressure of a typical carboy.
Just put a QD on the gas post with a tube to a blow off jar. Here's what I envision:
  • A blow off tube to a dry jar to catch any blow off
  • A tube to the liquid post of the keg
  • A tube from the gas post to a sanitizer filled blow off jar
  • Optional: a second jar to catch overflow from the sanitizer filled jar and prevent air back flow when cold crashing the fermenter
No more pressure in the carboy than from a normal blow off tube.

Brew on :mug:
 

RevKev

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I may experiment with this, I do larger batches in a conical and do daisy chain my blow off. Only problem is I use sanke kegs for large batches 1/4 and 1/6th and small 2.5 gal kegs for special batches
 
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Mer-man

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The post didn't generate a lot of comments, so wasn't sure anyone was really interested. If there is interest, it shouldn't take to long to clean it up for the public.

Brew on :mug:
Well I can step-by-step reuse your math, since you were kind enough to show your work.

But a spreadsheet would be tops :rockin:
 

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It's threads like this that keeps me coming back to HBT. What a cool concept and excellent response. Thank you!! I would love to see the spread sheet!
 
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Mer-man

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So I have a wrinkle here, that @doug293cz might be able to speak to: I liquid purged my serving keg before venting positive pressure and hooking it up to be gas purged.

How would I estimate the starting point from a liquid purged keg with your formulas?
 

doug293cz

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So I have a wrinkle here, that @doug293cz might be able to speak to: I liquid purged my serving keg before venting positive pressure and hooking it up to be gas purged.

How would I estimate the starting point from a liquid purged keg with your formulas?
A ball lock keg has a total volume of about 5.3 gal or 678 fl oz. Due to the lid design, there is a volume of 3 fl oz that cannot be filled with liquid (for an unmodified keg.) When you then blow the liquid out of the keg with CO2, that 3 fl oz of air is dispersed throughout the entire keg volume. Thus the "starting" O2 concentration is:
210,000 ppm * 3 / 678 = 930 ppm​
So, doing a liquid purge is almost equivalent to doing 5 purge cycles at 30 psi. Thus you still need 8 more 30 psi purge cycles to get down to the ~100 ppb O2 range.

Brew on :mug:
 
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Mer-man

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A ball lock keg has a total volume of about 5.3 gal or 678 fl oz. Due to the lid design, there is a volume of 3 fl oz that cannot be filled with liquid (for an unmodified keg.) When you then blow the liquid out of the keg with CO2, that 3 fl oz of air is dispersed throughout the entire keg volume. Thus the "starting" O2 concentration is:
210,000 ppm * 3 / 678 = 930 ppm​
So, doing a liquid purge is almost equivalent to doing 5 purge cycles at 30 psi. Thus you still need 8 more 30 psi purge cycles to get down to the ~100 ppb O2 range.

Brew on :mug:
So I just entered all your formulas to make my own worksheet. But I gotta say, is 25L realistic for total volume to flush? You'd have the keg internal volume, the connection to the next keg, and the tube to the airlock/blowoff ... but that I can work out myself. Just going from 25L to 23L makes a huge difference.

Also, water-purging is pointless, e.g. 7ppb v. 0ppb, since it wastes CO2 in the first place.

Thanks doug!
 

Murphys_Law

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A ball lock keg has a total volume of about 5.3 gal or 678 fl oz. Due to the lid design, there is a volume of 3 fl oz that cannot be filled with liquid (for an unmodified keg.) When you then blow the liquid out of the keg with CO2, that 3 fl oz of air is dispersed throughout the entire keg volume. Thus the "starting" O2 concentration is:
210,000 ppm * 3 / 678 = 930 ppm​
So, doing a liquid purge is almost equivalent to doing 5 purge cycles at 30 psi. Thus you still need 8 more 30 psi purge cycles to get down to the ~100 ppb O2 range.

Brew on :mug:
Great info, Doug.

So I'm clear - you're saying you can achieve close to the same results doing a liquid purge v. hitting an empty keg with 30 PSI of CO2 and doing purge cycles?

May seem like a dumb question, but how long do you open the PRV to purge? Will just a quick burst suffice?
 

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A ball lock keg has a total volume of about 5.3 gal or 678 fl oz. Due to the lid design, there is a volume of 3 fl oz that cannot be filled with liquid (for an unmodified keg.) When you then blow the liquid out of the keg with CO2, that 3 fl oz of air is dispersed throughout the entire keg volume. Thus the "starting" O2 concentration is:
210,000 ppm * 3 / 678 = 930 ppm​
So, doing a liquid purge is almost equivalent to doing 5 purge cycles at 30 psi. Thus you still need 8 more 30 psi purge cycles to get down to the ~100 ppb O2 range.

Brew on :mug:
Highlighted in Red above.

So instead it would be much more advantageous to fill your keg all the way, trying to reduce that leftover air space under the lid as much as possible. Then push a pint to a quart of water/Starsan out, and purge that small headspace 5-10 times at 30 psi, before pushing the remainder out. The cost of CO2 that way is only a little more than what the 5 gallon purge would take, and yielding a much more reduced residual O2 level.

8 full volume purges at 30 psi wastes a lot of CO2.
 
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doug293cz

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So I just entered all your formulas to make my own worksheet. But I gotta say, is 25L realistic for total volume to flush? You'd have the keg internal volume, the connection to the next keg, and the tube to the airlock/blowoff ... but that I can work out myself. Just going from 25L to 23L makes a huge difference.

Also, water-purging is pointless, e.g. 7ppb v. 0ppb, since it wastes CO2 in the first place.

Thanks doug!
The 25 L assumed 20 L for the keg and 5 L of headspace in the fermenter, so realistic for a simple arrangement. If you string kegs together, then the flushed volume will be much higher, and the results less impressive.

Brew on :mug:
 

doug293cz

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Great info, Doug.

So I'm clear - you're saying you can achieve close to the same results doing a liquid purge v. hitting an empty keg with 30 PSI of CO2 and doing purge cycles?

May seem like a dumb question, but how long do you open the PRV to purge? Will just a quick burst suffice?
I would never do multiple purge cycles on an empty keg. One 30 psi purge cycle on an empty keg uses about 2X the CO2 as a liquid purge.

But yes, you can get the same results as a liquid purge using a sufficient number of gas purge cycles.

On a purge cycle, you open the PRV until you can't hear gas escaping any more. The math assumes that at the end of venting, the pressure in the headspace is 0 psi gauge (14.7 psi absolute.)

Brew on :mug:
 
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doug293cz

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Highlighted in Red above.

So instead it would be much more advantageous to fill your keg all the way, trying to reduce that leftover air space under the lid as much as possible. Then push a pint to a quart of water/Starsan out, and purge that small headspace 5-10 times at 30 psi, before pushing the remainder out. The cost of CO2 that way is only a little more than what the 5 gallon purge would take, and yielding a much more reduced residual O2 level.

8 full volume purges at 30 psi wastes a lot of CO2.
Someone (don't remember who) cuts their gas "dip" tubes down so that they don't extend into the keg at all. Then by tilting the keg just right during liquid filling, they can get essentially all of the air out.

Your method of pushing out a pint or so, and then doing purge cycles will also be effective.

My kegging method is to liquid purge, then rack, and then do ~10 purges @ 30 psi. Seems to work ok, But if I want to get even more rigorous I might use your suggestion (but still purge after racking, just to take care of whatever might have sneaked in during racking.)

Brew on :mug:
 
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Murphys_Law

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Highlighted in Red above.

So instead it would be much more advantageous to fill your keg all the way, trying to reduce that leftover air space under the lid as much as possible. Then push a pint to a quart of water/Starsan out, and purge that small headspace 5-10 times at 30 psi, before pushing the remainder out. The cost of CO2 that way is only a little more than what the 5 gallon purge would take, and yielding a much more reduced residual O2 level.

8 full volume purges at 30 psi wastes a lot of CO2.
Are you only filling with water and not star san? I've seen a few posts of others doing this?? I supposed at this point it's beer and shouldn't get infected??
 

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I have a bit of a different way to purge a keg using Star-San. When the "donor" keg runs out of Star-San, the CO2 starts bubbling up from the bottom of the receiving keg.

That creates foam. That foam is filled with.....CO2. What I do is when I get that foam, I pull one off the QD on the receiving keg, otherwise I get huge overflow. I then dip the lid into the foam and reattach the QD to create more foam, which fills the space under the lid with....foam. Which is filled with CO2. I dip the lid into the keg which is full of foam coming out of the opening, and then fix it in place.

Doing that, I've almost perfectly purged the keg of O2. There is nothing but CO2-filled foam and Star-San in that keg except for, maybe, a little tiny bit of air in the "IN" dip tube. When it comes time to blow out that keg prior to filling from the fermenter, I'm blowing out CO2 and Star-San with Co2.

I think that's pretty close to perfect. Am I missing anything with that?
 

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I have a bit of a different way to purge a keg using Star-San. When the "donor" keg runs out of Star-San, the CO2 starts bubbling up from the bottom of the receiving keg.

That creates foam. That foam is filled with.....CO2. What I do is when I get that foam, I pull one off the QD on the receiving keg, otherwise I get huge overflow. I then dip the lid into the foam and reattach the QD to create more foam, which fills the space under the lid with....foam. Which is filled with CO2. I dip the lid into the keg which is full of foam coming out of the opening, and then fix it in place.

Doing that, I've almost perfectly purged the keg of O2. There is nothing but CO2-filled foam and Star-San in that keg except for, maybe, a little tiny bit of air in the "IN" dip tube. When it comes time to blow out that keg prior to filling from the fermenter, I'm blowing out CO2 and Star-San with Co2.

I think that's pretty close to perfect. Am I missing anything with that?
Depends on just how well you are displacing the air in the lid cavity with the foam, and not letting any air back in as you seal the lid. Also, the bubbles exposed to the air will quickly pick up some O2, since the liquid film forming the bubbles' shells is very permeable to O2. Don't know how big an effect this is tho. Overall, you should have considerably less O2 than just leaving the lid cavity full of air, but it's not really possible to figure out how much.

To get the air out of the gas dip tube, just put a gas QD on the post during filling. When bubbles or StarSan flow out of the QD, remove it.

Brew on :mug:
 

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Are you only filling with water and not star san? I've seen a few posts of others doing this?? I supposed at this point it's beer and shouldn't get infected??
I fill with Starsan, but others apparently use regular water. I guess it works fine.

Now the water (or Starsan) may again contain dissolved O2... especially when filled with lots of splashing or from an aeration faucet.
 

processhead

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I fill with Starsan, but others apparently use regular water. I guess it works fine.

Now the water (or Starsan) may again contain dissolved O2... especially when filled with lots of splashing or from an aeration faucet.
I am not sure that dissolved gases in the water or Starsan can degrade the CO2 purge, if that's where you are going with this.

If the O2 (air) is in solution and the solution is all displaced by the CO2, the amount of O2 that out-gases into the CO2 charge should be pretty insignificant.
 

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I am not sure that dissolved gases in the water or Starsan can degrade the CO2 purge, if that's where you are going with this.

If the O2 (air) is in solution and the solution is all displaced by the CO2 purge, then there should be insignificant amounts of dissolved O2 that out-gases into the CO2 charge.
Exactly where I'm going.

I indeed wonder how much dissolved O2 makes it out of the purging solution by the time it's replaced with CO2. The higher CO2 pressure in the purging space above surely helps to reduce outgassing. The process is fairly short too, but some of it must make it out.
 

processhead

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Exactly where I'm going.

I indeed wonder how much dissolved O2 makes it out of the purging solution by the time it's replaced with CO2. The higher CO2 pressure in the purging space above surely helps to reduce outgassing. The process is fairly short too, but some of it must make it out.
I would agree that the higher CO2 pressure in the head space would help inhibit out-gassing.

A question I keep asking myself throughout this interesting discussion is to what degree these trace amounts of O2 will detract from my enjoyment of the beer that resides within.;)


When I think about the whole brewing process and the things that can affect the quality of the beer, I tend to go after the low-hanging fruit first.

Trace amounts of O2 in the keg in the amounts we are talking about here are pretty close to the top of the tree, IMHO.

Oxidation of food and beverages becomes more of a concern when stored for longer periods under poor conditions, which doesn't happen very often at my brewery. YMMV

Still, it is absolutely good to understand the chemical and physical processes that go into brewing and packaging good beer. I think that 90% of home brewers would do well to keep things in perspective and focus on the more basic things that go into making good beer.
 

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...

I indeed wonder how much dissolved O2 makes it out of the purging solution by the time it's replaced with CO2. The higher CO2 pressure in the purging space above surely helps to reduce outgassing. The process is fairly short too, but some of it must make it out.
I would agree that the higher CO2 pressure in the head space would help inhibit out-gassing.

...
The CO2 pressure in the headspace does nothing to inhibit the desorption of O2 from the liquid. The rate of O2 desorption is affected only by the temp of the liquid, the O2 partial pressure in the headspace, the concentration of O2 in the liquid, and the surface area of the liquid/gas interface. The O2 partial pressure in the headspace will be very low once the headspace expands significantly, and certainly less than the equilibrium O2 partial pressure for the O2 concentration in the liquid. Thus O2 will come out of solution while the liquid is being pushed out of the keg. How much? I can't say.

Brew on :mug:
 

doug293cz

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A question I keep asking myself throughout this interesting discussion is to what degree these trace amounts of O2 will detract from my enjoyment of the beer that resides within.;)


When I think about the whole brewing process and the things that can affect the quality of the beer, I tend to go after the low-hanging fruit first.

Trace amounts of O2 in the keg in the amounts we are talking about here are pretty close to the top of the tree, IMHO.

Oxidation of food and beverages becomes more of a concern when stored for longer periods under poor conditions, which doesn't happen very often at my brewery. YMMV

Still, it is absolutely good to understand the chemical and physical processes that go into brewing and packaging good beer. I think that 90% of home brewers would do well to keep things in perspective and focus on the more basic things that go into making good beer.
A commercial brewery has found that over 150 ppb (that's billion) total packaged oxygen (TPO) will cause noticeable flavor change in their IPA's after just three weeks of room temp storage. For their non-hoppy beers the limit is 200 ppb. Things will be much better if the beer is kept cold, but as you can see, it doesn't take much O2 to cause noticeable affects. There is also a fair about of anecdotal info on HBT about improved O2 control post fermentation improving the life of kegged beer, especially hoppy beers. YMMV.

Brew on :mug:
 

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Someone (don't remember who) cuts their gas "dip" tubes down so that they don't extend into the keg at all. Then by tilting the keg just right during liquid filling, they can get essentially all of the air out.
Ahem. That would be me... link is in my sig below with pictures and some process details.

1. Shortened (flush inside) dip tubes. Alternatively take them out and re-attach at of sequence.
2. Fill to brim with liquid of choice
3. Attach lid
4. Open PRV
5. Attach water source through liquid side
6. Once water is coming out of the PRV attach plain gas QD.
7. Close PRV. Liquid will start comign out of gas QD.
8. Tilt to approx 45 degrees, gas port up, and gently rocking back and forth a few times until it doesn't sputter any more
9. Almost simultaneously detach water source and gas QD. Alternatively re-install gas dip tube and QD. if doing this step there's going to be a few mL of air in the post. If using that method i'd hit it with water through the liquid side one more time with a QD on the gas side.

Push water out with CO2 or inert gas of your choice.
 

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I use one of my kettles as the liquid reservoir and drain by gravity into the keg. After its full and the air worked out by tipping as outlined above, I then push the water back up into the kettle via CO2 pressure. This way I can reuse the sanitizer solution and do multiple kegs at once.
 

processhead

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A commercial brewery has found that over 150 ppb (that's billion) total packaged oxygen (TPO) will cause noticeable flavor change in their IPA's after just three weeks of room temp storage. For their non-hoppy beers the limit is 200 ppb. Things will be much better if the beer is kept cold, but as you can see, it doesn't take much O2 to cause noticeable affects. There is also a fair about of anecdotal info on HBT about improved O2 control post fermentation improving the life of kegged beer, especially hoppy beers. YMMV.

Brew on :mug:
The link with the industry study was an interesting read.
Since it was addressing DO issues with a bottling line, it occurred to me that their challenges could be significantly greater than those for beer kegging, although the same principles apply.

I did not see where they detailed the nature of the flavor changes that were observed on the beers with the higher DO numbers. Bad flavors? Or just different?

As a commercial brewery they are understandably concerned about product consistency and anything that detracts from uniformity or shortens the shelf life of the product are things they would want to avoid.

Another somewhat relevant question that occurred to me is, what is the relative head space volume to liquid volume ratio in a 12 oz bottle compared to a 5 gallon Cornelius keg? Does this factor into the discussion on container purging efficacy?

Keg storage temperature issue is a significant factor for me. I will keep kegs in cool or cold storage whenever possible prior to serving.
 

processhead

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The CO2 pressure in the headspace does nothing to inhibit the desorption of O2 from the liquid. The rate of O2 desorption is affected only by the temp of the liquid, the O2 partial pressure in the headspace, the concentration of O2 in the liquid, and the surface area of the liquid/gas interface. The O2 partial pressure in the headspace will be very low once the headspace expands significantly, and certainly less than the equilibrium O2 partial pressure for the O2 concentration in the liquid. Thus O2 will come out of solution while the liquid is being pushed out of the keg. How much? I can't say.

Brew on :mug:
I stand corrected.:)
 

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The link with the industry study was an interesting read.
Since it was addressing DO issues with a bottling line, it occurred to me that their challenges could be significantly greater than those for beer kegging, although the same principles apply.

I did not see where they detailed the nature of the flavor changes that were observed on the beers with the higher DO numbers. Bad flavors? Or just different?
Couldn't say. I only know what was in the presentation.

Another somewhat relevant question that occurred to me is, what is the relative head space volume to liquid volume ratio in a 12 oz bottle compared to a 5 gallon Cornelius keg? Does this factor into the discussion on container purging efficacy?
Interestingly enough, the headspace in both cases is about 6% of the liquid volume. I've measured both, since the question has been asked before.

Commercial bottlers pre-purge with CO2 immediately before filling, and since they are bottling fully carbonated beer, they get significant foam. The foam is full of CO2, and if they cap before the foam subsides, very little O2 gets into the headspace.

Brew on :mug:
 
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