Keg Cleaning Physics - How do you purge?

[Trickster]

I was considering the other day about how most keg washers work. They clean upside down for the cascading waterfall of chemicals and water to clean the keg. This part makes sense. The part that I am wondering if we need to rethink is the CO2 purge. It seems to me that the CO2 being a denser gas when the keg is purged upside down the CO2 would make it down to the gas out / in section of the tap.

Now if one were just pressurizing the keg without blowing out gas, technically the air and CO2 will stratisfy and the CO2 will be on the bottom of the keg to create a blanket and the air would be expelled when filling the keg to keep equal pressures and beer flowing.

The other possibility, one would be expelling gas when purging the CO2 out of the keg for the first part of the purge. I would think that part of this gas is a blend of CO2 and air, probably still leaving some air in the keg. Either way their is still very much a possibility of having air in the kegs possibly reducing shelf life.

When working at a previous brewery there did not seemed to be an issue with shelf life in their kegs but their beer never stayed on the shelf very long. Has anyone ever considered this? Has anyone noticed any oxidation in their kegs? Does anyone take their kegs off their washer and purge them upright? Is this even something I need to worry about because almost all gas will be expelled in the filling process?

Cheers

[CopperKettle]

I recommend getting rid of caustic keg cleaning (which requires purging with air not CO2 first) and going with an all acid and CO2 method, that way you never introduce O2 in the keg anyway. Birko chemical can get you hooked up.


As far as the traditional method I think most people just use a brute force approach and just blast the keg with CO2, wasting quite a bit, just to ensure all the O2 is out.

[CDangerBrew]

Five Stars acid #6 is also designed to work in a CO2 environment, we use this in our BBT. Every 4-5 cycles we totally evacuate CO2 and run PBW, and acid #5. It seems after 4-5 cycles of the acid#6 there is a "cloudy" look to the stainless inside the tank. For that reason we have not implemented this procedure with our kegs yet.

When we get around to the sanitizing/CO2 purge in our keg regimen, we run pressure up to 7-10 psi then evacuate while CO2 is flowing into the keg, and repeat this 3 times. While I am fairly certain all the O2 is evacuated, CopperKettle is correct in the wasting a lot of CO2 part. I may have to try a different chemical manufacturer to see if there is a difference. The benefit of an acid only keg cleaning regimen is very attractive.

[revnatscider]

Does no one fill the first keg with sani and push it out to another keg with CO2? Then you have a pure-CO2 empty keg. Repeat the process with each keg.

[Natrat]

It seems to me that the CO2 being a denser gas when the keg is purged upside down the CO2 would make it down to the gas out / in section of the tap.

Now if one were just pressurizing the keg without blowing out gas, technically the air and CO2 will stratisfy and the CO2 will be on the bottom of the keg to create a blanket and the air would be expelled when filling the keg to keep equal pressures and beer flowing.

Gentlemen, the gas will not stratify. It is GAS...meaning it is at a high state of energy, bouncing around and MIXING quickly. Even more quickly as it is above atmospheric pressure. It might stratify a bit as you reach the freezing point of CO2, but probably not much until you actually see CO2 snow coming out of the air. So yes, the compressed air (if that is what you are using) and CO2 mix during the purge cycle. And you could argue that there may be some residual oxygen after the purge cycle. But this is unlikely, as the purge usually exchanges several times the volume of the keg in CO2.

Yes, in a room of air which is not agitated much, the CO2 runs out of your tanks and collects on the floor. Remember how cold it is from chilling your tank and from adiabatic decompression. As it warms up to ambient, it mixes pretty quickly.

And, as you say, the compressed air is in fact purged by liquid prior to being purged by CO2.

I think, personally, that the orientation of the kegs during the cleaning cycle is a non-issue, as far as gases are concerned. As for residual oxygen, if there is any, it will be almost undetectable.

I remember going to a technical dive shop once, and seeing guys buying nitrox ROLLING their tanks on the floor to mix them.

[davidrubidge]

Neutral keg clean

You can clean effectively EDTA/Neutral products that will not be affected by the CO2 - Holchem do a good one.

[dick murton]

If you want to purge with CO2, then push it in through the beer inlet and vent the gas via the extractor tube. This cycle on big rackers is very short, for example, one of the machines I have worked with purges through for a total of 8 seconds and pressurises for 6 seconds - to get to about 21 psi.

[TL Services]

Very much in agreement with Dick Murton on this

The majority of DO2 issues I ever saw were due to pickup prior to keg fill, eg. fob tank or buffer tanks.

I do recall one instance where an attempt to improve line efficiencies by speeding-up fill rate resulted in gas entrainment into the beer (CO2 & N2 - kegs were purged with mixed gas); this then dissolved over the following few days, leading to quite worrying dispense problems...

...so time-efficiency doesn't always pay!

[kai]

The atmosphere in a clean keg doesn't matter too much if the keg is filled properly. The fob from a good fill pushes out the original atmosphere and leaves the beer in the keg with minimal DO pickup.

It's also a good reason to not hold onto that final half-keg from a racking run for too long. Best drink it quick.

[TL Services]

As long as a quiet fill is assured, ie. with no turbulence or 'fountain', then there shouldn't be much O2 pick-up.

However filling with the keg inverted is prone to a less-than-quiet fill, allowing gases to become entrained in the liquid as micro-bubbles.

These dissolve over time and - as per my previous post - can cause significant problems, whether CO2, N2 or O2.

[kai]

[golly], yes, good point. I was commenting around using a keg washer with a separate fill station and assuming the keg was upright upon filling.

The moral to every story is, never assume anything. And explain yourself clearly (yourself being me)!