'Auto-transfer' to secondary corny keg fermentors?

Question

I've been reading some topics about corny-keg fermentors and became interested in that. So I was wondering:

If I ferment in a sealed keg with the liquid side plugged into another keg, by its liquid side to, can I expect that the pressure built by the yeast in primary is able to transfer the beer to the secondary one?

If so, how much time that it would takes to happen? Can I expect some attenuation or it will be so fast that primary wouldn't be done yet? If so, how much time/attenuation should I wait with simple air lock in the gas side of the keg and even so be able to do this hypothetical 'auto-transfer' (transfer with the CO2 built by the yeast instead of CO2 tank)?

Considering that is possible, it seems to be a good idea?

Answer 1

You can use the pressure from fermentation to transfer from the fermenter to a serving keg.

First, you'll want a spunding valve on the fermenter to control the pressure by releasing gas after the target pressure has been reached.

When fermentation is complete, pressurize the serving keg with CO2 to slightly less pressure than what's showing on the spunding valve. Be careful with this part. If you pressurize too much, when you connect the kegs, gas will move from the serving keg to the fermenter, disturbing the sediment. If you pressurize too little, the beer will flow quickly into the serving keg, foaming up and making it difficult to transfer all the beer.

Connect the "out" side of the fermenter to the "out" side of the serving keg with a short length of beer line. If you've done the pressure balancing right, beer should flow slowly into the serving keg.

Now you need to slowly relieve the pressure in the serving keg. I use a length of line attached to a quick release at one end, with a ball valve in the middle. Open the valve slightly -- just until you can hear some gas escaping. If you relieve the pressure too quickly, the beer fill foam, and you'll have trouble filling the keg, so go slowly.

If the fermenter did not have adequate pressure when beginning the transfer, you might need to hook up a CO2 tank and regulator to finish the job. I've found that 35psi at room temperature is just about adequate to get the job done.

Also, my experience has been that some yeast strains do poorly in a high pressure environment. It's trial and error, as this is not documented anywhere that I can find.

Answer 2

I think what will end up happening is something like…

After lag and reproduction, the yeast will start to ferment, and pressure will build up on the fermenting corny. This will slowly push still-fermenting wort into the second carboy, though perhaps following some of the trub that will have settled out first. At some point, the two cornys will reach an equilibrium, where both are "full" of fermenting beer. Given one is sealed and one is air-locked, I don't think it will be 50/50, but maybe 40/60 or 30/70.

But, there will still be fermentation effects (flocculated yeast and trub) in both cornys, and still a requirement to transfer/rack … both of which seems to defeat the imagined purposes of the experiment.

I don't think it'll work out.

(But there's only one way to find out for sure. For science! ;)

Answer 3

I see a potential problem in that as the first waves of beer flow out into the secondary keg, they aren't yet done fermenting. So its like you're racking some of the beer to secondary on the very first day of fermenation. I would worry that this would shock the yeast somehow in the secondary keg and you'd get a stalled out fermentation there. Furthermore, you'd have a bunch of trub in the secondary, as the beer there finishes fermenting (assuming it does ferment out), so you'd have to rack again to a third keg to serve.