Max batch size to ferment in a corny without blowoff

[dnitty]

I'm considering fermenting in corny kegs. I don't really mind not having a full five gallons on draft and I would rather brew more often. I've ordered two modified keg lids from Adventures in Homebrewing (they are set up to use an airlock). I plan on fermenting just enough to avoid blowoff; anyone have an idea of what that ideal batch size would be? I understand that there are a lot of variable here but I'm looking for a ballpark batch size.

By the way, reasons for switching are as follows: easy transfer to secondary with minimal O2 exposure, safer than the glass carboys that I use for most batches, no light exposure, and harder to take samples (a bad habit of mine that my wife mocks me for).
Thanks,
Dean

 

[MannyEdwards]

At my first attempt I would leave 20% head space and see how that does. That's more than you need in a carboy, but it's narrower in the keg, so it probably needs to be taller.

 

[mucci2k]

If you use Fermcap you could hypothetically ferment 5 gallons in a corny.

But....

1. I would only do a low ABV beer.
2. Try and find a yeast that does not have a violent fermentation.
3. Keep the temp down.

 

[Tegra]

If you dare you could check out the monster long post on fermenting under pressure. It (along with fermcap) will allow almost a full keg.

I was using two kegs, with the gas ports teed together, to a spunding valve.

I have since moved the process to Sanke kegs.

Tom

 

[beersk]

Lagers, you can do 4.75-5 gallons, as they don't produce much krausen. Ales, especially hefe and Belgian styles, need a lot more. You don't want to blow off too much or you risk compromising your fermentation, ie it may under attenuate if there's too much blow off.
I ferment in kegs as well, but in the 3.5-4.5 gallon range. I don't mind having less than 5 gallons either.