Converting 1/6th kegs into fermenters. How much headspace to leave for krausen?

I'm considering fermenting in 20L kegs because a friend has several with damaged stems that I was thinking of converting. Just wondering how much space I should leave for the krausen.

 

pressurized fermentation or atmospheric? lager or ale?

 

I'm fermenting an ale. I'm not really sure what pressurized fermentation is, but there is an airlock on the top so I guess atmospheric? Thanks for the reply!

 

You probably need at least a gallon of headspace (for MOST beers). Some beers will require a blow off tube. Maybe 3.5 to 4 gallons in a 5 gallon keg.

Fermenting under pressure requires a pressure release valve (spunding valve) to be attached to the sealed keg. The pressure is supposed to keep the krausen from getting too crazy. My first fully closed keg fermented beer is about a week in right now, so I'll have a better idea of how well it works in a couple weeks. I split an 12 gallon batch between 3 - 5 gallon kegs and linked them together to work with 1 spunding valve. I use ball lock kegs, so I'm not sure how you would go about setting up the pressure release valve on your kegs.

 

I'll try to keep this simple...

For an atmospheric ale fermentation of a moderate strength wort, i'd start with about 4.25G and if it looks like you can go higher (i.e. minimal to no blowoff), try more the next time. For a lager you can usually go to about 4.5G.

After you do it a few times you can dial the ideal volumes in.

As another reference point, when i ferment a 1.050 lager cold (48F), under 3.5 psi of pressure, using a flush cut gas dip tube (i.e. another 1/2" of head space), i fill a corny keg just over the top weld and get 0 blow off, usually. That nets me just under 5G into the serving keg.

 

I always ferment 2.5 gal batches in 5 gal cornet kegs. Only once EVER has it come out the top. Have done a few 1.5 gal batches in 1.75 kegs, comes out all the time. 1.75 batches in 3 gallon kegs is no problem. I will let you do the math.