I do full volume BiaB currently and was entertaining the idea of doing 2 batches parti-gyle style (I'd use my old 40qt kettle for the first boil). I've read various things online with different percentages (60-40, 56-44, etc) but I'm sure they apply to doing traditional mashing with less than a full volume of water.
Has anyone done this? Is it a horrible idea? If you have, what were the approximate percentages?
Before anyone asks, I'm doing this because I want to start keeping a strong and a session on tap at all times, and this seems like an ideal way to do that.
What's the max grain bill you can mash BIAB style in your 10gal kettle? Once you know that, there is some math to do to figure out how you can split it to get 2 worts of the proper O.G. for your batches.
I don't know what my max is, the recipe I'm working with now has 14.75 pounds. Assuming I can get 60% efficiency (I have no idea though, this is the first time I've crushed my own) that'll give me 1.064 OG for the first beer. If the second beer were in the 1.030 range that would be ideal.
Make sure that you crush it finer than you think you should. I Biab and I had a lower crush and got mid 60's effs but with a finer crush with more flour I got much better efficiency. I would guess that you should soot for a bit more grain.
I have toyed with this idea too, I think that the main problem is the water to grain ratio is pretty high and those grains might be "rinsed" too much of their sugars by the time you move to the second gyle stage. If you lowered the initial volume and diluted to your wort to the desired gravity for your first batch, I am sure the second "sparge" stage would give better results. Try to mash for longer on the second stage and do a longer than normal mash out. Should work just fine then.