by warra48 » Wednesday Dec 22, 2010 11:14 am
Wow, 20 lbs of grains, that equates to approximately 9 kg.
I normally mash in the range of 2.5 to 3 litres per kg. In your case that would equate to a range of 22½ to 27 litres, or 25 litres avarage..
Assuming you'll batch sparge, you'll only drain about 16 litres of that initially, allowing 1 litre per kg absorption by the grains.
You can then add your sparge water to take you to whatever boil volume you desire. For a 23 litre batch you need around 30 to 32 litres pre-boil, which is my usual range. I lose about 5 litres to evaporation over 60 to 90 minutes, 1 litre to cooling shrinkage, and some to trub, giving me 23 litres into the fermenter.
If you BIAB, then you wouldn't need to use 40 litres necessarily, but I'm no expert on that. Others may be able to guide you better on that.
To me, boiling 40 litres down to 23 litres does not look like good practice.
BeerSmith with efficiency set at 80%, predicts an OG of 1.090 for a 23 litre batch with your 20 lbs of grains.
Do you really wish to brew something with a potential ABV of around 9%?
Maybe you can post your recipe, and give us some idea of your equipment in your system, so we can have a better look at it.