If batch sparging though it's not as straightforward as sparging until you reach your pre-boil. Considering this is brew #1 for you crd and you're doing the maths by hand instead of brewing software, you're doing very well.
I'd suggest brewing software for future brews but for now let's do the maths.
Go 3l/kg of grain as Stu suggests. If you are draining from the esky there will be some residual liquor depending on your false bottom. Work out how much this is as this will be your mash tun loss. Hopefully it's negligable, but let's assume it's 2 litres. Add this to the strike volume.
- MLT loss = 2 litres
- Strike volume = (4.31 x 3) + 2 = 12.93 + 2 = 14.93 litres
Allow 1l/kg for grain absorption as this is liquor that won't be transferred to your kettle. It will be thrown out with the grains.
- Losses to grain = 4.31 litres
If you are doing a 60 minute boil for a 23l batch, you will lose around 5 litres to evaporation but it all depends on your system.
To work out your batch sparge volume you work out the difference.
- Post boil volume = strike water + sparge - MLT loss - boil loss - grain loss
- Therefore sparge volume is -
= post boil - strike water + MLT loss + boil loss + grain loss
- = 23 - 12.93 + 2+ 5 + 4.31
- = 21.38 litres
So you could do two batch sparges of 10.7 litres each and you should end up with 28 litres into the kettle. If you batch sparged with too much water then once you reach 28 litres (your preboil volume) you will still have some sugary goodness left in your esky which means the starting gravity of your brew will be less than it could be.
Make sure you work out what your final volume is in the
kettle and in the
fermenter and record both. Also record your OG. Once you have all this you can input this into brewing software and work out your total system efficiency, so that next time you can brew the beer to the strength you want rather than just using a kit.
Good luck!