Batch Sparging a Large Grain Bill
For my first AG, I was able to split my sparge into two equal parts as suggested on this forum but with this large of a grain bill (17lbs)the two equal sparge volumes are to small. I am doing a 90 min boil and am using a preboil volume of 7 gallons. (Hopefully the 7 gal will get me 5 gallons)
Here is what I want to do. Do a mash out with 2 gallons close to boiling. Sparge with 1.85 gallons at 168. This will give me just a bit over 7 gal figuring 1.25 qts per lb of grain with an absorption rate of .12gal/lb of grain. Will this be an efficient way to sparge this large amount of grain or is there a better way? Thanks 
Sounds fine to me. You can jsut do 3 batches if you wanted to though.

Keep sparging until you have your boil volume...:mug:

Does it matter how small your sparge volume is?
And does 7 gal pre boil for 90 minutes look good for a 5 gallon batch? I am new to AG (2nd batch) and don't have my absorption and evaporation rates down yet. 
I'm finding that it takes time to figure out absorption and boil off rates. For reference, I'm doing a 5.5 batch today with 2.5oz hops and I'm starting with 7.5 gallons in the keggle...

What did you get last time?
All you can do is take the recommended for the first time and take measurments and records on your system and dial in the figures until it is right. This may help http://www.howtobrew.com/section3/chapter17.html 
If you want to maximize your efficiency, don't add the 2 gallon mashout water. If 2gallon mashout plus 1.85 sparge was going to hit your volume, I'd recommend taking a first running first, then sparge TWICE with 2 gallons each at 180F.
If you have an accurate way of measuring your runnings volume, just figure out how much you're missing when you take your first running. If you want 7gal preboil and your first running was 3 gallons, you're right on with two 2gal sparges. If you only get 2.5g out of first running, then make the sparges 2.25 each. Pretty simple in practice. 
I screwed up the first time and did not measure the wort collected to figure the absorption rate. I will this time. I boiled for 60 min for the first batch and used 6.5 gallons and ended up with around 5.25 gallons.

Bobby. So you are saying not to raise the first running temp to around 168, but instead  just collect the first running as is and then do two sparges of 2 gallons at 180? I am assuming that the two sparges at 180 will bring the grain bed temp up enough to maximize extraction.

You also need to know what the dead space is in your kettle.
If not you need to know how much you leave behind with the hops. If you have no dead space/loss then yo have an evaporation rate of ~20% hour. So you should loose 30% over 90 minutes 30% of 7g = 4.9g 
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