Watched the torture test by Genus that was posted in this thread and managed to find this post as well as I look into adapting my bigger AVB recipes to the AF. Thanks for sharing this, Noob! I've got three recipes I do pretty regularly that are right at 16lbs that I'm no longer worried about at all. I've also got a 10% stout recipe with 20.63lbs of grains that I feel should be doable.
I've typically mashed this stout in at 1.25qt/lb. Realizing that a full volume mash won't be possible I played around with the numbers tonight on Beersmith. It looks like a 1.45qt/lb ratio would put me at about 9 gallons total mash volume in the kettle and require a 1.3 gallon sparge. I'm not seeing any reason why this couldn't be done with the grain pipe in based on the Genus video and your post. When I did this stout on my previous system I had no issues with a stuck sparge, and that's without rice hulls. At 1.45qt/lb my mash should actually be slightly thinner even after you account for the dead space around the grain pipe.
I'd like to be able to use the grain pipe to keep it simple. I'd much rather sparge through the grain pipe then have to move wort to a 2nd kettle. In fact I'd scale the recipe down before I'd go that route.
So what am I missing?
So my first thought about this is...go for it! lol, I love to see peeps push the limits. Ive watched the genus video as well with their barleywine using 20lbs of grains (they later added rice hulls to). Special note is that they also had 3-4 stuck mashes throughout the mash even with recirculating at a seemingly appropriately slow rate. So just know, you will be battling this a little bit Id think. So from my experience with the anvil and using beersmith as well here are some things to think about:
1) As long as the total mash volume is at or below ~9.3 gallons (taking grains and mash water into account based on beersmith estimates), your water/wort level will be just below the malt ring where the recirculation disc sits. Ive hit this mark plenty and wouldn't recommend going above much otherwise the wash water will go above the disc. You stated you would be at 9gallons so you should be good to go.
2) I'm not sure how you calculated the mash thickness but (unless my equipment/mash profile is off) the beersmith calculated mash thickness takes into account ALL kettle water which gives the illusion of a thinner mash when its not. Make no mistake about it, if you bring 20+lbs of grain/hulls to the party, this is going to be THICK. So Ive estimated the volume of the malt-pipe to be ~6.97gallons based on the diameter of 10.125inches and 20inches in height if its filled to the brim (using volume of a cylinder equation). Its closer to 6.45gallons is you estimate the height of the malt-pipe to be 18.5inches (just below where the recirc disc sits). So Ill use one of my previous brews as an example here where I brought 19lbs of grain&hulls to the party (Double NEIPA) and beersmith tells me that 19lbs equates to taking up ~1.48gallons of space. So, even if you use my ~7gallons volume in the malt pipe which is generous, that leaves 5.5gallons space in the malt pipe for water. So 19lbs of grain + 5.5 gallons of water equates to ~1.16qt/lb inside the malt pipe. Beersmith told me that my mash thickness would be 1.7qt/lb because it uses all water in the kettle. In that recipe I had 7.86 gallons total of strike water and 1.5gallons of sparge. So my point is that your mash thickness inside the malt-pipe will be thicker than what you'd expect and it also substantiates why Genus had issues with stuck mashes too.
3) Remember, in comparison to your previous mash tun/brewery setup, your previous mash tun was wider for sure and the anvil is narrow (malt-pipe narrower). So even if the mash thickness is equal, your chances of grain compaction are still higher on the foundry because theres more grain stacked on itself vertically. So its difficult to compare mash thickness IMO on two different systems where the mash tun geometry is different, its not an apples to apples comparison.
Again, I say go for it despite all this!
Keep us posted. I think that worst case scenario, you have to stir more often on this to prevent/remedy the stuck mashes, it will be more work as you are at the limit of the anvil for sure. But you could also say screw it, stop recirc all together, wrap it up and just let it mash as well to avoid compaction.
BTW I can't remember, you are using a bag in the malt-pipe yes? I think this would help as well if stirring more so less flour gets through to the element on the bottom. But - you probably will have more flour than normal on the element due to more stirring and agitating the grains anyways.
In the end it will be beer! I started brewing only a little over a year ago and this anvil is my first and only system, so you have more experience overall than me. Hope my experiences here helps.
Cheers!