I'd say the issue would be the sheer size of the tun + the size of the manifold * the weight of grain used (25-30 odd kg of grain in the tun).
The flow for the initial few L it was perfectly fine but once we recirculated (even with the pump operating at 1/2 ball valve) the grainbed conpacted pretty quickly and the runoff rate became almost a trickle. We also tried without a pump and manual recirc with little sucess as well. The keg mastun on the otherhand (which was well overfilled with grain) and a dome falsie managed a fine runoff rate at full flow and 10 mins recirc. Personally i'd err on the side of caution and go for a 50L tun at maximum, especially if you plan on using the beerbelly falsie or something of similar size.
We also tried raking the mash to counteract any channelling or glueing of the grainbed. Still, no dice!
I beleive Fents always does triple batches in his 100L tun which should handle it ok, the only other thing id be concerned about is the deadspace and temperature losses because of this, especially if you are only doing single batches. Once you try and work it to capacity like we did, thats when you will run into a world of hurt. I will be the 1st to admit we where pushing the boundaires of his tun. 3/4 full with grain, once 2.8L:Kg of strike water was added, we where almost overflowing.
My evaluation would say anything up to triple batches as fents performs you should be good to go with a 100L box. (although i'd probably go a 70L).
If i was buying one of these bad boys, i'd look at a max of 60L. Capable of double batches and probabaly not too much deadspace if you are doing single batches. Also, you can do a single batchspage if you are so inclined. :icon_cheers:
Now if it looks like i am bad mouthing these things, im not. If i was interested in upgrading my tun i'd probabaly buy up too, however make sure its kitted out with an appropriate sized manifold so you dont run into the probalems we had, this is a warning for any mash tun really.
Note: Some issues may have eventuated due to a possible uneven flour:husk ratio between the tuns, this was just a theory we had for the poor runoff rate but are unsure if this is really the case.
Cheers!