Do you know where the 120 microns came from? Is it just based on trial and error (sometimes it's the best way!) or is it based on some calculations?
I am thinking of using the bag but putting it in a stainless steel strainer with an added mesh filter of '?' microns. This would help me removing the bag and prevent the bag from touching the bottom of the kettle.
I read that somewhere:
"In a study in a German brewery (2), hot trub particles varied in size from 30 to 80 microns."
I dont know where 120 microns came from... and i actually dont know the 120 microns to which you refer? Did some one (did I?) measure the size of the gap in the voile material and say it was 120 microns?
I dont think I actually know the density of the weave on my BIAB bag - I dont really care to know. I describe it as a "very fine mesh" it should be on the order of a pair of ladies nylons, the mesh in a french press coffe filter, the mesh in a "fine" hop bag. If you tip water into it, it would fall straight through with basically no resistance, sand wouldn't fall through, flour would if you jiggle it for long enough. Basically, bordering, but not at the point where it stops being mesh and starts just being cloth. The bag material was first worked out by a smart guy walking into a fabric shop and thinking to himself that "this stuff looks just about right" and then later by dozens of people trying to come up with something better, and failing.
Bag inside another perforated pot - if you like. A few brewers I know of do a similar thing. It works pefectly well. I've tried it and personally find the bag a hell of a lot easier to handle without the pot. And then i have to actually have another pot; and then clean another pot; and by then I might as well just have a multi vessel system.... so its not something that i think is a great idea, but it certainly works OK.
Mesh to strain out hot break..... more or less doesn't work. If the mesh is actually fine enough to filter it out, then it will more or less instantly clog up with break and liquid will go nowhere. The filters that are able to effectively filter break are all depth filters. Grain beds, hop backs - all rely on 3 dimensional depth filtration, not surface filtration which is what you get with a mesh.
You can do it - but be prepared to wait quite a while for the liquid to come out of your mesh filter if you make it fine enough to do the job you want it to.
And as I have repeatedly said.... you dont need it. the break that makes it through the bag, just comes out in the boil like all the other break.
I suggest you try BIAB as detailed in this thread, lots of thought and lots of brewing has gone into making it just about the easiest way to skin this particular cat. If for some reason it doesn't give you results you are happy with, then
try modifying it. Of course if you feel the irresistable urge to tinker with it.. why the hell not? If you give it a little thought, virtually anything you come up with can be made to work - after that its just about your personal preferences.