BeerBorg

One of the limits of BIAB is mash size. Big beers, or big batches, require more grain and water than fit in the pot. Big mashes are also hard to lift. Some use pulleys and step ladders or similar. Today I'm making two batches with one mash and boil. Both Belgian Pale Ales, just with different yeasts, one five gallon with WLP530, one three gallon with Safbrew Abbaye. There are 17# of grain in this mash. I decided to line my 52qt Igloo cooler with a polyester voile curtain to make a mash tun. There is already a drain valve on it. That way I don't have to lift the heavy hot grain bag. I'm sure I can sparge in it too. There are no pipes or mesh added, just the curtain. I've seen a cooler with voile in it on a blog, I don't know if he added any pipes. I'll let you know how it goes.

Sounds like fun to me.
FWIW, I have done 22 lbs in my 15 gallon pot, BIAB style. No way to lift that bag and I don't have a pulley system so I use strainers to removed the grain into Lowes 5 gallon pots lined with paint strainer bags. Actually, I usually use 2 so that 1/3 goes into each of the 2 buckets and the last 1/3 stays in the BIAB bag. I can squeeze each bag to get the goodness out of it and also sparge in it too (which I rarely need to do). Only takes just a few minutes and I almost always hit my numbers or better.

That sounds creative. My biggest pots are both 10 gallon, so I wanted to do something different. I've also been using 5 gallon paint strainer bags, which will not fit the ten gallon pot.
My method worked. The bag and grains blocked the outlet, reducing flow to a trickle. Stirring helped, but the flow was erratic, spilling wort! Finally I just pulled the bag away, getting good flow. At the end I raised one side of the cooler on a steel bowl, and closed the lid on the bag to keep it at the high side. Temperature dropped 5' in an hour without preheating the cooler. Next time I'll heat the water 2 or three degrees higher.

I've done a "split BIAB" in a 16Q and 12Q pot, then combined the results back into the 16Q pot for the boil before. That worked fine even though it sounds weird.

I do things like that on the stove, Mash. I'll refine this method, possibly using my inside process to get ten gallons from a ten gallon pot and 52qt cooler. I may also use this process to get two more varied batches from one boil. If I use a neutral bittering hop, I can put half in another pot after the boil to whirlpool with American hops. Then pitch 05 in one and 530 in the other. One somewhat busy brew day results in two 5 gallon kegs of different beer.

I'm a ladder and pulley brewer too. I have to admit when using a 60 quart kettle 23 pounds of grain is about all that can fit in the mash without it overflowing. For now wrestling with that much wet grain is not an issue, it's no more strenuous than lifting two fermentors filled with wort into a chest freezer. In the future, when safely lifting that much weight becomes unmanageable, I'll have to consider other options. Thanks for sharing your idea it sounds like a good process.

Thank you all for your feedback. I might add a simple strainer on the inside of the outlet to keep it from blocking. I may also look at other coolers to dedicate one for mashing.

I am a mashtun user, but I bought a BrewBag that is designed to fit the 10 gallon round coolers. This works great, I still use the manifold in the cooler found out yesterday I still do, I knocked the tubing between the manifold and ball valve fitting off and had a "stuck sparge", lifted the bag out and hung over 5 gallon bucket and reattached back in business in a matter of a minute or so. With the 4 heavy duty seams and looped handles when it comes time to get the last of the wort, I lift it up off the bottom with a rope around the top off the step ladder, tie it off and retrieve the rest of the liquid about half way thru the boil. Easy cleanup of the mash cooler and the grain rinses off the bag nicely. BrewBag also makes them for rectangular coolers, very nice quality.
www.brewinabag.com

Mine is just a Walmart voile curtain. But I plan to sew a drawstring pocket into it.

I own bags from brewinabag.com and biabbags.webs.com (wilserbrewer) and though different, both are very high quality bags. If I'm worried about wieight, I'd use the brewinabag version but I've got a bag from biabbags.com that I've used for 4 years and other than stains from the malts, it's just as good today as it was 4 years ago but I have a spare just in case.

Check this out for strength on the wilserbrewer bags.