It is difficult to setup a reliably functioning recirculating BIAB without a specifically designed grain bag.
I love recirculating my wort during mashing: it provides clear wort and allows consistent temperatures and mash step automation. I also love BIAB, and use it whenever I want to shave some time off my brew day, or setup my brewery outside on a nice day, or am feeling too lazy to clean up a lot of equipment. I really want to combine the two into a recirculating BIAB system. No so much for clear wort, more for temperature stability and step mash automation. I'll just call this style "re-BIAB" for short from here on out.
TL;DR: it proves more difficult than I imagined using a standard grain bag, but there are some steps to take to increase your success rate. The ultimate solution however is using a 400 micron bag from The Brew Bag!
The very first time using re-BIAB I dry fired my heating element! Luckily I caught it early and could save the heating element with a stainless steel wire brush. It turns out there is such a thing as the BIAB version of a stuck mash, where the permeability of the filter (the bag in this case) is greatly reduced, forming a vacuum underneath the false bottom as the pump drains the wort out from under the mash bed. If you lift the bag off the false bottom, flow immediately returns to normal, sometimes with a sucking sound as the vacuum is released.
The Sight Glass Manometer
There are a number of factors that I experimented with to try and resolve the problem:
- pump flow rate
- false bottom porosity
- grain crush size
- Grain rest before pumping, and,
- the addition of rice hulls to the mash.
One key piece of equipment that I used in my experiments is a prototype sight glass that, with its opening under the false bottom, acts as a simple manometer. As the porosity of the filter decreases and the vacuum under the false bottom increases, the water level in the sight glass will fall. So here is how the experiments went - qualitative results only!
Flow rate. If you try to recirculate full bore at 15-19L/Min with your pump in this system, I can pretty much guarantee that the last thing you'll hear before dumping the batch because it got burned is that "giant sucking sound" when you lift the bag and release the vacuum beneath - notice how heavy that bag feels now? You certainly don't need that kind of flow rate just to maintain mash temperatures. I found that reducing the flow to about 5L/min greatly helps to minimize the stuck bag problem, but not completely! I nearly burned a batch flowing at about 6L/min. At least you can walk away relatively safely for a short time - and I even had batches that were fine throughout the entire duration! So definitely keep the pump rate on the low side.
False Bottom Porosity. I tested various false bottoms to see if increasing the porosity of the false bottom would help increase flow. I tested the standard 1.3 mm Craft Hardware false bottom, an RV2-3.5mm perforated sheet, and a custom made perforated sheet with 12mm holes and a calculated open area of almost 50%. Surprisingly, none of these helped. Even with the 12 mm hole false bottom, the system stuck again and again (I tested at higher flow rates). I could not discern any difference between the different versions.
A word here on baskets. A stainless steel basket in lieu of a mesh bag may help solve the problem, depending on the mesh size. A basket also allows drainage out the sides of the basket. Unfortunately I did not have time to acquire or make one, so I can't say for sure but it makes sense that if there is space between the basket mesh and the kettle wall, there will always be a free return path for wort to reach the outlet, bypassing the grain. At this point I don't plan on offering a basket product, but if you have one and use it in a re-BIAB system, please share your experience in the comments!
Grain crush size. In the forums several users suggested that a finer grain crush contributes to the problem; a very fine crush creates more powder which more easily clogs the bag. My own tests on the matter seem to support this, to some extent. I ran 3 batches crushed at .025", .04", and .05" (.63mm, 1.0mm, and 1.27mm) . The finely ground batch did indeed seem to clog faster than the courser crushes, but both the .04" and .05" crush also created circulation problems. It may have taken a bit longer but the underlying problem was still there. Furthermore, I experienced a significant drop in mash efficiency with the .05" crush, so I probably will stick with .04" in the future.
Grain rest. The idea here is to mash-in with the pump and heater off, then let the system rest for 5-10 minutes while the grains absorb water. if the particles enlarge with water absorption, maybe the grain bed will be porous enough to allow sufficient flow even with the bag in place. Now, my experience here was mixed, as the first time I tried this I was convinced it helped, but the next time it did not. The theory is good, however, and I'd recommend this step if you want to give re-BIAB a try.
Rice Hulls. I've used them frequently in my 3v HERMS system when brewing with adjuncts or high wheat percentage grain bills, and they do help in that scenario. Unfortunately, I didn't find them helping very much in the re-BIAB scenario. Do they make it worse? No. Are they expensive? Not at all. So go ahead and use them if you have them, especially if you are using other adjuncts or a high percentage of wheat.
So where does this leave us? I was ready to stop recommending re-BIAB completely! Until, that is, I saw that The Brew Bag started selling a BIAB bag with 400 micron mesh size - specifically made for recirculating systems. I had a few sent over from the US and got to brewing.
Good news: the 400 micron bag fixed all the problems! Using a standard grain crush (1mm) I am now able to recirculate at full flow, without rice hulls, using the standard slotted false bottom. Turns out the 210 micron bags are really just not applicable to a reBIAB system.
I would like to start supplying these 400 micron bags in the shop, but I'm just not sure how high the demand is going to be here in Europe. Do you have experience with or would you like to use a re-BIAB system? Let me know in the comments!