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Maximum batch size for a kettle


Feb 15, 2020
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I have an 8 gallon kettle which I'm using for BIAB. The Kettle acting as both Boiler and Tun.

I have brewed several beers using Partial Mash (DME being added after the boil) with this setup and am transitioning to a Full Grain recipe.

I now realise that my 8 Gallon kettle is a restriction given that the water, plus displacement of grains for most 5 Gallon recipes exceeds the "Mash Volume Needed" of my Kettle. I want to brew as much beer as possible for the volume of the Kettle.

I can create an Equipment Profile of say 2.5 gallons and then use "Scale Recipe", find I can squeeze a little more in and so create a 3 gallon profile etc, but is there a way to have BeerSmith dynamically scale a recipe to maximise the batch size for the recipe/equipment?
A kettle that's twice the volume of the batch size is a rule of thumb for full volume mash BIAB. You can get by with a smaller kettle by holding the excess water back and sparging with it. The most common ways are either pouring it over the grain bag while it's suspended over the kettle or dunking the bag in the sparge water several times. Even with the proper size kettle you'll need to do this when brewing high gravity batches. My personal preference is to reduce the batch size to half the volume of the kettle.
This is not a direct answer to your question, but another way to go about this would be to use a gallon or two of "kettle top-up". You would mash all your grains in less than the full amount of water, and after removing and draining the grains you top-up the kettle with the extra water to achieve  your desired pre-boil volume. The kettle top-up is in the equipment profile, and for every gallon of kettle top-up the mash volume required goes down by 1 gallon. Just keep an eye on your mash water/grain ratio, but if you are starting with a full-volume BIAB mash setup then you can reduce your mash volume quite a bit before you get to the 1.25 gts/lb ratio that is typical of traditional mashing.

The only complication that I know about with this technique is water treatment. You want to treat the mash water to achieve the proper mash pH and flavor profile, but treat the top-up water only with the appropriate "flavor salts" with no need to worry about pH there. When I did this I took a bit of a shortcut. I would fill the kettle with all the water and treat it all at once with the "flavor salts" (i.e. Na, Cl, SO4), then remove a couple of gallons to a separate container, then add acid to the kettle if needed to get the right mash pH.

I am thinking along the same lines as Gigafemto, but with maybe a different twist to it. 

Let's try this for spit and giggles.  Make your recipe for 5 gallons batch size and plan your water salts accordingly.  As Gigafemto suggested, add a standard amount of water to your kettle for mashing, dough in with the grain you will use and then top off with additional water in the mash (this may mean you need to heat the water separately).  Once you remove the grains, you can use one of the sparge methods that Bob357 suggested and use those runnings to top off your boil kettle to the pre-boil volume you need.  You may find that your mash pH predictions become a little more erratic, but if your water is treated, you should still be somewhere in the safe zone for enzyme activity and tannin extraction.

Another way to approach this is to forget the fact that you are mashing in a bag and set up your system as if you were doing a single infusion with batch sparge.  Again, you can use the sparge water as Bob suggested, but this will keep your mash volume a little more consistent and predictable.  Just because you are doing a BIAB style mash, does not mean you need to label it as such for the software.  It is just a program and is unimpressed by labels.

Still another approach is to figure out how much grain you can handle, mash with that amount of grain, and add DME to bring your pre-boil gravity up to what you want. I have a 10-gallon kettle and can do 5-gallon BIAB batches with all-grain for most beers, but when I did a Russian Imperial Stout recently I added 4 pounds of DME to achieve the 1.092 OG without overloading my kettle (or bag or back).

Thank you all for your insightful advice.

I brew fairly low ABV beers so a 2 Gallon "dunk" Sparge would work with my "Mash Volume".

In my case, a Fuller's London Porter clone from "CAMRA's Brew Your Own British Real Ale" with the 2G "Kettle Topup" Profile, ends up calculated as:

8.88 Gallons "Water Needed"

6.88 Gallons "Total Mash Water"
7.69 Gallons "Mash Volume Needed" (just fits)

and then:

2.00 Gallons "Kettle Topup"

I'm assuming 151F for the Sparge water (or higher to "Mash out" with this? does it matter about "Mash out" anyway? given that the grains are eventually removed from the wort?).

With regards to the "dunking" - how long should I perform this for? or should I continue until a specific Refractometer reading, for instance?
Dunk sparging is akin to batch sparging, so shouldn't be a problem if you treated your mash to a reasonable pH. As for the process, just dunk the grain bag a few times to wet the contents well and allow it to drain, or squeeze into the kettle.
How long you allow the grains to remain in the batch sparge is really up to you.  If you want a consistent process then you can either (a) allow the bag to remain in contact with the sparge water until you reach a given or consistent gravity, though this value will change based upon the recipe and target gravity desired, or (b) allow the grains to remain in contact for a given time and hold that time constant for all recipes.  Either approach is fine but remember to open up the bag once you have placed it in the sparge water and stir to incorporate the sparge water into the grain slurry.  If you do not do this, then you are really only rinsing the sugars from the outer surface of the bag and layers of grain.

Temperature of the sparge water is also up to you.  I usually heat it up to mash out temperature of around 168F, but some people use cooler water to make the bag easier to handle and squeeze the free wort from between the grain particles.