Max amount of grain in my eBIAB pot?

[whovous]

I do continuous re-circulation eBIAB in a six gallon pot. I typically mash with about 4.2 gallons of water, and then move the bag to a strainer in a large bowl where I do a pour-over sparge to get my volume back up. When all is said and done, I usually wind up with about 3.5 gallons to my fermenter due to boil and equipment loss. My gravity is usually higher than planned as well, so I top off with RO water (I salt it first) and get to the 4 to 4.25 gallon range in the fermenter, where trub and other losses follow.

I know what I want to do, but I am having trouble wrapping my brain about how to calculate it. I want to do all grain and I want an OG of 1.065. 1.070 would be even better. Between sparging and simply adding more water post-boil (i.e., partial boil) I want five gallons in the fermenter. Five gallons post-trub loss would be even better.

Is this possible? There comes a point where adding more grain to my six gallon pot will lead to lower and lower efficiencies, but I do not know how to determine where that point is. How much grain is too much grain in a six gallon pot?

 

[wilserbrewer]

Your kind of asking for someone to guess your best efficiency max grain bill in your 6 gallon pot?

This is not an easy or answerable question because a lot of the variable are unique to you.

When trying to get higher gravities, especially with a smaller kettle, you need not be all concerned w efficiency imo.

An extra 50 cents or dollars to worth of grain is a minimal expense.

 

[doug293cz]

Grain takes up about 0.08 gal / lb. Then if you define:

Usable volume = Vol[usable] = Vol[pot] - Required Freeboard Vol
Max Mash Thickness = t[max] in qt/lb,​

Then equations become:

(1) Strike Vol [gal] + 0.08 [gal/lb] * Grain Weight [lb] <= Vol[usable] [gal]
(2) 4 [qt/gal] * Strike Vol [gal] / Grain Weight [lb] >= t[max] [qt/lb]​

Equation (2) can be rearranged to:

(3) Strike Vol [gal] >= Grain Weight [lb] * t[max] [qt/lb] / 4 [qt/gal]​

If we substitute Equation (3) into Equation (1) the result is:

(4) (t[max] / 4) [gal/[lb] * Grain Weight [lb] + 0.08 [gal/lb] * Grain Weight [lb] <= Vol[usable] [gal]​

This reduces to:

(5) Grain Weight [lb] <= Vol[usable] [gal] / (0.08 + t[max] / 4) [gal/lb]
​

Say you want a freeboard volume of 0.7 gal (so you have room to stir) and want a mash thickness less than 1.3 qt/lb, then max grain weight would be:

(6 [gal] - 0.7 [gal]) / (0.08 + 1.3 / 4) [gal/lb] = 5.3 [gal] / 0.405 [gal/lb] = ~13 lb
​

Different required freeboard and/or max mash thickness will change the max grain weight allowed.

Brew on

 

[whovous]

Your kind of asking for someone to guess your best efficiency max grain bill in your 6 gallon pot?

This is not an easy or answerable question because a lot of the variable are unique to you.

When trying to get higher gravities, especially with a smaller kettle, you need not be all concerned w efficiency imo.

An extra 50 cents or dollars to worth of grain is a minimal expense.

But I do need to be concerned with efficiency if I am trying to create a recipe for the highest gravity five gallon brew I can wind up with from a six gallon pot. Beyond a certain point, adding grain reduces efficiencies, and at some point those added grains mean my beer is going to get smaller, not larger, if I insist on winding up with a five gallon batch.

I realize the obvious answers are to either get a larger pot or to keep doing what I've been doing, which is to focus on batch sizes smaller than five gallons, but where is the challenge doing those things?

 

[whovous]

Grain takes up about 0.08 gal / lb. Then if you define:

Usable volume = Vol[usable] = Vol[pot] - Required Freeboard Vol
Max Mash Thickness = t[max] in qt/lb,​

Then equations become:

(1) Strike Vol [gal] + 0.08 [gal/lb] * Grain Weight [lb] <= Vol[usable] [gal]
(2) 4 [qt/gal] * Strike Vol [gal] / Grain Weight [lb] >= t[max] [qt/lb]​

Equation (2) can be rearranged to:

(3) Strike Vol [gal] >= Grain Weight [lb] * t[max] [qt/lb] / 4 [qt/gal]​

If we substitute Equation (3) into Equation (1) the result is:

(4) (t[max] / 4) [gal/[lb] * Grain Weight [lb] + 0.08 [gal/lb] * Grain Weight [lb] <= Vol[usable] [gal]​

This reduces to:

(5) Grain Weight [lb] <= Vol[usable] [gal] / (0.08 + t[max] / 4) [gal/lb]
​

Say you want a freeboard volume of 0.7 gal (so you have room to stir) and want a mash thickness less than 1.3 qt/lb, then max grain weight would be:

(6 [gal] - 0.7 [gal]) / (0.08 + 1.3 / 4) [gal/lb] = 5.3 [gal] / 0.405 [gal/lb] = ~13 lb
​

Different required freeboard and/or max mash thickness will change the max grain weight allowed.

Brew on

I am still wrapping my head around some of this, but it seems like the main thing I need to decide is what mash thickness I want and the rest takes care of itself.

Sorta. But does a false bottom alter the equation? My brew bag (Wilser, of course!) rests on a false bottom. That means my grain cannot occupy about 10% of the pot. Does that alter the equation, or is it just irrelevant? I think it is irrelevant, but I am really not certain of this.

 

[doug293cz]

I am still wrapping my head around some of this, but it seems like the main thing I need to decide is what mash thickness I want and the rest takes care of itself.

Sorta. But does a false bottom alter the equation? My brew bag (Wilser, of course!) rests on a false bottom. That means my grain cannot occupy about 10% of the pot. Does that alter the equation, or is it just irrelevant? I think it is irrelevant, but I am really not certain of this.

Yes, a false bottom will alter the equation. Max mash thickness is really about how thick a mash you can stir. All of the liquid below the false bottom does nothing to reduce the viscosity of the grain bed/water mix. Thus the volume below the false bottom needs to be accounted for prior to figuring out the water volume available for participation in mash thickness determination. I'll have to think about how to properly do the false bottom correction before I give a quantitative (new equation) reply.

You are correct that increasing grain to water ratio reduces lauter efficiency (and mash efficiency, since mash efficiency equals conversion eff time lauter eff.) But this is a continuous effect, it doesn't suddenly start happening at some specific grain bill size, but rather efficiency drops off with any increase in grain bill. Here's a chart that shows the effect for batch and no-sparge processes at two different grain absorption ratios (0.12 & 0.06 gal/lb.) Fly sparging will have a similar behavior, but is too complicated to model accurately for the general case, so no chart for fly sparge.

I don't think you ever get to a point where adding more grain reduces the SG of the resulting wort.

Brew on

 

[wilserbrewer]

In my experience, the thickest mash one can handle is about 1qt / lb.

With 5 gallons of usable mash volume this will be 15 lbs

With 6 gallons usable mash space this is 18 lbs.

Of course a sparge will be required. What gravity and efficiency you can pul is somewhat unique to you and your system.

Sounds like plenty of grain to me to yield a pretty big abv 5 gallon batch, ymmv.