Flat vs. Domed false bottom - Home Brew Forums
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Old 04-21-2011, 04:10 PM   #1
nostalgia
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My Google-fu has not been strong enough to find if this has been discussed.

My MLT will have a bottom drain, so diptubes are not an issue here. What I'm curious about is if there is a difference in operation between a flat and domed false bottom.

Is a flat false bottom better for fly sparging because it gives a more consistent grain bed depth? Does it not matter?

It would seem without a diptube to hold it down that the domed bottom would want to wander around the kettle, where a flat one should stay in place, but I'm just speculating. Anyone with experience?

Are there any other pros/cons of each design I'm missing?

Thanks,

-Joe
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Old 04-21-2011, 05:51 PM   #2
JuanMoore
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I can't see any major advantages or disadvantages to either shape. I suppose that combined with the bottom shape of the MLT it could make a difference in minimizing the amount of free liquid not in contact with the grains. For example, I would choose a domed FB for a flat bottom MLT (cooler), but a flat FB for a concave bottom MLT (keggle).

 
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Old 04-21-2011, 09:47 PM   #3
JohnTheBrewist
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I'm just guessing here, but I think the advantage to a domed bottom is that it won't collapse under the weight of the grain. This allows a thinner sheet of metal to be used.
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Old 04-21-2011, 10:03 PM   #4
JuanMoore
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Quote:
Originally Posted by BierHerr View Post
I'm just guessing here, but I think the advantage to a domed bottom is that it won't collapse under the weight of the grain. This allows a thinner sheet of metal to be used.
The grain is near bouyant during the mash (some of it actually is bouyant), so there's almost zero weight or downward pressure on the false bottom. I once used a flimsy mesh kitchen strainer when I had some equipment malfunction, and it worked fine underneath a grist with 24# of grain. The problem was actually how to keep it from floating upwards since the rim/handle was made of plastic.

 
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Old 04-21-2011, 10:36 PM   #5
Catt22
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I favor a flat false bottom. It provides for a more uniform sparge as the grain bed is the same depth everywhere. It's true that the grain is suspended in the mash and there will be no force on the false bottom so long as the space below is fully flooded. Things change dramatically under dynamic conditions when you open the drain valve. The FB then acts as a coarse filter and it will present some resistance to flow no matter what. The grain bed will gradually compact as you drain the MT. This may or may not be readily visible, but it is happening to some degree. Worst case, obviously, would be a completely compacted grain bed and a stuck sparge. No big deal though as most times you only need to stir the grain bed to get it going again. Add a pump and start applying suction to the FB and all of this becomes more critical. I've witnessed the collapse of one FB that was not well supported. The force pushing down on the FB can be hundreds of pounds even with relatively low suction applied. I also like that it's easier to stir the mash thoroughly with a flat FB, but that's a rather minor consideration IMO.

 
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Old 04-21-2011, 10:39 PM   #6
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Quote:
Originally Posted by Catt22 View Post
I favor a flat false bottom. It provides for a more uniform sparge as the grain bed is the same depth everywhere. It's true that the grain is suspended in the mash and there will be no force on the false bottom so long as the space below is fully flooded. Things change dramatically under dynamic conditions when you open the drain valve. The FB then acts as a coarse filter and it will present some resistance to flow no matter what. The grain bed will gradually compact as you drain the MT. This may or may not be readily visible, but it is happening to some degree. Worst case, obviously, would be a completely compacted grain bed and a stuck sparge. No big deal though as most times you only need to stir the grain bed to get it going again. Add a pump and start applying suction to the FB and all of this becomes more critical. I've witnessed the collapse of one FB that was not well supported. The force pushing down on the FB can be hundreds of pounds even with relatively low suction applied. I also like that it's easier to stir the mash thoroughly with a flat FB, but that's a rather minor consideration IMO.
I hadn't considered a pump causing suction. Even when draining the flimsy mesh I used held it's shape, but I can see how it would be different had the grain bed not set well.

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Old 04-21-2011, 10:49 PM   #7
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Quote:
Originally Posted by JuanMoore View Post
I hadn't considered a pump causing suction. Even when draining the flimsy mesh I used held it's shape, but I can see how it would be different had the grain bed not set well.
A set grain bed is actually a partially compacted grain bed. Another term for a "settled" grain bed. The grain particles become fully saturated and close packed forming the filter bed. This happens naturally and inevitably when you begin the vorlaugh or sparge. This is desirable and beneficial up to a point. It's not a problem until it compacts enough to inhibit the flow rate significantly. At that point, some channeling is mostly unavoidable. A mesh screen can also collapse when suction is applied. A lot of guys put a SS coil spring or stiff wire inside the braided mesh to prevent collapse.

 
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Old 04-21-2011, 10:57 PM   #8
JohnTheBrewist
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Quote:
Originally Posted by JuanMoore View Post
The grain is near bouyant during the mash (some of it actually is bouyant), so there's almost zero weight or downward pressure on the false bottom. I once used a flimsy mesh kitchen strainer when I had some equipment malfunction, and it worked fine underneath a grist with 24# of grain. The problem was actually how to keep it from floating upwards since the rim/handle was made of plastic.
With a heavy grist, once the wort is drained, you could have 30 or more lbs of wet grain sitting directly on the FB. With a full sized FB (15") that is 16ga or less, it could potentially dome in with the concave keggle, rendering your FB useless for future brews. An upward dome shape would keep this from happening.
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Old 04-22-2011, 01:30 AM   #9
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Quote:
Originally Posted by BierHerr View Post
With a heavy grist, once the wort is drained, you could have 30 or more lbs of wet grain sitting directly on the FB. With a full sized FB (15") that is 16ga or less, it could potentially dome in with the concave keggle, rendering your FB useless for future brews. An upward dome shape would keep this from happening.
It's really not an issue with a properly supported FB. One of my friends collapsed a dome type FB not long ago. Not sure which one he was using, but said he bought it from NB. The flat FB in my Polarware kettle hasn't collapsed yet, and believe me, I have tried.

 
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