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Old 06-15-2011, 11:25 AM   #1
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Default Evaporation rate

So, I've been brewing with a 15 gallon Mega Pot for the last couple years. The pot itself is great, but my evaporation rate has been huge...so I thought. It wouldn't be unheard of to boil off almost 2 gallons in a 60 minute boil. I have just made the switch to a keggle, thinking that I would lose considerably less liquid during my boil because of the tall/narrow shape as compared to the short/wide shape of the Mega Pot. I did a test run last night with 7 gallons of water for 60 minutes and lost 2.5 gallons. I understand that water and wort boil differently, but c'mon. 2.5 gallons?!

Does anyone else have this problem?

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Old 06-15-2011, 11:36 AM   #2
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Same thing here. 2.25G per hour in my 10G pot. Don't really think it is a problem.....at least you know your boil-off rate.


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Old 06-15-2011, 01:45 PM   #3
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With my keggle I regularly boil off about 2 gallons for 60 minutes (it changes every time though cause I boil outside). It's prob the only thing left that really annoys me about my brewing process). I just know I'm not going to boil off less than 1.5 gallons so I always aim for that and top off as needed.
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Old 06-15-2011, 02:04 PM   #4
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1.75 to 2 G boil off is about normal for a 60 minute boil in my keegle. Maybe a little less, but as mentioned is varies slightly with the weather. I think consistancy is the key. Once you know the boil off it's a lot easier to hit your final volume.

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Old 06-15-2011, 02:15 PM   #5
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Maybe I'm not looking at it in a very pragmatic way, but if I recall my thermodynamics of boiling liquids:

(Rate of heat absorbed from burner/coil) = (Rate of heat lost to ambient) + (Rate of heat used in vaporization of liquid)

The term on the left is a function of burner power (easy to tweak) and setup geometry.
The first term on the right is a function of pot geometry, ambient temperature, wind speed, etc., and is not-so-easy to tweak
The last term is directly proportional to the boil-off rate.

(Note that this equation only applies to the steady-state boil, and not the temperature rise of the liquid up to the boiling point.)

So if you want to reduce the absolute amount of boil-off, reduce the burner power until just barely at a boil?

However, I do agree with others who have posted that consistency is the key aspect to the process.
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Old 06-15-2011, 02:23 PM   #6
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I think those numbers are reasonable. I don't know where you're located but relative humidity of the environment will play a role in your evaporation rate as well. I'm in southern AZ and I would expect to boil off 4-5 gallons for a 10 gal batch. I'd probably loose a gallon to evaporation if I just sat it there for awhile and didn't even boil it!
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Old 06-23-2011, 06:50 AM   #7
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Put a lid on it to cover the BK but not completely. You can control your boil-off completely doing this but remember, never cover the boil completely.
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Old 06-23-2011, 07:53 AM   #8
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Boil off is mainly dependent on two variables, the surface area of the liquid, and the vigor of the boil. Ambient conditions can also play a role, but have a much less significant impact. Batch size has no effect on boil-off amount. If you're boiling off more than 2 gal/hr your boil is probably more vigorous than it needs to be. I boil-off ~1.5 gal/hr in my keggle with a good rolling boil.

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