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Old 01-17-2013, 03:29 PM   #1
SimonB
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Default Extremely High Boil-Off %

Hi all

I'm just setting up my new electric kettle BIAB setup and trying to test parameters before actually performing a brew.

It's a 28litre urn that I am using. My target bottling volumes are around 15litres.

I performed a 15litre test boil for 60 minutes. The boil off % was monumental 30.2% , with a vigorous boil (but not one that would cause risk of boil over).

This seems very high compared to a lot of what I have read which range from 4-15%, especially considering my urn is a pretty tall narrow one (30cm diameter)

I guess I have to do some serious topping up to reach my target volumes... or have I missed something?

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Old 01-17-2013, 03:35 PM   #2
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That's 1.1 gallons/hr. That a pretty normal boiloff rate. The percentage doesn't really matter. I lose 1 gallon/hr in my setup whether I have 6 gallons or 8 gallons in the kettle.

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Old 01-17-2013, 03:51 PM   #3
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Thanks Pablo...

I just realized that the boil % was a pretty dumb measure to go by...

If I had started with 10 gallons and ended up with 8.9 it would have been 11%

So I guess that if 1.1 gallons an hour is pretty normal then I shouldn't be concerned...

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Old 01-17-2013, 04:52 PM   #4
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Quote:
Originally Posted by SimonB View Post
Thanks Pablo...

I just realized that the boil % was a pretty dumb measure to go by...

If I had started with 10 gallons and ended up with 8.9 it would have been 11%

So I guess that if 1.1 gallons an hour is pretty normal then I shouldn't be concerned...
Yep, you can either account for it preboil, or just top off at the end. No worries.
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Old 01-17-2013, 06:33 PM   #5
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As a matter of my own interest, how would one scale one's boil off to other setups without experimentation.

I mean a 20 gallon setup isn't going to lose a gallon an hour (or would it if the urn was the same diameter as mine)?

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Old 01-17-2013, 06:56 PM   #6
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Quote:
Originally Posted by SimonB View Post
As a matter of my own interest, how would one scale one's boil off to other setups without experimentation.

I mean a 20 gallon setup isn't going to lose a gallon an hour (or would it if the urn was the same diameter as mine)?
I'm not sure there's a really practical way to do that. There are so many variables: diameter of vessel, strength of boil, altitude/atmospheric pressure, humidity, etc. I think testing is the only way to get a real accurate picture.
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Old 01-18-2013, 01:22 AM   #7
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Quote:
Originally Posted by pabloj13 View Post
I'm not sure there's a really practical way to do that. There are so many variables: diameter of vessel, strength of boil, altitude/atmospheric pressure, humidity, etc. I think testing is the only way to get a real accurate picture.
This^


Just for reference, I use a 15" keggle, and my boil off is 2 gallons per hr.
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Old 01-18-2013, 05:43 PM   #8
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In the amount of time it would take me to figure out the math involved to calculate the boiloff rate based on the shape of a vessel so that I could scale the calculation to a different system, I could put a few gallons of water into the pot and boil it for an hour. Somebody else that is a lot better in physics or engineering may be able to do it more quickly, but testing it would be the easiest way in my book to figure it out.

Boiloff is also not an exact science when it comes to brewing, especially outdoors. Simple things like wind, air humidity, rate of boil, etc. can affect boiloff as well even from batch to batch on the same system.

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Old 01-18-2013, 06:35 PM   #9
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My boiloff rate in my 10 gallon Sams club stock pot (~18" wide) is around 2 gallons an hour as well with a ~75k btu burner.

-Steve

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Old 01-18-2013, 06:42 PM   #10
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We just had a similar discussion on the AHA site, excepting that it also had to do with figuring how much minerals to add to the water.

If you do find that your system does boil off significantly more than the typical rate of around 1 gal/hr when dealing with 5 gal batches, then figure the mineral additions based on a volume of water needed when the boil off rate is at the typical rate. Then add plain water to the kettle to make up for that additional water that your system is going to boil off.

Low mineralized water like RO or distilled water would be ideal for that 'top-off' water, but it shouldn't matter too much unless your tap water is really highly mineralized.

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