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Old 10-05-2012, 04:27 PM   #1
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Default Does boil off rate change as your wort level drops?

If you have a 15 gallon boil kettle, would your boil off rate change if you had 5 gallons in the kettle versus lets say 12? My thought is, is that it would not change but want to confirm this.

I tested my boil off rate recently using 5 gallons of water over 30 minutes and it boiled off 1.5 gallons, so in 60 minutes I'd be looking at a 3 gallon boil off.

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Old 10-05-2012, 04:31 PM   #2
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No.

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Old 10-05-2012, 04:37 PM   #3
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>>I tested my boil off rate recently using 5 gallons of water over 30 minutes and it boiled off 1.5 gallons, so in 60 minutes I'd be looking at a 3 gallon boil off.

Holy cow thats a lot of boil off. I think thats unnecessarily high. Try turning down the heat.

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Old 10-05-2012, 04:40 PM   #4
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I know, I was surprised too. Thing is, the way my propane burner is it's pretty hard to turn it down and still be able to get a rolling boil. I just compensate for it by having a larger pre-boil volume.

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>>I tested my boil off rate recently using 5 gallons of water over 30 minutes and it boiled off 1.5 gallons, so in 60 minutes I'd be looking at a 3 gallon boil off.

Holy cow thats a lot of boil off. I think thats unnecessarily high. Try turning down the heat.
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Old 10-05-2012, 04:52 PM   #5
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Physics says that changing the state of water from liquid to gas requires a certain amount of kilocalories per liter. If you add that much energy you boil off a certain amount of liquid. In more detail it also depends a little on the partial pressure of the gas but in our brewing case that won't matter much. So short answer is no.

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Old 10-05-2012, 04:57 PM   #6
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Physics says that changing the state of water from liquid to gas requires a certain amount of kilocalories per liter. If you add that much energy you boil off a certain amount of liquid. In more detail it also depends a little on the partial pressure of the gas but in our brewing case that won't matter much. So short answer is no.
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Old 10-05-2012, 05:36 PM   #7
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Originally Posted by RM-MN View Post
Physics says that changing the state of water from liquid to gas requires a certain amount of kilocalories per liter. If you add that much energy you boil off a certain amount of liquid. In more detail it also depends a little on the partial pressure of the gas but in our brewing case that won't matter much. So short answer is no.
The overall mass of the liquid is changed. If you're applying the very same amount of heat, the boiloff rate will increase.

However, most of us will adjust the heat down to keep a consistent rolling boil during the 60+ minutes, hence, the boil off rate stays roughly the same.

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Old 10-05-2012, 07:35 PM   #8
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Quote:
Originally Posted by Misplaced_Canuck View Post
The overall mass of the liquid is changed. If you're applying the very same amount of heat, the boiloff rate will increase.

However, most of us will adjust the heat down to keep a consistent rolling boil during the 60+ minutes, hence, the boil off rate stays roughly the same.

MC
Nope, for water to boil the entire mass of the water has to be ~ at the boiling point so you are not heating up more or less water, you are simply changing its state. Takes the same to boil off a gallon (ignoring the heat loss through the sides of the vessel) from a hundred gallons as it will for 5 gallons.
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Old 10-05-2012, 07:46 PM   #9
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Thank you as well.


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Nope, for water to boil the entire mass of the water has to be ~ at the boiling point so you are not heating up more or less water, you are simply changing its state. Takes the same to boil off a gallon (ignoring the heat loss through the sides of the vessel) from a hundred gallons as it will for 5 gallons.
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Old 10-05-2012, 08:27 PM   #10
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I dont understand why people are saying it doesnt change. Obviously its easier to boil a smaller amount of liquid than a larger amount. I am not sure it is necessary to worry about but your only question is does it change....yes. If i am boil a full pot it does so less vigourously then a small pot. There is a limit to how much you can boil with a given output so getting close to that limit will rapidly change the boil off. I think the difference most see in brewing is too small to worry about.

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