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 Home Brew Forums > What's an accurate evaporation rate for the hour long boil?
05-11-2012, 09:50 PM   #31
tenbricks
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Quote:
 Originally Posted by Bobby_M Percent doesn't work for me. Boiloff is usually a fixed amount no matter how much liquid is in the same kettle. I lose about 1.5 gallons in 70 minutes in my keggle regardless if I start with 13 or 8 gallons.
This.

Boiloff rate is determined by (heat in) = (heat out), the temperature remaining a constant 212. In practice I think heat out is almost entirely the boiling (vaporization) of the wort, much less so by surface area/size/shape. Heat in is determined by rate of propane burning and how efficiently it's heating the kettle, which will be a constant regardless of boil size. Thus, mostly it will be constant amount per time, not constant percentage.

Quote:
 Originally Posted by EdWort The Blichmann Boilermaker 20 gallon kettle will boil off 2 gallons in 90 minutes. I'm attributing that to the larger diameter of the kettle compared to a keggle.
I have the same pot, and see a bit more than a gallon/hour for 5 or 10 gal batches. Must not have the propane as high up as EdWort does.
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05-12-2012, 12:30 AM   #32
wailingguitar
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Quote:
 Originally Posted by Tankard I'm tweaking the Beersmith settings and the default evaporation rate is 11%. So I would collect 7 gallons of wort and boil down to 5.5 gallons in an hour. That sounds a bit high. Has anyone ever calculated this based on a turkey fryer setup? I have a 30 qt aluminum pot. Or, what do you have your Beersmith evaporation rate set at? How close is it, typically?
That is about what I am getting on mine (33qt tamale pot on a Bayou Classic)... about a gallon and a half (7 to 5.5) boiled off in an hour as opposed to a gallon (6.5 to 5.5) when done on the electric stove top. I have never used Beersmith or anything like that, so I have no idea about it's setup or defaults.
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11-08-2012, 05:23 PM   #33
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I would agree with the arguments for a volumetric boil off rate related to the surface area of the water rather than a percentage of total volume.

I also would think that environmental conditions would have an impact on that number too.

A hot dry day would allow more water loss, versus a hot humid day would absorb less.

If you are always working indoors and have a relatively constant temperature and humidity, you are in the best case scenario.

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03-16-2014, 08:48 PM   #34
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Listen to @blacklab and @bobbrews. The evaporation rate depends on the surface area of your brew kettle, the rate of boil (e.g., rolling boil), and to a lesser extent, the altitude. A percentage can be accurate if you always boil the same volume in the same kettle at the same rate, but as soon as you change one of these variables that percentage will be wrong.

If your kettle is tall and thin, it will generally have lower evaporation rate than a short and wide kettle. If you use a simmering boil, your evaporation rate will be much lower than a vigorous rolling boil. It doesn't matter how much you are boiling, because the boiling rate is the same (as judged from the surface - simmer, rolling, etc) and the surface area for the pot is the same.

If all we cared about is minimizing the evaporation rate (which would save water and energy), we might choose a tall, think kettle and simmer for an hour. But beer quality usually trumps these concerns, so the choice of brewpot and boiling rate is not so straightforward.

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02-28-2015, 10:47 AM   #35
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I concur on the physics. The evaporation rate depends on the heat balance.

The amount of heat that goes in the kettle is your BTU's which are almost directly proportional to the flow of gas to the burner (how high you set the flame) or the electrical power supplied to the heating element.

The amount of heat that is lost to the surroundings via convection and radiation is proportional to the surface area of your kettle and its temperature. Even if your kettle is half-full, its walls are going to have about the same temperature from the bottom to the top as if it were full, because of the high thermal conductivity of the metal.

The difference between the supplied heat and lost heat is spent on the latent heat of vaporization, and that's what determines your evaporation rate.

How much of the actual wort area is exposed is not so much of the factor as it would be in the case of evaporation at room temperature.

That being said, the amount of wort in the kettle does not have a significant effect on your evaporation rate. Hence it is a fixed value rather than a percentage.

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