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Old 01-18-2008, 09:43 PM   #1
skifast1
 
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I'm many years removed from my engineering classes - will the evaporation rate in a full boil be higher or lower at cold ambient temps (assuming equal humidity)? I'm celebrating MLK day by brewing a British Bitter - close enough
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Old 01-18-2008, 10:05 PM   #2
Scimmia
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Hmm, that's a good question. First off, though, when you say equal humidity, are you talking about relative humidity or absolute humidity? Assuming relative humidity, I think the partial pressure of water vapor would be lower at the lower temperature, so I think that boil off rates would be higher.

 
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Old 01-18-2008, 10:09 PM   #3
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Ha - I meant relative. There's another subject that made a quick exit out of my brain after the final exam.
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Old 01-20-2008, 06:11 PM   #4
modenacart
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Warm air can hold more mosture so a high ambient temperature should have more boiled off vapor. That is why you see condisation on you window when you have ac blowing on you winshield.

 
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Old 01-20-2008, 06:34 PM   #5
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There will be a slightly greater wort loss when it is cold, but wind is far more important. Wind carries off heat from the flame, the kettle and the blowing over the wort surface accelerates evaporation and heat loss.
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Old 01-20-2008, 06:39 PM   #6
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The wind blowing makes sense because of the moisture gradient, but cold air does not hold more moisture then hot air. It does not make sense that the air being cold will lead to more evaporation.

 
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Old 01-21-2008, 02:49 PM   #7
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modenacart, air doesn't "hold" moisture at all. Water vapor can exist, regardless of the presence of air. But yes, more water can exist as vapor in higher temperatures. Don't forget, though, that we're talking about boiling water (HOT), not water at ambient temperatures, and evaporation depends SOLELY on the temperature of the liquid, not the temperature of the air; and with the lower partial pressure of water vapor at lower temperatures, there would be less recondensing on the surface, making me think that boil off would be higher. Now when that water vapor cools to ambient temperatures, a lot of it will recondense, but who knows where that will be. Very little of it in the pot, I'm sure.

 
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Old 01-21-2008, 02:59 PM   #8
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Woohoo - thermodynamics cage fight!!
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Old 01-21-2008, 03:13 PM   #9
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The more I think about it, though, I'm not sure that enough of the ambient humidity will be reaching the surface of the liquid to make any difference at all. hmm, doesn't seem like we have any experts in the house, how about personal experiences?

Like david_42 said, though, wind would be absolutely critical. It would carry away the boiled vapor quickly, not giving it a chance to recondense into the kettle.

 
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Old 01-21-2008, 04:10 PM   #10
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Quote:
Originally Posted by Scimmia
modenacart, air doesn't "hold" moisture at all. Water vapor can exist, regardless of the presence of air. .
When water becomes a vapor its in the gas form and becomes part of the "air".

 
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