Evaporation sensitivity to ambient temp?

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skifast1

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:D
 
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.
 
Ha - I meant relative. There's another subject that made a quick exit out of my brain after the final exam.
 
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.
 
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.
 
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.
 
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.
 
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.
 
Scimmia said:
modenacart, air doesn't "hold" moisture at all. Water vapor can exist, regardless of the presence of air. .
Click to expand...

When water becomes a vapor its in the gas form and becomes part of the "air".
 
Ok, but the air doesn't "hold" water, and the only thing that matters here is the temperature of the water vapor, which is well above ambient.

Don't forget that we're also talking net evaporation rates here, as evaporation is always happening at a constant rate relative to the water temperature, the only thing that changes is the rate that moisture is recondensing on the surface.
 
I don't think that humidity has as much of an affect on the boil-off as many brewers believe. The main effect on the boil-off is now much of the energy of the burner are you getting into your wort. The more energy that is available for evaporation, the more water you will loose. ambient temperature and humidity should not have a signigicant affect, but air pressure does.

You should build youself a shield for the burner and pot to make sure that the wind is not blowing away the flame. Once I did this I started saving on propane and had much more consistent boil-off rates.

Kai
 
Scimmia said:
Ok, but the air doesn't "hold" water, QUOTE]

You are arguing semantics with air not holding water. A colder ambient temperature would increase heat losses thus less energy going into the boil, thus less boil off. I am sure this is almost negligible anyways.

Also the moisture gradient and capacity of the air to hold water vapors does matter because it gives the water vapors a place to go allowing for more to boil off more readily. Any way you look at it, a warm ambient temperature will have more evaporation holding all else the same for both conditions.
Click to expand...
 
modenacart said:
You are arguing semantics with air not holding water. A colder ambient temperature would increase heat losses thus less energy going into the boil, thus less boil off. I am sure this is almost negligible anyways.
Click to expand...

Call it semantics if you want, but the only effect the air has is it's ability to affect the temperature of the water vapor.

modenacart said:
Also the moisture gradient and capacity of the air to hold water vapors does matter because it gives the water vapors a place to go allowing for more to boil off more readily.
Click to expand...

Well, only if you disregard the diffusive properties of gases. The air doesn't give the water vapor "a place to go", it's a vapor that will diffuse on it's own, regardless of the supposed "holding capacity" of the air. This is especially true when you consider that water vapor is actually lighter than air at the same temperature, so it will rise and diffuse even quicker at boiling temperatures.

Again, evaporation is a constant at a given temperature, it's the condensation that matters. You could make an argument that the cooler air will cool the water vapor quicker, making it recondense back into the kettle, but when you have rising steam from the boiling wort, I don't think it will be significant.
 
My master's project was modeling a peanut roaster and moisture gradients most definitely matter. There is no way you can argue that the moisture content of the air doesn't matter.
 
If moisture graidents didn't matter then rain would not condense out of the air and water vapor wound not just come from the surface.
 

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