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Home Brew Forums > Home Brewing Beer > Beginners Beer Brewing Forum > Temperate/humidity on evaporation rate?
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Old 05-14-2009, 01:43 PM   #1
chode720
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Default Temperate/humidity on evaporation rate?

I am brewing again this weekend and it will be my first time brewing outside. The weather here in Northeast Ohio has not been very conducive for brewing outside and I have been in my garage with my propane burner. I keep the door cracked in the garage to have a little airflow, but its not open very much.

Well I am planning on brewing on my patio this weekend and I was wondering how much of an effect I will see in my evaporation rate. I know that the temperature, humidity, and wind can effect my evaporation rate, but I was wondering how much extra or less wort I need to collect to compensate.

I have been starting with 6 to 6.1 gallons of wort and boiling down to 5 gallons.


Thanks!

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Old 05-14-2009, 02:30 PM   #2
Pangea
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The 2 biggest factors influencing evaporation are not included in your list of what may affect your evaporation. The only things that really matter are pressure and heat applied to the kettle.

Your evaporation rate will be effectively the same outdoors as it was indoors, unless the wind blows your flame out... Yes, wind can wick heat away from the kettle, but unless you're in hurricane katrina it will be negligible. This is my opinion speaking as a chemical engineer.

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Old 05-14-2009, 05:55 PM   #3
menschmaschine
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Many things can affect your evaporation rate significantly, not just atmospheric pressure and heat energy. Here is a thread discussing it.

chode, you are correct in that relative humidity and temperature can affect your evaporation rate. A more accurate measurement is the moisture in the air while removing temperature out of the equation. This is called humidity ratio as I posted in the link above.

You are also correct in wind affecting your evaporation rate. The higher the wind speed over your boil ("boundary layer" between the liquid and the atmosphere), the more moisture will be carried off.

This is very difficult to predict/calculate, so I would just plan on a percent or two more for outside. Unless it's really windy, then maybe 2 or 3 percent more than inside.

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Old 05-14-2009, 11:07 PM   #4
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Quote:
Originally Posted by menschmaschine View Post

This is very difficult to predict/calculate, so I would just plan on a percent or two more for outside. Unless it's really windy, then maybe 2 or 3 percent more than inside.

With all due respect, I don't think that the wind will increase evaporation rate by that much. I'm not going to do a mythbusters experiment to prove it, but it seems logical that while the wind can affect the boundary layer, improving mass transfer away from the boiling wort - it can also impact the amount of energy being transferred from the burner to the kettle as well as the amount of energy transferred from the kettle surface area to atmosphere.

This is why I don't think that wind will make a significant (or even predictable, without a supercomputer) impact.

A burner operating in zero wind is being more efficiently directed at the bottom of the kettle. When the wind kicks up, a greater amount of heat from the flame is being transferred to the air and away from the kettle.

Also, wind increases flow at the boundary layer around the entire surface area of the kettle as well, not just at the boiling liquid/air interface. This change at the entire kettle surface boundary layer can dramatically increase energy transfer from the kettle to the air, thus DECREASING boil off rate.

As far as humidity affecting the boil - it is really affecting evaporation. The impact of humidity on evaporation rate is irrelavent of the heat induced evaporation (vaporizing water by adding energy). If we fill the kettle up with water, and turn the burner off, there will be some mass transfer that occurs between the water and atmosphere - and that occurs at the boundary layer. If there is less moisture in the atmosphere, there will be greater mass transfer of water from the kettle, thus greater evaporation. If there is more moisture in the atmosphere (higher humidity) the evaporation rate will decrease. Now go fill your kettle with water and see how much water evaporates to the air only attributed to mass transfer during a typical brewing day 60 minute boil..... its not very much. Yes, increased humidity decreases evaporation. But the evaporation was so small to begin with (compared with the vaporization due to heat) that it is negligible.

Leave a cup of water out for a while and see how long it takes to evaporate. This illustrates my point that evaporation due to mass transfer is so small during a 60 minute boil that it's not worth considering.

In summary, I totally agree that wind and humidity affect boil-off rate, as do many other factors. However I still don't think they are significant.

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