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Q & A: water vapor

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Most recent answer: 10/22/2007
Q:
We currently add hot water to a kettle at ~170 degrees F. The vapors from this "hot" water rise up and collect onto an overhead auger where dry material drops into the kettle, causing a "build-up" due to caking and the presence of water at that junction. The question is: If we add cooler water, 170, 160, 140, will the amount of vapor rising be significantly less? Logic would say yes but if it is not boiling we question how much less vapor we would actually get. We can’t go below 140. I guess I’m looking for a graph or equation?
- Rich (age 31)
Manufacturing Plant, MN
A:
Anyway, although there are sevral complications in getting the rate of water evaporation, probably the most imporatnt factor here is the equilibrium vapor pressure of the water. You can look that up in tables. Here's some numbers for how that vapor pressure depends on temperature:

100 F: 51 (mm Hg units)
140 F: 150
150 F: 191
160 F: 244
170 F: 297.

There's an equation, called Clausius-Clapeyron, which captures this behavior well, but for your purposes these numbers should suffice. You can do better by about a factor of two by dropping to 140 F.

Mike W.

(published on 10/22/2007)

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