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If I have a strong vessel that I've cooled to say -20°C, and lowered the pressure below 0.006 atmospheres, the water in food placed inside will sublimate and freeze dry the food.

But what happens to the water vapor? Does the moisture rise to the top or fall to the bottom of the vessel itself? Or does it just expand to fill the container equally?

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""Actually, that suggests another question."" Maybe, but my impression is that I should not answer any more. I deleted my answer. –  Georg May 18 '11 at 19:55

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I do believe that freeze-dryers have to have a condenser that actively removes the water vapor. It would depend on the size of the vessel, but the water vapor from the food is likely to be enough to raise the pressure to near atmospheric, and certainly enough to destroy the low pressure required. So a condenser is actively removing the moisture by exposing a surface at a much lower temperature than the atmosphere. But come to think of it, I'm not quite sure how this works considering that the condensed water would be ice.

Directly to your question, I am fairly positive it fills the entire vessel. It is, after all, a gas. There is little in the way of other competing gases and there are negligible effects from things like gravity. I would imagine that the gas would be a near homogeneous mixture of the water vapor and other sparse gases.

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Yes, my assumption was that the expanding water vapor would need to be evacuated out. I'm thinking you could pump out the water vapor into a chamber where you would warm it enough so it wouldn't harm the pump. (figuring that would be easier to build than a pump that could handle -20C water vapor!) –  Garrett Serack May 18 '11 at 19:05
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I agree with the answer, but to be explicit: it will fill the vessel uniformly in the limit that mgh << kT. If you were to build a vessel that was a few 10's of kilometers tall (it wasn't specified) the answer would change. –  Anonymous Coward May 19 '11 at 1:17

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