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I know boiling point at vacuum is far below the room temperature. But I still wondering if the humidity reaches the saturated humidity (100% relative humidity), will the vapor condense to liquid, although it is probably going to evaporate again soon?

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Fundemental questions here: are you positing that the vacuum is actively maintained (or equivalently that the volume of the chamber is very large compared to the STP volume of that much water vapor) or are you assuming that a volume of water is exposed in a (small) closed vacuum volume and we wait to see what equilibrium state is reached? –  dmckee Aug 18 '13 at 4:16

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While dmckee points out your question is too vague, I'll take a shot and guess you're asking from a pop-sci standpoint. Since you mention vacuum chamber, I'm going to assume it's actively maintained. For example put a wet shirt in a vacuum chamber and pump it down.

The water will boil off but it has to be removed from the system because you are trying to maintain a vacuum. Thus the pump will condense the water (along with other gas molecules) outside of the chamber. By the time you get down to a good low vacuum pressure (< 1 mTorr) there will be no water or very few molecules left. Release the vacuum and your shirt will be dry.

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Actually vacuum pumping to dry something out isn't nearly as efficient as the second paragraph seems to be claiming. It works, but it takes a long time, e.g., maybe overnight to dry out a vacuum flask with water drops beaded on the inside. The liquid evaporates, but the evaporation is a slow process. –  Ben Crowell Aug 19 '13 at 2:27
    
@BenCrowell I don't really know. I did look at designing a dryer system like this once, but the pressure vessel was too expensive for the budget. Naked scientist did some rough calculations...thenakedscientists.com/forum/index.php?topic=25084.0 –  user6972 Aug 22 '13 at 0:25
    
I don't really know. The second paragraph of my answer isn't speculation. I've actually done this. It really is a slow process. –  Ben Crowell Aug 22 '13 at 0:27
    
What was the vessel's average pressure? I saw a video where a shirt was dried in about 16 minutes from wet, to vacuum to back out. That's what started the research into the cost. –  user6972 Aug 22 '13 at 2:28
    
This was with a cheap mechanical pump that probably only reached 5% of an atmosphere. The figure of 16 minutes doesn't seem consistent with the claim in your answer that "By the time you get down to a good low vacuum pressure there will be no water or very few molecules left." –  Ben Crowell Aug 22 '13 at 2:38

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