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Obtain Saturated Air from Unsaturated Air Without Adding Any Moisture

05/28/2017 5:48 PM

Hi, I saw this question on the book Thermodynamics- Cengel

The first answer I thinked was rise the pressure of the mixture.

The solutions manual says the way to do it is colling the air at constant pressure.

On this discussion in yahoo answers I saw an instersting answer:

"It is much easier to raise the percent of saturation by cooling than by compression since compression also raises the temperature. If you raise the pressure and the temperature then you still have to cool it to make it saturated"

What do you think about the answer given in yahoo?

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Guru
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#1

Re: Obtain Saturated Air from Unsaturated Air Without Adding Any Moisture

05/30/2017 12:22 AM

Multiple solutions were offered that would do the trick, and cooling the air is easier than having to compress it and potentially cool it.

What scale are you talking about, can you give an example and application? Is this perhaps for water harvesting from the air in a desert environment or similar?

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Guru
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#2

Re: Obtain Saturated Air from Unsaturated Air Without Adding Any Moisture

05/30/2017 5:12 AM

It makes perfect sense. After all, that's why it rains.

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#3

Re: Obtain Saturated Air from Unsaturated Air Without Adding Any Moisture

06/02/2017 4:30 AM

Yahoo is correct. If you compress it isothermally the actual vapour pressure rises, so the % saturation (= actual VP/saturated VP) rises.

If it is compressed adiabatically, the temperature rises, and the saturated VP rises by more than the actual VP rise due to compression. So it becomes less saturated (until it cools down!)

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