What happens to moisture when air compressed?

  • #1
On an automotive forum, a member made a statement that compressed air won't hold as much moisture as ambient pressure air because the water is "squeezed" out by the pressure and is also, therefore, "drier" air. He pointed to the need to drain water from the tank as proof of the squeezing process. What is the effect of pressure on air's ability to hold water? Does it result in "drier" air? My intuitive reasoning is that "freshly" compressed air can hold more water vapor since it is warmer. The air would not be drier since the volume of air and water in the tank is proportional. I also believe the water in the tank is a result of condensation of the heated compressed air cooling over time, not because it was squeezed out. Who's correct?
 
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Answers and Replies

  • #2
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I believe you are correct. The pressure of the air has no influence upon its ability to hold water (unless you are squeezing it an awful lot), however the temperature has a very strong effect. Compressing a gas willl heat it up, allowing it to pick up more water, which will condense when the gas in exposed to a cold surface. I think your analysis is correct. As this was from an automotive forum, don't forget that burning gasoline creates water vapor in the exhaust, which can condense if the exhaust is cooled.
 
  • #3
LURCH
Science Advisor
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My initial reaction is that the truth is exactly opposite fo what the poster states, since cloud chambers work by decreasing pressure to liquidise water vapor. But I seem to remember something about a chart called a "Mollier Chart", used for calculating the amount of water vapor a certain volume of air will hold at a certain temp. Now I'm not educated enough to be able to generate such a chart, but I do remember the basic concepts behind it. One of those concepts is that a given volume of air will hold a certain mass of water vapor regardless of pressure. So if water vapor content is measured in kg/m3, the amount remains constant. But if water vapor content is measured in kg of water per kg of air, the ratio decreases as pressure increases. For example, if the pressure in a certain area is increased from one atmosphere to two, the amount of water vapor that area of air will hold remains constant, but the amount of air in the same area doubles. So the amount of water vapor that can be evaporated into one m3 of air has remained the same, but the amount of water that can be evaporated into one kg of air is only half as much (because the same amount of volune that once held 1 kg of air now holds 2, but the same amount of water vapor).

I seem to recall one of our PFers is an engineer who works on heating systems for the insides of buildings (it may be Integral). He would know how to make a Mollier Chart and the exact princi[ples at work here. I'll PM him a link to this Topic.
 
  • #4
Integral
Staff Emeritus
Science Advisor
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I am not the one you are thinking about, (may be Russ?) Lurch, I think you are on the right track. The answer may lie in the definition of due point and relative humidity. I am not sure of the exact factors which determine these quantities. If you do some research on them you may find some key ideas which will lead to a correct physical explanation.

While "squeezed out" may be a reasonable analogy, it certainly does not cut it as an actual physical mechanism.
 
  • #5
I found a website for industrial compressed air dryers that addressed this matter, although only fundamentally. They state that moisture will condense out of air when sufficiently cooled (which is what I thought) OR when under enough pressure (what the poster thought). With respect to an air compressor, the heat created by friction during the compression process offsets the propensity of moisture to condense due to pressure. Does a Mollier chart take into account pressure or just temperature? At 100 psi tank pressure, I wonder how much, if any, moisture condenses out due to pressure?
 

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