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Good afternoon,

I am really not a physics guy but I am trying to calculate the effect that pressure has in the dewpoint value.

So I have a machine that refrigerates atmospheric air and dries it. This gives me around 12ºC dry bulb temperature, RH of 62% and a dewpoint of around 4.88ºC

Next this air will be compressed to 7bar and temperature will be around 40ºC. Which I know will affect my dewpoint. I have seen a tremendous amount of online calculators that give me the new dewpoint value, but I want to calculate this and at least understand how it is being made so I can compute this into a formula.

Anyone up to explain to me how is pressure been taken into consideration?

Do I need to use the saturation air pressure and vapour pressure formula? I am still not getting there.

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  • $\begingroup$ Are you familiar with Raoult's Law (en.wikipedia.org/wiki/Raoult%27s_law) and how the vapor pressure of a pure substance is calculated (en.wikipedia.org/wiki/Antoine_equation)? $\endgroup$
    – David White
    Commented Apr 9, 2020 at 17:15
  • $\begingroup$ if you increase the total pressure of the air to 7 bars, you are also increasing the partial pressure of the water vapor in the air by the same factor of 7. The dew point is the point at which the partial pressure of water vapor in the air is equal to the equilibrium vapor pressure (which increases with increasing temperature). So the dew point will be higher. $\endgroup$
    – Chet Miller
    Commented Apr 9, 2020 at 21:53
  • $\begingroup$ OK - so I need to check the Raoult's Law as I am not familiarized with it. Ihave used the Antoine equation that I believe helps me calculate the partial vapor pressure of water ? or I am mistaken? $\endgroup$
    – Antonio Peres Gomes
    Commented Apr 10, 2020 at 13:02
  • $\begingroup$ I also assume that the dewpoint is when partial vapor pressure = saturation vapor pressure where the air cannot hold more water. This assumption lead me to think when (partial vapor pressure / partial vapor pressure saturation) = 1 or 100 % relative humidity - is where i have my dewpoint . But maybe i am missing a step here to calculate something, as I dont know where to factor in the pressure of the air (7bar) that @ChetMiller commented so how do i relate my partial vapour pressure saturation value to find out the TºC that this means ? (getting my dewpointºC) $\endgroup$
    – Antonio Peres Gomes
    Commented Apr 10, 2020 at 13:11

1 Answer 1

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Here's a sample calculation.

At 12 C dry bulb, the equilibrium vapor pressure of water is 0.014 bars; and, so, for 62% relative humidity, the partial pressure of the water vapor is (0.62)(0.014)=0.0087 bars. The dew point for a partial pressure of 0.0087 bars is 5 C.

Now you increase the total air pressure from 1 bar to 7 bars. This causes the partial pressure of the water vapor in the air to increase by a corresponding factor of 7, to a value of (0.0087)(7)=0.061 bars. The temperature at which this value is the equilibrium vapor pressure of water is about 36 C. So that is the dew point of the compressed air.

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  • $\begingroup$ Thank you - that really simplified it. the Antoine equation gives the equilibrium vapor pressure, which i was struggling to "understand" what was. really appreciated the help $\endgroup$
    – Antonio Peres Gomes
    Commented Apr 10, 2020 at 14:17

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