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I don't understand why evaporative cooling can't go to temperatures as low as fridges (like 3°C)? For a link to the design I'm talking about: https://engineering.stackexchange.com/questions/35452/how-effectively-will-this-evaporation-setup-cool-down-the-water

As you see, the accepted answer tells me it is NOT possible to bring down the temperature to 3°C because of "dew point consideration".

I calculated the dew point for 5°C at a relative humidity of 85% and the dew point is 2°C. How does that mean that the water mist cannot cool the container that far?

EDIT: I think one reason why the guy answered it's impossible (in the link above) is that he thought a constant water spray volume is going on. That would mean that the relative humidity goes to 100% eventually, as the water-holding capacity of air goes down with temperature. The thing is, I gradually decrease the water spray volume, so that the relative humidity stays constant at 85%.

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  • $\begingroup$ Since the original question was on Engineering SE, perhaps this follow up might be better there as well? $\endgroup$
    – Jon Custer
    Commented May 8, 2020 at 14:37
  • $\begingroup$ Yes, but this is more theory than practical... $\endgroup$
    – El Flea
    Commented May 8, 2020 at 14:37
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    $\begingroup$ The engineering link provides good advice. In addition, note that the closer your cold temperature approaches the dew point temperature, the lower the rate of heat transfer will become. Once that rate of heat transfer matches the rate at which heat leaks in from the environment, you will reach an equilibrium temperature that you can't go below. Note that because a temperature difference is the driving force for heat transfer, this equilibrium temperature will take quite a long time to reach. $\endgroup$
    – David White
    Commented May 8, 2020 at 15:38
  • $\begingroup$ @DavidWhite So in short, you're saying that it IS possible to cool something down from 40°C to 3°C in atmospheric pressure? $\endgroup$
    – El Flea
    Commented May 9, 2020 at 6:10
  • $\begingroup$ @ElFlea, it's theoretically possible, if you have PERFECT insulation that prevents all heat flow from the outside world. You will, of course, be unable to find insulation that can do that, so you will find that it is practically impossible to cool something down to the dew point via evaporative cooling. Why are you so concerned with proving that such a feat can be done? $\endgroup$
    – David White
    Commented May 9, 2020 at 17:00

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The rate of evaporation is going to be roughly proportional to the "driving force," determined by the difference between the equilibrium vapor pressure of the water and the partial pressure of water in the gas phase. In terms of relative humidity, this reduces to: $$driving force=P_{vap}\left(1-\frac{RH}{100}\right)$$So, higher RH is going to reduce the driving force. But, even more important, lowering the temperature reduces the equilibrium vapor pressure, and thus the driving force. As an example, near 0 C, the equilibrium vapor pressure is about 611 Pa, while at 25 C, the equilibrium vapor pressure is 3170 Pa. So, for the same RH, the evaporation rate at 25 C is about 5X the evaporation rate near 0 C.

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  • $\begingroup$ Yes, but I don't see how this means that evaporative cooling can't cool something down from 40°C to 3°C in atmospheric pressure? $\endgroup$
    – El Flea
    Commented May 9, 2020 at 6:07
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    $\begingroup$ It can, provided the air is very dry. It has to have an absolute humidity that is lower than the absolute humidity for 100% RH at 3 C. In other words, the vapor pressure of the water vapor in the 40 C air would have to be less than about 700 Pa. That would mean that the 40 C air would have to have an RH of less than 10%. $\endgroup$
    – Chet Miller
    Commented May 9, 2020 at 11:40
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If the air is gradually cooled while maintaining the moisture content constant, the relative humidity will rise until it reaches 100%. This temperature, at which the moisture content in the air will saturate the air, is called the dew point. If the air is cooled further, some of the moisture will condense.

As I understand it, as long as the air temperature is above the dew point, evaporative cooling should be possible. However, at temperatures below the dew point, the rate of condensation will be greater than that of evaporation, and evaporative cooling should cease.

Assuming you did your calculation correctly, evaporative cooling should occur if the air temperature is above 2$^0$C. As far as I know unless the relative humidity is 100% at 3$^0$C there should be no reason for evaporative cooling to cease at 3$^0$C.

Hope this helps.

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