I am an architect and am trying to understand the effects of evaporative cooling in a humid climate for buildings. I would like to use the example of a zeer pot for my question. I have read everywhere that evaporative cooling is not possible in a humid climate simply because humid ambient air is already approaching 100% saturation. Obviously when saturated air cannot take any more moisture – evaporation does not take place. I get that. However if humid air is heated then the saturation decreases quite a lot. For instance I remember looking at a chart that shows if air temperature is doubled then its ability to hold moisture increases about ten fold. In a tropical climate there is a lot of sun and heating air is easy. If air were heated to increase its moisture holding capacity and then passed over an extremely porous, moist surface will cooling take place or will the whole system gain heat? If a zeer pot for instance were placed inside a simple black metal chimney in the sun, open on the bottom and top, where humid air is heated and the heated air rises and passes over the pot will the pot cool or will it gain heat? Is there a formula (something I can understand) I can use to calculate heat gain and loss in this example? Roughly speaking is the concept of evaporative cooling possible in a humid climate if the humid air is heated?
[Physics] Evaporative cooling in a tropical environment
thermodynamics
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No, the temperature of the water is not that important for the performance of an evaporative cooler. This is basically because the energy needed to increase the temperature of liquid water (its specific heat capacity) is very small compared to the energy needed to evaporate the same amount of water (its enthalpy of evaporation).
At room temperature the specific heat of liquid water is 4.18 J/(g$\cdot$K) while the ethalpy of evaporation is 44.0 kJ/mol. Since the molar mass of water is roughly 18 g/mol, this means that approximately 585 times as much energy is needed to evaporate an amount of water as to increase the temperature of the same amount of water by 1 K. This means that even if the water starts at freezing temperature, is heated to 40 $^\circ$C (104 $^\circ$F) and then evaporates, less than 7% of the energy absorbed by it is used for increasing the temperature.
The temperature of the water might effect the rate at which evaporation occurs, but I guess evaporative coolers are designed to achieve 100% relative humidity in the wet bulb regardless of the temperature of the water, so this probably doesn't affect the performance.
Exactly the experiment you describe is shown in this video around 8 minutes in.
This is called evaporative cooling and indeed has been used for cooling things for all of recorded history. indeed, assuming you possess sweat glands your own body uses evaporative cooling. Actually your refrigerator uses evaporative cooling as well, but it keeps the evaporating liquid inside a sealed system.
Reducing the pressure increases the effectiveness of the evaporative cooling, but you have to use energy to pump out the air so you don't get the extra cooling for free.
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I haven't gone through this carefully, but I believe the coldest temperature that can be achieved by a zeer pot is the wet-bulb temperature, which is equal to or higher than the dew point. Changing the temperature of saturated air changes its relative humidity, but not its dewpoint.
If you have a pot of water at the same temperature as air at 100% relative humidity, and you heat the air, I strongly suspect you will also heat the water.
It's a very interesting idea, though!