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?
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The effect of evaporative cooling in a humid environment is best summed up as "Not very #@~&#^"% much!". Lived with one of those things in New Mexico for a few years. It did a sweet and inexpensive job most of the time, but on the few even moderately humid days we did get It wasn't worth a thing. – dmckee --- ex-moderator kitten Jul 13 '14 at 21:28
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somewhat related http://physics.stackexchange.com/questions/114869/does-uchimizu-watering-streets-in-summer-help-reduce-temperature-in-an-area – user1306322 Jul 13 '14 at 22:06
2 Answers
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!
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The second law of thermodynamics comes into play here, heat cannot move from a cooler to a hotter body without doing work, i.e., expending energy. In this case the work is done by convection currents carrying the heated air away from the pot. It will definitely be more effectve in a windy spot (but out of the sunlight!) than some place with stagnant air. Your idea of using heated air should work, think of a hair drier, since it will increase evaporation and carry the heat of vaporization away.
Here's an idea for improving the cooling of the zeer pot. Make a small chimney with a small fire or coals at the bottom. The hot air will rise inside the chimney. Now make a second chimney with the zeer pot at the bottom and connect it to the heated chimney. The rising hot air will draw the air over the pot up with it too, increasing convection and improving the cooling. It would take some experimentation to make this work, insulating the pot from the coals is one obvious problem.
