How does evaporation generate cooling? Swamp coolers edition

[russ_watters]

Yes, but if the cold coil was outside, you could have a situation where it's losing more heat to the environment than it's pulling in....

No, that doesn't even make any sense/it's a self contradiction. It's called "the cold coil" because it is colder than the environment it is in and colder than the warm coil.

Something else I'm thinking of now, is some of the condensation on the cold coil coming from vapor in the outdoor air, or is it all from vapor in the indoor air that's being recirculated?

The cold coil is outside. All of the air flowing over it is outside air so all of the condensation is from outside air. There's no way for inside air to get to it.

It may help if you draw or look at a diagram of the process. It should be immediately obvious why what you are saying makes no sense.

 

[sophiecentaur]

But this doesn't account for the extra 540C heat of vaporization that was mentioned here, it only accounts for the difference between 100C in the vapor molecules leaving and the liquid water temperature, divided by how many molecules are still left in liquid form.

You are right, of course. My description misses out a significant factor. The 'energy' that air, passing over the surface, removes will consist of the KE plus the PE of each molecule.* However, I think I'm correct in saying that only the fraction of water molecules with initially high KE will be taken far enough from the surface to be removed 'into the air'. It's the loss of the KE , shared with the air, that increases the temperature of the air but, losing the PE, will require more energy to be supplied to the surface than is delivered into the air. I liked the old term "sensible heat" which is heat that's associated with the temperature rise of the steam from a boiler after it's boiled with latent heat.

I've been thinking about Orbital Physics more than Thermal Physics, lately but there's a parallel with the energy used by a rocket to 1. Escape from a planet's gravitational PE and 2. the KE left for interplanetary travel. The bonds at a water surface corresponds to the g from a massive planet where the bonds on the surface of, say Methanol, correspond to the g from a small planetoid.

 

[swampwiz]

Evaporative coolers (i.e., "Swamp Coolers") work by having water put onto a surface and then blowing air over it so that it evaporates; the equilibrium temperature of this action is the Adiabatic Saturation Temperature or "Wet Bulb" temperature. In a high humidity environment, the wet-bulb T is close to the regular (or "Dry Bulb") temperature, so this doesn't do much, but in a low-humidity environment, the wet-bulb temperature is much lower, so it works fine, also adding in some humidity so that the controlled environment is not so dry. The difference between the dry- & wet- bulb temperatures is basically due to the heat of evaporation of water vapor - such that energy that goes into the evaporation is taken from the water on a wet surface.

The biological action of sweating basically uses this idea, as sweated skin has the equilibrium temperature as the wet-bulb (ironically, sweating in a dry environment doesn't leave any sweat as it evaporates away, while sweating in a humid environment leaves sweat drops).