Massive heat wave here and I don't have an air conditioner. I was investigating how commercial air conditioners work and to my surprise found that they use merely evaporation rather than a compressor/radiator cycle like fridges.

I have this fan on my desk so I did a quick test by putting a plastic shroud around it and funneling the air through a little damp sponge. To my surprise the effect was much better than expected, the air flow was rather low but the air that did come through was quite chilly!

Now I'm planning to make a bigger version. I got this 500w bigass fan that I'm going to leave outside, shroud it and run the air through a few layers of ~0.5m by 0.5m thin cotton, dapened(hopefully) by capillary action from a vat below. Do you think that will give any kind of tangible effect for cooling a single 5m by 5m room?

EDIT: Wait a minute, I had another thought. I checked the energy figures for air and water and came up with this: If I have a closed system with 1kg of dry air and 100g of water at 20C and sea level pressure and then if the water should spontaneously evaporate consuming 226000j of energy and expanding the volume by 0.18m3, it should suck that energy out of the air and give a mixture of

  1. 0.18m3 of 100C water vapor
  2. 0.19m3 of -206C air

for the temperature average by volume of -57C so the concept should still work or..? Obviously not all of the water will spontaneously evaporate but every bit that does should cool the overall system no?

  • $\begingroup$ Its correct that the sponge cools down due to the evaporation. But the fan blows only the evaporated water to you. I guess the pleasant effect is originated rather by the evaporation of the damp that was blown to your skin by the fan. Actually what you built is not an air conditioner. Physically it is more the other way round: You cool down the sponge and blow the waste heat to you. $\endgroup$
    – thyme
    Commented Jul 27, 2014 at 9:04
  • $\begingroup$ well that is a bummer :/ looks like I built a humidifier. Correct me if I'm wrong but I think that aside from cooling the sponge, I also cooled the water in the sponge? And if so then I could make water flow through the sponge constantly by putting a reservoir under it and pumping the water up and thus should be getting a source of cooled water which I could then use to cool air with a separate radiator+fan. Thats a bit too complicated to just throw together without knowing much anything about the efficiency of the processes going on in there.. I think I'll try and figure out some other design $\endgroup$
    – user81993
    Commented Jul 27, 2014 at 9:51

1 Answer 1


What you have built is an Evaporative Cooler. They work by evaporating water. This requires energy, which is extracted from the air, thus reducing its temperature. Unlike air conditioners they do not require an outside radiator, and they do make the air more humid, whereas ACs make it drier. These coolers are commercially available, so don't try to patent the idea. Google "Evaporative Cooler" to find some.

Real air conditioners work differently. They use compression & expansion to cool the air inside, and release the resulting heat (plus waste heat) outside at a higher temperature. They work in the same way as your fridge: the inside gets cold, and the radiator at the back gets hot.

  • $\begingroup$ Are you sure that the slightly cooled air around the sponge has any noticeable effect when it is blown to you by a fan? I would agree that this principle works if he wants to cool something next to the sponge. But I still think that the only effect he feels is the damp on his skin. $\endgroup$
    – thyme
    Commented Jul 27, 2014 at 10:43
  • $\begingroup$ @thyme Evaporative coolers are still used in some dry areas (I had one for a few years in southern New Mexico). They really do cool the air as well as moistening it as long as the air is dry enough. The air is forced through the fiberous pad continuosly so there are two competing effects: the pad is cooled by evaporation and warmed by contact with the air. The result is an output that is both cooler and moister than the outside air. When the humidity rises much above 30% they start to get a lot less efficient, but in that part of the world it is often 10-15%. $\endgroup$ Commented Jul 27, 2014 at 15:21
  • $\begingroup$ the colloquial term is "swamp cooler" ! $\endgroup$ Commented Jul 27, 2014 at 15:48

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