Supposed I wanted to cool a cage by draping a wet towel over it and letting the water evaporate. Also suppose that I periodically re-wet the towel to prevent it from drying out. Would it be better cooled with refrigerated or just regular tap water? I thought refrigerated at first but now I’m not so sure. Anyway I failed physics so help me out on this one.
Just thinking aloud here (well, typing, but you know what I mean). When you put the wet towel on the top of the cage, you provide a cold body for heat from the cage to be transferred to. Then, the (now warmer) water in the towel evaporates, taking the heat away from the system.
You want to be able to get as much heat out of the cage before all the water evaporates, so use as cold water as possible. It will then be able to absorb more heat per weight of water before it floats away. Therefore, you won’t have to re-moisten the towel as often.
Also, as Cecil points out here: Does stuff cool faster in the freezer than in the fridge?, the colder your cold body is, the faster heat will be transferred from the hot body.
Speaking of which, who’s in the cage?
Colling down with chilled water ought to be better, since the change in temperature required to approch the ambient temperature will be greater, leading to more removal of heat from your object. (You’ll also remove some from the surrounding air, which won’t help you, but there’s nothing you can do about that.)
Why do you think using cooled water wouldn’t be better?
By the way, evaporative cooling of heated structures has become a major new method with removal of heat from operating systems. The idea is that you spray the liquid onto the thing to be cooled, you get a bit of cooling fro the expansion of the spray, and the evaporation of it cools the hot machinery down. You don’t need bulky refrigeration units and coils, and you don’t heat the ambient air with your heat exchanging. The downside is that you run out of water and have to keep refilling the tank.
Well… no REAL reason, all my logical thinking capabilities are saying “Dude, use chilled” but my ditzy side is going “Hey, heat loss is from EVAPORATION!” Eh. Maybe I should give it a good kick and be done with it. Are there any conditions under which tap water would be better?
As for the contents, they are two furry little guinea pigs. For hot days. My mother will murder the three of us if I bring them in, you see.
I see. But you gotta explain to yiour ditzy side that, while you heat things up in order to get them to evaporate more readily, starting with something cooler doesn’t stop it from evaporating. In fact, it forces the item itself to use more of its heast energy to make the evaporation happen, which is your goal.
Chilled water would be better but I don’t think it would be enough better to make it worth the chilling. Assume the air you want to cool down is at 80 F.
Evaporating 1 pound of water takes 970 Btu. Raising it from say 32 F to 80 F takes 48 Btu. Raising tap water at say 60 F to 80 F takes 20 Btu. So with water cooled to 82 you are removing 1018 Btu and with tap water 990 Btu. It’s better but only by 28 Btu.
And even that is with the towel at 80 F. As water evaporates the towel will cool down so the temperature increase before evaporation takes place will be even smaller.
You also need a way to move air through the towel. If you do that, though, it really works. I went up to the swimming pool one day when the temperature was about 115 F. So I dipped my terrycloth, knee length robe in the water and put it on. I though I was going to freeze to death.
That’s cooled to 32.
I appreciate your predicament but a wet towel is no substitute for air conditioning. I have had a guinea pig, though kept it indoors all the time. From this site:
Yeah, I know. But they get cold packs to lie on, cool veggies and spritzings with a spray bottle every now and then. Keeping them inside simply isn’t an option. Perth is windy so there will usually be a breeze moving through the towel.