40
$\begingroup$

My parents bought this "air conditioner", but I am very skeptical that this can cool a room, or even cool anything.

I doubt that it even has a cooling element, I suspect that it is just a fan + humidifier.

But even if this device had a cooling element, it still couldn't cool a room:

If air is cooled, the resulting heat can't just vanish, it has to go somewhere, because of the 1st law of thermodynamics (energy conservation). In a normal full-sized air conditioner, the air is cooled and the resulting hot air is blown outside. But in this mini "air conditioner", the heat cant go outside, it can only stay in the room, keeping the room at the same temperature.

Am I missing something or is this a scam as I suspected?

In response to a comment: I'm interested in using this cooler in Germany, where the relative humidity is typically 70%.

Improve this question
$\endgroup$
2
  • 2
    $\begingroup$ Comments are not for extended discussion; this conversation has been moved to chat. $\endgroup$
    – tpg2114
    Commented Aug 8, 2019 at 20:44
  • $\begingroup$ Friendly reminder: comments are intended to be a temporary place to suggest improvements to the question. For extended back-and-forth, use the chat link above. $\endgroup$
    – rob ♦
    Commented Aug 10, 2019 at 4:46

4 Answers 4

Reset to default
63
$\begingroup$

I doubt that it even has a cooling element, i suspect that it is just a fan + humidifier.

The fan+humidifier is the cooling element for this unit. It uses purely evaporative cooling to reduce the temperature of the system. It can do this because the phase change between liquid and vapour requires energy. By just passing a convective current of relatively dry air over a liquid water reservoir, heat is taken from the air to evaporate the water. This results in the humidified air being a lower temperature than before it entered the humidifier.

In this case, the heat doesn't just vanish. The heat lost is stored in the latent heat of vaporization of the water. If the vapour in the room were to begin condensation, the heat in the room would start to increase.

Basically, you're just using the humidity as a sort of thermal battery. You're able to store some of the heat in the room in the form of increased relative humidity, instead of having it go towards increasing temperature directly. The energy doesn't leave the system; it's just taken a different form as internal energy of the phase.

You can only remove so much heat this way, and the rate of heat removal decreases as the room's relative humidity approaches 100%. If you want to use that for constant cooling, you will need some way to remove the moist air and replace it with dry air (one that doesn't involve a dehumidifier that puts heat back into the room).

Improve this answer
$\endgroup$
19
  • 12
    $\begingroup$ @SinOfficial "Average" humidity varies a lot with location. $\endgroup$
    – JMac
    Commented Aug 7, 2019 at 19:01
  • 23
    $\begingroup$ Note: The evaporative cooler does exactly the same thing as human skin does by sweating, evaporating water to get rid of heat. Thus, increasing the humidity of the air in a room will reduce the ability of the body to cool itself down via transpiration. Now, the body's method is quite a bit more efficient in removing heat from the body itself, it does not need to reduce the heat of the air at all. Thus, I'd much rather just put up a fan and trust my own body to evaporate as much water as it sees fit. $\endgroup$
    – cmaster - reinstate monica
    Commented Aug 7, 2019 at 21:39
  • 46
    $\begingroup$ @SinOfficial, in a location like Arizona, where the average humidity is in the single digits, it's incredibly effective at cooling. In a location like Florida, where the average humidity is 100%, it has no effect. $\endgroup$
    – Mark
    Commented Aug 8, 2019 at 2:23
  • 22
    $\begingroup$ To expand on what @cmaster is saying, evaporative cooling is worse than useless except in bone-dry climates like Arizona. They prevent your body from cooling by making the air miserably humid. This is obvious from a theoretical standpoint but I've verified it from experience in Delhi as well. $\endgroup$
    – R.. GitHub STOP HELPING ICE
    Commented Aug 8, 2019 at 4:29
  • 3
    $\begingroup$ @R.. They also work reasonably well in ventilated rooms - the humidity doesn't build up, and the cooling is very noticeable. Obviously, this is worse than useless in 100% humidity, but works fine under 60% or so. Also obviously, you keep competing with fresh warm air coming from the outside, so don't expect miracles. $\endgroup$
    – Luaan
    Commented Aug 8, 2019 at 11:02
11
$\begingroup$

One this size is more of a personal cooler, placed right in front of you it will probably keep you a little cooler, it will do little to nothing to cool a normal sized room. But the principle is sound, as water evaporates it becomes cooler than the liquid water. Adding ice will cool the water, so the water vapor will be even cooler. Growing up in the 1960s, in Texas, all we had to cool our house were evaporative coolers (also called water coolers, or swamp coolers), These were large and blew a lot of air with a "squirrel cage" blower inside a box with vented padding on 3 sides which had water pumped over them. They were placed outside of a window so the humidified, cooler air was forced into the room. They would usually keep a large area comfortable even in the middle of summer(usually 20 to 30 degrees F, or more, cooler than the outside temperature). These work best in "dry heat" where humidity is low, as water can evaporate faster. They do not cool as well on rainy days or other times of high humidity.

Improve this answer
$\endgroup$
1
  • 1
    $\begingroup$ +1 for the searchable term "swamp cooler". $\endgroup$
    – arp
    Commented Aug 9, 2019 at 3:41
3
$\begingroup$

It's an evaporative cooler: You fill it with water, it blows the room's air across the water, and the energy required for evaporation is heat that is thus removed from the room.

I don't know how well a small one like that will work, and in any case it is only going to work if the air is fairly dry; but in principle, at least, it is plausible and not a scam.

Improve this answer
$\endgroup$
5
  • $\begingroup$ And how much degrees could a room with average humidity and 30°C be cooled down with this technique? $\endgroup$
    – SinOfficial
    Commented Aug 7, 2019 at 18:52
  • 4
    $\begingroup$ @SinOfficial The achievable output temperature is called the wet-bulb temperature (en.wikipedia.org/wiki/Wet-bulb_temperature). Which is actually the most interesting measurement of heat with respect to the human body: It is the limit to which your skin can cool itself down via transpiration. Nice values are somewhere between 10°C and 20°C, horribly humid values are somewhere between 20°C and 30°C and positively dangerous values are between 30°C and 35°C. Anything above that is plain deadly. I'd say, as long as you can have fun on your bike, your evaporative cooler achieves <20°C. $\endgroup$
    – cmaster - reinstate monica
    Commented Aug 7, 2019 at 22:05
  • $\begingroup$ @SinOfficial Reading the wikipedia article more closely, it seems that wet-bulb temperatures over 30°C are very rare and are only reached in exceptional heat-waves. So, I guess it starts getting dangerous at significantly lower wet-bulb temperatures. $\endgroup$
    – cmaster - reinstate monica
    Commented Aug 7, 2019 at 22:26
  • $\begingroup$ @cmaster Follow-up question: does an evaporative cooler decrease the wet-bulb temperature, or does it remain exactly constant? $\endgroup$
    – Sanchises
    Commented Aug 8, 2019 at 9:43
  • 1
    $\begingroup$ @Sanchises Actually, I'm not that sure. I think that it should be constant, but I may be wrong because of the way that the wet-bulb temperature is defined. In any case, the wet bulb temperature is much closer to the dew point than to the air temperature. Since the dew point rises as you evaporate water, and since it is a hard lower limit to the wet-bulb temperature, there is not much room for uncertainty anyways. I think it's safe to assume that the wet-bulb temperature is constant for any practical purposes. $\endgroup$
    – cmaster - reinstate monica
    Commented Aug 8, 2019 at 19:23
0
$\begingroup$

Evaporation cools while making the air more moist. The heat is regained on condensation which will usually happen on the walls (which may be isolated well enough for the heat not being able to escape). Of course, unless you are living in very dry climate, this is a recipe for mold. Usually, air in need of humidifying is air that has a higher temperature than the outside in which case you would not want to have it cooled down. Cooling down air by evaporation will have a double effect of relative moisture, adding more humidity and decreasing the air's ability to contain vapour.

So the device's combined effects of humidification and cooling are rarely desirable at the same time.

Improve this answer
$\endgroup$

Not the answer you're looking for? Browse other questions tagged or ask your own question.