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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%.

rob
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SinOfficial
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    Comments are not for extended discussion; this conversation has been moved to chat. – tpg2114 Aug 08 '19 at 20:44
  • 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. – rob Aug 10 '19 at 04:46

4 Answers4

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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).

Greg Schmit
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JMac
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  • And how much degrees could a room with average humidity and 30°C be cooled down with this technique? – SinOfficial Aug 07 '19 at 18:52
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    @SinOfficial "Average" humidity varies a lot with location. – JMac Aug 07 '19 at 19:01
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    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. – cmaster - reinstate monica Aug 07 '19 at 21:39
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    @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. – Mark Aug 08 '19 at 02:23
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    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. – R.. GitHub STOP HELPING ICE Aug 08 '19 at 04:29
  • @SinOfficial In my experience it's less than half a degree. Does feel a bit cooler than a regular fan but I'm not sure if it's real or my mind trying to justify having spent money buying the thing. I can tell you one thing though. The more buyer's remorse you have the less the cooling effect feels – slebetman Aug 08 '19 at 08:26
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    @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. – Luaan Aug 08 '19 at 11:02
  • @Luaan: If the room is well-ventilated, then in order to get any cooling, heat must transfer from masses with high heat capacity in the room to the cooled air before the air can leave. This is the same principle as why turning on the AC in your car with windows open does almost nothing. – R.. GitHub STOP HELPING ICE Aug 08 '19 at 14:04
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    @R.. But if the air is cooled, that's exactly what you would expect to happen. The air would be cooler than everything in the room, and thus should be able to take heat away from the objects. As long as you keep removing moist air, you should be able to reach an equilibrium temperature lower than it was without the cooler. Thermal comfort is pretty complicated, but having both airflow and lower temperature could easily cause a more significant cooling sensation than being in drier, warmer air. – JMac Aug 08 '19 at 14:15
  • @JMac: It'd be interesting to see numbers for such a model, but (1) in order to get significant movement of heat out of objects with high heat capacity, the rate at which air leaves by ventilation is going to need to be fairly low (see how little car AC works even with windows just slightly open) which means the equilibrium humidity will need to be high. And (2) from experience being in non-Arizona-like environments where people think evaporative cooling works at all reasonably, it doesn't. – R.. GitHub STOP HELPING ICE Aug 08 '19 at 15:15
  • @R..: They are not "worse than useless" except in bone-dry climates. They work quite well in places with moderate humidity, e.g. most of the US east/south of the Cascades and west of the Mississippi. (Friends have a somewhat larger one, and it works quite well with humidity in the 15-60% range that is typical hereabouts.) – jamesqf Aug 08 '19 at 16:09
  • @R.. Why does the rate of air leaving from ventilation need to be low? Ideally it should match approximately the rate of dry air flow coming in. Assuming air at the same temperature, the faster the airflow, the greater the convection coefficients, and therefore (all else equal), if you're not depressurizing the room, greater airflow out means greater airflow in general, which means more heat is taken from the room and exhausted through the ventilation. – JMac Aug 08 '19 at 16:13
  • @jamesqf: Evaporative cooling absolutely does not work in the US southeast. Even the humidity before you go adding to it with evaporative cooling is unbearably awful. In order to have a chane of being effective for cooling a space for human living (or for keeping equipment/materials/structures from suffering damage due to heat or moisture) the initial humidity must be well below spec so that it doesn't go over from adding to it. – R.. GitHub STOP HELPING ICE Aug 08 '19 at 16:16
  • An informative way to try to quantify this based on existing model of human body cooling/comfort might be to look at how the change in temperature and humidity from evaporative cooling affects the heat index. – R.. GitHub STOP HELPING ICE Aug 08 '19 at 16:23
  • @R.. Heat index is a really quick and dirty way to look at it. It you wanted better information related to human comfort and ventilation systems, ASHRAE has quite extensive documentation about thermal comfort. "For thermal comfort - this is the standard." . There's really a lot going on; but saying it only works in bone dry conditions isn't necessarily completely accurate (and very vague). It's a spectrum varying from quite effective, to detrimental. – JMac Aug 08 '19 at 16:34
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    This will probably work well in hot and dry areas, such as in the U.S. southwest. In Arizona, restaurant patios have misters that just spray water into the air above patron's heads: you can hold your hand a foot from the outlets and you don't feel dampness, just coolness. But in areas where heat is accompanied by high humidity--which is pretty much any non-desert area--humidifying a hot muggy space is probably going to make everyone just feel clammy. – CCTO Aug 08 '19 at 16:50
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    @R.. Jamesqf specified south/east of a western landmark, and west of an eastern landmark. He never said "US southeast". – T.J.L. Aug 08 '19 at 19:48
  • Ok, I misread that. – R.. GitHub STOP HELPING ICE Aug 08 '19 at 20:16
  • @R..: Isn't that what I said? Most of the US southeast is (if the times I've been there are typical) horribly hot and humid, so much so that I don't see how people can live there. But in more salubrious climes (or even too hot but not humid ones), evaporative cooling works quite well. – jamesqf Aug 10 '19 at 03:46
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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.

Adrian Howard
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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.

Flyto
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  • And how much degrees could a room with average humidity and 30°C be cooled down with this technique? – SinOfficial Aug 07 '19 at 18:52
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    @SinOfficial The achievable output temperature is called the wet-bulb temperature (https://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. – cmaster - reinstate monica Aug 07 '19 at 22:05
  • @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. – cmaster - reinstate monica Aug 07 '19 at 22:26
  • @cmaster Follow-up question: does an evaporative cooler decrease the wet-bulb temperature, or does it remain exactly constant? – Sanchises Aug 08 '19 at 09:43
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    @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. – cmaster - reinstate monica Aug 08 '19 at 19:23
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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.