I work in an office where the airconditioning is not working, so I brought a small mobile airconditioning device: it's a small machine, emitting cooled air. It looks like this:

enter image description here

However, according to the laws of thermodynamics, the total amount of heat should always increase in a closed system. Therefore, regular airconditioning systems vent their waste heat to the outside, but this little mobile machine does not have any connection to the outside, so it has no means to discard its waste heat.

So my question is: how can such a machine even exist?

  • 11
    $\begingroup$ A Google image search tells me that your photo shows a "heater", which I guess also has an unheated fan mode. So basically, in your use case you just have a fancy fan, not an AC. $\endgroup$
    – Ruslan
    Jun 14 at 8:16
  • 4
    $\begingroup$ "emitting cooled air" - citation needed. Heat pumps (aka air conditioning) do nothing except make heat and move it around. It's gimmick season again, eh? $\endgroup$
    – Mazura
    Jun 14 at 18:05
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    $\begingroup$ "how can such a machine even exist?" that's your fault because you "brought a small mobile airconditioning device" then typed that on the internet instead of a rant with a bunch of swear words starting with 'public service announcement'. This belongs on Law.SE . If you can qualify heating the air as conditioning it, then there wouldn't need to be an H in HVAC. $\endgroup$
    – Mazura
    Jun 14 at 18:09
  • 2
    $\begingroup$ You bought a snake oil machine. $\endgroup$
    – DKNguyen
    Jun 14 at 18:31
  • 1
    $\begingroup$ How could you not know you brought a heater, not an AC? $\endgroup$
    – RonJohn
    Jun 15 at 6:36

4 Answers 4


Correct that to be able to cool the entire room, you need a way to be able to expel heat from the room. This is done with a hose of some sort for most portable air conditioners. Without a hose, heat cannot be exchanged and it would be a violation of thermodynamic laws if the entire room could be made cooler.

However, there are evaporative cooling systems (I'm not sure if that is what you have in the image above) and as in the link, they work to cool the local area by blowing cooler air that is generated by water evaporation.

It is also possible that what you have is a fan, and a fan blowing air on your body can make you feel cooler simply by carrying away the hotter air near your skin (which is replaced by the slightly cooler air in your surroundings) and increasing the speed of moisture evaporation on your skin.


According to Google image search that's the picture of a space heater. Those can usually also act as fans. A fan does not cool air down, it merely creates an airflow that leads to a higher rate of evaporation of sweat. Since the evaporation process requires heat, this natural mechanism does cool the human body down. There are similar looking evaporative air coolers that have a water reservoir. Those machines trade (very slightly) lower air temperature for higher air humidity. Since the optimum humidity range is between 30-60%, there is a small window of conditions in which such an evaporative air cooler can slightly increases the comfort level, assuming that it is being used in a dry, hot climate.

  • 12
    $\begingroup$ In a dry climate like here in Colorado, evaporative coolers can cool the air significantly, much more than "slightly increasing comfort level". And the additional moisture increases comfort too. They are actually really effective. $\endgroup$ Jun 14 at 15:57
  • 5
    $\begingroup$ Evaporation isn't the only (or perhaps even the main) reason why moving air feels colder even when it's not: moving air also disrupts the thin, insulating layer of still air that tends to form around you. $\endgroup$
    – gidds
    Jun 14 at 16:33
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    $\begingroup$ @flattermann comfort vs temp/humidity varies widely from person to person, but in the places they work (western/mountain US timezone areas with high temp and low humidity) they can be extremely effective. in Arizona where you routinely have 100+ temps and 10% humidity you will see around 30-40 degrees of cooling, but once you get to the central/east timezones they are worse than useless $\endgroup$
    – eps
    Jun 14 at 22:06
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    $\begingroup$ @CrisLuengo "And the additional moisture increases comfort too." Perhaps up to a point, but at some point, high humidity makes heat worse (see "heat index"). The human body is itself, to some extent, an evaporative cooler (the skin gets rid of heat by having sweat evaporate off it), and the higher the humidity, the less well this works. Evaporative coolers can be dangerous, as once they're raised the humidity and the temperature has equalized with the surroundings, you now have hot, humid air. $\endgroup$ Jun 15 at 0:19
  • 4
    $\begingroup$ @Acccumulation You are right, I’m more than familiar with high-humidity climates. But going from 5-10% humidity to 30-40% increases comfort. Note also that this humid air is not hot, it’s been cooled to a pleasant temperature. $\endgroup$ Jun 15 at 0:50

There is a company that's trying to market a free-standing outdoor cooling device. It doesn't add humidity to the air, and it doesn't require a hose to dump heat externally.

What it does need is periodic refills with liquid nitrogen. So it's still venting heat externally (at the liquefaction plant); it's just introducing new stages into the "circulation loop", with the cold side involving trucks and Dewar flasks, and the hot side involving atmospheric circulation.


Normally the small machine must contain a reservoir of water to humidify the air (humidifier), a small pump which circulates the water on wires,..., between which passes the current of air, a quantity of water evaporates by absorbing a quantity of heat, the temperature decreases by a few degrees for the largest humidifiers in buildings(from 1 to 2 degrees max if I remember correctly).


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