Why would a household air cooler not cool when running in a closed room?

A household air cooler works by using heat from the air to evaporate water. The process of converting water from liquid to vapor takes a lot of energy, which shows up in heat lost from the air.

If you run a household air cooler in a sealed room, you will see a drop in temperature for a short time, until the humidity gets up high enough. How high is that? It's a temperature called the "dew point". At this temperature, the relative humidity reaches 100%, and water in the air will start to condense on any available surface. Forcing more water, say, from inside the household air cooler, to evaporate will push the vapor content in the air higher, which will force vapor already in the air to condense out.

Now, when vapor condenses into liquid, it reverses the process of turning liquid into vapor. Instead of absorbing heat from the environment to convert liquid to vapor, the vapor dumps heat back into the environment when it condenses into a liquid.

So when the temperature and the humidity meet at the equilibrium point, you reach a state where all the household air cooler is doing is moving heat from the cooling unit to the available surfaces in the rest of the room, using water vapor to carry this heat energy. So the temperature will drop a little, and the humidity will rise to 100% at the equilibrium dew point, and then temperature and humidity will be constant.

Except for one little thing.

It takes energy to run the fan, and possibly also to pump water through the household air cooler. This energy has to come in from outside, and represents a net addition to the energy inside the room. The ultimate fate of this energy is going to be heat. So the room will slowly heat up. Even if you're powering this thing with a super-duper battery, you are converting energy from a room-temperature storage unit into mechanical energy, which then converts into heat, so the room still warms up over time.

This article comes from quora edit released