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In essence, I am wondering if adding water to a system helps or hinders the power of an air conditioner to remove heat from that system?

There is a box of 27 cubic meters with an air conditioner in its only opening. Outside the box it is constantly temperature T, inside the box it is constantly temperature T-5 with relative humidity of 25%, and the air conditioner is constantly working at maximum capacity.

Q1: You add 20 liters of temperature T water to the inside of the box, everything else remains constant. When the last drop of water has evaporated, is the inside the box temperature above, below, or at T-5?

Q2: You add 20 liters of temperature T-5 water to the inside of the box, everything else remains constant. When the last drop of water has evaporated, is the inside the box temperature above, below, or at T-5?

Q3: Does the form of the water matter (a pool of liquid water vs. vapor water)?

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closed as off-topic by Wolpertinger, heather, Qmechanic Aug 14 '16 at 18:17

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  • $\begingroup$ I would expect it to hinder the cooling. Water holds lots of heat, so it would take a lot more energy to cool a volume of humid air than dry air. $\endgroup$ – CoilKid Aug 14 '16 at 16:07
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For Question 3, the form of the water definitely matters as the vaporized water would affect the humidity of the air whereas liquid water (not vaporized) obviously does not.

If the air conditioner is already working at max capacity to keep the box at steady state before anything is added, then the temperature will rise as the water added is at a higher temperature than the air inside, regardless of the state the water is added in.

After the last drop has evaporated the humidity would have increased inside due to the evaporation but of course the air conditioner would also decrease the humidity as part of its function. The answer should be left to someone else.

But the answer to your initial question depends on whether by "hinder" you mean the function of the air conditioner in extracting heat or the function of the air conditioner in keeping the air constant temperature and humidity because they don't necessarily have to be the same thing. If I introduce water at 1 kelvin it will obviously lower the temperature inside, thus making heat extraction easier for the air conditioner, since there's less of it, but would still cause more work for the air conditioner as it will eventually evaporate and the humidity would increase unless the air conditioner does more work.

So under that premise adding any water at all will hinder the air conditioner.

EDIT: Therefore adding water below T-5 would both help and hinder. Adding water > or = T-5 would only hinder.

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