As explained in Why does water evaporate spontaneously at room temperature despite ΔG > 0? water evaporates in room temperature because partial pressure of water is less than "standard" conditions of air in normal atmosphere.

If I need to evaporate or boil 100 g of water to artificially increase humidity of air over time, does it make any difference in total energy comsuption if I slowly evaporate the water or if I boil it rapidly? Common sense seems to indicate that rapidly boiling the water takes much more energy but is it really so? I would not be surprised if slowly evaporating the water would take exactly the same amount of energy (which would cause slight temperature drop in room temperature) but I don't know how to compute or demonstrate this. Does it make a difference if the room is considered big but closed or the room is open to atmosphere? If it makes difference for this discussion let's say that humidity of air in room is 25% RH and the room temperature is 25°C.

  • $\begingroup$ If the humidity of air is less than 100%, then water will evaporate by itself with no effort on your part, given enough time. The energy needed for that will be taken from the surroundings. You may add some heat if you want the water to evaporate faster. $\endgroup$ – Ivan Neretin Nov 27 '17 at 7:51
  • $\begingroup$ I consider the "energy needed for that will be taken from the surroundings" as part of the total energy comsuption. Consider trying to increase humidity of air inside a house when air outside the house is much colder than the air inside the house. If energy is taken away from the room to allow evaporation then more energy is needed to make the room warm again. $\endgroup$ – Mikko Rantalainen Nov 27 '17 at 8:43
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    $\begingroup$ Then the total energy consumption for water evaporation is the same no matter what. $\endgroup$ – Ivan Neretin Nov 27 '17 at 8:46
  • $\begingroup$ Is it safe to say that the claim that warm mist humidifiers use more energy than cold mist humidifiers is false? Pretty much all cold mist humidifier manufacturers are claiming that their product is better because it saves energy. $\endgroup$ – Mikko Rantalainen Nov 27 '17 at 9:05
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    $\begingroup$ No. Besides water evaporation, there is some energy spent on heating the air in the room. So the claim can pretty well be true. $\endgroup$ – Ivan Neretin Nov 27 '17 at 9:27

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