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Question

Are some ways of vaporizing 1kg of pure water at ATP more efficient? Specifically cold vs hot methods of vaporizing water.

Background

There are many ways of vaporizing water. Some ways humidifiers can accomplish this are:

  • Ultrasonic vibration
  • Impeller methods using a rotating disk
  • Passive evaporator methods using a fan through a wet membrane
  • Steam vaporizers

Attempts to answer my question

According to these other questions on SE:

The vaporization of water can be calculated to have a heat of vaporization value of 40.65 kJ/mol which is about 586 calories.

Also according to another question the relative humidity of the room will not have an effect on the heat of vaporization.

But I guess this question is lacking a time constraint. Just waiting until the water evaporates on its own would be the most efficient, but this isn't what I am looking for in an answer.

I don't know how to better ask my question to communicate that I am only curious about active methods to force the vaporization of a body of water.

Or does my question not make any sense assuming it might take the same about of energy (40.65 kJ/mol as provided above) to vaporize water regardless of the method?

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2 Answers 2

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Yes. The most efficient way would be the method that loses the least amount of energy. So you want to minimize the heat radiated away as you heat the water. Two methods of doing this are to minimize the time to vaporization and to apply thermal shielding to the container holding the water.

You can see this by considering what happens if you slowly heat the water. The heat will be lost to its surroundings at a rate proportional to the difference in temperature. By insulating the container, less heat is lost and the heat supplied to the water will go into heating it instead of escaping into surroundings.

Another factor to increase efficiency is to heat the water in a very warm room or furnace. Since heat loss is proportional to the temperature difference in the water and its surroundings, by not cooling the room where the heating takes place this temperature difference will be smaller. Heating the room requires energy and will lower overall efficiency.

Also, you would want a method that quickly transfers heat to the water. This could be a flame underneath the water or a heating element submerged into the water. The more heat you can add to the system the quicker it will reach the boiling point.

Finally, you can decrease the pressure of the water you are heating. This will decrease the temperature at which the water boils. At higher altitudes, which have lower atmospheric pressure the water can boil at a significantly lower temperature. In Denver, CO, for example, water will boil at around $94.5 degrees^1$.

  1. https://www.engineeringtoolbox.com/boiling-points-water-altitude-d_1344.html
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  • $\begingroup$ so you are saying that the steam method is more efficient than the cold vaporization methods? $\endgroup$ Dec 2, 2019 at 4:00
  • $\begingroup$ No, I'm not saying that. I don't know anything about the steam method nor the cold vaporization method. Many of the methods I mentioned could be used in conjunction with those to make them more efficient. I assume cold vaporization is just heating cold water. $\endgroup$
    – Natsfan
    Dec 2, 2019 at 20:53
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Some humidifiers that use ultrasonic or impellers as their operating principle only produce a fine mist of water droplets and the water they vaporize is because of the increased surface area of the water that is exposed to the atmosphere. My personal experience with this type of humidifier is that they produce more mist than vapor, and the amount of water removed from the original supply is not equal to the amount of vapor produced.

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