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I am not sure why some hand dryers are blowing hot air and not just air at room temperature.

To me, hair dryers are just a way to dry one's hands using the same principle as when we shake our hands in the air, or when we blow some air over a hot drink.

Given that the blown air temperature's is not enough (is it ?) to vaporize water, why is hot air used?

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    $\begingroup$ This enables to waste more energy. Another reason is that not to make you feel cold. $\endgroup$
    – Val
    Jul 21 '14 at 14:31
  • $\begingroup$ LOL, it is not true that the hair dryer throws droplets just like if you shake your hand. Most of the water from the hair evaporates, one H2O molecule after another - evaporation doesn't require one to reach boiling point - and a higher temperature surely increases the evaporation rate, doesn't it? The relative humidity of the air decreases which encourages the hot air to absorb more H2O molecules. $\endgroup$ Jul 21 '14 at 15:06
  • $\begingroup$ Experiment seems to contradict here. I have a blowdryer with hot and cool settings. Whether I'm using it to dry hair or anything else, the hot setting easily dries things faster. Hot air holds more water, and the vapor pressure of water goes up fairly quickly with temperature, so you'd expect hot air to do the job faster. $\endgroup$ Jul 21 '14 at 15:44
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Fact:

By increasing the air's temperature, one also increases the amount of water vapor it can hold before saturating.

Speculation:

Therefore, the osmotic pressure on water molecules in your hair increases and the water vaporizes more quickly than with room temperature air.

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To take an extreme case, suppose your hand dryer is located in a room without climate control, such as in an isolated restroom in the middle of a park. Occasionally the temperature in the room will fall to the dew point. In that case a room-temperature hand dryer would blow saturated air and have zero drying effect. However, warming the air brings it above the dew point and allows it to hold moisture from your hands.

There's probably also a comfort issue. Evaporation absorbs heat. Blowing unwarmed air on wet hands might dry them, but would make them feel cool. By blowing warmed air, the heat of vaporization can come from the air rather than from your hands.

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