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What's the most efficient way to place your hands under the hand dryer?

Let's assume that dryer creates simple downward flow of hot air.

Here are some examples:


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The leftmost one. In the other two your hands are overlapped so that area isn't getting any hot air. :) –  Pratik Deoghare Feb 8 '12 at 13:21
Of course, if you are talking about energy efficient , then its a moot point as most bathroom hand dryers stay on for a while after you finish (or turn off too early, in which canse you have to turn them on again). –  Manishearth Feb 8 '12 at 13:54
@Manishearth I'm talking about speed. The goal is to minimize time you need to dry your hands –  Poma Feb 8 '12 at 14:28
@PratikDeoghare You can hold your hands this way without overlapping. And of course those variants are just an example, feel free to provide your own solutions. –  Poma Feb 8 '12 at 14:30
@Poma I was kidding. –  Pratik Deoghare Feb 8 '12 at 15:36

3 Answers 3

If you want to dry your hands completely, you need to turn them over and over anyways.

But the best approach would be if you have your palms facing up/down (can't tell which of these diagrams it is). You need to maximise the surface area of the exposed part, as heat you receive will be $\propto (surface\space area)\times(time)\times\cos(angle\space of\space tilt)$.

So the best way would be to keep your palms separately, and rotate them slowly. Surprisingly, the heat needs to fall on the sides of your hand for the same amount of time as it needs to fall on your palms. This is because the sides of your hands absorb less heat (less surface area), but they have less water that needs to be evaporated. So just turning your hands in circles is the best idea. No need to touch them together and try to squeeze the water out (You're drying your hands, not washing them with soap).

EDIT: If you want the most efficient way to dry your hands, then yes, rubbing them together helps (not squeezing, though). The most efficient way would be that you dry your palms, on both sides. Then, you rub your palms against the sides of your hands. Now repeat. Spreading the water around speeds up drying, as the water film reduces its thickness uniformly. Shaking your hands before drying also speeds it up.

In the comments, you mentioned that "air flow" causes evaporation, and not heat. This is only partially true. When water evaporates, it "steals" heat from the surroundings to do so (even if it doesn't heat up to 100 degrees C, it still has to take latent heat). If the air is stagnant, it will slowly lose heat and cool down, reducing the rate at which it can give heat, thus reducing evaporation. Aside from that, it will become more humid, again reducing the rate. When you have a wind, the old air (cold and humid) is constantly replaced by the new air (warm and dry), so you get a good constant rate of evaporation. Yes, a faster airflow will increase the rate of evaporation (mainly due to the surface humidity that it manages to carry away), but temperature is also a big factor.

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When I place them perpendicular to floor I feel air flow on both sides but it has less pressure –  Poma Feb 8 '12 at 14:32
First thing, its heat you want, not pressure. Anyways, I agree that its not effective in drying your palms, but it is required if you want to dry the sides of your hands. Read the third paragraph carefully. –  Manishearth Feb 8 '12 at 14:42
The important thing is that any part of your hand should spend the same time underneath the dryer, regardless of the size. Of course, when rotating your hands, the shape of your hands does come into the picture (due to the $\cos\theta$ term) and makes the optimization complicated. –  Manishearth Feb 8 '12 at 14:45
When palms become dry I can rub my hands to redistribute water –  Poma Feb 8 '12 at 14:55
And also I think the main thing that causes water to evaporate is actually air flow and not heat –  Poma Feb 8 '12 at 14:56

I believe that both answers so far are partially completed. So, I will try to complete then.

Two determinant factors must be considered. On one hand, the water evaporation. On the other hand, the blowing of the water to the floor (by air flux). A typical drier uses them both.

As @Alexander pointed out, AirBlade technology is the current most efficient way of drying hands, and it basically uses air blowing. The pertinent point here is that the hands are parallel to the gravity force vector. That way, the work needed to make the water to leave is minimized.

For the case of a 'old' dryer, this idea stands: hands must be the closer to parallel the better: it minimizes the total needed work to pick all the elements of water and send them to the floor.

With this said, the correct answer will be the left one: the hands are parallel to the air flux, thus minimizing the work necessary to transport an element of water to the ground. All the other cases will just increase the necessary work to be done by the air flux.

However, I guess there will be a point where blowing does not compensate, mainly due to water viscosity. On that point, evaporation should be the main drying factor. On that case, @Manishearth point is the rule of tomb: perpendicular to the air flux, maximizing evaporation.

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In my experience they don't blow water droplets off your hand. And if they do, its more efficient to shake the bulk water off first manually. –  Manishearth Feb 26 '12 at 16:49
(Also @Manishearth) I'm not entirely convinced that putting your hands perpendicular to the flow is best for evaporation. For a cube at least, I've seen the correlation that $\text{Nu}=0.71\,\text{Re}^{0.52}\ $ for the front and $\text{Nu}=0.12\,\text{Re}^{0.70}\ $ for the sides. So for sufficiently turbulent flow, it'd make sense that the parallel arrangement offers better convection of heat and humidity. –  rdhs Feb 26 '12 at 18:19

Neither of these examples are efficient by any standards. Instead of heating up the air which is consuming a lot of energy it is much better to increase the pressure and blow of the water instead of evaporating it. The currently most efficient hand dryer is the Dyson Airblade.
There are similar dryers available but the idea is always the same.

You can try this in any workshop that has compressed air. With a relatively small amount of air with a pressure of 8 bar you can dry your hands within seconds.

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The OP was asking for the most efficient way to dry your hands under a dryer. I doubt we are allowed to install a new dryer in the process.. –  Manishearth Feb 8 '12 at 14:48
Yes, the goal is to minimize time you need to dry your hands using given bathroom dryer –  Poma Feb 8 '12 at 14:58
The airblade is awesome! –  Lagerbaer Feb 23 '12 at 6:34

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