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Why Sweating Cools You Down
posted August 9 2012 13:53.57 by Giorgos Lazaridis




There is this wild phenomenon that many people are not aware of: Water is a liquid that has an extreme high Specific Heat. Actually, there are no many other liquids that have higher specific heat i than water. The term "specific heat" is a value that determines the amount of energy required from a substance to absorb, in order to change its temperature by one degree. It is usually measured kJ/kgK (Kilo-Joules per weight and degree Kelvin). To heat one kilo of water by one degree, the energy required is 4.19 Kj, because the specific heat of water is 4.19 Kj/kgK.

And here is the interesting part. When the water boils at 100 degrees C, it starts to evaporate. At that point, the calculations are made by the Latent Heat of Evaporation, which is the energy required to evaporate the liquid. For water, the latent heat of evaporation is times higher than the specific heat, namely it is 2.257 Mj/kg (yes that is MEGA-Joules).

My mother once was cooking spaghetti. She put the water to boil, she had like 3 liters of water in the pan... and then she forgot it and the water just evaporated. How much energy did she spoiled?

Let's start by heating up the water... Suppose that the water temperature was 20 degrees, so it had to climb 80 degrees to reach 100 degrees. We know that the specific heat of water is 4.19 Kj/kgK, so the energy required was 4.19 x 3 x 80 (assuming that 1 liter weights 1 kilo) = 1005.6 Kj = 1Mj.

Now let's evaporate it. The latent heat of evaporation is 2.257 Mj/kg, and since we had 3 kilos, the energy required to evaporate it was 2.257 x 3 = 6.771 Mj. So the total energy spent was 7.771 Mj. We can change units to KWHours (which we are more familiar with). 7.771Mj are equal to 2.15 KwH. So, my mother forgot the pan with the water for 1 hour on the 2 KWatts heater...

To give you an analogy of how much energy my mother spent to evaporate just 3 liters of water, i'll give you a more dramatic example. A WW2 PanzerII tank weights 25 tones, that is 245.2 kN on our beautiful earth. To lift it to the 3rd floor of a building (10mt), you need to apply a work of:

W = 245.2 kN x 10 = 2452 KJ

My mommy just evaporated the energy required to lift 3 of these Panzer II tanks onto my apartment....

So, now, you should be able to understand why the power plants use evaporating water to cool down and condense the super-heated vapors. These gigantic towers that you see in the power plants which emit a thick white smog are the cooling towers. Inside these towers there are pipes through which the super-heated vapors run. Springlers spray water on these pipes, the water evaporates and dissipates some 2.3 MJoule of energy per kilo of water.



Power plant cooling tower. Source: wikipedia


And here is also a video which explains why humans use sweating to cool down.



[Link: Khan Academy]
 
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