A question of thermodynamics.

[megalomania]

Here is a question that has irritated me for some time now that I forgot about until the beginning of the summer heat.

Consider a fast moving fan blowing air directly on you. This is intended to keep you cool because the fast moving air takes up some of your heat and is rapidly moved away. However, what if the air blowing through the fan is significantly warmer than you? Shouldn’t fast moving air that is hot actually make you warmer, or is the fact the air is moving at such a great speed and making momentary contact enough for it to momentarily get even hotter. Does this not go against the technical rules of thermodynamics, or is this allowed by the fact the air is moving so fast in can’t reach equilibrium with your body?

This problem occurred to me a couple summers ago when I was working at the steel mill. They have these large shop fans on the floor which is fine in some areas, but I wonder how much good they do for the guys around the furnaces where the ambient air is over 130 degrees F in the summer. The heat might actually be greater depending on if a furnace door is open, hot steel is being moved, how the fans may be positioned, and the time of day.

As I sit now at my computer, my fan in the window blowing on me, it looks to be another 90+ degree day. Is it actually counterproductive to have heat so much warmer than me blowing right on me? Would it be better to move the fan on the other side of me and blow the interior room air on me?

The theory would seem to indicate blowing air hotter than you should make you hotter. In my experience I am cooler sitting in front of the fan blowing on high. Of course in the winter when I go over to my parents house, as they have a fireplace that gets used all the time, I like to stand in front of the blower to get warm. It is blowing very hot air, albeit not as fast as a fan on high, but the blower still goes rather fast.

Is there some rule that defines where heat energy is allowed to go against the standard thermodynamics and make a system temporarily not at equilibrium? If so, Is there a limit to how hot the air can be and still take away heat? I would think at some point, if it got hot enough, the hot air blowing on my through my fan would actually make me warmer.

[Charles Owlen Picket]

Interesting.....My take on this is the human cooling system. We sweat and the sweat is body temp. The moving air (of the fan, in our example) drops the level of temp of the sweat droplets as it forms, producing a "feeling" of coolness.

In point of fact it possibly does us no good what so ever as it evaporates the water from the body at a greater rate. There is also the acclimation of the body to conditions where we live. I live and have lived for MANY years in a place (you know) where it typically rivals Death Valley for median temp. I'm used to it; I wear long sleeves to keep the UV from giving me skin problems. - I go to a friend's State for Holiday and it feels COLD to me at 60 F !!! (I'm really NOT kidding.)

So with this example we have two issues, one thermodynamic, the other is the human cooling system. We've all seen picture of peoples who developed in climates of EXTREME humidity.....I've read that the level of sweat glands for indigenous persons who lived in such areas for multiple generations gets less and less....

[Telkor]

Well, the increased air speed will make the sweat vaporize faster, cooling down the skin, while the direct transport of heat from the warm air will not be increased.

However, as the production of sweat is limited, you'll reach a certain temperature where the air stream won't cool you any more, no matter of speed.

The higher the air humidity, the less cooling effect will be achieved.

[sbovisjb1]

I went to the Bushveld for the first time in 10 years. I do not believe how hot it got (my body got unused to the extreme temps.) The farm laborers were used to it and they laughed at me sweating in the shade. They were very kind and got me drinks, etc, but after a few weeks I got used to it and once I left, I wished for the heat and hated cooler temperatures.

[Barnacles]

I think how it cools you has less to do with temperature than it does the density. The way I look at it is that the water that your body excreets is fairly warm, but also 20x denser than air. Water transfers heat much better, this is why 50 degree air does not burn like 50 degree water, so what it is doing is blowing the hot water off of you and replacing it with Air that transfers the heat less effeciently.

[tranquillity]

The way the body cools is the heat of vaporisation of water (sweat) on the skin. It takes energy to turn liquid water into a gas and that energy comes from the body and is removed at the water floats away as steam. Also at a particular temperature the air can only hold so much steam and you will find localised around your body an area of higher humidity than the rest of the air preventing, slightly, further evaporation and cooling. The fan keeps dry air around the body as the steam is carried away faster and thus you can evaporate sweat more quickly.

There would be a point where blowing hotter air onto you from, say, a furnace would do more heating than cooling but it would be above the capacity sweat has to remove heat from the body.