[quote]vroom wrote:
Mert,
You refuse to cite anything or back up any of your numbers.
Then you criticize me for pointing to actual information, in support of my viewpoint? Outstanding!
I don’t suppose you’d like to back up your latest load? Or do I have to take it on your word because I’m too lazy to get out my calculator and double check everything you simply assume?
[/quote]
First off, how does the specific heat capacity of air have to do with this problem? Metals have very low specific heat capacity but they are excellent conductors of heat, and will transfer heat very quickly. The important fact about air in my proposition is that there is just so much of it passing by a person in a set time frame. Given an approximate cross sectional area of a person as 1 square meter, a insensible 1 kilometer/hour breeze will send 1000 cubic meters, or a billion mls, or a million liters of air past someone per hour. A mole of air is about 28 Liters at typical living conditions, and a mole of air is about 28 grams (based on the molar mass of nitrogen. This means that 1000 kilograms of air would pass by in close proximity to the skin per hour. The Cp of a human is a little less than 1.0 cal/g deg, or around 4 times greater than air, therefore, the air that passes by someone even at undetectibly slow speeds per hour has the total heat capacity to absorb all the heat from 250 kilograms of human material. My only point right now is that its not the Cp, but the conductivity and available volume of a material that allows it to transmit heat. The specs for the air flow in my classroom are that it should completely replace room air 4-6 times/hour. The room is 10 x 7.5 x 2 meters or 1500 cubic meters, or 1.5 million liters x 4-6 x per hour=6-9 million liters=about 6-9 thousand kilograms replaced per hour, and the in vent is smaller than the surface area of a person (about 2 square meters). The air speed in the room due to ventilation is still almost undetectible, unless you stand right under the in or out vent, in which case it is still a fairly calm breeze. In other words, at least 6000 kilograms of air (60 times my bodyweight) passes by in close proximity per hour.
Now for a moment, I want to remind you that I am not out to question whether this much air can effectively cool the body, but whether:
Q: can circulating air supply a significant amount of the heat of vaporization which vaporizes sweat.
Any kinesiology textbook will tell you that evaporation of a kilogram of sweat removes 540-580 calories of heat from the body, and no kinesiology textbook will consider the possibility that sweat can be evaporated by absorbing energy from the air. In other words, the question is very radical in its implications for kinesiology. I will mention, as I am sure I have in other threads before, that I do have a master’s degree in kinesiology, as well as 2 years lab experience. I don’t bring this up to set myself up as an authority on the question posted above, but rather that I know from experience that the measurements of human metabolism present in the science of kinesiology are flawed and simplified. There are often significant variations between calorie expendature measured by gas exchange, and by work and heat producton. I also bring up my experience to help clarify why I sometimes throw out numbers without citations. Many of the values I have put out there are just so commonly used in kinesiology that to me, I forget that not everyone knows them. You also know that I have taught highschool chemistry and physics for 9 years up to the college freshmen level (although my first degree is in biology). I again bring this up only to confirm that this problem is ALWAYS oversimplified in chemistry and physics. It is really a complex engineering problem which depends greatly on the conformation of the system (shapes, temperature gradients etc.) If it sounds like I’m just falsely appealing to authority here, well, its only a false appeal if I’m lying about my credentials. I want to make it clear that I am only appealing to my experience to demonstrate that you will not find a simple straightforward answer to the question. No book, or site is going to say anything other than that evaporation of sweat removes 540-580 calories per kilogram. If the answer to the question is YES, then the evidence will have to be synthesized from a series of facts, it is not just going to appear in black and white. I do not know the answer to the question, and from the beginning I stated that “I think that one would have to conclude that…”
Coming up I will attempt to provide evidence for one or more of the three basic ways I can see to answer the question:
One is to determine if AIR is SIGNIFICANTLY cooled as it passes over water of the same temperature. I will look for support for this, although I think we all know it is cooled somewhat. It is really just a question of whether it is significant compared to the amount of heat absorbed from a person.
Two is to show that evaporation of water from a system in which there is no extraneous heat production (ie calories burned) is significant compared to the amount of sweat produced by a person under exertion.
Three is to show that humans can have much more evaporation of sweat than would possibly be needed to remove all surplus heat.
If you want to call this “talking out of my butt” then go ahead. Again, my critique was that blanket responses like this are not critically evaluated responses. Critical evaluation is about questioning the specific points made, NOT just saying “I evaluated them and found them lacking.”