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Questions about insulation and heat transfer

  1. Dec 31, 2009 #1
    Hi. I am not in any way proficient in thermodynamics, but I was thinking about insulation and heat transfer and a few questions occurred to me.

    Since insulation blocks heat transfer, shouldn't a winter coat block heat in a hot climate? If you could only choose between wearing almost nothing and wearing a winter coat, in a hot desert (during the day) which option would heat you up the most? What's the most significant factor determining the person's temperature? Does the heat generated by the body keep building up underneath the insulation?

    If one were to perfectly insulate oneself (so no heat could ever enter or escape), would one's temperature continuously rise? Does the body's heat maintain an equilibrium with it's environment, or does it simply generate heat to counter act the normal environmental conditions it's used to? Would you remain at normal body temperature if you're body was perfectly insulated?

    Who would likely remain cooler, a knight wearing extremely shiny and reflective heavy steel armor in a desert, or a person wearing all black light-weight cloth (that "breaths well") in a desert? (Assume it's a hot desert, and it's during the day.) Which person would have the most important factors for staying cool on their side, if either?

    In general, does an article of clothing that protects you well against one extreme of temperature protect you well against the other too?

    I might write more, but it's late, so I'm leaving it at that for now. Any help is appreciated.
     
  2. jcsd
  3. Dec 31, 2009 #2

    Danger

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    Too many questions... too much alcohol... but I'll give it a shot anyhow.
    As far as I know, a perfectly insulated human body will maintain its normal operating temperature of approximately 37°.
    I would put my money against the knight. Armour doesn't 'breathe'. The knight's own sweat would work against him. His environmental humidity would rise because of that, and the armour itself would act like an oven. His own body heat would be retained, plus whatever solar radiation made its way in. That would put him at serious disadvantage against someone versed in the ways of desert warfare.
     
  4. Dec 31, 2009 #3

    Mapes

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    I'd try to post sober from now on if I were you. :smile: The human body produces waste heat, so the temperature would not be stable. Perfect thermal insulation, even with an air supply, would kill you.

    Even if the temperature were 35°, say, it's still cooler than you. Insulation would only block dissipation of your waste heat (I've seen this estimated on the order of 100W) and exacerbate the situation.

    If I'm facing a fire at >1000°C or touching a hot surface at 200°C, though, I'd appreciate the insulation. That, not desert temperatures, is the other extreme.
     
  5. Dec 31, 2009 #4

    russ_watters

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    Deserts are dry so the primary mode of heat dissipation is evaporation. That's why you wear light clothes or even a long robe - it blocks the sun while letting sweat evaporate.
    your body can successfully maintain temperature in a surprisingly hot climate if the humidity is low enough.
     
  6. Dec 31, 2009 #5
    Insulation doesn't block heat transfer - if something is generating x watts of heat, that heat will travel through whatever is insulating it regardless (at steady state that is). What insulation does do is create a temperature gradient. For a person wearing clothes you have heat generation (by metabolism) and insulation from the clothes, so the more insulation the higher temperature of the person.

    You're basically trading radiation and convection resistance. The shiny armor will be resistive to radiation heat transfer from the sun, but will also be resistive to convection which your body relies on to cool itself. The black clothing will be less resistive to radiation from the sun, but will allow more heat transfer from the body by convection. Without qualitative values I can't really answer which would be better, though the rather obvious "both" would be best - white (reflective) and breathable clothing which would provide lower heat input through radiation and allow your natural cooling system (sweating) to work through convection.
     
  7. Dec 31, 2009 #6

    russ_watters

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    ...and if it is hot enough that you need insulation to keep from getting hotter, then you are on a clock. Firemen can't stay in a fire for very long.
     
  8. Dec 31, 2009 #7
    Yeah, try wearing a zipped-up winter coat while walking along a sunny, 95 degree Florida beach in August.
    Besides the strange looks, you will definitely end up dead or in the ER within an hour or so.
     
  9. Jan 2, 2010 #8
    But at the same time, you might want to wear some mitts when taking a 450 degree glass plate out of the oven...
     
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