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B How does sweating cool a person down?

  1. Jan 12, 2019 #1
    I have probably known the stock answer to "How does sweating cool a person down" since elementary school. I'm looking for a far deeper answer to the question on this thread. I googled the question, and the first answer google provided is this answer from the website www.explorit.org: "Imagine each of those as a drop of sweat that will evaporate taking a bit of your body heat away. So sweat helps cool you down two ways. First, it makes your skin feel cooler when it's wet. And when it evaporates it removes some heat."

    I actually don't fully understand the first way that explorit mentions how sweating helps cool a person down, but I'm not much interested in it either. I'm interested in exporit's second way that sweating cools a person down.

    I'm going to restate what explorit said without the pronouns: "When sweat evaporates, the evaporation of sweat removes some heat." Before just two days ago, I thought that the evaporation of sweat removes heat only because the sweat is hotter than a person's body. And I thought when the hot sweat evaporates, the hot sweat is removed from the person's body, cooling the person down. Now I'm not so sure that the sweat is hotter than a person's body heat. I don't think it's as simple as I thought before. In my thread, "How does a metering device on an A/C lower the pressure" on the Mechanical Engineering subforum, PF member Russ Waters made a response to me that casts a lot of doubt on my initial conceptions of this issue.

    I'm a student attending a HVAC program at a trade school. My instructor and I were discussing how the metering device on an A/C lowers the pressure in the A/C. On my thread "How does a metering device on an A/C lower the pressure", I mentioned how my instructor gave me an explanation that sounds like it is saying that the sensible heat of a refrigerant is decreased because part of the refrigerant's sensible heat is converted to latent heat. I mentioned that I have never heard of this before my instructor mentioned it. Russ waters replied "Sure you have. Every morning when you get out of the shower and feel cold. Every time you sweat." I know that it takes a relatively large amount of heat to convert water from liquid to vapor without even increasing the temperature at all. I will call this amount of heat necessary to change the state of a liquid to vapor the "Change of State Heat". Russ waters is saying that sweating cools a person because a person's body heat is used to make the change of state heat to convert liquid sweat to vapor when it evaporates away.

    I see two potential causes of how sweating can cool a person down:
    #1 Sweat is hotter than a person's body heat. When a person's sweat evaporates, the evaporation cools a person down because it gets that hot sweat off a person's body
    #2 When sweat evaporates, the human body heat is used to "power" the change of state of sweat from liquid to vapor. The sensible heat of the human body is decreased because the sensible heat of the human body goes to form latent heat in the sweat vapor.

    Is Russ Waters correct that sweating cools a person because the sensible heat in a person's body powers the sweat's turning into vapor?

    Is part of the reason that sweat cools a person down because sweat is hotter than a person's body heat and when a person's sweat evaporates the evaporation removes that hot sweat?
     
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  3. Jan 12, 2019 #2

    Drakkith

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    Yes, Russ is correct that sweating cools a person down because changing water from a liquid to a gas requires energy. This energy comes from the body, so the evaporation of sweat removes energy from the body, cooling it.

    No, that's not part of the reason. Sweat is not usually warmer than the rest of the body. Sweat is in direct contact with the skin, so there can't be much of a temperature difference. As far as I know at least.
     
  4. Jan 12, 2019 #3

    tech99

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    It is interesting that if the ambient temperature is above that of the body, sweating is the only available method of losing heat.
     
  5. Jan 12, 2019 #4

    sophiecentaur

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    This thread is common to the other one, posted by the OP.
    Change of state in involves energy transfer - for puddles or sweaty bodies.
     
  6. Jan 12, 2019 #5
    Sweating works very well. The Badwater 135 is run at temperatures up to 130 deg F. In the sun. https://en.wikipedia.org/wiki/Badwater_Ultramarathon.

    When riding a motorcycle through the desert at temperatures over 110 degrees F, I had to wear a windbreaker to keep from overheating. The goal is to have just enough airflow over the skin to carry away heat by sweating. Increasing the airflow heats you up more than sweat carries the heat away.

    Water will evaporate whenever the vapor pressure of the liquid water is greater than the vapor pressure of the water vapor in the atmosphere. The rate of evaporation is a function of the vapor pressure difference between the liquid water and the atmospheric water vapor pressure, and the air velocity past the liquid water surface.
     
  7. Jan 12, 2019 #6

    russ_watters

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    Let me amplify that a bit: the sweat has to be cooler than body temperature (even if only a little) because if it were warmer than body temperature it would be warming the body.

    Or to reiterate the mechanism: evaporation cools the sweat that remains on your body below ambient and body temperature, cooling the body.
     
  8. Jan 13, 2019 at 1:20 AM #7

    DaveC426913

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    Isn't it a simpler matter to say that
    Evaporation is an endothermic process. It requires energy, and it gets that energy ultimately by stealing heat from the body.
    ?
     
  9. Jan 13, 2019 at 4:50 AM #8

    sophiecentaur

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    How could it be? That would need a refrigeration process (heat pump) in the body and we don't have one. It is produced right at the surface so it will be the same temperature as the sweat glands in the skin. Having an internal temperature higher than it should be will stimulate sweating.
     
  10. Jan 13, 2019 at 6:23 AM #9
    Sweat is cooler than the surface of the skin. I don’t know exactly why it is cooler, but when it contacts the skin, the thermal energy of sweat increases while the surface of your skin’s thermal energy decreases. This is, at its core, due to the kinetic energy of particles vibrating. Basically that’s it. Sometimes it will evaporate if it’s conections are broken and sometimes no, but it will always tend towards a thermal equilibrium.
     
  11. Jan 13, 2019 at 7:47 AM #10

    Bandersnatch

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    I'm not sure it has to be below ambient temperature to cool the body down. As long as the partial pressure of water vapour close to the skin is low enough, the evaporation will proceed, carrying away the thermal energy.
    E.g. an astronaut on Mars exposing some skin would be subject to evaporative cooling. Other examples could be sweating during winter workouts, or evaporation cryostats.
     
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