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Heated coat

  1. Oct 30, 2011 #1
    I am thinking about doing a coat that could help keeping someone alive when sleeping in the streets in the winter. Where I live, temperatures can easily reach -30 degrees celsius.

    Assuming the coat is well insulated and that the heating element is all over the inside of the jacket, what kind of power (watts) is needed to keep the inside of the coat at 37 degrees celsius for 6 to 8 hours?

    How to even approach this problem to have a rough estimate to know if it is even possible or not?

    Thank you
     
  2. jcsd
  3. Oct 31, 2011 #2
    does it really need to be powered? Seems overly complicated...are you planning to use batteries? or instruct the homeless guy to connect his coat to the lamp post?

    how do they sleep when they climb the Everest? Tent (to eliminate windchill) and a good sleeping bag should be enough...

    my 2 cents.
     
  4. Nov 2, 2011 #3
    A typical person puts out around 100 watts of heat energy. If you insulate that properly, you will do much better than providing a heating element. However, insulating that properly and still allowing them to breathe is the challenge.

    Saying that the coat is "well insulated", and then asking how much heating is needed is like saying that a bucket is "sufficiently not leaky" and then asking how much water is needed to keep it full. When you define "well insulated" (a typical well insulated coat may have a thermal conductivity of 0.05 W/K m) then you will have the answer to your question.

    Heat transfer = thermal conductivity * Area * temperature difference / thickness of insulation.

    Remember that it is unlikely that you will cover the entire person, although that may be an option!
     
  5. Nov 2, 2011 #4

    AlephZero

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    What you need is a "survival bag". Cheap ones are available for less than $10.

    Why do you think the inside temperature needs to be 37C? That's the core body temperature, not the air temperature you need to feel comfortable. In fact it would be much too hot for most people to feel comfortable. You can sleep perfectly well in an air temp of 0C, if you are protected from wind and rain.
     
  6. Nov 3, 2011 #5
    to answer the physics question, estimate the surface area, then estimate the thickness of the coat, estimate the conductivity of your insulator, and use newton's cooling law with a heat transfer convective coefficient of 10 (if there is no wind) and solve the simple 1D conduction problem. Should get you in the ballpark.

    If you want more accurate you need to do FEA but you'll need a computer/software and I dont know if you have the latter.
     
    Last edited: Nov 3, 2011
  7. Nov 3, 2011 #6

    Bobbywhy

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    Fishingspree2:

    Here are three selected paragraphs about “space blankets” from Wikipedia:

    First developed by NASA in 1964 for the US space program,[1] the material consists of a thin sheet of plastic (often PET film) that is coated with a metallic reflecting agent, making it metallized polyethylene terephthalate or MPET, usually gold or silver in color, which reflects up to 97% of radiated heat.[2][3]

    In their principal usage, space blankets are included in many emergency, first aid, and survival kits because they are usually waterproof and windproof. That, along with their low weight and ability to pack into a small space, has made them popular among outdoor enthusiasts and emergency workers. Space blankets are often given to marathoners at the end of races. The material may be used in conjunction with conductive insulation material and may be formed into a bag for use as a bivouac sack (survival bag).

    In addition to the space blanket, the United States military also uses a similar blanket called the "casualty blanket". It uses a thermal reflective layer similar to the space blanket, backed by an olive drab colored reinforcing outer layer. It provides greater durability and warmth than a basic space blanket at the cost of greater bulk and weight. It is also used as a partial liner inside the layers of bivy sacks in very cold weather climates.
    http://en.wikipedia.org/wiki/Space_blanket

    One example of where to find space blankets:
    https://www.amazon.com/s/ref=nb_sb_...pace+blanket+heavy+duty&sprefix=space+blanket

    Your local city government or homeless advocacy groups may help pay the (small) cost. If you can find a way to help our homeless brothers and sisters who are suffering from cold stay warm you will truly be a “Good Samaritan” in the biblical sense. And if you help those people without expecting anything in return you will experience the joy of altruism.
     
  8. Nov 4, 2011 #7
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