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Convection, Conduction, Or Radiation. You decide

  1. May 20, 2008 #1
    Hello everyone, new guy here.

    I'd like some other minds on an on-going debate i have with quite a few people.
    The subject is "Radiant Floor Heat". I'm not going to cover any formulas at this point as i need to convey the conceptual side of things first.

    We know that heat is transfered via three mechanisms. Namely conduction, convection and radiation.
    Lets apply this to a cement slab which is insulated in principle from the ground by a reflective bubble insulation. In most applications a hydronic tube is coiled in various geometric patterns around the floor and the cement is poured.

    Case studies have shown that this bubble insulation is crushed by the overwhelming force of the cement which effectively nulls any conduction and convection insulating properties it may have had(mostly conduction). Fundamentally the ground is a rather large heat sink that varies greatly depending on geological conditions such as ground rock and flowing ground water (both of which increase this heat sink capacity) and temperature difference.

    At this point the so called radiant heat system has a great deal of convection loss.

    Now let us consider the heat gain mechanisms.
    Conduction.
    The cement may be in contact with furnature, walls, and equipment, ambient air and people.
    Convection.
    The ambiant air which is heated by conduction creates convection currents that are distributed through the heated space.
    Radiation.
    Once again conduction plays the key roll in heating the cement. As the cement increases in temperature so does the radiation.

    It seems to me that for this heating system to be technically correct it would operate principly from radiant transfer (electro magnetic wave propagation) However we know that there is quite a bit of conduction and convection transfer with this system.

    Suppose we were to construct a small test unit one of which uses a glass vacuum chamber to effectively stop all convection and conduction. Next we test the systems ability to condition a heated space by simply heating it. Without conduction or convection this system would be forced to operate in principle from electro magnetic radiation.

    If a system operates from x % conduction, y % convection and z % radaition is it technically correct to state that it is radiated?

    I tend to think that z must be very near to unity for this statement to hold true.
    It would also be very interesting to find a case study on this matter.

    All thoughts are greatly appreciated.
     
    Last edited: May 20, 2008
  2. jcsd
  3. May 20, 2008 #2

    russ_watters

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    Staff: Mentor

    Your point seems to be that a "radiant" heater isn't really just a radiant heater. That's correct. Heck, one of the biggest benefits is if you install them in a bathroom, your feet won't get cold when you stumble to the bathroom in the morning.

    Regarding the failure of the insulation below, that's a huge problem if it is really as bad as you say.
     
  4. May 21, 2008 #3
    It depends on a lot of things. Emissitivity of the floor, walls and such. reflectivity and in real cases transmission of the incident EM-waves.

    Often in real cases you have both convection and radiation. IT depends on a lot of things, are you building a furnace, or a home? Are you burning fuel in the furnace or heating a material? and so on.

    contact between surfaces, then it is mostly convection. If you place a metalslab in a furnace, then mostly radiation.

    Then you must also go through the geometry, it is very important in radiating heat-problems. If you got a bend/curve in your material that you are heating, then it radiates on itself and sruff like that, these problems can be very very complex.

    I hope this helped.
     
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