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Thermal losses through single glazing

  1. Jan 18, 2013 #1
    Hello,
    I have looked through various websites, which show single glazing heat losses (U-factor) to be 5.7 W/m^2.K.
    But when I use thermal conductivity basic formulas, the yielded results are quite different:
    k of glass is 1.05 W/m.K; L -> glass thickness is 4 mm = 0.004 m; so:
    U = k/L = 1.05 / 0.004 = 262.5 W/m^2.K

    I tried transparent vynil sheets 800 microns thick (vynil-crystal, used as wind shields) and it yields better results, at k = 0.19 for PVC, U factor is about 190 W/m^2.K.

    This is for an outside "winter garden", which I "single glazed" with vinyl crystal. Currently I am not able to heat it up above 10 degrees celsius, the roof is polycarbonate multi-wall sheet, supposed U factor given from manufacturers 3.4 W/m^2.K. 50 m^2 of room area.

    So, my main question is, what is the CORRECT heat loss from single glazing? The one from the internet or the one that I have calculated?
     
  2. jcsd
  3. Jan 18, 2013 #2

    AlephZero

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    Your result is wrong (too high) because you ignored the convective heat transfer between the glass and the air, on both sides.

    If the room air temperature is say 20 degrees C and the outside air temperature is 0, the surface temperatures of the glass will be less than 20, and greater than 0. In fact if your number and the website are both "right", the glass surface temperatures are more like 10.2 and 9.8 degrees than 20 and 0.

    I don't know if the web site value is "right". An accurate value would depend on the wind speed outside the window, for example.

    There is some information on convective heat transfoer from a flat plate to air here:
    http://www.thermal-wizard.com/tmwiz/convect/natural/vp-isot/vp-isot.htm
     
  4. Jan 18, 2013 #3
    Well, I've tried to understand what you've said, but I'm a non-physicist... Do I need to get the temperature difference in a different manner? I have filled the calculator you gave me with the following data:

    specific heat: 840 J/kg.K
    thermal expansion: 0.0000059 m/m.K
    conductivity: 1.05 W/m.K
    dynamic viscosity: 10 kg/m.s
    density: 2800 kg/m^3
    emissivity: 0

    width: 1.0
    height: 1.0
    plate temperature: 15
    ambient: 2

    The results yielded 253 W as convective heat transfer, and 19 W/m^2.K for the coefficient;
    I don't know for sure if this is the holy truth, and still need a more clear explanation on how convection can actually LOWER the heat loss.
    Any further explanation I welcome :)
     
  5. Jan 18, 2013 #4

    AlephZero

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    The numbers you supply are supposed to be the properties of air, not of the glass. The defailts are for air at 20 C which is probably near enough.

    Basically you have three thermal resistances in series.

    Convection between the air and glass inside the room
    Conduction through the glass
    Convection between the air and glass outside the room.

    If you have the three "U" values, the overall "U" value is
    $$\frac{1}{\frac{1}{U_1} + \frac{1}{U_2} + \frac{1}{U_3}}$$

    So assuming your 19 W/m^2 K was right (it isn't, because the material propertires are wrong) the overall U value would be
    1 / (1/19 + 1/262.5 + 1/19) = 9.2 W/m^2 K

    Because the glass is a much better conductor (U = 262) than the convection between the glass and the air (U = 19), you can make the approximation that the temperature of both sides of the glass is mid way between the inside and outside air temperatures.

    You will probably find the "U" values for the convection change a lot depending on the temperature difference. To compare with the glazing web site, you need to know what temperature difference they use as a standard value for comparing different types of glazing.
     
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