Surface Heat Transfer Coefficients Equations for Windows

Heat Transfer Engineering
Thermodynamics

Interior and Exterior Surface Heat Transfer Coefficients Equations for Windows

Heat transfer through a window is also affected by the convection and radiation heat transfer coefficients between the glass surfaces and surroundings. The effects of convection and radiation on the inner and outer surfaces of glazings are usually combined into the combined convection and radiation heat transfer coefficients hi and ho, respectively. Under still air conditions, the combined heat transfer coefficient at the inner surface of a vertical window can be determined from:

Heat Coefficient

where Tg = glass temperature in K, Ti = indoor air temperature in K, εg = emissivity of the inner surface of the glass exposed to the room (taken to be 0.84 for uncoated glass), and σ = 5.67 x 108 W/m2 · K4 is the Stefa Boltzmann constant. Here the temperature of the interior surfaces facing the window is assumed to be equal to the indoor air temperature. This assumption is reasonable when the window faces mostly interior walls, but it becomes questionable when the window is exposed to heated or cooled surfaces or to other windows. The commonly used value of hi for peak load calculation is:

hi = 8.29 W/m2 · °C = 1.46 Btu/h · ft2 · °F (winter and summer)

which corresponds to the winter design conditions of Ti = 22°C and Tg = -7°C for uncoated glass with εg = 0.84. But the same value of hi can also be used for summer design conditions as it corresponds to summer conditions of Ti = 24°C and Tg = 32°C. The values of hi for various temperatures and glass emissivities are given in the Table below. The commonly used values of ho for peak load calculations are the same as those used for outer wall surfaces (34.0 W/m2 · °C for winter and 22.7 W/m2 · °C for summer).

Combined convection and radiation heat transfer coefficient hi at the inner surface of a vertical glass under still air conditions (in W/m2 · °C)*

Ti
°C
Ti
°C
Glass emissivity, εg
20
17
2.6
3.5
7.1
20
15
2.9
3.9
7.3
20
10
3.4
4.2
7.7
20
5
3.7
4.5
7.9
20
0
4.0
4.8
8.1
20
-5
4.2
5.0
8.2
20
-10
4.4
5.1
8.3

Notes:

* Multiply by 0.176 to convert to Btu/h · ft2 · °F.

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