Hi

Does anyone know what is the reference doc for the black body temperature we use when a line is sun exposed. It is normally 85-90 deg C, but is it something calculted or there is a ref. for it?

Thanks

I was unable to find out a reference so I'll try to explain bellow.

A temperature of 90 Celsius it’s only a reference figure based on the fact an object (subject only to radiation heat exchange) that is at thermal equilibrium radiates and absorbs thermal energy at the same rate, and thus has a constant temperature.

The flux of solar "shortwave" radiation at the top of the atmosphere (the solar constant) is 1367 W/m^2.

It is interesting that based on this figure and conservation of energy to the solar energy crossing two concentric spheres (one having the radius of the Sun and the other having the radial distance from the edge of the Earth’s atmosphere to the center of the Sun) you can evaluate the emissive power of Sun and the surface temperature of Sun of 5800K (by Stefan-Boltzmann law). It is worth to say Sun is not in such thermal equilibrium i.e Sun does not radiate and absorbs thermal energy at the same rate; Sun produces radiative energy by the nuclear fusion of hydrogen into helium in its core. However, Earth can be considered in radiative equilibrium and using again the Stefan-Boltzmann law one can even evaluate the Earth temperature of 279 K, but here the result is not so accurate due to the shielding effect of the Earth’s atmosphere (not so „transparent” to longer wavelength Earth emission and BTW the correlation wavelength- thermodynamic temperature is done by Wien's law).
Anyway these details are here only to give an idea on how the conservation of energy and Stefan-Boltzmann law are applied in calculations, "calibrating" this radiative model.

A similar calculation can be used for a "blackbody" on the Earth Surface, so your pipe is supposed to be in such eternal radiative thermal equilibrium i.e. radiates and absorbs radiation energy at the same rate. Which one? A radiative thermal flux of 1367 W/m^2 is exaggerated since the measurement on the maximum ground-level flux for continental and maritime locations varied from 1090 to 1130 W/m^2; however clouds act reflecting and absorbing it, so a thermal flux of 1000 W/m^2 is widely considered.

Applying now the Stefan-Boltzmann law:

1000 [W/m^2]= 5.67*10^(-8) [W/m^2/K^4]*T^4

we obtain the result as T= 364.4 [K]= 91.3 [C]

You may now reconsider the assumptions.
Is the pipe in such thermal equilibrium as radiates and absorbs thermal energy at the same rate and nothing else?

My best regards.

Thanks champ!

Visit this website for subject on "radiation heat transfer", http://www.engineeringtoolbox.com/radiation-heat-transfer-d_431.html

Regards.