Example 5.18: Two very large walls are at the constant temperature of 800 °F and 1000 °F.
(a) Assuming they are black bodies, how much heat must be removed from the colder wall to maintain a constant temperature?
(b) If these two walls have emissivities of 0.6 and 0.8, respectively, what is the net heat exchange?
Solution: It is important that we use absolute temperatures.
Assuming that heat transfer areas of both walls are one square foot each.
Temperature of hot surface, T1 = 1000 °F = 1460 °R,
Temperature of cold surface, T2 = 800 °F = 1260 °R
(a) For black bodies, emissivity values are equal to 1.0
Now the radiation heat transfer can be calculated to be
(b) For a case, where bodies have different values of emissivities, ε1 = 0.6, and ε2 = 0.8, Fε can be found as
And the radiation heat transfer in this case is