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This page calculates the net radiant heat exchange for two isothermal (constant temperature) parallel flat black plates of equal size. 

The heat flow (q) from the plate is calculated as:

q = 56.69x10^-9 x VF_(1-2) x L₁ x W  (T₁⁴ - T₂⁴)

Where 56.69x10^-9 is the average Stefan-Bolzmann constant, L₁ is the length of plate 1, W the common width of both plates, T₁ is the plate 1 temperature and T₂ is the plate 2 temperature. The view factor VF_(1-2) (also known as radiation shape factor, angle factor, and configuration factor) is defined as:

Where x, y, and z are defined as:

x = L₁ / W
y = L₂ / W
z = x² + y²

Where L₁ is the plate 1 length, L₂ is the plate 2 length, and W is the width of the plates.

References

Ellison G. N., Thermal Computations for Electronic Equipment, Van Nostrand Reinhold Company, New York, 1984. p. 78 - 80.

Holman, J.P., Heat Transfer, 7th ed., McGraw Hill Book Company, New York, 1990. p. 385 - 405.