This page calculates the net radiant heat exchange for two isothermal (constant temperature) parallel flat black disks.

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

q = 56.69x10^{-9} x VF_{(1-2)} x A_{1} x (T_{1}^{4} - T_{2}^{4})

Where 56.69x10^{-9} is the average Stefan-Bolzmann
constant, *A _{1}*
is the area of disk 1,

Where *r _{1}*,

r_{1} = rad_{1} / D

r_{2} = rad_{2} / D

X = 1 + ( (1 + r_{2}^{2}) / r_{1}^{2 })

Where *rad _{1}* is the disk 1 radius,

**References**

Rosenow, W. M., J. P. Hartnett, Y. I. Young, *Handbook of
Heat Transfer*, 3rd Ed., McGraw-Hill Handbooks, New
York, 1998. p. 7.3 - 7.19, 7.82.