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 A1 x (T14 - T24)
Where 56.69x10-9 is the average Stefan-Bolzmann constant, A1 is the area of disk 1, T1 is the disk 1 temperature and T2 is the disk 2 temperature. The view factor VF(1-2) (also known as radiation shape factor, angle factor, and configuration factor) is defined as:
Where r1, r2 and X are defined as:
r1 = rad1 / D
r2 = rad2 / D
X = 1 + ( (1 + r22) / r12 )
Where rad1 is the disk 1 radius, rad2 is the disk 2 radius, and D is the distance between the disks.
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.