This page calculates the net radiant heat exchange for two isothermal (constant temperature) parallel square flat black plates of different edge lengths.

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

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

Where 56.69x10^{-9} is the average Stefan-Bolzmann
constant, *L _{1}*
is the length of plate 1,

Length of plate 1 divided by the distance between the plates (L_{1}
/ D) must be greater than 0.2 for this calculation.

VF_{(1-2)} =

Where:

Where *X* and *Y* are defined as:

X = (C) x ( 1 +E
)

Y = (C) x ( 1 - E )

C = L_{1} / D

D = L_{2} / L_{1}

Where *L _{1}* is the plate 1 length,

**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, 7.81.