1. Enclosure is adiabatic: Refractory surface  does not allow any heat which means all incident flux is re- radiated or reflected.
2. Surfaces  and  are black.

Heat flows directly from  to  and to  As a result of incident radiation,  is heated and also begins to radiate. Of the total energy leaving from  to  fraction is absorbed by  and is controlled by . is view factor and corresponds to faction of total energy leaving  which is absorbed by surface2. The remaining fraction  is intercepted by surface
After a while surface  is heated up. Of the total energy leaving the surface   is absorbed by surface  and  is intercepted by  and  are related by

(2)

(3)

Direct heat exchange between  and

(4)

Heat exchange between  to  and  to  is

(5)

(6)

Since there is no heat flow to the refractory

(7)