Effect of emissivity of surfaces

For most general calculations, it is necessary to take into account the effect of emissivities of the source and sink surfaces. If  and  are not black but gray surfaces with emissivities  and

(13)

Where

(14)

Equation 14 reduces to    when
The following procedure may be adopted to calculate the rate of heat transfer between two surfaces within a furnace:

Determine or estimate the temperatures of surfaces  and  and emissivities  and

Find  which is the fraction of energy from  intercepted by

Calculate  by equation 12 and then determine F by equation 14

Calculate by equation 11