Good morning.

Let us consider an electric furnace with a parallelepiped chamber of dimensions (H = 1m, l = 1m, L = 4m). The collecting surface is constituted by the load arranged uniformly on the furnace hearth.
The vault of the furnace "hot surface" is at the uniform temperature T2.
The surface S3 consists of 4 refractory side walls, which will be considered T1 = 800K, and T2 = 1200K.

1) Express F13 and F23 as a function of F12.

Calculate F12 using the chart.

2) Deduce the heat flow exchanged between 1 and 2.

And now,the only formula I know for this exercices is
Someone can help me please?