Thermal role of refractory surface:

Refractory surfaces play an important role in keeping the furnace temperature constant by minimizing the heat loss to the surrounding. Consider a furnace which is at an average temperature  and is enclosed by the refractory. The intensity of radiation striking a refractory surface is
Of this incident radiation, is absorbed and is reflected. Here is reflectivity. Due to incident flux the refractory surface is heated to a temperature T_R and refractory surface will also emit energy at the rate of . At steady state, heat balance is:

[Heat loss/unit area through the furnace wall to the surroundings = rate of absorption of heat from the                                                                furnace ─ Rate of emission of radiation back into the furnace]  

(1)