3. A furnace wall consists of 250 mm fire brick, 125 mm insulation brick and 250 mm building brick. The inside wall is at temperature of 600°C and the atmospheric temperature is 20 °C. Calculate the heat loss per m² of wall area and the temperature of the outside wall surface of the furnace. The heat transfer coefficient for the outside surface is 10 W/m²K and the thermal conductivities of the fire brick, insulation brick and the building brick are 1.4, 0.2 and 0.7 W/mK respectively.

Fig: 1