A layer of 5 cm thick insulating brick having conductivity of 1.5 W/m K is placed between two 0.5 cm thick steel plates. The conductivity of mild steel is 50 W/m K. The faces of brick adjacent to the plates are rough having solid-to-solid contact of 30% of the total area. The average height of the asperities is 0.1 cm. If the outer plate surface temperatures are 100°C and 500°C respectively, calculate the rate of heat transfer per unit area. The conductivity of air is 0.02 W/m K.