A layer of refractory material (k=1.8 W/.m.oC) of 50 mm thickness is located between two steel plates (k = 50.0 W/m.oC) of 6.3 mm of thickness. The faces of the refractory layer adjacent to the plates are roughened so that only 30% of the total area is in contact with the steel. The voids are occupied by air (k=0.015 W/m.oC) and the average roughness thickness is 0.8 mm. Considering that the temperatures of the external surfaces of the steel plate are 477 oC and 91 oC, respectively. Calculate the rate of heat transfer established in the composite wall. NOTE: In roughness, the air is still (only consider the conduction)