1. A furnace wall consists of 200 mm of refractory reclay brick, 100 mm of kaolin brick, and 6 mm of steel plate. The re side of the refractory is at 1150 DC , and the outside of the steel is at 30 DC , An accurate heat balance over the furnace shows the heat loss from the wall to be 300 W/ m2 . It is known that there may be thin layers of air between the layers of brick and steel. To how many millimeters of kaolin are these air layers equivalent