3. The walls of an oven are made from steel sheets with insulating
board between them of thermal conductivity 0.18 J m-1 s
-1 °C-1
. If the maximum internal temperature in the oven is 300°C and
the outside surface of the oven wall must not rise above 50°C,
estimate the minimum necessary thickness of in sulation
assuming surface heat transfer coefficients to the air on both
sides of the wall are 15 J m-2 s
-1 °C-1. Assume the room air
temperature outside the oven to be 25°C and that the
insulating effect of the steel sheets can be neglected.