The wall of a drying oven is constructed by sandwiching an insulation material of thermal conductivity k = 0

The wall of a drying oven is constructed by sandwiching an insulation material of thermal conductivity k = 0.05 W/m · K between thin metal sheets. The oven air is at T8,i = 300°C, and the corresponding convection coefficient is hi = 30 W/m2 · K. The inner wall surface absorbs a radiant flux of q rad = 100 W/m2 from hotter objects within the oven. The room air is at T8, 0 = 25°C, and the overall coefficient for convection and radiation from the outer surface is ho = 10 W/m2 · K.

(a) Draw the thermal circuit for the wall and label all temperatures, heat rates, and thermal resistances.
(b) What insulation thickness L is required to maintain the outer wall surface at a safe-to-touch temperature of To = 40°C?