2. A membrane type electrical heater of 20,000 W/m^2 capacity is sandwiched between an insulation of 25 mm thickness with thermal conductivity of 0.029 W/mK and a metal plate with k = 12.6 W/mK of thickness 15 mm. The convection coefficient is 150 W/(m^(2) k ). The surroundings are at 5°C. Determine the surface temperature of the heater and the flow on either side .
3. The inside of a furnace wall is at 1000°C and with the existing wall of material with thermal conductivity of 1.7 W/mK looses 2.5 kW/m^2 when the outside is exposed to convection to air at 30°C with h = 27 W/(m^(2)K). Determine the wall thickness.