The walls of a typical industrial furnace used for the heat treating of the metal are usually a composite structure made up of layers of fire brick, high temperature insulation, steel plates, and outer surface cladding. Radiant tubes/beams and/or combustion burners inside the furnace provide the requisite heat. Consider plain surface of such furnace wall where the inner wall surface is heated to 850°C by radiant heating from hot gases at 2000 °C with a heat flux of 45,000W/m2 . The convection heat transfer coefficient of hot gases inside the furnace is estimated to be is 15 W/(m2 K), the thermal conductance per unit area of the composite wall is 250 W/(m2 K), and there is convection at the outer surface of the furnace to the ambient air at T?. Sketch the thermal resistance circuit

Question

for the heat transfer in this system, and calculate the total heat flow per unit area through the composite wall and the exterior wall surface temperature. Also if outside air temperature is T?=30°C, estimate the average convection heat transfer coefficient at the exterior surface.



GIVEN



FIND

(a) Draw the thermal circuit

(b) Rate of heat flow per unit area (q/A)

(c) The exterior surface temperature (Two)

(d) Average convection heat transfer coefficient at exterior surface.

ASSUMPTIONS

- Heat flow through the wall is one dimensional

- Steady state prevails

SKETCH

Answer 1

The thermal circuit for the furnace wall is shown below



The rate of heat flow per unit area through the wall is equal to the rate of convective and radiative heat

flow to the interior wall



We can calculate the outer surface temperature of the wall by examining the conductive heat transfer

through the wall given