The side surfaces of a 3-m-high cubic industrial furnace burning natural gas are not insulated and the temperature at the outer surface of this section is measured to be 110C. The temperature of the furnace room including its surfaces is 30C and the emissivity of the outer surface of the furnace is 0.7. It is proposed that this section of the furnace wall be insulated with glass wool insulation (k = 0.038 W/m.K) wrapped by a reflective sheet ( = 0.2) in order to reduce the heat loss by 90 percent. Assuming the outer surface temperature of the metal section still remains at about 110C determine the thickness of the insulation that needs to be used. The furnace operates continuously throughout the year and has an efficiency of 78 percent. The price of the natural gas is $1.10/therm (1 therm = 105 500 kJ of energy content). If the installation of the insulation will cost $550 for materials and labor determine how long it will take for the insulation to pay for itself from the energy it saves.
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