The inner wall of a furnace is at a temperature of 700°C. The composite wall is made of two substances, 10 and 20 cm thick with thermal conductivities of 0.05 and 0.1 W.m-1.°C- 1 respectively. The ambient air is at 30°C and the heat transfer co-efficient between the outer surface of wall and air is 20 W.m-2.°C-1. The rate of heat loss from the outer surface in W.m-2is__________________?
Correct answer: A. 165.4
- A. 165.4
- B. 167.5
- C. 172.5
- D. 175
Explanation
The total thermal resistance per unit area is 0.10/0.05 + 0.20/0.10 + 1/20 = 4.05 m²·°C/W. Hence heat loss is (700 − 30)/4.05 = 165.4 W/m², giving option a.
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