Subtopic: Calculation of power, torque, fuel consumption, heat load and power losses
Question Statement:
The wall of a cold storage is made up of four layers; concrete, brick, cardboard and paint with respective thickness of 5, 60, 8 and 1 mm, and their corresponding thermal conductivities are 0.8, 0.7, 0.04 and 0.15 W m⁻¹ K⁻¹. The overall resistance of the wall to conduction heat transfer in m² K W⁻¹ is _____.
Official Answer Key:
0.29 to 0.31
Step-by-Step Solution & Explanation
For layers in series, total resistance R = Σ(Lᵢ/kᵢ):
R = 0.005/0.8 + 0.060/0.7 + 0.008/0.04 + 0.001/0.15
= 0.00625 + 0.08571 + 0.20000 + 0.00667 = 0.2986 m²K/W. This lies within the accepted range 0.29–0.31 (the cardboard layer, despite being thin, dominates the total resistance owing to its very low conductivity).
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