The critical radius of insulation for a spherical shell is (where, K = thermal conductivity of insulating material h0 = heat transfer coefficient at the outer surface) ?
Correct answer: B. 2K/h0
- A. K/h0
- B. 2K/h0
- C. h0/K
- D. h0/2K
Explanation
For a sphere, balancing the insulation's added conduction resistance against the increased outer area gives the critical radius, r_c = 2K/h₀. Hence the factor 2 distinguishes the spherical result from the cylindrical result K/h₀.
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