Correct Answer - Option 3 : 5.67 cm
Concept:
The critical radius of insulation for a cylindrical body:
\({r_{cr,cylinder}} = \frac{k}{h}\)
The critical radius of insulation for a spherical shell:
\({r_{cr,sphere}} = \frac{{2k}}{h}\)
Calculation:
Given:
k = 0.17 W.m-1.°C-1, and h = 3 W.m-2.°C-1
The critical radius of insulation for a cylindrical body:
\({r_{cr,cylinder}} = \frac{k}{h}\)
\({r_{cr,cylinder}} = \frac{0.17}{3}\times 100=5.67\;cm\)