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What is the critical radius of insulation for asbestos (thermal conductivity = 0.17 W.m-1.°C-1) surrounding a circular pipe and exposed to room air at 20°C with heat transfer coefficient 3 W.m-2.°C-1?
1. 7.21 cm
2. 6.37 cm
3. 5.67 cm
4. 6.93 cm
5.

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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:

\({r_{cr,cylinder}} = \frac{k}{h}\)

\({r_{cr,cylinder}} = \frac{0.17}{3}\times 100=5.67\;cm\)

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