Question Bank • QB_SUB_DFE_CON_001 NAT • 2 Marks

Section 7: Dairy and Food Engineering

Steady-State Conduction & Shape Factors

Subtopic: Critical radius of insulation for cylindrical pipe

Question Statement:

An electrical steam pipe of outer radius $r_o = 25\text{ mm}$ is covered with insulation of thermal conductivity $k = 0.15\text{ W/(m}\cdot\text{K)}$. If the convective heat transfer coefficient with ambient air is $h = 5.0\text{ W/(m}^2\cdot\text{K)}$, the critical radius of insulation (in mm) is:

Answer Key:

30.0

Step-by-Step Solution & Explanation

Critical radius of insulation for a cylinder: $r_{cr} = \frac{k}{h} = \frac{0.15}{5.0} = 0.03\text{ m} = 30.0\text{ mm}$
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