Question

A pipe of 25 mm outer diameter carries steam. The heat transfer coefficient between the cylinder and surroundings is 5 $W/m^{2}K$. It is proposed to reduce the heat loss from the pipe by adding insulation having a thermal conductivity of 0.05 W/mK. Which one of the following statements is TRUE?

• A. The outer radius of the pipe is equal to the critical radius..
• B. The outer radius of the pipe is less than the critical radius .
• C. Adding the insulation will reduce the heat loss.
• D. Adding the insulation will increase the heat.

Answer 1

The correct option is C Adding the insulation will reduce the heat loss.

Given data :
Outer diameter of pipe,
$d_o=25mm$
$\therefore Radius:r_o=\frac{d_o}{2}=\frac{25}{2}=12.5mm$
$h=5W/m^{2}K$
$k=0.05W/mK$
critical radius of insulation,

$r_c=\frac{k}{h}=\frac{0.05}{5}=0.01mm=10mm$
As $r_o>r_c$, then adding the insulation will reduce the heat loss.