First slide

Heat transfer

Question

In an Air conditioner compartment of a train if a material of thermal conductivity K and thickness 10 cm is required to fulfill the purpose. To do so there is a need of two glass each of thickness 1 cm with air gap of 8 cm between them. If thermal conductivity of glass and air are 1 W/mK and 0.01 W/mK then the value of K is:

Moderate

A

0.0125W / mK
B

0.05W / mK
C

0.255W / mK
D

0.33W / mK

Solution

$T h e r m a l R e s i s \tan c e, R_{t h} = \frac{L}{K A}$
$\frac{L}{K A} = \frac{L_{1}}{K_{1} A} + \frac{L_{2}}{K_{2} A}$
$\begin{array}{l} \Rightarrow \frac{10}{K A} = \frac{1}{(1) A} + \frac{8}{0.01 A} \Rightarrow K = \frac{10}{800 + 1} \\ \Rightarrow K = 0.0125 \end{array}$

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