Discussion
A brick wall (k = 0.9 W/mk) of thickness 0.18 m separates the warm air in a room from the cold ambient air, on a particular winter day, the outside air temperature is -5°C and the room needs to be maintained at 27°C.The heat transfer coefficient associated with outside air is 20 W/m2k. Neglecting the corrective resistance of the air inside the room, the heat loss in (W/m2) ,is:
Answer: C
The heat loss is \(128\ \text{W/m}^{2}\).
Neglecting the inside air resistance, the heat passes through the wall and the outside air film in series. Per square metre:
\[ \begin{aligned} R &= \frac{0.18}{0.9} + \frac{1}{20} = 0.20 + 0.05 = 0.25 \\ q &= \frac{27 - (-5)}{0.25} = \frac{32}{0.25} = 128 \end{aligned} \]
Using the wall resistance alone gives \(32/0.2 = 160\), which forgets the outside film. 88 and 110 do not follow from these resistances.
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