Q11. An aluminum hot air duct has outer diameter 0.9 m and negligible wall thickness. The air in the duct is at a temperature of 50°C and the surrounding air temperature is 22°C. The inner surface heat transfer coefficient is 20 W/m2K and the outer surface heat transfer coefficient is 15 W/m2K.
It is proposed to lag the duct with rock wool to a thickness of 200 mm. The rock wool is provided in flat sheets 100 mm thick, each square meter having a quoted thermal resistance of 2.25 K/W. When the lagging is applied to the duct, the outer surface heat transfer coefficient may be assumed to be unchanged. Calculate EACH of the following:
(a) The rate of heat loss per meter run without lagging
(b) The rate of heat loss per meter run after the lagging is applied.
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