Q7. A freezer locker has a 200 mm air gap between the inner aluminum lining and a 10 mm thick outer steel shell.
Some of this gap is to be used by coating the inner surface of the steel with a layer of foam insulation which cannot be used below a temperature of 0°C.
The aluminum lining is at a uniform temperature of -20°C and the atmospheric temperature is 35°C.
Calculate EACH of the following per of surface area:
(a) the rate of heat flow without the insulation; (4)
(b) the maximum thickness of the foam insulation; (8)
(c) the percentage reduction in heat flow when the insulation is used. (4)
Note: thermal conductivity of air = 0.026 W/mK.
thermal conductivity of the foam insulation = 0.013 W/mK
thermal conductivity of steel = 52 W/mK.
outer surface heat transfer coefficient = 8 W/ K
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