Q7. A cold store has a 150 mm air gap between the inner steel lining and the 8 mm thick outer steel casing. Some of the air is used by coating the steel lining with a layer of insulating foam 75 mm thick, as shown in Fig Q7.
The minimum temperature of the reduced air gap is to be 15℃.
The lining is at a uniform temperature of -25℃ and the ambient air temperature is 35℃, both of which remain constant.
(a) Calculate EACH of the following for 1 m2 of surface area:
(i) The rate of heat flow without the foam insulation; (4)
(ii) The required thermal conductivity of the foam insulation; (5)
(iii) The percentage reduction in heat flow when the foam insulation is used.(2)
(b) Sketch the thermal gradient diagram for the insulated wall showing ALL the temperatures.(5)
Note: Thermal conductivity of air = 0.026 W/mK
Thermal conductivity of steel = 52 W/mK
Outer surface heat transfer coefficient = 10 W/m2 K
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