A composite wall is to be used to insulate a freezer chamber at - 350C. Two insulating materials are to be used with conductivities of 0.04 W/mK and 0.1 W/mK. If the outside surface temperature of the inner layer (0.04 W/mK) should not go below zero and if the exposed surface temperature should not go below (the expected wet bulb temperature) 220C determine the insulation thicknesses. The heat gain is to be limited to 10 W/m^2. Also estimate the value of convection coefficient at such a situation. Outside is at 250C.

Question

Dwayne Bolton

Engineering

30 December, 02:14

A composite wall is to be used to insulate a freezer chamber at - 350C. Two insulating materials are to be used with conductivities of 0.04 W/mK and 0.1 W/mK. If the outside surface temperature of the inner layer (0.04 W/mK) should not go below zero and if the exposed surface temperature should not go below (the expected wet bulb temperature) 220C determine the insulation thicknesses. The heat gain is to be limited to 10 W/m^2. Also estimate the value of convection coefficient at such a situation. Outside is at 250C.

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Answers (1)

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Petey · Answer 1

thickness1=1.4m

thickness2=2.2m

convection coefficient=0.33W/m^2K

Explanation:

you must use this equation to calculate the thickness:

L=K (T2-T1) / Q

L=thickness

T=temperature

Q=heat

L1=0.04 * (0--350) / 10=1.4m

L2=0.1 (220-0) / 10=2.2m

Then use this equation to calculate the convective coefficient

H=Q / (T2-T1)

H=10 / (250-220) = 0.33W/m^2K