Question

The walls of an industrial freezer are constructed from a composite of three materials, an inner layer of plastic, a layer of insulation material, and an outer layer of stainless steel. Heat transfer occurs into the interior of the freezer from the external atmosphere.

a) Provide a sketch to illustrate the variation in temperature from the interior of the freezer compartment to the outer ambient air. In your sketch clearly identify the individual thermal resistances and identify the mode of heat transfer for each resistance. You may assume there are no contact resistances at the interfaces

b) Determine the rate of heat transfer (per unit area) through the walls of the freezer. State any assumptions that you make.

c) What thickness of insulation material is required such that the rate of heat transfer per unit area is reduced to 18 W/m2 ?

Data :
Temperature of freezer compartment, 253 K Ambient temperature, 27 °C
Internal heat transfer coefficient, 13 W/(m2 K) External heat transfer coefficient, 6 W/(m2 K)

Plastic material: Thickness, 5 mm Thermal conductivity,

Insulation material: Thickness, 100 mm Thermal conductivity,

Stainless steel: Thickness, 1.5 mm Thermal conductivity,

1 W/(m K)

0.07 W/(m K)

16.5 W/(m K)

Answer 1