Reconsider Prob. 10–157. It is desired to insulate the wall in order to decrease the heat loss by 85 percent. For the same inner surface temperature, determine the thickness of insulation and the outer surface temperature if the wall is insulated with (a) polyurethane foam (k = 0.025 W/m ∙ °C) and (b) glass fiber (k = 0.036 W/m ∙ °C).
Prob. 10-157
A 6-m-wide, 2.8-m-high wall is constructed of one layer of common brick (k = 0.72 W/m ∙ °C) of thickness 20 cm, one inside layer of light-weight plaster (k = 0.36 W/m ∙ °C) of thickness 1 cm, and one outside layer of cement based covering (k = 1.40 W/m ∙ °C) of thickness 2 cm. The inner surface of the wall is maintained at 23°C while the outer surface is exposed to outdoors at 8°C with a combined convection and radiation heat transfer coefficient of 17 W/m2 ∙ °C. Determine the rate of heat transfer through the wall and temperature drops across the plaster, brick, covering, and surface-ambient air.
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