A dormitory at a large university, built 50 years ago, has exterior walls constr

Question

A dormitory at a large university, built 50 years ago, has exterior walls constructed of Ls 25-mm-thick sheathing with a thermal conductivity of KS 0.1 W/m2 K. To reduce heat losses in the winter, the university decides to encapsulate the entire dormitory by applying a Li =25-mm-thick layer of extruded insulation characterized by KI = 0.029 W/m2 K to the exterior of the original sheathing. The extruded insulation is, in turn, covered with an LG= 5-mm-thick architectural glass with KG= 1.4 W/m2 K. Determine the heat flux through the original and retrofitted walls when the interior and exterior air temperatures are T? i= 22C and T? o= -20C, respectively. The inner and outer convection heat transfer coefficients are hi= 5W/m2K and ho=25 W/m2K, respectively.

Explanation / Answer

Heat Flux = (Ti -To) / (1/hi + 1/ho + K1/L1 + k2/L2 +K3/L3)

                  = (22-(-20)) / ( 1/5 + 1/25 + 0.025/0.1 + 0.025/0.029 + 0.005/1.4 )

                 = 30.98 W/m^2

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