3.7 The walls of a refrigerator are typically constructed by sandwiching a layer of insulation between sheet metal panels. Consider a wall made from fiberglass insulation of thermal conductivity k₁ = 0.046 W/m K and thick- ness L₁ = 50 mm and steel panels, each of thermal con- ductivity k = 60 W/m K and thickness L₁ = 3 mm. If the wall separates refrigerated air at T, = 4°C from ambient air at T = 25°C, what is the heat gain per unit surface area? Coefficients associated with natural convection at the inner and outer surfaces may be approximated as h₁ = h = 5 W/m²K.