A flue passing hot exhaust gases has a square cross section, \(400 \mathrm{~mm}\) to a side. The walls are constructed of refractory brick \(200 \mathrm{~mm}\) thick with a thermal conductivity of \(1.5 \mathrm{~W} / \mathrm{m} \cdot \mathrm{K}\). Calculate the rate of heat loss from the flue per unit length when the interior and exterior surfaces are maintained at 350 and \(25^{\circ} \mathrm{C}\), respectively. Use a grid spacing of \(100 \mathrm{~mm}\).