Hw2 Solution e4i8

1. The walls of a refrigerator are typically constructed by sandwiching a layer of insulation between sheet metal s. Consider a wall made from fiberglass insulation of thermal conductivity ki = 0.046 W/m K and thickness Li = 50 mm and steel s, each of thermal conductivity kp = 60 W/m K and thickness Lp = 3 mm. If the wall separates refrigerated air at T∞,i = 4ºC from ambient air at T∞,o = 25ºC, what is the heat gain per unit surface area? Coefficients associated with natural convection at the inner and outer surfaces may be approximately as hi = ho = 5 W/m2 K.

2. Two stainless steel plates 10 mm thick are subjected to a pressure of 1 bar under vacuum conditions for which there is an overall temperature drop of 100ºC across the plates. What is the heat flux through the plates? What is the temperature drop across the plane? Use Table 31 and Table A-1 for thermal properties.

3. A stainless steel (AISI 304) tube used to transport a chilled pharmaceutical has an inner diameter of 36 mm and a wall thickness of 2 mm. The pharmaceutical and ambient air are at temperatures of 6ºC and 23ºC, respectively, while the corresponding inner and outer convection coefficients are 400 W/m2 K and 6 W/m2 K, respectively. (1) What is the heat gain per unit tube length? (2) What is the heat gain per unit length if a 10-mm-thick layer of calcium silicate insulation (k = 0.050 W/m K) is applied to the tube?