A deep freezer for hazardous chemicals is to be maintained at -20oF to insure a safe environment for the chemicals. The freezer is in a room that is kept at 60oF, and the inner and outer walls of the freezer are made of 26-gauge (0.0184-in) steel. The insulation in the walls of the freezer is 2 inches of unpacked glass wool, and the convective heat-transfer coefficients are: ho = 4 Btu/h ft2 oF and hi = 2 Btu/h ft2 oF.
a) Determine the overall heat transfer coefficient U.
b) What is the heat-transfer rate (W) from the room into the freezer?