Water (cp = 1.0 Btu/lbm-°F) is to be heated by solar- heated hot air (cp = 0.24 Btu/lbm-°F) in a double-pipe counter- flow heat exchanger. Air enters the heat exchanger at 190°F at a rate of 0.7 lbm/s and leaves at 135°F. Water enters at 70°F at a rate of 0.35 lbm/s. The overall heat transfer coefficient based on the inner side of the tube is given to be 20 Btu/h-ft2-°F. Determine the length of the tube required for a tube internal diameter of 0.5 in.