Water (c_p = 1.0 Btu/lbm·°F) is to be heated by solar-heated hot air (c_p = 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.