Water (cp = 4180 j/kg·k) is to be heated by solar-heated hot air (cp = 1010 j/kg·k) in a double-pipe counter-flow heat exchanger. air enters the heat exchanger at 90°c at a rate of 0.3 kg/s, while water enters at 22°c at a rate of 0.1 kg/s. the overall heat transfer coefficient based on the inner side of the tube is given as 80 w/m2·k. the length of the tube is 12 m and the internal diameter of the tube is 1.2 cm.