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A counterflow, twin-tube heat exchanger is made by brazing two circular nickel tubes, each 70 m long, together as shown below. Hot water flows through the smaller tube of 15mm diameter and air at atmospheric pressure flows through the larger tube of 50mm diameter. Both tubes have a wall thickness of 8 mm. The thermal contact conductance per unit length of the brazed joint is 300 W/m K. The mass flow rates of the water and air are 0.07 and 0.28 kg/s, respectively. The inlet temperatures of the water and air are 125 and 33°C, respectively. Employ the & - NTU method to determine the outlet temperature of the air. Hint: Account for the effects of circumferential conduction in the walls of the tubes by treating them as extended surfaces.