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A counter flow double pipe heat exchanger is to heat water from 20 degrees Celsius to 80 degrees Celsius at the rate of 1.2 kg/s. The heating is to be accomplished by geothermal water available at 160 degrees Celsius at a mass flow rate of 2 kg/s. The inner tube is thin-walled and has a diameter of 1.5 cm. If the overall heat transfer coefficient of the heat exchanger is 640 W/m^2K, determine the length of the heat exchanger required to achieve the desired heating.