A counter-flow double-pipe heat exchanger is to heat water from 20C to 80C at a rate of 1.2 kg/s. The heating is to be accomplished by geothermal water available at 160C 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 C, determine the length of the heat exchanger required to achieve the desired heating. Cold water Hot geothermal water 2 kg/s ,160C 20C 80C 1.2 kg/s D= 1.5 cm