A counter-flow double-pipe heat exchanger is to heat water from 20°c to 80°c at a rate of 1.2 kg/s. the heating is to be accomplished by geothermal water available at 160°c 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/m2 · °c, determine the length of the heat exchanger required to achieve the desired heating. take the specific heats of water and geothermal fluid to be 4.18 and 4.31 kj/kg · °c, respectively.
