A concentric-pipe parallel-flow heat exchanger is to heat water (c p = 4.18 kJ/kg·K) from 20°C to 80°C at a rate of 1.2 kg/s. The heating is to be accomplished by geothermal water (Cp =4.31 kJ/kg·K) available at 160°C at a mass flow rate of 2.0 kg/s. The inner pipe 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 ·K, determine the length of the pipe required to achieve the desired heating using the ε -NTU method.