A double-pipe parallel-flow heat exchanger is to heat water (Cp = 4180 J/kg ( (C) from 25(C to 60(C at a rate of 0.2 kg/s. The heating is to be accomplished by geothermal water (Cp = 4310 J/kg ( (C) available at 140(C at a mass flow rate of 0.3 kg/s. The inner tube is thin-walled and has a diameter of 0.8 cm. If the overall heat transfer coefficient of the heat exchanger is 550 W/m2 ( (C, determine the length of the heat exchanger required to achieve the desired heating.