A double pipe counterflow heat exchanger is to heat water from 20 at 80C at a rate of 1.2 kg/s. The heating is to be carried out with geothermal water that is available at a temperature of 160 degrees C and is characterized by mass flow rate 2 kg/s. The inner tube is characterized by the existence of thin wall and has a diameter of 1.5 cm. Its overall heat transfer coefficient heat exchanger is equal to 640 kW/m2 K. Using her method efficiency, calculate the length of the heat exchanger required for the achieving the desired heating. Estimate the variance in its calculation length if instead of the efficiency method, you use the waist method log temperature difference.