In a concentric tube counterflow heat exchanger water flows through the inner tube (Di = 25 mm) at the rate of 12 kg/min while oil flows through the outer annulus (Do = 45 mm) at the rate of 6 kg/min. The inlet temperature of oil and water are 100°C and 30°C respectively. What should be the length (m) of the exchanger for an oil outlet temperature of 60°C? Take Ui =  37.8 W/m2k. Cp(oil) = 2130 J/kg K & (Cp)water = 4178 j/kg k. [answer correct up to one decimal place]